Tuning my '79 w/ unknown cam
08-23-2015, 06:13 PM
#1
Burning Brakes
Thread Starter
Tuning my '79 w/ unknown cam
All-
I've owned my '79 for about 2.5 years now and never managed to get it to idle and run as well as I'd like. Here are the facts.
So far I'm pretty happy with the Summit 600. I didn't expect it to cure my tuning issues, but after doing a lot of reading I decided that a vacuum secondary and electric choke is best for me. Summit boasts that it should be ready to go out of the box, but it definitely was not. I changed the power valve from 6.5 to 3.5 before installing it due to my low idle vacuum.
Using my vacuum gauge, I adjusted the idle mixture screws and timing to get the best vacuum. I can get around 7-8hg at around 900rpm with the idle screws 1/4 turn out from bottoming out. I don't know where the timing is set, there is no timing gauge (or whatever its called) at the front of the engine. I have always just adjusted the timing to best vacuum and backed off if the starter complains, today I didn't need to back off at all.
The carb's primary jet is 67, secondary jet is 73. I have a full set of Holley jets and can swap out to anything as needed, but haven't done so yet.
Here are my existing symptoms:
I'm not sure what I should expect out of this engine as far as street manners go. Ideally it would fire up and idle without dying when cold, not idle above 1,000rpm, not smoke me out when idling in the garage, and not bog on me when trying to take off from a stop.
Right now I can't drive it much since the single plane manifold along with the thick carb to manifold gasket that came with the carb to mitigate heat soak, prevent me from closing the hood with the air cleaner installed. I'm not sure yet what to do about that. I'm thinking about cutting the hood and installing a scoop later. I like the idea of pulling in cold air from outside of the engine bay at all times, especially since I live where it regularly gets above 100 degrees outside.
I'm not necessarily opposed to swapping out the super vic single plane intake manifold for a dual plane that sits lower, but I don't think that'll actually cure my issues unless I have a manifold vacuum leak somewhere.
I've owned my '79 for about 2.5 years now and never managed to get it to idle and run as well as I'd like. Here are the facts.
- Car is an original L48 according to the VIN. It has a 4-speed.
- Engine block was cast in 1975. It has flat top pistons with a 4-bolt main.
- The heads don't appear to be GM. I've tried to ID them to no avail. Stamping under the valve cover doesn't help. They are cast iron.
- Someone put in a healthy cam, roller rockers, and stud girdles. I've pulled 7,000 rpm with this beast maybe 7-10 times and it hasn't exploded yet.
- Intake manifold is a super vic.
- I installed a new Holley 650 Double Pumper when I bought it. Just recently swapped it for a Summit 600 which is based on the old Autolite 4100 design. Summit part # sum-rm08600vs
- I just installed an MSD module and coil. I intend to set the rev limiter to 6,500 rpm.
- Idle vacuum has always been somewhere between 5-9hg.
- The car has a Flex-a-lite 398 Syclone 16" fan that advertised 2500cfm @ 17 amps of draw. The original fan and shroud were gone when I bought the car.
- I recently installed Doug's side pipes with the medium noise/flow inserts. I haven't managed to get it to run quite the same ever since.
So far I'm pretty happy with the Summit 600. I didn't expect it to cure my tuning issues, but after doing a lot of reading I decided that a vacuum secondary and electric choke is best for me. Summit boasts that it should be ready to go out of the box, but it definitely was not. I changed the power valve from 6.5 to 3.5 before installing it due to my low idle vacuum.
Using my vacuum gauge, I adjusted the idle mixture screws and timing to get the best vacuum. I can get around 7-8hg at around 900rpm with the idle screws 1/4 turn out from bottoming out. I don't know where the timing is set, there is no timing gauge (or whatever its called) at the front of the engine. I have always just adjusted the timing to best vacuum and backed off if the starter complains, today I didn't need to back off at all.
The carb's primary jet is 67, secondary jet is 73. I have a full set of Holley jets and can swap out to anything as needed, but haven't done so yet.
