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idle low several minutes then comes up and smooths out

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idle low several minutes then comes up and smooths out

 
Old 06-03-2019, 10:15 AM
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vince vette 2
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Default idle low several minutes then comes up and smooths out

First, my apologies for posting this - I posted before on this before but can't find the old post.

Situation is 1980 L82, original engine, Q-jet with electric choke professionally rebuilt (Cliff Ruggles - apologies to Lars - I wasn't a forum member back then). After starting cold and kicking off the cold idle, the RPM are low and if I put the car into drive I need to ride the throttle if the brake is on to keep it from stalling. Several minutes later the rpm comes up a couple hundred and life is good. Previous advice in earlier post all focused on the carb. Tried various tuning items based on advice provided but with no results at that time (many months ago).

I finally got all the rear end stuff back in and started the car yesterday. I just ran it in the driveway letting everything warm up to ensure no surprises as the water pump was replaced. I kept the air cleaner cover off to keep an eye on the carb choke plates and this also takes the snorkel dampers out of the equation. The primary chock plate opened properly in a normal time and when fully opened the RPM in part was about 600. Then sometime at least 5 minutes later but no more than 10 minutes, the engine came up about 200 RPM over a 4 or 5 seconds. Nothing visibly changed on the carb though if the primaries opened slightly I probably would not have seen that.

Questions:
1 - is there anything else in the carb controlled by a TVS which might possibly activate after that much time?
2 - could this be a vacuum advance issue with the distributor, in which case what TVS might be involved?
3 - could it relate to the exhaust restrictor valve - though I disconnected the vacuum line to that some time ago?

This may be totally unrelated, but just to mention, when first started the passenger side of the engine compartment had some smoke very after starting, too soon to be burning oil film off the block and it didn't seem like an oil smell. So I suspect it was exhaust. It was there for several minutes. When the idle came up this was gone, but I don't know for certain if it stopped at the same time or some time sooner.
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Old 06-03-2019, 01:08 PM
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MelWff
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That really doesnt sound abnormal to me. Even though the choke is open the intake is still relatively cool and fuel will drop out of the air/fuel mixture until the engine is up to full operating temperature.
The other thought is how did you adjust the idle speed and what RPM did you set it for?
I chock the wheels on an automatic, set the parking brake hard, and adjust the mixture screws and idle speed screw in drive.
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Old 06-03-2019, 02:05 PM
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Originally Posted by MelWff View Post
That really doesnt sound abnormal to me. Even though the choke is open the intake is still relatively cool and fuel will drop out of the air/fuel mixture until the engine is up to full operating temperature.
The other thought is how did you adjust the idle speed and what RPM did you set it for?
I chock the wheels on an automatic, set the parking brake hard, and adjust the mixture screws and idle speed screw in drive.
I can understand it gradually getting better as it warms up. But this a rapid transition - several minutes of a low idle around 600 in neutral, not enough to keep running in drive with the brakes on and then right up to 800 RPM in a few seconds.

For idle mixture I set them with the car warmed up but in park. For idle speed I set it when warmed up for 800 in park which, when in drive with brakes gives me 600.

I didn't mention this issue to a friend of mine who has owned several C2's and C3's. He came over when I first got the car running back running a couple years ago. I started it and as we were chatting it did this transition and his comment was "something just changed". Admittedly, he's more of a mechanical fuel injection guy who has been making some pretty intricate repro parts for that system for a couple decades. But he knows his way around a Q-jet too. He thought it was more outside the the carb than inside.
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Old 06-03-2019, 03:26 PM
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(Just looking at my 77 diagram).... Does your engine have the emissions hoses hooked to the top of the thermostat neck and routed to the heat riser on the exhaust manifold? This may be a source of choking off your engine (with restricted exhaust flow) if the heat riser is blocked in the intake manifold.
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Old 06-03-2019, 07:04 PM
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Originally Posted by sstocker31 View Post
(Just looking at my 77 diagram).... Does your engine have the emissions hoses hooked to the top of the thermostat neck and routed to the heat riser on the exhaust manifold? This may be a source of choking off your engine (with restricted exhaust flow) if the heat riser is blocked in the intake manifold.
I thought about that possibility. But sometime ago I popped the vacuum line off that to see what would happen. Nothing changed. It still took several minutes before that quick transition happened. But reading a little more today leads me to a couple questions. Is the vacuum supposed to hole it open or closed? Does it also use a bi-metallic spring to control opening and closing? Why do I ask? Whether vacuum were to hold it opened or closed, the fact that I pulled the line and it made no difference would suggest that the vacuum and hear riser valve have no part in this. But, if GM combined the vacuum and bi-metallic spring then maybe there's and answer there. E.g. some set up where a drop in RPM due to load would change vacuum enough to at least partially overcome the spring when cold and get the RPM back up. I could debate this or I could go try to quickly wire the valve open (I just checked and it does move) and start it and see.
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Old 06-03-2019, 11:09 PM
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Default apparently not the hear riser valve?

