I'm stumped.
Thread Starter
Intermediate
Joined: Dec 2020
Posts: 44
Likes: 16
I'm stumped.
Hello all, thanks for the help in the past. I can't figure this out and its driving me nuts. 3 things and I'm not sure if they are in cahoots. 1. My idle is too high, around 13k. My idle screw is bottomed out. All 3 carburetors are freshly rebuilt, not my first rodeo on carburetors. I am very thorough. The butterflys are where they should be. No vacuum leaks that I can find with carb/brake cleaner. If i unplug the intake vacuum and plug it with my finger there is no change.
2. My timing is a little erratic I try to set it around 10 btdc, but it bounces all around 10 to about 15 btdc.
3. When I turn the key off the car diesels for a uncomfortable amount of time.
I just replaced a head gasket and adjusted the valve lash to 24 and 28.
The car is all original 1969 L71, 427/435 car with 53k original miles. It has all new plugs and wires. It has the transistorized magnetic distributor.
I'll be glad to check and respond with and ideas or readings.
Thanks, Jason
2. My timing is a little erratic I try to set it around 10 btdc, but it bounces all around 10 to about 15 btdc.
3. When I turn the key off the car diesels for a uncomfortable amount of time.
I just replaced a head gasket and adjusted the valve lash to 24 and 28.
The car is all original 1969 L71, 427/435 car with 53k original miles. It has all new plugs and wires. It has the transistorized magnetic distributor.
I'll be glad to check and respond with and ideas or readings.
Thanks, Jason
Racer
Joined: Dec 2016
Posts: 353
Likes: 161
With erratic timing the first place I would look is at the distributor. As Lars says, 90% of all carburetor problems are timing problems.
Check to see if all components of the distributor are working properly. Are the weights moving? Is there a broken spring in the distributor. Do you have a worn bushing? The list goes on and on with that one piece, but erratic timing, high idle and dieseling would all point me to the distributor.
Check to see if all components of the distributor are working properly. Are the weights moving? Is there a broken spring in the distributor. Do you have a worn bushing? The list goes on and on with that one piece, but erratic timing, high idle and dieseling would all point me to the distributor.
Race Director
Joined: Mar 2017
Posts: 11,960
Likes: 4,516
From: Virginia
Where did you get your carbs? Perhaps your vacuum advance is connected to a ported vacuum source, and that source isn't quite obscured by the throtte plate. Take a look at Lars' breakdown of a commercially built carb to see what he found, and adjusted, to get it to run right.
Otherwise, I think @Devs77 nailed it, timing is the likely problem. Perhaps the advance weight springs are too weak to keep mechanical advance from hitting at idle.
The dieseling problem in my 80 went away with a proper ignition tune. I don't know if the carbon deposits cleaned themselves up, or it was simply due to the overheating caused by retarded ignition timing, but timing (and idle mixture adjustment) fixed it.
Otherwise, I think @Devs77 nailed it, timing is the likely problem. Perhaps the advance weight springs are too weak to keep mechanical advance from hitting at idle.
The dieseling problem in my 80 went away with a proper ignition tune. I don't know if the carbon deposits cleaned themselves up, or it was simply due to the overheating caused by retarded ignition timing, but timing (and idle mixture adjustment) fixed it.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
vacuum leaks. carb cleaner is no longer flammable. but at least it makes clean spots as you spray it around the carbs. starting fluid is better but dangerous. propane is good and fairly safe. where in florida?
Drifting
Joined: Dec 2017
Posts: 1,306
Likes: 226
From: PA
I agree with checking out the distributor first. I'm not an expert at all, but just based on a lot of thread reading over the last few years. I'll throw in also timing chain. Though the miles are low if it's original then it's had a lot of years under tension to stress relax. Simple check as I learned recently, turn the engine by had in one direction watching for rotor motion on the dist. Then reverse turning the engine and see how many degrees of turn it takes for the rotor to move.
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,235
Likes: 9,376
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
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the dieseling is a clue to the timing...I would disconnect the out board carbs to make sure they arent letting any air through. They shouldnt have an idle circuit either. check your choke as well. the high idle cam may be sticking or miss adjusted, giving you more fuel and bringing your idle up which wold take you out of the idle circuit , which then wouldnt be effected by the idle vacuum screw.....
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
The timing is bouncing around due to weak mechanical advance weight springs most likely. Get this sorted with Mr. Gasket springs in the 928G kit....
Once this is sorted.....it is vary likely that your timing is several retarded....casing you to have to crack the throttle blades open so far to keep it running that you are out of the idle circuit completely......
E-Mail Lars and get his timing papers....set the timing for 38 degrees total with a dial back light. Hook your vacuum advance to full manifold vacuum......this will improve the idle and allow you to close the throttle blades even further.......dieseling is caused by throttle blades that are open too far, allowing fuel in the engine to be compression ignited even after the key is shut off. Keeping the center carb blades cracked just slightly will make this go away IF.....the front and rear carb are shut all of the way. The throttle blades on the front and rear carbs need to be closed completely at idle.....this could be a cause of your high idle but I believe you have a few things going on here.......check them all.
What fuel are you running in this? L-71 was 11 to 1 compression......
Jebby
Once this is sorted.....it is vary likely that your timing is several retarded....casing you to have to crack the throttle blades open so far to keep it running that you are out of the idle circuit completely......
E-Mail Lars and get his timing papers....set the timing for 38 degrees total with a dial back light. Hook your vacuum advance to full manifold vacuum......this will improve the idle and allow you to close the throttle blades even further.......dieseling is caused by throttle blades that are open too far, allowing fuel in the engine to be compression ignited even after the key is shut off. Keeping the center carb blades cracked just slightly will make this go away IF.....the front and rear carb are shut all of the way. The throttle blades on the front and rear carbs need to be closed completely at idle.....this could be a cause of your high idle but I believe you have a few things going on here.......check them all.
What fuel are you running in this? L-71 was 11 to 1 compression......
Jebby
Last edited by Jebbysan; May 4, 2021 at 02:35 PM.
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Thread Starter
Intermediate
Joined: Dec 2020
Posts: 44
Likes: 16
Thanks, I'll review Lars post. I rebuilt my own carbs. I had no Idea that carb/break cleaner was no longer flammable. I went in the house last night thinking that I eliminated the possible vacuum leak. Thanks for that info.
Last edited by 4speed69; May 4, 2021 at 05:34 PM.
Thread Starter
Intermediate
Joined: Dec 2020
Posts: 44
Likes: 16
Found the source for the high idle. It was my fast idle cam. As many times as I looked at it, I once again removed the middle carburetor and began staring at it, making sure that the butterflys were closed. When I installed it, I noticed that I had it wrong.
I still have the erratic timing and dieseling. Now on to the distributor and the T.I box.
I still have the erratic timing and dieseling. Now on to the distributor and the T.I box.
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,235
Likes: 9,376
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
getting there....
