Need Engine Diagnostic Guru Help
05-03-2008, 11:26 PM
#1
Melting Slicks
Thread Starter
Need Engine Diagnostic Guru Help
EDIT: Caution - long old post. See last post for final update.
I would very much appreciate some carburetor, timing and vacuum diagnostic help for the ’73 I recently acquired. While these may all be separate issues, I’m concerned they could be related. Sorry for the length of the post but I want to give all you experts a thorough description of the issues. Yeah, yeah I know – an expert is just a former drip under pressure but I trust y’all anyway.
Here is what I know about the engine (N.O.M.) which was built about 5,000 miles ago:
Block: 1973 350 ci w/ 4-bolt mains
Heads: Camel Hump Heads – 2.02 intake valves.
Exhaust valves may be 1.60 but not sure.
cc is not known at this time.
Hardened valve seats with 3 angle valve grind
Cam: Crane 967601 BluePrint Hydraulic Lifter Camshaft
RPM range 2000 – 5600
Duration @ 0.050” lift: 222/222
Valve lift: 0.447”/0.447”
Replaces GM 350 HP 3863151
Intake: Holley Contender Dominator II Model 300-36
Carb: Holley 650 Double Pumper Model 4777
Choke: Electric
Crank: Forged and balanced, believed to be standard stroke
Timing Chain: Edelbrock Performer
Pistons: TRW L2490F-30 Forged Pistons
Exhaust: Sanderson Cast Headers model QP1000 into 3” exhaust w/ turbo mufflers
Ignition: GM HEI
Fuel Pump: Holley 12-834 – 80 GPH, 7.5 PSI Max
Transmission: TH400
I started down this road because of a rough idle (particularly on cold start) and the belief that the carb was dumping fuel into the intake after shutdown.
I’d better back up a bit and set the stage. I picked up my ’73 about a month ago in Arizona and drove it back to Texas. When I test drove the car, the previous owner was frustrated because he said it had just started to flood and stall at stops especially after a hard run. He called a mechanic and described the problem and was told one likely culprit was a blown power valve. So he bought a Holley kit and rebuilt the carb in his garage before I left town. This did not completely cure the problem but I was able to get back to Texas with it stalling only a couple of times at stops but the idle seemed rough. Other than that it ran very good. Once I got back I discovered the carb was dripping fuel into the intake from the secondary. I was unable to get the fuel level in the secondary to remain constant and ended up replacing the secondary float. This seemed to cure the dripping but I still suspected the carb was dumping fuel after shutdown. When cold starting it would choke properly and start very easily but the idle would be extremely rough and it seemed as if the engine needed to “clear its throat” like maybe it was flooded. A few romps on the go pedal would send a couple of small puffs of white smoke out the exhaust, then the engine would begin to run better and the idle would improve somewhat. I can smell gas after parking the car in the garage. The smell will last a couple of days but goes away if I don’t drive the car for several days. The fuel level in the primary is not remaining constant. It isn’t as bad as the secondary was but I may need to replace the primary float. More often than not the primary fuel level ends up higher than I set it.
Before tackling the carb any further, I decided the first task should be to get the timing set properly since the previous owner had it retarded in order to pass an emissions check. I was able to determine today that the engine was getting maximum advance at about 2100 RPMs. This seemed too early based on the papers I read by Lars and others. I purchased the Mr Gasket spring kit Lars recommended and installed the gold springs as he suggested. With the new springs, maximum advance came in about 2500 RPMs. I then set the timing at 36 degrees at 2500 RPM. So far so good but when I checked the intial advance, it was 22.5 at 700 RPM. This seems too high. I then reconnected the vacuum advance and checked again. I was getting NO vacuum advance at any engine speed. I pulled out my Mityvac hand pump and checked the vacuum advance. It would not let me pull a vacuum on it so I am assuming it is bad. Correct?
When I installed the new springs in the distributor I made note of the numbers on the centrifugal advance if they mean anything to anybody: Center plate – 363, weights – 106, bottom plate – 108.