OK, didn't wire the valve open, but watched it during warm up and didn't see the mechanism move. However, if it is being heat activated on a metal spring, it would be possible to set up link which could push the valve open against the spring force but wouldn't have to follow the spring if it was moving on it's own. So I don't know for sure yet.
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Old 06-04-2019, 09:19 AM
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Actually, I guess the question I should ask, regardless of whether the heat riser is involved in this is does vacuum hold it open or closed?
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Old 06-04-2019, 11:03 AM
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I think vacuum holds it open.....not 100% sure

To confirm if it's anything vacuum related, pull (and plug the cab ports) and see if anything changes.
Is your vacuum advance on the distributor hooked to ported or full vacuum, and what shape and strength are your throttle return springs?

Last edited by sstocker31; 06-04-2019 at 11:08 AM.
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Old 06-04-2019, 11:31 AM
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Does your exhaust have that heat valve in it? It used to be on the passenger side just below the manifold if i remember correctly. If its not open then does open, creating back pressure , the releasing it would effect idle

Last edited by Rescue Rogers; 06-04-2019 at 11:56 AM.
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Old 06-04-2019, 12:04 PM
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Originally Posted by Rescue Rogers View Post
Does your exhaust have that heat valve in it? It used to be on the passenger side just below the manifold if i remember correctly. If its not open then does open, creating back pressure , the releasing it would effect idle
Yes, it has a heat valve.I guess the next logical question is am I sure there's actually a valve plate in it. The answer is yes. I haven't visually checked recently, but I've had the car since new and the only time the cross over pipe as off was when I put headers on it in 1984 and then again when I removed them and put the crossover back on sometime around 1990. I can't say I checked to see if all the mechanism was there, but unless GM didn't put it in, then it is as it was intended to be when stock. The mechanism itself is free to move as I moved it by hand last night.

As for moving with vacuum, I haven't had the vacuum line hooked to it for some time.I guess I should just pop it on next time I start it and see if it actuates it in either when cold or warmed up.

Looking to the core question, is it involved in this long time between initial start up and a final smooth idle high enough not to stall when under load in drive, brakes on, - the linkage did not move when the car did its transition from lower to higher idle after about 5 minutes warming up last night. So, unless there is a bimetallic spring in there that is able to open it up when warm but connected in a way which would not move the outside mechanism, then it is not involved in the idle change issue.

I did remember today that I do have a vacuum gage/tool which I can apply vacuum with - shows A) how rarely I've used it and B) why I need to work faster on this project to get it drivable before I forget how to drive. I can test few things with that including finding out if the TVS to that line is actually functional.
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Old 06-04-2019, 12:25 PM
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Originally Posted by sstocker31 View Post
I think vacuum holds it open.....not 100% sure

To confirm if it's anything vacuum related, pull (and plug the cab ports) and see if anything changes.
Is your vacuum advance on the distributor hooked to ported or full vacuum, and what shape and strength are your throttle return springs?
If it needs vacuum to hold it open then I'd better either put the line back on or tie it open. I'll look for further confirmation - maybe on next start I'll put pull vacuum directly on it while still running at the pre full warm up and see if anything changes.