Here are my existing symptoms:
- If I pump the gas once, it fires up nicely, when hot or cold.
- When cold, immediately after startup, it will die after about 10 seconds unless I nurse the throttle. I haven't tried adjusting the choke. All I have done since installing the carb is adjust the idle speed, idle mixture, and changed the power valve.
- When it warms up, the fan kicks on. This puts a load on the engine similar to putting an automatic car into drive. I adjusted the idle speed to compensate by increasing the idle RPM.
- Although the idle mixture screws are turned in nearly all the way, when warm and the fan turns off, the idle increases and I get a lot of eye-tearing rich exhaust.
- There is a heavy hesitation when accelerating from a stop.\
- Adjusting the idle mixture screws do change the idle speed, when I then compensate with the idle speed screw. No matter what I do, I seem to always get the same vacuum readings. I turned them in as far as they could go w/o sacrificing vacuum in an attempt to get rid of the intermittent black smoke coming from the tailpipe.
I'm not sure what I should expect out of this engine as far as street manners go. Ideally it would fire up and idle without dying when cold, not idle above 1,000rpm, not smoke me out when idling in the garage, and not bog on me when trying to take off from a stop.
Right now I can't drive it much since the single plane manifold along with the thick carb to manifold gasket that came with the carb to mitigate heat soak, prevent me from closing the hood with the air cleaner installed. I'm not sure yet what to do about that. I'm thinking about cutting the hood and installing a scoop later. I like the idea of pulling in cold air from outside of the engine bay at all times, especially since I live where it regularly gets above 100 degrees outside.
I'm not necessarily opposed to swapping out the super vic single plane intake manifold for a dual plane that sits lower, but I don't think that'll actually cure my issues unless I have a manifold vacuum leak somewhere.
Last edited by AboveTheLogic; 08-24-2015 at 10:27 AM.
08-23-2015, 07:00 PM
#2
Burning Brakes
Thread Starter
I found this post on another forum which I found interesting:
Maybe I can try retarding the timing a bit and see if that'll allow me to close down the idle screw? It is worth trying if the next time I'm trying to tune it I get smoked out again.
improper air/fuel ratio, low ignition timing, and low idle rpm all cam lower idle vacuum.
It sounds like low ignition timing.
The engine will need at least 25 degrees of timing at idle with a cam big enough to only produce 7 or 8 inhg.
What happens is low timing makes you turn up the idle screw to get the idle rpm to the right level. And too much idle screw will open the throttle blades past the idle transfer slots which makes the engine run very rich at idle. Not so bad when cold but gets out of hand when the engine warms up.
plus a cam that big will require 1000 rpm idle speed.
You will need to reduce the amount of timing in the distributor advance. Don't run over 40 degrees mechanical advance. 25 initial plus 15 mechanical advance is 40. Add another 10 with the vacuum advance if the engine will make enough vacuum.
It sounds like low ignition timing.
The engine will need at least 25 degrees of timing at idle with a cam big enough to only produce 7 or 8 inhg.
What happens is low timing makes you turn up the idle screw to get the idle rpm to the right level. And too much idle screw will open the throttle blades past the idle transfer slots which makes the engine run very rich at idle. Not so bad when cold but gets out of hand when the engine warms up.
plus a cam that big will require 1000 rpm idle speed.
You will need to reduce the amount of timing in the distributor advance. Don't run over 40 degrees mechanical advance. 25 initial plus 15 mechanical advance is 40. Add another 10 with the vacuum advance if the engine will make enough vacuum.
08-23-2015, 07:04 PM
#3
Melting Slicks
Member Since: Apr 2009
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Okay, for starters you need to:
1. Install a timing tab onto your timing chain cover and use a timing light to set the timing.
2. Open your mixture screws to 1-1/2 turns.
3. Set the initial timing to 10-12 degrees with the vacuum advance disconnected and set your idle speed in neutral to 800 rpm.
4. You'd be much better off using a 180 degree dual plane intake with your fairly mild cam and (probably) high gears. As the dual plane intake manifolds have longer runners they produce better torque way down low where street engines need it the most.
