Low manifold vacuum?
11-17-2005, 12:53 AM
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Low manifold vacuum?
Hey everyone, it's been a while since I've been here...college has been killing me these days. Anyways, I've been trying to get my 1981 corvette running a little better, but I seem to be having some issues that I hope you guys can help with? First some engine specs...
I had this engine was built about four years ago (and rebuilt about four months ago because of piston ring problems)...the specs are:
383 (stroker), 10.5:1 compression
comp cam XR288HR
Duration Advertised: 288° Intake / 294° Exhaust
Duration @ .050'' Lift: 236° Intake / 242° Exhaust
Valve Lift w/1.5 Rockers: .520'' Intake / .540'' Exhaust
Lobe Separation Angle: 110°
Dart Pro 1 Aluminum Heads
Roller rockers
Keith Black hypereutectic pistons
Edelbrock 750 cfm carb (Performer Series)
Edelbrock Performer Air Gap Intake
MSD unilite electronic dist.
Dynomax headers, straight exhaust to flowmaster
TH-350 tranmission with 3000rpm stall, torque converter
Well, ever since I've had this engine, it has never quite performed like I want. It always seems to have an "off idle stumble." I've had countless mechanics work on it, but every time they adjust something, it just seems to get worse. I think the last guy who worked on it put metering rods and jets that make it much to lean, because it almost always backfires back up through the carb whenever I touch the gas (I'm also at 5,000 ft in Prescott, AZ, so the air density is much lower up here).
So, I've decided to try to fix this thing on my own. I bought a book called "carter carburetor's" where I learned quite a bit about these edelbrock carburetors (which are just like carter AFB's), and I've fiddled around with it quite a bit.
First of all, I put the accelerator pump to it's max setting (so that it will put out the most fuel possible), which seemed to help the stumble situation a bit. Then I bought the idle step up spring set for the carb and tried all the springs. The springs included are: 3" Hg (blue), 4" Hg (yellow), 5" Hg (orange), 7" Hg (pink) and 8" Hg (plain) springs. The pink, 7"Hg spring seem to work the best, but still, the off-idle stumble is there. The lower ones (blue and yellow) are just horrible. The engine died every time I even touch the gas with the blue and yellow ones. A friend of mine couldn't even drive it out of the parking lot, it died so many times.
Well, the book also says that the "idle enrichment circuit" may have something to do with it, so I bought an manifold vacuum gauge and adjusted the idle screws until I get the highest vacuum reading (which for me, is about 7"hg). The book says that I should expect to have 16 to 17" Hg in idle! So it seems that I'm 10"Hg too low at idle? What could explain this? Could this be part of my problem?
I'm going to buy one of the kits that includes a large selection of metering rods and jets and play around with them a bit, but it just seems that my manifold vacuum is way too low? I checked for vacuum leaks, but there are none. When I disconnect anything that is using vacuum pressure (such as the brakes or headlights), and expose the port to the atmosphere, the idle skyrockets and the manifold vacuum goes way up! Should that happen? What's up with this engine? Is the cam just too wild to create the required manifold vacuum at idle? I would appreciate any comments and help I can get!
I had this engine was built about four years ago (and rebuilt about four months ago because of piston ring problems)...the specs are:
383 (stroker), 10.5:1 compression
comp cam XR288HR
Duration Advertised: 288° Intake / 294° Exhaust
Duration @ .050'' Lift: 236° Intake / 242° Exhaust
Valve Lift w/1.5 Rockers: .520'' Intake / .540'' Exhaust
Lobe Separation Angle: 110°
Dart Pro 1 Aluminum Heads
Roller rockers
Keith Black hypereutectic pistons
Edelbrock 750 cfm carb (Performer Series)
Edelbrock Performer Air Gap Intake
MSD unilite electronic dist.