So, is intial timing of 22.5 at 700 RPM okay? This seems a lot higher than what other folks are getting. Could the timing mark be off? I do plan to replace the vacuum advance unit with one recommended in Lars paper (Echlin VC1838). Any reason I would want to run any other? You may want to read about my vacuum below before answering the timing questions. The vacuum advance now on the car has the numbers 508 and 15 stamped on it.
Now, back to the carb and the fuel delivery. The engine has an inline fuel pressure gauge installed between the pump and the primary feed. Most of the time the gauge reads 8-9 psi but sometimes after idling for awhile it will read about 5 psi. I pulled the line to the secondary and put my Actron vacuum/pressure gauge on it. That gauge read 10 psi at idle. I don’t think I should be getting that kind of pressure out of this fuel pump (see specs above). I assume the fuel pressure is too high but I don’t know if the pump is bad or not. It looks new. Should I replace the pump or put in a regulator? Note that the return line is not connected to anything. If I put in a regulator, should I put in something like a Mallory 4309 which has provisions for a return line? Here’s a link to the Mallory unit: http://store.summitracing.com/partde...5&autoview=sku
If I replace the pump, do any come with provisions for a return line? Do any regulate the pressure? Any other advice? I’m assuming the higher pressure has something to do with the fuel level problem I’m having in the primary but if that’s true, why did replacing the secondary float seem to solve that fuel level issue?
My gut tells me I should have the return line operational. Correct? After setting the timing, I set the carb using my vacuum gauge and the only issue I noted was that my idle drops about 200 RPM when I shift into reverse or drive. That is, my idle was about 700 RPM in park and dropped to 500-550 in reverse or drive. Is this normal? The air/fuel mixtures screws are both set about 1.25 turns out which gave me maximum vacuum at idle.
On to vacuum. After setting the timing, at idle my vacuum gauge reads 10” and is steady. According to information I found on another website, that reading means “Late Valve Timing”. When I blip the throttle the vacuum drops completely to zero and then rebounds to about 20” before returning steady to 10”. The info I found suggests this is due to poor rings but the previous owner did a leak down test while chasing another problem and said the results were excellent (less than 3% leak down). His other problem turned out to be a missing baffle in a valve cover allowing oil through the PCV. Another observation that may be related: I used about ¾ of a quart of oil on the drive back to Texas (750 miles). I read that using oil can also be a symptom of late valve timing. Unfortunately, I don’t know anything about valve timing or how it becomes late or how to fix it. Any advice?
Other observations:
I averaged 14.7 MPG on the trip back to Texas.
Running 91 octane and hearing no pinging.
Oil pressure is never below 35.
Engine temp stays below the 210 mark on the temp gauge, probably no more than 205.
Engine pulls strong and is fun to drive. No problems when standing on the go pedal but I suspect the issues above are causing some performance to be left on the table.
I checked for vacuum leaks using a propane torch and could find none. I retorqued all the carb, intake and exhaust bolts and replaced a few vacuum lines anyway.
I do have one headlight that has a bad actuator rod seal.
Battery is new, charging system is fine.
When I bought the car, I originally planned to replace the engine with a fuel injected engine. However, if I can get this engine tuned and running properly, I might just try to add fuel injection instead.
If you’ve read this far and have any suggestions, I would love to hear them. Thanks!
DC
I would very much appreciate some carburetor, timing and vacuum diagnostic help for the ’73 I recently acquired. While these may all be separate issues, I’m concerned they could be related. Sorry for the length of the post but I want to give all you experts a thorough description of the issues. Yeah, yeah I know – an expert is just a former drip under pressure but I trust y’all anyway.
Here is what I know about the engine (N.O.M.) which was built about 5,000 miles ago:
Block: 1973 350 ci w/ 4-bolt mains
Heads: Camel Hump Heads – 2.02 intake valves.
Exhaust valves may be 1.60 but not sure.
cc is not known at this time.
