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won’t idle/ running lean.

Posted By: 8T7AERO

won’t idle/ running lean. - 11/12/21 08:36 PM

Hi I was out driving my car and made a few quick stops last weekend, after the last one I started the car and she wouldn’t idle, had to keep my foot on the gas to keep from stalling and it also started to miss when driving back home. Got home and let it sit for awhile, started it back up ( which it’s start right up) Checked for vacuum leaks, loose spark plug wire etc, but it wouldn’t idle below 1500 rpm or It would stall. I then noticed the driver side headers were glowing bright red !!!! Turned it off at that point, the temp never went over 180 and the fans were coming on also. Today I started it up and idled fine till the motor warmed up and then she wouldn’t idle again and the headers were starting to glow red again which I think means a lean condition. The carb Is a Holley 750 ultra pumper. Also running 101 unleaded gas.
Posted By: Buick Runner

Re: won’t idle/ running lean. - 11/12/21 09:58 PM

Vacuum leaks, probably gets worse as the parts expand from heat. Probably a vacuum leak on the driver's side of the intake manifold.
Posted By: 8T7AERO

Re: won’t idle/ running lean. - 11/21/21 07:50 PM

Hi Buick Runner, I tried to check for vacuum leaks using the carb cleaner way, with a cold and warm engine, sprayed around the heads/intake area, base of carb and vacuums lines which I only have 2 the PVC and the fuel vapor canister hooked up. No increase in RPM’s when i did this method, but i did see the passenger headers glowing RED also and it made a loud backfire out the exhaust this time, do u think the idle circuit in the carb could me blocked? or something else with the carb?
Posted By: Buick Runner

Re: won’t idle/ running lean. - 11/21/21 08:27 PM

I don't know much about Holleys. I am mostly familar with the stock Qjet. It stills sounds like a bad vacuum leak, try using an unlit propane torch if you can. Are you sure that you are using a vacuum advance dist with your non stock carb? The stock dist is computer controlled and will only work with the stock computer controlled carburetor. Also check your ignition timing as retarded timing can also make an engine run hot.
Posted By: 8T7AERO

Re: won’t idle/ running lean. - 11/21/21 09:17 PM

I’m using MDS ignition, 6AL box, coil, and non-vacuum MDS dist. p/n 85551. I’ll check the timing, Dist. isn’t loose i checked it and I’ve been driving it for a year with no issues until a few weeks ago. Thanks
Posted By: Chargedbill

Re: won’t idle/ running lean. - 11/28/21 05:47 AM

From my experience with Holley carbs if the engine backfires out the exhaust it generally means a rich condition- if it backfires through the intake its running lean. This assumes your problem is carb related.
Posted By: BadSS

Re: won’t idle/ running lean. - 11/29/21 09:44 PM

I’m a little late to the party but the vast majority of the time when headers are glowing red it’s due to retarded timing and the unburnt fuel is igniting in the headers. It would have to have a big time serious vacuum leak for them to glow red due to a lean condition.

If you know your timing tab is correct and the timing looks good it could be an issue with the MSD. If you’re not sure about the timing mark loosen the distributor, rev the engine up around 3,000 rpm and hold it steady. Then move the timing back and forth to see if the engine increases with the movement. Where it revs the highest is where your engine wants the timing with the springs you have in the distributor.

If the base timing ends up being too low or too high after doing this you’ll need to play around with the springs and or the vacuum advance pod if you have one to get the base timing and the total timing where they need to be.
Posted By: tom demarco

Re: won’t idle/ running lean. - 11/29/21 10:31 PM

The best test is propane enrichment at idle, put propane in the air filter area and see it the colors goes back to normal, if it does you know it is not a rich issue, it could be lean, or retarded timing but you said you where driving it for a bit and it was fine. do you have spark plug wires that can handle MSD, you did not say it was skipping so I am going to assume your wires are for MSD, I am thinking the MSD has gone bad or the distributor, or the wiring to and from, A timing light may give you an indication but that is not a solid diagnosis. If the propane makes it idle and no red headers odds are all is good with your MSD. I have seen carbs go bad and cause this issue but usually both banks. how much vacuum does a gauge say? Good Luck 18 inches is what you want.
Posted By: 88ssBrent

Re: won’t idle/ running lean. - 11/29/21 11:41 PM

Badss is correct in what he is saying. When I built my engine the first time I fired up I didn't have my distributor where it needed to be. Headers where so hot that it flaked off my new ceramic coating, glowing red, and melted anything and everything within 6". It was about a $600 mistake, timing is the first thing I would check. I seem to think that there might be a issue with the msd.
Posted By: 8T7AERO

Re: won’t idle/ running lean. - 12/14/21 05:56 PM

Hello, Thanks for all the info, I put a brand new Holley 600 carb on it that I had for another project, It did the same thing, once it warmed up it wouldn’t idle and headers started to glow red ( but not as bright) but it took a little longer to do so. I’m using MDS 8.5mm super conductor wires. I normally run 91 octane unleaded and mix it with 101 octane unleaded about 50/50 but this time I used a more of the 101 octane, this the only thing different I did before I drove it.. My vacuum at idle is 10” do to the cam size. My timing light died on me, when I get my new I’ll check it. Also I’m trying to make up a propane tester. Thx everyone
Posted By: 8T7AERO

Re: won’t idle/ running lean. - 01/05/22 01:10 AM

Hello, checked my timing and it was set 22 deg. total advance. I retarded the timing and the headers still stayed red. So at this point I figured the intake gasket might be the problem, so I started to disassemble it, put the #1 cylinder at TDC and pulled the distributor cap and seen that the reluctor paddles and magnetic pickup assembly are covered in rust/corrosion. Anybody think this could be cause of the issues ? Thanks
Posted By: 8T7AERO

Re: won’t idle/ running lean. - 02/05/22 06:08 PM

Thanks for the help, it was the intake manifold gasket that was the problem.The Monte is up and running. Also the corrosion on the MSD distributor is a know problem with that model according to MSD.
Posted By: Buick Runner

Re: won’t idle/ running lean. - 02/06/22 12:03 AM

Originally Posted by 8T7AERO
Thanks for the help, it was the intake manifold gasket that was the problem.The Monte is up and running. Also the corrosion on the MSD distributor is a know problem with that model according to MSD.


Probably late, but did you check the intake manifold for warpage? I had intake vacuum issues and it turned out the manifold was warped.
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