As for what port it's coming from, it is whatever was stock. I'll need to look tonight. I could maybe check on line to find a schematic but they seem to be scarce. I'll rectify that tonight by posting one here. The linkage springs on the carb are original but in good shape. They snap right back when being pulled by hand.
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Old 06-04-2019, 12:28 PM
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Vacuum holds the heat riser closed. It fails safe, open. If you have a vacuum pump, please prove this to yourself, but this has been my experience.
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Old 06-04-2019, 02:49 PM
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Originally Posted by Bikespace View Post
Vacuum holds the heat riser closed. It fails safe, open. If you have a vacuum pump, please prove this to yourself, but this has been my experience.
It would make sense to fail opened, but there are so many things that don't make sense on these cars .... like why are half the damned mounting and adjusting bolts on the accessories and lots of other items metric. I'm OK with metric in and of itself. But when working on this I can't move 2 inches or 5.08 cm without having to swap from a Brit Imperial to System Internationale. Hell, back on the power antenna the head of the mounting bolt was metric and the nut was standard or vice versa. I have no idea what the thread is.
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Old 06-04-2019, 09:57 PM
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Originally Posted by sstocker31 View Post
I think vacuum holds it open.....not 100% sure

To confirm if it's anything vacuum related, pull (and plug the cab ports) and see if anything changes.
Is your vacuum advance on the distributor hooked to ported or full vacuum, and what shape and strength are your throttle return springs?
Here's the schematic off the fan cowling itself "restored" where needed due to age. All the hoses are routed as seen except for those no longer required. The EFE (heat valve) comes off a ported source, at least that's my take.


1980 L48 and L82

Also, I pulled vacuum on the heat valve and it moves. I'm assuming that was from open to closed.
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Old 06-04-2019, 10:53 PM
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I think these two photos say it all. This is a 1980 heat riser from my 80. It is still functional (and for sale). Vacuum applied: Closed. Vacuum off: Open. Not only does this fail safe (open), but at WOT, it should open up as well with the loss of vacuum.



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Old 06-05-2019, 06:53 AM
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Originally Posted by Bikespace View Post
I think these two photos say it all. This is a 1980 heat riser from my 80. It is still functional (and for sale). Vacuum applied: Closed. Vacuum off: Open. Not only does this fail safe (open), but at WOT, it should open up as well with the loss of vacuum.

Yep

Which then brings us back to, what is engaging 5 minutes into the warm up to cause a sudden increase of a 200 or so RPM. Basically, with the heat valve and air pump both removed from the vacuum system the only components left are the snorkel dampers and vacuum advance. I know the dampers are not involved because the same sudden transition occurs with our without the air filter cover on. That leaves vacuum advance. That vacuum circuit is kind of confusing in that it has two sources, one from the front of the carb and one from the back. Each has its own TVS. The front takes its temp signal from the coolant and the one from the back takes its temp signal from inside the air cleaner. My guess is that it's the coolant temp sensor opening to advance the timing. Lots of fun to track this down.

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Old 06-05-2019, 11:48 AM
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Why don't you temporarily put the vacuum advance to full manifold vacuum (and eliminate all the BS in between) and see if it runs and idles better?
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Old 06-05-2019, 01:23 PM
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Originally Posted by sstocker31 View Post
Why don't you temporarily put the vacuum advance to full manifold vacuum (and eliminate all the BS in between) and see if it runs and idles better?

That's pretty much step one of following Lars' timing papers (who you should email for a copy). I'd imagine that one of the vacuum switches either opens or closes, and either causes or fixes a vacuum leak (likely causes). You can check to see if timing changes with a digital timing light.

Bypass the TVSs to isolate the change. If you don't have a visual emissions inspection, you don't really need any of the TVSs, or half of the vacuum lines. Your engine will run better, and look cleaner, with a new water neck and/or plugs in place of the TVSs.

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Old 06-05-2019, 10:52 PM
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Originally Posted by sstocker31 View Post
Why don't you temporarily put the vacuum advance to full manifold vacuum (and eliminate all the BS in between) and see if it runs and idles better?
Originally Posted by Bikespace View Post

That's pretty much step one of following Lars' timing papers (who you should email for a copy). I'd imagine that one of the vacuum switches either opens or closes, and either causes or fixes a vacuum leak (likely causes). You can check to see if timing changes with a digital timing light.

Bypass the TVSs to isolate the change. If you don't have a visual emissions inspection, you don't really need any of the TVSs, or half of the vacuum lines. Your engine will run better, and look cleaner, with a new water neck and/or plugs in place of the TVSs.
Thanks gents, that'll be this weekends project. I already had Lars's tech notes on all this and was reviewing them today. I pretty much convinced that the issue is just the one TVS waiting so long to open up the vacuum advance. I'll walk through the set up and see what happens.
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Old 06-07-2019, 03:03 PM
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Keep us informed of your progress....
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