[Need Help] Is anyone willing...
Thread Starter
Safety Car
Joined: Apr 2008
Posts: 3,908
Likes: 10
From: Charleston SC
[Need Help] Is anyone willing...
to help me get my carb (and distributor/timing) tuned on my '68?
I've read the manual, tried my best, and still have problems. For instance, after finally getting the idle at a reasonable level, whenever I slightly (or at all) press the gas pedal, the RPMs jump back up to ~2K and stay there..even when driving around. And sometimes if I let it sit there, it will slowly go back down to normal..but one prod of the gas pedal and it jumps back up again.
I also want to make sure that the vacuum advance on the distributor is set right, and that my timing is optimal.
I will drive it to anyone in SC of reasonable distance to get this thing working right..and will pay (beer, money, whatever).
I've read the manual, tried my best, and still have problems. For instance, after finally getting the idle at a reasonable level, whenever I slightly (or at all) press the gas pedal, the RPMs jump back up to ~2K and stay there..even when driving around. And sometimes if I let it sit there, it will slowly go back down to normal..but one prod of the gas pedal and it jumps back up again.
I also want to make sure that the vacuum advance on the distributor is set right, and that my timing is optimal.
I will drive it to anyone in SC of reasonable distance to get this thing working right..and will pay (beer, money, whatever).
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
I'll bet your fast idle is set too high. I'm assuming q-jet. On the passenger's side of the carb toward the bottom, there's a hidden adjustment screw. Turn it counter-clockwise a bit.
Pressing the gas pedal resets the choke and high idle function.
Pressing the gas pedal resets the choke and high idle function.
Drifting
Joined: Mar 2011
Posts: 1,271
Likes: 1
From: PA
That also sounds like it could be weak springs in the distributor not pulling the weights back in. What advance kit did you put in there ? Set the timing with the advance disconnected and plugged. Rev the engine to like 2k with a light on it and see what you get. Plug advance back in and repeat.
Last edited by boltnut; Apr 14, 2011 at 06:08 AM.
Thread Starter
Safety Car
Joined: Apr 2008
Posts: 3,908
Likes: 10
From: Charleston SC
Dohhh, I should have added more info.
It's all aftermarket.. Carb is a Holly Demon with vacuum advance. Distributor is a Summit HEI. Intake is a Weiand Stealth Air Strike. Engine is 327.
But what I really wanted was to know if there was anyone local...a shop, or anything.
It's all aftermarket.. Carb is a Holly Demon with vacuum advance. Distributor is a Summit HEI. Intake is a Weiand Stealth Air Strike. Engine is 327.
But what I really wanted was to know if there was anyone local...a shop, or anything.
Thread Starter
Safety Car
Joined: Apr 2008
Posts: 3,908
Likes: 10
From: Charleston SC
That also sounds like it could be weak springs in the distributor not pulling the weights back in. What advance kit did you put in there ? Set the timing with the advance disconnected and plugged. Rev the engine to like 2k with a light on it and see what you get. Plug advance back in and repeat.
So is it my timing or distributor advance that I need to adjust?
Drifting
Joined: Mar 2011
Posts: 1,271
Likes: 1
From: PA
you set the transfer slots on the primaries correctly ? yes ?
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Thread Starter
Safety Car
Joined: Apr 2008
Posts: 3,908
Likes: 10
From: Charleston SC
like i said it very likely could be a crappy advance kit in the distributor. that does not sound like the vac advance can to me. sounds like the weights are sticking open and not returning to "zero" when you let off the gas. you could try swapping a known good distributor to rule that out.
you set the transfer slots on the primaries correctly ? yes ?
you set the transfer slots on the primaries correctly ? yes ?
And I don't even know what transfer slots are..
Safety Car
Joined: Sep 1999
Posts: 4,930
Likes: 11
From: AnyTown NJ
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
You'll probably get a better response if you posted in the Southeast section of the forum.
http://forums.corvetteforum.com/sout.../?daysprune=-1
http://forums.corvetteforum.com/sout.../?daysprune=-1
Drifting
Joined: Mar 2011
Posts: 1,271
Likes: 1
From: PA
maybe not crappy but suspect and need to be checked.
transfer slot on a 4bbl must be set correctly in relation to the throttle blades or you will have all sorts of idle issues. running rich ?
brand new demon ? there are threads all over the internet about quality
control issues with those out of the box...metal shavings in passages, ect.
transfer slot on a 4bbl must be set correctly in relation to the throttle blades or you will have all sorts of idle issues. running rich ?
brand new demon ? there are threads all over the internet about quality
control issues with those out of the box...metal shavings in passages, ect.
Drifting
Joined: Mar 2011
Posts: 1,271
Likes: 1
From: PA
not trying to be a jerk but the carb and distributor are the "brains" of the N/A engine and many folks often overlook the importance of calibrating the advance and fuel curves after spending big bucks on other parts. this must be done and is hard for the hobbyist to get perfect. lots of variables and all engines respond different. you would be well ahead of the game to have a pro tune the carb and distributor for the rest of your combo. not just a backyard beer session. unless you are interested in learning to read plugs and swap out accel pumps and cams on the carb. fine tuning a car can be really fun and rewarding but usually only after a bunch of frustration is spent first ! maybe try a dyno session ?
Melting Slicks
Joined: Apr 2007
Posts: 3,189
Likes: 111
From: Tolland CT
to help me get my carb (and distributor/timing) tuned on my '68?
I've read the manual, tried my best, and still have problems. For instance, after finally getting the idle at a reasonable level, whenever I slightly (or at all) press the gas pedal, the RPMs jump back up to ~2K and stay there..even when driving around. And sometimes if I let it sit there, it will slowly go back down to normal..but one prod of the gas pedal and it jumps back up again.