With your timing being set by "ear" its very likely way too far advanced or retarded and the only way you'll ever get the timing right is by using a timing light. It's imperative the engine idles with the idle circuit only so fatten up your mixture screws and drop the idle speed down.
1. Install a timing tab onto your timing chain cover and use a timing light to set the timing.
2. Open your mixture screws to 1-1/2 turns.
3. Set the initial timing to 10-12 degrees with the vacuum advance disconnected and set your idle speed in neutral to 800 rpm.
4. You'd be much better off using a 180 degree dual plane intake with your fairly mild cam and (probably) high gears. As the dual plane intake manifolds have longer runners they produce better torque way down low where street engines need it the most.
With your timing being set by "ear" its very likely way too far advanced or retarded and the only way you'll ever get the timing right is by using a timing light. It's imperative the engine idles with the idle circuit only so fatten up your mixture screws and drop the idle speed down.
08-23-2015, 07:09 PM
#4
Burning Brakes
Thread Starter
Thanks.
It'll idle with the screws 1-1/2 out. I screwed them in to try to get rid of the rich condition when warm, but I'm not sure it helped. I need to read up on exactly what those mixture screws do. To me, right now, they're just magic screws that somehow rich or lean out the engine at idle somehow.
The car does have 3.36 gears and what I think is a close ratio 4-speed (tall 1st gear). A previous owner had installed 225/60/15 tires all around and since putting 275/60/15 tires in the rear I've really felt the desire to set up the engine for low end torque. I'm fairly happy with the torque it has now with the single plane, but I'm sure it could be better.
It'll idle with the screws 1-1/2 out. I screwed them in to try to get rid of the rich condition when warm, but I'm not sure it helped. I need to read up on exactly what those mixture screws do. To me, right now, they're just magic screws that somehow rich or lean out the engine at idle somehow.
The car does have 3.36 gears and what I think is a close ratio 4-speed (tall 1st gear). A previous owner had installed 225/60/15 tires all around and since putting 275/60/15 tires in the rear I've really felt the desire to set up the engine for low end torque. I'm fairly happy with the torque it has now with the single plane, but I'm sure it could be better.
08-23-2015, 07:24 PM
#5
Don't know if its the proper way to do it but this is how I adjust my holley carb. With the car warmed up and idling and parking brake on, turn in the idle mixture screws one at a time til the car starts to stumble. At that point back the screw back out a half turn. Do one side then the other. After that adjust the idle speed with the set screw on the throttle shaft. All this is based on having proper timing though. Also without knowing the cam specs you can have normal vacuum for the cam or you can have a vacuum leak. Just a FYI and not meant to be a cure all or necessarily the proper way.
08-23-2015, 07:26 PM
#6
Le Mans Master
I think I'd pull the carb and see where the throttle plates are in relation to the transition slots in the main body. If the throttle plates are open too far, the carb will be into main metering. If it's into the main metering, you can adjust the idle mixture screws until you wear them out and still be way too rich. 35+ years ago, we used to add a small hole to the throttle plates to "bleed" air into the system. Kind of a known vacuum leak. That let us get the throttle closed a bit and down to the transition slots. Took care of the eye watering idle and the hesitation at the same time.
I put a Demon carb on my 69 with their Idle-Eze system. Same effect, but more adjustable.
I put a Demon carb on my 69 with their Idle-Eze system. Same effect, but more adjustable.
08-23-2015, 07:27 PM
#7
Drifting
You need to know what your actual timing is.period.
Trying to tune an engine with a large cam and a single plane intake is battle of compromise at best.
A dual plane intake will improve your idle characteristics and low speed response.
It kind of sounds like a smaller cam would benefit you all around.
Trying to tune an engine with a large cam and a single plane intake is battle of compromise at best.
A dual plane intake will improve your idle characteristics and low speed response.
It kind of sounds like a smaller cam would benefit you all around.