Hardened valve seats with 3 angle valve grind
Cam: Crane 967601 BluePrint Hydraulic Lifter Camshaft
RPM range 2000 – 5600
Duration @ 0.050” lift: 222/222
Valve lift: 0.447”/0.447”
Replaces GM 350 HP 3863151
Intake: Holley Contender Dominator II Model 300-36
Carb: Holley 650 Double Pumper Model 4777
Choke: Electric
Crank: Forged and balanced, believed to be standard stroke
Timing Chain: Edelbrock Performer
Pistons: TRW L2490F-30 Forged Pistons
Exhaust: Sanderson Cast Headers model QP1000 into 3” exhaust w/ turbo mufflers
Ignition: GM HEI
Fuel Pump: Holley 12-834 – 80 GPH, 7.5 PSI Max
Transmission: TH400
I started down this road because of a rough idle (particularly on cold start) and the belief that the carb was dumping fuel into the intake after shutdown.
I’d better back up a bit and set the stage. I picked up my ’73 about a month ago in Arizona and drove it back to Texas. When I test drove the car, the previous owner was frustrated because he said it had just started to flood and stall at stops especially after a hard run. He called a mechanic and described the problem and was told one likely culprit was a blown power valve. So he bought a Holley kit and rebuilt the carb in his garage before I left town. This did not completely cure the problem but I was able to get back to Texas with it stalling only a couple of times at stops but the idle seemed rough. Other than that it ran very good. Once I got back I discovered the carb was dripping fuel into the intake from the secondary. I was unable to get the fuel level in the secondary to remain constant and ended up replacing the secondary float. This seemed to cure the dripping but I still suspected the carb was dumping fuel after shutdown. When cold starting it would choke properly and start very easily but the idle would be extremely rough and it seemed as if the engine needed to “clear its throat” like maybe it was flooded. A few romps on the go pedal would send a couple of small puffs of white smoke out the exhaust, then the engine would begin to run better and the idle would improve somewhat. I can smell gas after parking the car in the garage. The smell will last a couple of days but goes away if I don’t drive the car for several days. The fuel level in the primary is not remaining constant. It isn’t as bad as the secondary was but I may need to replace the primary float. More often than not the primary fuel level ends up higher than I set it.
Before tackling the carb any further, I decided the first task should be to get the timing set properly since the previous owner had it retarded in order to pass an emissions check. I was able to determine today that the engine was getting maximum advance at about 2100 RPMs. This seemed too early based on the papers I read by Lars and others. I purchased the Mr Gasket spring kit Lars recommended and installed the gold springs as he suggested. With the new springs, maximum advance came in about 2500 RPMs. I then set the timing at 36 degrees at 2500 RPM. So far so good but when I checked the intial advance, it was 22.5 at 700 RPM. This seems too high. I then reconnected the vacuum advance and checked again. I was getting NO vacuum advance at any engine speed. I pulled out my Mityvac hand pump and checked the vacuum advance. It would not let me pull a vacuum on it so I am assuming it is bad. Correct?
When I installed the new springs in the distributor I made note of the numbers on the centrifugal advance if they mean anything to anybody: Center plate – 363, weights – 106, bottom plate – 108.
So, is intial timing of 22.5 at 700 RPM okay? This seems a lot higher than what other folks are getting. Could the timing mark be off? I do plan to replace the vacuum advance unit with one recommended in Lars paper (Echlin VC1838). Any reason I would want to run any other? You may want to read about my vacuum below before answering the timing questions. The vacuum advance now on the car has the numbers 508 and 15 stamped on it.