Dynomax headers, straight exhaust to flowmaster
TH-350 tranmission with 3000rpm stall, torque converter
Well, ever since I've had this engine, it has never quite performed like I want. It always seems to have an "off idle stumble." I've had countless mechanics work on it, but every time they adjust something, it just seems to get worse. I think the last guy who worked on it put metering rods and jets that make it much to lean, because it almost always backfires back up through the carb whenever I touch the gas (I'm also at 5,000 ft in Prescott, AZ, so the air density is much lower up here).
So, I've decided to try to fix this thing on my own. I bought a book called "carter carburetor's" where I learned quite a bit about these edelbrock carburetors (which are just like carter AFB's), and I've fiddled around with it quite a bit.
First of all, I put the accelerator pump to it's max setting (so that it will put out the most fuel possible), which seemed to help the stumble situation a bit. Then I bought the idle step up spring set for the carb and tried all the springs. The springs included are: 3" Hg (blue), 4" Hg (yellow), 5" Hg (orange), 7" Hg (pink) and 8" Hg (plain) springs. The pink, 7"Hg spring seem to work the best, but still, the off-idle stumble is there. The lower ones (blue and yellow) are just horrible. The engine died every time I even touch the gas with the blue and yellow ones. A friend of mine couldn't even drive it out of the parking lot, it died so many times.
Well, the book also says that the "idle enrichment circuit" may have something to do with it, so I bought an manifold vacuum gauge and adjusted the idle screws until I get the highest vacuum reading (which for me, is about 7"hg). The book says that I should expect to have 16 to 17" Hg in idle! So it seems that I'm 10"Hg too low at idle? What could explain this? Could this be part of my problem?
I'm going to buy one of the kits that includes a large selection of metering rods and jets and play around with them a bit, but it just seems that my manifold vacuum is way too low? I checked for vacuum leaks, but there are none. When I disconnect anything that is using vacuum pressure (such as the brakes or headlights), and expose the port to the atmosphere, the idle skyrockets and the manifold vacuum goes way up! Should that happen? What's up with this engine? Is the cam just too wild to create the required manifold vacuum at idle? I would appreciate any comments and help I can get!
11-17-2005, 02:53 AM
#3
Drifting
What is the timing set at? You should have at least 12 degrees BTDC. If the timing is retarded it will affect the idle quality and the driveability. Have you done a compression test?
If the idle goes up when you create a vacuum leak it means the engine is starved for air at idle. I don't know if these things have an idle air bypass system but it sounds like that is what it needs. Big camshafts will do this. The fix in the old days was to drill a 1/16" hole in each primary throttle blade to get more air and then to recalibrate the idle restrictions to get the engine to idle. You really need to talk to Lars before you do this however.
That big camshaft will lower the manifold vacuum but it should do more than 7". A stock engine will pull around 17" to 18". With that cam I would think you should be somewhere around 12" to 14". Are you sure the valves are adjusted correctly? Valves that are too tight will give a very low vacuum reading. We had a guy do that already this last summer.
I would start with the timing. You need to get the initial advance up to 12 to 18 degrees BTDC then set the centrifugal advance curve to be all in at about 2800 RPM. Run the vacuum advance off the manifold port. Then start on the carburetor.
Cut and paste all the info in your post into a PM to Lars. He is the brains here on the forums when it comes to carbs and ignitions.
BigBlockk
Later.....
If the idle goes up when you create a vacuum leak it means the engine is starved for air at idle. I don't know if these things have an idle air bypass system but it sounds like that is what it needs. Big camshafts will do this. The fix in the old days was to drill a 1/16" hole in each primary throttle blade to get more air and then to recalibrate the idle restrictions to get the engine to idle. You really need to talk to Lars before you do this however.
That big camshaft will lower the manifold vacuum but it should do more than 7". A stock engine will pull around 17" to 18". With that cam I would think you should be somewhere around 12" to 14". Are you sure the valves are adjusted correctly? Valves that are too tight will give a very low vacuum reading. We had a guy do that already this last summer.
