idle low several minutes then comes up and smooths out
#1
Drifting
Thread Starter
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.
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.
#2
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.
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.
#3
Drifting
Thread Starter
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.
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.
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.
#4
Melting Slicks
(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.
#5
Drifting
Thread Starter
(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.
#6
Drifting
Thread Starter
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.
#8
Melting Slicks
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?
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 OMF; 06-04-2019 at 11:08 AM.
#9
Dementer sole survivor
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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.
#10
Drifting
Thread Starter
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.
#11
Drifting
Thread Starter
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?
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?
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.
#12
Le Mans Master
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.
#13
Drifting
Thread Starter
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.
#14
Drifting
Thread Starter
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?
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?
1980 L48 and L82
Also, I pulled vacuum on the heat valve and it moves. I'm assuming that was from open to closed.
#15
Le Mans Master
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.
Last edited by Bikespace; 06-04-2019 at 10:55 PM.
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vince vette 2 (06-05-2019)
#16
Drifting
Thread Starter
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.
Last edited by vince vette 2; 06-05-2019 at 07:09 AM.
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4SPDL81 (06-05-2019)
#18
Le Mans Master
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.
Last edited by Bikespace; 06-05-2019 at 01:26 PM.
#19
Drifting
Thread Starter
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.