Now, back to the carb and the fuel delivery. The engine has an inline fuel pressure gauge installed between the pump and the primary feed. Most of the time the gauge reads 8-9 psi but sometimes after idling for awhile it will read about 5 psi. I pulled the line to the secondary and put my Actron vacuum/pressure gauge on it. That gauge read 10 psi at idle. I don’t think I should be getting that kind of pressure out of this fuel pump (see specs above). I assume the fuel pressure is too high but I don’t know if the pump is bad or not. It looks new. Should I replace the pump or put in a regulator? Note that the return line is not connected to anything. If I put in a regulator, should I put in something like a Mallory 4309 which has provisions for a return line? Here’s a link to the Mallory unit: http://store.summitracing.com/partde...5&autoview=sku
If I replace the pump, do any come with provisions for a return line? Do any regulate the pressure? Any other advice? I’m assuming the higher pressure has something to do with the fuel level problem I’m having in the primary but if that’s true, why did replacing the secondary float seem to solve that fuel level issue?
My gut tells me I should have the return line operational. Correct? After setting the timing, I set the carb using my vacuum gauge and the only issue I noted was that my idle drops about 200 RPM when I shift into reverse or drive. That is, my idle was about 700 RPM in park and dropped to 500-550 in reverse or drive. Is this normal? The air/fuel mixtures screws are both set about 1.25 turns out which gave me maximum vacuum at idle.
On to vacuum. After setting the timing, at idle my vacuum gauge reads 10” and is steady. According to information I found on another website, that reading means “Late Valve Timing”. When I blip the throttle the vacuum drops completely to zero and then rebounds to about 20” before returning steady to 10”. The info I found suggests this is due to poor rings but the previous owner did a leak down test while chasing another problem and said the results were excellent (less than 3% leak down). His other problem turned out to be a missing baffle in a valve cover allowing oil through the PCV. Another observation that may be related: I used about ¾ of a quart of oil on the drive back to Texas (750 miles). I read that using oil can also be a symptom of late valve timing. Unfortunately, I don’t know anything about valve timing or how it becomes late or how to fix it. Any advice?
Other observations:
I averaged 14.7 MPG on the trip back to Texas.
Running 91 octane and hearing no pinging.
Oil pressure is never below 35.
Engine temp stays below the 210 mark on the temp gauge, probably no more than 205.
Engine pulls strong and is fun to drive. No problems when standing on the go pedal but I suspect the issues above are causing some performance to be left on the table.
I checked for vacuum leaks using a propane torch and could find none. I retorqued all the carb, intake and exhaust bolts and replaced a few vacuum lines anyway.
I do have one headlight that has a bad actuator rod seal.
Battery is new, charging system is fine.
When I bought the car, I originally planned to replace the engine with a fuel injected engine. However, if I can get this engine tuned and running properly, I might just try to add fuel injection instead.
If you’ve read this far and have any suggestions, I would love to hear them. Thanks!
DC
Last edited by DC3; 08-15-2008 at 11:04 PM. Reason: Add manifold model #
05-04-2008, 12:22 AM
#2
Le Mans Master
Quick observations. The initial problem of rough idle could have been caused by the two problems that you found. The HL vacuum leak and the vacuum advance leak. These are the first thing I always check when a car misses at idle. It also would be more pronounced cold because the 5-8 runner is lean. I would suggest that you remove all vacuum devices and stop up the ports to isolate that.
Your vacuum advance is definitely bad, and with that cam, you need a fast and quick acting advance or the engine will not idle steady and will stumble off idle.
From the description of the fuel level problems and the post nasal drip, I would say the carb needs some professional attention.
Your vacuum advance is definitely bad, and with that cam, you need a fast and quick acting advance or the engine will not idle steady and will stumble off idle.
From the description of the fuel level problems and the post nasal drip, I would say the carb needs some professional attention.
05-04-2008, 12:39 AM
#3
CFOT Attention Whore
i would say also the carb should be a VS. there's no need to have a DP on that heavy a car with that "mild" an engine. either a 650 or 700VS would be better.
otherwise, as said above, plug all vacuum leaks and get the timing set correctly. everything else you do before that is pretty much pzzing in the wind.
otherwise, as said above, plug all vacuum leaks and get the timing set correctly. everything else you do before that is pretty much pzzing in the wind.