08-23-2015, 07:32 PM
#8
Burning Brakes
Thread Starter
I've read in the past about single vs dual plane. I just came across this article:
http://www.hotrod.com/events/coverage/0601em-intake/
...which breaks it down even further. I didn't realize until today just how drastically a single plane intake can impact street manners. I took the advice of a buddy back in 2013 who recommended the super vic, before doing any of my own research. Now that I think about it, he ran his super vic and double pumper on a relatively high stall (3,500+) auto where low RPM quite literally didn't matter at all.
I like the idea of pulling the carb to see where the butterflies are now. I'll see if I can get that done later tonight, if not I'll report back later in the week.
I'm fairly convinced that installing a decent dual plane intake is going to make quite a difference. This is a good example of how my double pumper / super vic intake performed with a 3,000 rpm clutch dump. Even on that day, the idle quality was poor and the engine didn't like being under 2,000 rpm:
If I drove it everyday like that, the old setup would have been fine
http://www.hotrod.com/events/coverage/0601em-intake/
...which breaks it down even further. I didn't realize until today just how drastically a single plane intake can impact street manners. I took the advice of a buddy back in 2013 who recommended the super vic, before doing any of my own research. Now that I think about it, he ran his super vic and double pumper on a relatively high stall (3,500+) auto where low RPM quite literally didn't matter at all.
I like the idea of pulling the carb to see where the butterflies are now. I'll see if I can get that done later tonight, if not I'll report back later in the week.
I'm fairly convinced that installing a decent dual plane intake is going to make quite a difference. This is a good example of how my double pumper / super vic intake performed with a 3,000 rpm clutch dump. Even on that day, the idle quality was poor and the engine didn't like being under 2,000 rpm:
If I drove it everyday like that, the old setup would have been fine
Last edited by AboveTheLogic; 08-23-2015 at 07:37 PM.
08-23-2015, 08:23 PM
#10
I think I'd pull the carb and see where the throttle plates are in relation to the transition slots in the main body. If the throttle plates are open too far, the carb will be into main metering. If it's into the main metering, you can adjust the idle mixture screws until you wear them out and still be way too rich. 35+ years ago, we used to add a small hole to the throttle plates to "bleed" air into the system. Kind of a known vacuum leak. That let us get the throttle closed a bit and down to the transition slots. Took care of the eye watering idle and the hesitation at the same time.
I put a Demon carb on my 69 with their Idle-Eze system. Same effect, but more adjustable.
I put a Demon carb on my 69 with their Idle-Eze system. Same effect, but more adjustable.
This. And change the jets to 72 front, 76 back to start. I think your way too fat.
Hi Tim.
08-23-2015, 08:25 PM
#11
Use new Felpro gaskets, and silicone around the water jacket holes, and the China wall. DON'T use the gaskets on the China walls, JUST a fat bead of silicone RTV.
08-23-2015, 10:06 PM
#12
Dr. Detroit
Member Since: Mar 2012
Location: New Braunfels Texas
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You need to know what your actual timing is.period.
Trying to tune an engine with a large cam and a single plane intake is battle of compromise at best.
A dual plane intake will improve your idle characteristics and low speed response.
It kind of sounds like a smaller cam would benefit you all around.
Trying to tune an engine with a large cam and a single plane intake is battle of compromise at best.
A dual plane intake will improve your idle characteristics and low speed response.
It kind of sounds like a smaller cam would benefit you all around.
Check timing.....initial and total.
Post your results.
Jebby
08-24-2015, 12:08 AM
#13
Burning Brakes
Thread Starter
I see the Edelbrock Performer on Summit for $142. I was quick on the draw and picked up the weiand for $80 cash from the next town over and I hope I don't regret the decision.
This engine makes power over 5,500 rpm and I didn't want to choke out the high end with a manifold that isn't made for that range. The $142 Edelbrock has an idle-5,500 rpm range.
The Weiand I just bought has a 1,500-6,700 range, compared to the Super Vic 2925 that's on there now with a 3,500-8000 range.