I would start with the timing. You need to get the initial advance up to 12 to 18 degrees BTDC then set the centrifugal advance curve to be all in at about 2800 RPM. Run the vacuum advance off the manifold port. Then start on the carburetor.
Cut and paste all the info in your post into a PM to Lars. He is the brains here on the forums when it comes to carbs and ignitions.
BigBlockk
Later.....
11-17-2005, 01:29 PM
#4
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Well, I'm not entirely sure about the timing. I was told that it's set somewhere around 21 or 22º at base and 36º at 3000rpm? But, I'm not sure if that's what it's actually set at...so I'm thinking about buying a timing light and checking it. I've never messed with the timing, so I'm not entirely sure what I'm doing.
I'm pretty sure I know how to adjust the base timing (just turn the dist. cap), but how is the centrifugal advance curve set?
I think I'll also buy a compressoin checker and see if there are any problems there too.
About the valves, I'm not sure at all. I had them adjusted about two months ago, but I'm not sure if the mechanic even knows what he's doing at this point. I've never adjusted the valves myself before...seems like something I could really screw up if I don't know what I'm doing? I'm pretty sure they are the type that use the "allen hex head" and are just screwed down until zero lash, but other than that, I have no idea on how to adjust them. Any recommendations on how to adjust the valves, maybe something I can read to figure it out?
Also, on the carburetor, I'm quite certain there are idle air bleed ports. I sprayed them all down with carb cleaner a few times, but it didn't affect it any. I think you're right about the engine starving for air...I think we may have pinpointed the problem with this engine. If we can figure out what's causing this low manifold vacuum, maybe it will get this engine running like it should!
I'm pretty sure I know how to adjust the base timing (just turn the dist. cap), but how is the centrifugal advance curve set?
I think I'll also buy a compressoin checker and see if there are any problems there too.
About the valves, I'm not sure at all. I had them adjusted about two months ago, but I'm not sure if the mechanic even knows what he's doing at this point. I've never adjusted the valves myself before...seems like something I could really screw up if I don't know what I'm doing? I'm pretty sure they are the type that use the "allen hex head" and are just screwed down until zero lash, but other than that, I have no idea on how to adjust them. Any recommendations on how to adjust the valves, maybe something I can read to figure it out?
Also, on the carburetor, I'm quite certain there are idle air bleed ports. I sprayed them all down with carb cleaner a few times, but it didn't affect it any. I think you're right about the engine starving for air...I think we may have pinpointed the problem with this engine. If we can figure out what's causing this low manifold vacuum, maybe it will get this engine running like it should!
11-17-2005, 02:16 PM
#5
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Originally Posted by lanceh007
Also, on the carburetor, I'm quite certain there are idle air bleed ports. I sprayed them all down with carb cleaner a few times, but it didn't affect it any. I think you're right about the engine starving for air...I think we may have pinpointed the problem with this engine. If we can figure out what's causing this low manifold vacuum, maybe it will get this engine running like it should!
if you find out what it is..
let me know i am curiouse to find out what your problem was... i think mine has to do with a single plane low vaccume intake and a carb that runs a little too rich... does your exuast have heavy fumes in it??? best of luck to you!
11-17-2005, 11:28 PM
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Well, I think I figured it out! Sure enough, it was the timing. I bought a timing light and it was running at 8º BTDC! The instant I turned the rotor cap, the engine came alive! I can't believe it, all that time, just way off timing! I set the timing to 21º and it performed beautifully. Smooth acceleration, great idle, wow...what a difference. Thanks for your helps guys! You've saved me some major headaches!
11-18-2005, 08:13 AM
#8
Drifting
Originally Posted by lanceh007
Well, I think I figured it out! Sure enough, it was the timing. I bought a timing light and it was running at 8º BTDC! The instant I turned the rotor cap, the engine came alive! I can't believe it, all that time, just way off timing! I set the timing to 21º and it performed beautifully. Smooth acceleration, great idle, wow...what a difference. Thanks for your helps guys! You've saved me some major headaches!
ESU