05-04-2008, 11:59 AM
#4
Melting Slicks
Thread Starter
Starts @ "Hg = 7-9
Max Adv (Distr. Degrees @ "Hg) = 7 @ 10-12
Would this be a good fit for my engine? Any other recommendations?
05-04-2008, 12:06 PM
#5
Melting Slicks
Thread Starter
i would say also the carb should be a VS. there's no need to have a DP on that heavy a car with that "mild" an engine. either a 650 or 700VS would be better.
otherwise, as said above, plug all vacuum leaks and get the timing set correctly. everything else you do before that is pretty much pzzing in the wind.
otherwise, as said above, plug all vacuum leaks and get the timing set correctly. everything else you do before that is pretty much pzzing in the wind.
When you say "mild", is there anyway to determine an approximate horsepower from the information I've provided on the engine? The previous owner is looking for the paperwork on the engine rebuild which may tell me more but right now the volume of the heads and confirmation of the exhaust valve size is a mystery. He was estimating the horsepower at 325.
At this point I am most concerned with the low vacuum reading at idle and what the vacuum gauge does when I blip the throttle. I haven't been able to locate a vacuum leak but I have not checked the line running from the carb to the transmission.
Thanks for the help.
DC
05-04-2008, 09:59 PM
#6
Melting Slicks
Thread Starter
Update
Today I removed all vacuum lines from the carb and manifold and plugged the ports. Vacuum at idle dropped from 10" to 9" but aside from that there was no other change. I checked again for a vacuum leak and could find none.
While in this configuration I unplugged various vacuum ports to see what would happen. When I unplugged the manifold vacuum port, the vacuum would rise from 9" to 12". When I unplugged the vacuum port at the back of the carb that went to the PCV, the engine would speed up and then immediately die.
Put it all back together and went for a drive. The timing changes I made yesterday were noticeable. The throttle response was better and it just seemed in general to run better. The idle quality was still the same though. Drove for couple of hours this afternoon with the tops off. What a gorgeous day.
I also checked the fuel pressure gauge at various times. I'm now thinking that gauge has issues. It would read anywhere from 3 to 9 psi at idle but the engine never ran any differently at any of the various pressures reported by the gauge.
Thanks for the responses so far. Please keep 'em coming.
DC
While in this configuration I unplugged various vacuum ports to see what would happen. When I unplugged the manifold vacuum port, the vacuum would rise from 9" to 12". When I unplugged the vacuum port at the back of the carb that went to the PCV, the engine would speed up and then immediately die.
Put it all back together and went for a drive. The timing changes I made yesterday were noticeable. The throttle response was better and it just seemed in general to run better. The idle quality was still the same though. Drove for couple of hours this afternoon with the tops off. What a gorgeous day.
I also checked the fuel pressure gauge at various times. I'm now thinking that gauge has issues. It would read anywhere from 3 to 9 psi at idle but the engine never ran any differently at any of the various pressures reported by the gauge.
Thanks for the responses so far. Please keep 'em coming.
DC
05-04-2008, 11:41 PM
#7
Le Mans Master
Today I removed all vacuum lines from the carb and manifold and plugged the ports. Vacuum at idle dropped from 10" to 9" but aside from that there was no other change. I checked again for a vacuum leak and could find none.
While in this configuration I unplugged various vacuum ports to see what would happen. When I unplugged the manifold vacuum port, the vacuum would rise from 9" to 12". When I unplugged the vacuum port at the back of the carb that went to the PCV, the engine would speed up and then immediately die.
Put it all back together and went for a drive. The timing changes I made yesterday were noticeable. The throttle response was better and it just seemed in general to run better. The idle quality was still the same though. Drove for couple of hours this afternoon with the tops off. What a gorgeous day.
I also checked the fuel pressure gauge at various times. I'm now thinking that gauge has issues. It would read anywhere from 3 to 9 psi at idle but the engine never ran any differently at any of the various pressures reported by the gauge.
Thanks for the responses so far. Please keep 'em coming.