I've read the manual, tried my best, and still have problems. For instance, after finally getting the idle at a reasonable level, whenever I slightly (or at all) press the gas pedal, the RPMs jump back up to ~2K and stay there..even when driving around. And sometimes if I let it sit there, it will slowly go back down to normal..but one prod of the gas pedal and it jumps back up again.
RE: Pro tuning.... I'm thinking the same for mine when I get to it. Needn't be dyno tuning but probably farm it out to someone who can look at the whole ignition timing/carburetor systems to get optimal performance. A Corvette shop commented about the advance weight springs in my rebuilt distributor being weaker than stock, advancing fast than they feel it should. They recommended original springs... As another poster stated, many variables to balance.
Drifting
Joined: Mar 2011
Posts: 1,271
Likes: 1
From: PA
there are guys that can tune your distributor for your combo if you send it to them, but you have to know all your engine specs. there are also guys who can tune your carb real close if you ship to them.....not cheap. think like 500.00.
Thread Starter
Safety Car
Joined: Apr 2008
Posts: 3,908
Likes: 10
From: Charleston SC
Today I setup an appointment with a race shop to tune my carb and distributor. Hopefully it doesn't take long/much money?
However, after talking to a few people today all yielded the same diagnosis....vacuum leak.
Checked all my vacuum lines, and re-tightened the valve covers (loose..) and started up the car a few times...no high idle. Interesting. Wait, the other variable that was changed is that I had the air filter mounted this time, when before I had nothing on the carb.
Drifting
Joined: Mar 2011
Posts: 1,271
Likes: 1
From: PA
Usually a vacuum leak is not sensitive to throttle position and the vacuum leak won't vary much due to engine speed. It is either leaking or it isn't. The valve covers and air cleaner should not cause a leak unless maybe the air cleaner is binding up the carb linkage somehow and holding the throttle open ? There are easy ways to check for vacuum leaks, like using propane or carb cleaner and spray small amounts near possible leak sites and listen for rpm changes. Also, check your plugs to see if one cylinder is running lean.
Don't let that shop touch your carb if they have not asked specific info on your engine combo.....like auto vs. Stick, rear gearing, cam duration, cubic inches, head flow, ect. Otherwise they are just stabbing in the dark.
When checking vacuum lines did any look Ill fitting, loose, or cracked ? I could see a cracked vacuum line giving an intermittent leak due to engine flex or other vibration/movement under the hood. Even a small leak in the right place will raise idle speed. Carb bolted down good ? Good gasket under it ? Many demon carbs have an idle bypass air system that is adjustable and is located under the air cleaner mounting stud....FYI...
Don't let that shop touch your carb if they have not asked specific info on your engine combo.....like auto vs. Stick, rear gearing, cam duration, cubic inches, head flow, ect. Otherwise they are just stabbing in the dark.
When checking vacuum lines did any look Ill fitting, loose, or cracked ? I could see a cracked vacuum line giving an intermittent leak due to engine flex or other vibration/movement under the hood. Even a small leak in the right place will raise idle speed. Carb bolted down good ? Good gasket under it ? Many demon carbs have an idle bypass air system that is adjustable and is located under the air cleaner mounting stud....FYI...
Last edited by boltnut; Apr 15, 2011 at 03:00 AM.
Thread Starter
Safety Car
Joined: Apr 2008
Posts: 3,908
Likes: 10
From: Charleston SC
When checking vacuum lines did any look Ill fitting, loose, or cracked ? I could see a cracked vacuum line giving an intermittent leak due to engine flex or other vibration/movement under the hood. Even a small leak in the right place will raise idle speed. Carb bolted down good ? Good gasket under it ? Many demon carbs have an idle bypass air system that is adjustable and is located under the air cleaner mounting stud....FYI...
Carb bolted down good, but dad had reused the old gasket instead of putting on the new one..could be a possibility..?
The idle bypass system is a new one to me...
One more idea...could it be the vacuum canister on the HEI distributor was in the wrong position? It was turned almost all the way in instead of all the way out (counter-clockwise) like it says in the directions..
Drifting
Joined: Mar 2011
Posts: 1,271
Likes: 1
From: PA
http://www.barrygrant.com/fromBarryG...e-Addendum.pdf
No, the vac can will no cause a vacuum leak. But you could have too much advance added with it. Is it hooked to ported or manifold source ?
No, the vac can will no cause a vacuum leak. But you could have too much advance added with it. Is it hooked to ported or manifold source ?
Thread Starter
Safety Car
Joined: Apr 2008
Posts: 3,908
Likes: 10
From: Charleston SC
http://www.barrygrant.com/fromBarryG...e-Addendum.pdf
No, the vac can will no cause a vacuum leak. But you could have too much advance added with it. Is it hooked to ported or manifold source ?
No, the vac can will no cause a vacuum leak. But you could have too much advance added with it. Is it hooked to ported or manifold source ?
Drifting
Joined: Mar 2011
Posts: 1,271
Likes: 1
From: PA
if its hooked into the carb above the throttle blades its also called ported. i havent ever heard of a vac advance can holding the rpms up after hitting the gas. not sure how it could. most of them only add 10-13 degrees of timing but yours could be way off i guess. if its hooked into ported you should really only see its effect at part throttle any way. my bet is still the weights sticking open for some reason in the distributor. do you have a gas grill ? you can hook a length of hose to the tank and sparingly "spray" possible leak areas and listen for rpm changes if you still suspect a vacuum leak.
or
http://www.aa1car.com/library/vacleak.htm
or
http://www.aa1car.com/library/vacleak.htm