1969 L46 350/350 Won't Idle

[David W.]

Re: 1969 L46 350/350 Won't Idle

The old fuel is gone. Running fresh RV non-ethanol fuel and yes it is 90 octane, not 93 as I mentioned earlier. I haven't had time to change out the carbs but one thing that popped to mind based on all the great feedback is the choke mechanism. The rebuilder included a new choke unit and I did have to mess around with it to get the rod to attach properly but I'm now thinking I didn't hook that up properly but it does seem quite simple. The rod from the butterfly goes into the top of the rectangular choke unit and inserts inside the loop of the 'spring'. One the manifold heats up the choke should open. But even at startup, shouldn't it stay running at idle?

[Duke W.]

Re: 1969 L46 350/350 Won't Idle

I'm not intimately familiar Quadrajets, but the choke system works pretty much the same as earlier "divorced choke" Holley carburetors. The thermostatic coil mounted on the inlet manifold pad above the heat riser passage closes the choke valve shaft via a link and another choke shaft link on the other side of the carb sets the fast idle cam when you perform the normal cold start procedure.

Upon initial startup the choke vacuum break will open the choke valve slightly and as the thermostatic coil heats up the choke will slowly open further, which also slowly relieves the fast idle cam. By the time the coolant temperature shows about half way to thermostat opening temperature the choke should be full open and the fast idle cam should be fully relieved, so the engine will be at normal hot idle.

If the various settings that you will find in your 1969 CSM are not properly adjusted the choke/fast idle system may not operate properly.

It's like trying to start a cold engine with no functioning cold start system. Even if you do get the engine to fire without the fast idle cam properly set, it will almost immediately die unless you add throttle.

With the air cleaner off on a cold engine it would take me about five seconds to determine if the cold start system is functioning properly, and the required time to correct any maladjusted items would not be long assuming you have the required service documentation.

It's not that hard to test and correct adjustment issues, but incorrect or defective parts (like a leaky choke vacuum break) could also be an issue that might take some time and effort to diagnose.

Duke

[David W.]

Re: 1969 L46 350/350 Won't Idle

Vacuum advance not modified from original.

[Mark E.]

Re: 1969 L46 350/350 Won't Idle

The only way i can think of the choke causing these symptoms is if it did not open when warm or somehow closes with a warm engine. Not likely, but worth checking that the choke stays open when things act up.

[Duke W.]

Re: 1969 L46 350/350 Won't Idle

It's likely not the cause of your starting issue, but have you ever checked the VAC to see if it meets spec. A lot of decades old original VACs do not, and some are replaced with a functionally incorrect part due to errors in both GM and aftermarket parts books that have been there for decades.

Few are ever checked as part of a good periodic inspection and maintenance program. In fact, few owners understand why it's there, what it actually does, and why it's important.

Duke

[Duke W.]

Re: 1969 L46 350/350 Won't Idle

The only way i can think of the choke causing these symptoms is if it did not open when warm or somehow closes with a warm engine. Not likely, but worth checking that the choke stays open when things act up.

Have you ever tried to cold start an engine whose choke would not close?

Duke

[Mark E.]

Re: 1969 L46 350/350 Won't Idle

Have you ever tried to cold start an engine whose choke would not close?

Duke

I get that. My impression is the symptoms happen while driving with a warm engine ("It would keep an idle but at 30mph or so it was lurching."). Maybe Dave can clarify this for us.

[David W.]

Re: 1969 L46 350/350 Won't Idle

I've been getting a lot of great suggestions as to potential issues so I decided to take the carb back off and restart the process. The car is now running great due to several factors, none of which I can particularly pinpoint as you can see below.

Initially when I removed the carb there were two fiber gaskets and one metal gasket. I initially installed the rebuilt carb with the gaskets the rebuilder supplied which was one fiber and one metal. When I redid it just now, I took one of the pieces of advice and reinstalled with the same configuration as when I removed the carb; two fiber gaskets with a metal gasket on top. I reinstalled the choke unit very carefully. Lastly I took more care in tightening the carb bolts and made them probably more snug than initially. Car started well and idles well. Conclusions- perhaps a slightly warped intake manifold was cured with the extra gasket. Perhaps I failed to tighten the carb bolts properly causing a vacuum leak. Lastly, the choke unit may have been the issue but this one I would say is unlikely as I had previously started the car by manually adjusting the choke and the stalling issue was still there.

Lessons learned and a huge thanks to all for their suggestions/recommendations. Time for a drive.