DC
While in this configuration I unplugged various vacuum ports to see what would happen. When I unplugged the manifold vacuum port, the vacuum would rise from 9" to 12". When I unplugged the vacuum port at the back of the carb that went to the PCV, the engine would speed up and then immediately die.
Put it all back together and went for a drive. The timing changes I made yesterday were noticeable. The throttle response was better and it just seemed in general to run better. The idle quality was still the same though. Drove for couple of hours this afternoon with the tops off. What a gorgeous day.
I also checked the fuel pressure gauge at various times. I'm now thinking that gauge has issues. It would read anywhere from 3 to 9 psi at idle but the engine never ran any differently at any of the various pressures reported by the gauge.
Thanks for the responses so far. Please keep 'em coming.
DC
Where in the heck are you checking that vacuum? Removing and stopping vacuum ports will not lower vacuum. Unplugging them certianly will not raise the manifold vacuum.
05-05-2008, 10:19 AM
#8
Melting Slicks
Thread Starter
With all hoses connected the first 3 ports read 10" at idle. Port 4 showed zero at idle but did show vacuum once the throttle was applied.
With all hoses removed and all ports plugged, the first 3 ports all read 9" at idle. The 4th was the same as before.
With all hoses removed and all ports plugged, connecting the vacuum gauge to either of the bottom ports on the carb (2 or 3), and then unplugging the manifold port (1), it would increase the reading from 9" to 12".
I did not try connecting the vacuum gauge to the manifold port and then unplugging any of the carb ports to see what would happen.
DC
05-05-2008, 12:24 PM
#9
Drifting
What vacuum advance would you recommend? In Lars' Distributor Vacuum Advance paper, he recommends Echlin VC1838 as "the most versatile and user-friendly unit for a good performance street engine". The specs on this unit are:
Starts @ "Hg = 7-9
Max Adv (Distr. Degrees @ "Hg) = 7 @ 10-12
Would this be a good fit for my engine? Any other recommendations?
Starts @ "Hg = 7-9
Max Adv (Distr. Degrees @ "Hg) = 7 @ 10-12
Would this be a good fit for my engine? Any other recommendations?
05-05-2008, 01:34 PM
#10
Le Mans Master
You have too much fuel pressure for the needle and seat. Put a stock replacement fuel pump on there and forget it. This could be an issue that is giving you fits. No need for a return line as you now have a holley DP. The return line is good for a Qjet but not needed for a Holley.
The vaccum advance can you have is too stiff for a car that only pulls 9-10 inches of vacuum at idle. You want it to be fully advanced at idle vacuum. There is a can that works well with engines using the SHP camshafts and I think it might be the VC1610 if memory serves me right. Verify this at NAPA when you go ask for a can for an early 60's 327/375 SHP Corvette engine. That is the right one to have if you only pull 9" of idle vacuum. It will give you 16 degrees of vacuum advance.
Start there and see how it runs for you.
-Mark.
The vaccum advance can you have is too stiff for a car that only pulls 9-10 inches of vacuum at idle. You want it to be fully advanced at idle vacuum. There is a can that works well with engines using the SHP camshafts and I think it might be the VC1610 if memory serves me right. Verify this at NAPA when you go ask for a can for an early 60's 327/375 SHP Corvette engine. That is the right one to have if you only pull 9" of idle vacuum. It will give you 16 degrees of vacuum advance.
Start there and see how it runs for you.
-Mark.
05-05-2008, 09:00 PM
#11
Melting Slicks
Thread Starter
The vaccum advance can you have is too stiff for a car that only pulls 9-10 inches of vacuum at idle. You want it to be fully advanced at idle vacuum. There is a can that works well with engines using the SHP camshafts and I think it might be the VC1610 if memory serves me right. Verify this at NAPA when you go ask for a can for an early 60's 327/375 SHP Corvette engine. That is the right one to have if you only pull 9" of idle vacuum. It will give you 16 degrees of vacuum advance.