I think I can put aside a couple of evenings this week to get the Weiand installed and report back how it makes a difference. I'll re-tune it and report back my results.
I don't have a timing light or timing tab yet. I have that on the to-do list.
Thanks again for all the replies, everyone.
This engine makes power over 5,500 rpm and I didn't want to choke out the high end with a manifold that isn't made for that range. The $142 Edelbrock has an idle-5,500 rpm range.
The Weiand I just bought has a 1,500-6,700 range, compared to the Super Vic 2925 that's on there now with a 3,500-8000 range.
I think I can put aside a couple of evenings this week to get the Weiand installed and report back how it makes a difference. I'll re-tune it and report back my results.
I don't have a timing light or timing tab yet. I have that on the to-do list.
Thanks again for all the replies, everyone.
08-24-2015, 09:50 AM
#14
Had a 1976 L-82, 4-sp
Member Since: Mar 2011
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Royal Canadian Navy
You probably have a solid lifter cam if your pulling to 7000 rpm without issue. A dual plane would be counter productive imo. Lots of us are running mech'l double pumpers and SP intakes on the street without issue. Save your money and work with what you have for now and then decide if your not happy with the performance.
08-24-2015, 10:11 AM
#15
Burning Brakes
Thread Starter
I'm not sure that it has solid lifters, I want to say that they are hydraulic or at least some event in the past led me to believe they are hydraulic. I've pulled top RPM recall at 7,200rpm once, and during the video above it pulled 7,000rpm while the wheels were spinning. If the tires have hooked and I'm doing a normal acceleration, I won't bring it past 6,500rpm. It only likes 7k when it gets up there quickly. Under normal hard acceleration, it doesn't like to be above 6,300-6,500. I've wondered if the fuel supply can't keep up unless it hits that high RPM quickly.
I hear you guys loud and clear about the timing thing. I'll get that piece sorted out with a timing tab and go from there.
I hear you guys loud and clear about the timing thing. I'll get that piece sorted out with a timing tab and go from there.
Last edited by AboveTheLogic; 08-24-2015 at 10:14 AM.
08-24-2015, 10:50 AM
#16
Dr. Detroit
Member Since: Mar 2012
Location: New Braunfels Texas
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Get an adjustable timing tab in aluminum....and use a piston stop to determine absolute TDC.
The car revs out well in the video so it can't be too far off.....
Correct timing and curve is the #1 issue with any performance car that was ever brought to me.
Post your results.....it is interesting and informational what people find.
Jebby
The car revs out well in the video so it can't be too far off.....
Correct timing and curve is the #1 issue with any performance car that was ever brought to me.
Post your results.....it is interesting and informational what people find.
Jebby
08-24-2015, 12:44 PM
#17
Burning Brakes
Thread Starter
Ah, never heard of a piston stop! Very cool looking gadget. I'll pick one up.
That video is quite old and I've fudged with a lot of things since then. I just wanted to show what it can do, and how well I got it to run at least once with my limited tuning skills
That video is quite old and I've fudged with a lot of things since then. I just wanted to show what it can do, and how well I got it to run at least once with my limited tuning skills
08-24-2015, 01:06 PM
#18
Race Director
You need a street cam...
Why are you trying to do this, that and everything else to make the car streetable with a high-overlap drag strip cam? The only thing you know about it is the vac it pulls is way too low. If you have an ET bracket on the side window it is one thing. If you actually wanna go places in it, get a streetable cam.
08-24-2015, 01:20 PM
#19
Burning Brakes
Thread Starter
I'm not sure just how radical the cam is, and I'm not sure if the cam is the problem or if it is something else. Its very possible that someone who actually knows what they are doing could come along and make it run nicely, I'm trying my best. I've thought about taking it into a shop to get tuned but I'm determined to be able to do it on my own.
08-25-2015, 06:02 PM
#20
Race Director
trust me. you have a vacuum gauge and know how to use it. it there was a vac leak, you woulda found it by now. that much vac at idle is a 106 to 109 lobe sep cam. full throttle use. not worth a crap below 3500 rpms.