1. I'm assuming VC1610 is for a points distributor but I couldn't find it listed in Lars paper. I did find a VC1810 listed for midyear high performance 327s. The specs are
Starts @ "Hg = 3-5
Max Adv (Distr. Degrees @ "Hg) = 8 @ 5.75-8
Would this be the one? Note that my distributor is HEI. Shouldn't I be looking for an HEI can instead with specs that match the 1810?
2. Should I be more concerned about the low vacuum at idle before deciding on a new can? All of the vacuum diagnostic information I've found says a low reading around 10" is a symptom of late valve timing. The only thing I been able to find related to late valve timing mentioned a slack timing chain or a cam that wasn't dialed in correctly. Is it possible that my engine is fine with the low vacuum reading or should I be chasing "late valve timing" before moving on? What else could cause this?
Thanks Mark.
Whew, I'm gonna owe everyone a beer when this all over.
Well, maybe not everyone, but at least those who helped.
DC
05-05-2008, 10:01 PM
#12
Le Mans Master
This makes sense and I was thinking along these lines myself. I now remember the previous owner telling me at one time he thought he had a fuel starvation problem but it turned out to be a bad coil. I'll bet he put on this fuel pump to try and cure that problem and it must be putting out more than than the stock pump. The existing Holley pump specs are 80 GPH and 7.5 psi max. Anyone know the specs for a stock replacement pump? And wouldn't the stock replacement pump have a return line?
Two things:
1. I'm assuming VC1610 is for a points distributor but I couldn't find it listed in Lars paper. I did find a VC1810 listed for midyear high performance 327s. The specs are
Starts @ "Hg = 3-5
Max Adv (Distr. Degrees @ "Hg) = 8 @ 5.75-8
Would this be the one? Note that my distributor is HEI. Shouldn't I be looking for an HEI can instead with specs that match the 1810?
2. Should I be more concerned about the low vacuum at idle before deciding on a new can? All of the vacuum diagnostic information I've found says a low reading around 10" is a symptom of late valve timing. The only thing I been able to find related to late valve timing mentioned a slack timing chain or a cam that wasn't dialed in correctly. Is it possible that my engine is fine with the low vacuum reading or should I be chasing "late valve timing" before moving on? What else could cause this?
Thanks Mark.
Whew, I'm gonna owe everyone a beer when this all over.
Well, maybe not everyone, but at least those who helped.
DC
Two things:
1. I'm assuming VC1610 is for a points distributor but I couldn't find it listed in Lars paper. I did find a VC1810 listed for midyear high performance 327s. The specs are
Starts @ "Hg = 3-5
Max Adv (Distr. Degrees @ "Hg) = 8 @ 5.75-8
Would this be the one? Note that my distributor is HEI. Shouldn't I be looking for an HEI can instead with specs that match the 1810?
2. Should I be more concerned about the low vacuum at idle before deciding on a new can? All of the vacuum diagnostic information I've found says a low reading around 10" is a symptom of late valve timing. The only thing I been able to find related to late valve timing mentioned a slack timing chain or a cam that wasn't dialed in correctly. Is it possible that my engine is fine with the low vacuum reading or should I be chasing "late valve timing" before moving on? What else could cause this?
Thanks Mark.
Whew, I'm gonna owe everyone a beer when this all over.
Well, maybe not everyone, but at least those who helped.
DC
HEI? - My bad - nevermind about the VC1810 NAPA vacuum advance can as it is for the points style distributors only. Just go get the Crane adjustable vacuum advance kit for the HEI and use that.
There is an advance limiting plate feature in the Crane kit that you can use to limit the vacuum advance to around 12-16 degrees. You will also want to adjust the spring pressure in the can so it will be fully advanced at about 8" of vacuum. This will help the idle so it will be firm and steady at your idle vacuum of 10 inches or so. This is ideal so shoot for that.
I would start with this stuff and go from there. You may have other issues but these are the issues we can identify for now.
-Mark.
05-06-2008, 05:31 PM
#13
Drifting
Just don't throw away any of the GM HEI stuff you take out of your dist. When you're all done swapping out vacuum cans and installing advance stops you'll find that the stock unit will most likely give the best performance set up the way it came from the factory. One thing you might benefit from is changing the springs on the advance to alter the advance curve to suit your needs better but the rest of the unit performs very well the way it is.
05-06-2008, 07:44 PM
#14
Melting Slicks
Thread Starter
So, is 10" of vacuum at idle okay then or do I need to consider fixing it?
Also, my current timing is all in at about 2500 rpm so I set it to 36 degrees at that rpm. That makes my initial timing around 22 degrees at 700 rpm. Initial seems high. Is it okay?
DC
05-06-2008, 07:56 PM
#15
Melting Slicks
Thread Starter
Would you happen to know how to tell if the weights in the distributor are stock? Here are the numbers I found stamped on the various components of the centrifugal advance: Center plate – 363, weights – 106, bottom plate – 108
Got any thoughts about the low vacuum at idle and whether or not I need to address it?
DC
05-07-2008, 10:55 AM
#16
Drifting
Thanks for all of your advice as well.
Would you happen to know how to tell if the weights in the distributor are stock? Here are the numbers I found stamped on the various components of the centrifugal advance: Center plate – 363, weights – 106, bottom plate – 108
Got any thoughts about the low vacuum at idle and whether or not I need to address it?
DC
Would you happen to know how to tell if the weights in the distributor are stock? Here are the numbers I found stamped on the various components of the centrifugal advance: Center plate – 363, weights – 106, bottom plate – 108
Got any thoughts about the low vacuum at idle and whether or not I need to address it?
DC
05-07-2008, 09:41 PM
#17
Drifting
05-07-2008, 10:17 PM
#18
Melting Slicks
Thread Starter
DC
05-07-2008, 10:37 PM
#19
Race Director
The only problem that you may have with the 22 initial is starting problems. If you don't have starting problems, then don't worry about it.
If you do, if so inclined, you can use a different GM hei module that allows you to to retard the timing electronically by 10° just for starting.
Check your timing at a much higher rpm than 2500. Sometimes the hei units will jump much higher than 36 at a much higher rpm.
Also make sure your vacuum advance doesn't take you over around 50 at cruise.
GM had several hundred different plate/weight combinations, so if your curve is right, don't sweat it.
If you do, if so inclined, you can use a different GM hei module that allows you to to retard the timing electronically by 10° just for starting.
Check your timing at a much higher rpm than 2500. Sometimes the hei units will jump much higher than 36 at a much higher rpm.
Also make sure your vacuum advance doesn't take you over around 50 at cruise.
GM had several hundred different plate/weight combinations, so if your curve is right, don't sweat it.
05-08-2008, 02:38 PM
#20
Melting Slicks
Thread Starter
The only problem that you may have with the 22 initial is starting problems. If you don't have starting problems, then don't worry about it.
If you do, if so inclined, you can use a different GM hei module that allows you to to retard the timing electronically by 10° just for starting.
Check your timing at a much higher rpm than 2500. Sometimes the hei units will jump much higher than 36 at a much higher rpm.
Also make sure your vacuum advance doesn't take you over around 50 at cruise.
GM had several hundred different plate/weight combinations, so if your curve is right, don't sweat it.
If you do, if so inclined, you can use a different GM hei module that allows you to to retard the timing electronically by 10° just for starting.
Check your timing at a much higher rpm than 2500. Sometimes the hei units will jump much higher than 36 at a much higher rpm.
Also make sure your vacuum advance doesn't take you over around 50 at cruise.
GM had several hundred different plate/weight combinations, so if your curve is right, don't sweat it.
When I get a working vacuum advance, I'll also double check the timing at various rpms to see what impact the new can has.
Got any thoughts about the low vacuum at idle? Even with all vacuum hoses disconnected and sources plugged, it reads about 10". Could this cam be dialed in correctly and the low vacuum reading is normal for it?
Thanks,
DC