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A quick question, hopefully with a quick answer.
Once the engine is warmed up, the RPMs on my small block vary by about 50-60 RPMs. They barely move when on fast idle, steady at about 1,350, but once it kicks off they range from 790 to 850 in park.. It does it smoothly, over 5-10 seconds, so if I don't have the digital timing light hooked up it would almost not be noticeable. Is this normal?
Thanks!
A little fluctuating is normal, but you can get even more if your vacuum advance isn’t all in at 2 inches of mercury fewer that what your engine produces on average. What are your timing and vacuum advance set up to?
Actually, I am also trying to get that sorted out. I just installed a new HEI (A Chinese knock-off, but got good reviews) the tach drive gear was damaged in the old (maybe original) distributor and this was less expensive than any of the other options I looked at. It does have an adjustable vacuum advance, though I haven't gotten around to adjusting it yet. After I set the timing (per Lars' papers) and plugged the vacuum advance back in, it really started to surge. Vacuum is steady at about 18-20 in/hg. So I will have to check what the vacuum advance is set at.
Also, relatively new is a Lars rebuilt Quadrajet and Edelbrock 2101 intake that I installed earlier this year. I was trying to get that set up and the divorced choke installed properly. I'm pretty sure that is all good now..Except I still have to make sure there is no interference from the gasket, the throttle dosen't want to close completely. If I push on it will drop another 100 RPMs.
do you have a stock L48 or an L82? Or something else entirely?
My 73 L82 has edelbrock heads but its stock otherwise, factory cam, pistons, intake, carb, etc... The idle does bounce a little when warmed up, about the same 50 RPM as what you get. I think the L48 has a less aggressive cam profile and would prob idle a little smoother. Thats been my experience anyway.
YMMV
It does not have an idle solenoid.. It is a factory A/C car, but it's currently inoperable. That's another problem to fix for another day. Only a rare occasion will it diesel for a second or two, so I'm not to worried about that.
I'm not aware of any other functions the solenoid provides, is it necessary? Does it pull, as well as, push on the throttle arm?
You said Lar’s rebuilt the carb.
If so, I would contact him directly and ask if the model carb you have should have the idle solenoid or not and how it functions.
If you are getting dieseling when you shut down the engine, I would suspect the ignition timing is not set correctly, which will cause the idle rpm to vary as you have explained.
Read Lar’s timing and carb setup papers and get the timing and carb dialed in correctly before you do anything else.
do you have a stock L48 or an L82? Or something else entirely?
My 73 L82 has edelbrock heads but its stock otherwise, factory cam, pistons, intake, carb, etc... The idle does bounce a little when warmed up, about the same 50 RPM as what you get. I think the L48 has a less aggressive cam profile and would prob idle a little smoother. Thats been my experience anyway.
YMMV
It's actually an engine from 1978/79 c/k-20 truck. I don't know many specs about it, but it has heads from a '72 Corvette. They're larger combustion chamber heads, 75cc, I think. It may have a very mild cam, so nothing special. That year should have only produced 165hp, but I had dyno'd before I installed the Q-jet and 2101 intake it was producing 190hp at 5,400rpm. It wouldn't breathe well at all past that. Now it will keep going strong up to 6,500rpm.
I'm thinking the little bit of bounce at idle might be normal.
You said Lar’s rebuilt the carb.
If so, I would contact him directly and ask if the model carb you have should have the idle solenoid or not and how it functions.
If you are getting dieseling when you shut down the engine, I would suspect the ignition timing is not set correctly, which will cause the idle rpm to vary as you have explained.
Read Lar’s timing and carb setup papers and get the timing and carb dialed in correctly before you do anything else.
Yes, I'm still sorting the timing issues. As mentioned above, I need to verify and adjust the vacuum advance on the new distributor. I have and am following his papers. Let's just say that I am still learning how a Quadrajet works, and it is assumed by some that I should already know exactly what I am doing and that it won't be explained me. The dieseling hasn't happened recently, hopefully it has been taken care of.
Thanks for the help!
From: At my Bar drinking and wrenching in Lafayette Colorado
If the throttle does not want to close completely, you have an installation problem. You have had installation errors with your carb as related to your divorced choke, as we have discussed. Also, your carb had multiple problems and issues with regards to altered & damaged throttle linkages, all of which were corrected and repaired. It sounds like you now also have a timing problem. Your carb, when tested, held rpm within +/- 10 rpm - that's the acceptance criteria that I use when testing the carbs, which is likely tighter than any factory setting. According to the test data sheet on your carb, your carb, when tested, was operating at 845 - 855 rpm, which is well within a normal stable range. Most carbureted engines don't idle within 30-50 rpm. If you are fluctuating significantly more than that, it's likely that you have a vacuum advance control unit that does not meet the "2-inch rule" requirement, so your timing is fluctuating, causing an rpm fluctuation. You might want to try unplugging the vacuum advance and seeing if the idle rpm stabilizes without the vacuum advance hooked up. The lack of a solenoid will not cause idle rpm instability. Have you adjusted your idle mixture screws to see if you have a mixture issue? Slight variations in engine vacuum at idle will cause changes to the idle mixture setting, and there is no assurance that your engine produces the same idle vacuum as my test engine. Try backing each screw out 1/2 turn and see if it has any effect on idle rpm stability. If not, return to original setting, and then screw them IN 1/2 turn and see if the issue changes. This is outlined in the instructions with the carb that I sent you. If you feel there is a carb problem, just send it back and I'll re-test it to see if I can duplicate the issue.
Sorry for the delay writing back.
Yes, that's why I was trying to get info on the swings in idle rpm to see if there was a problem with the installation of either the carb or the distributor. It was doing this before the either was replaced and I was expecting that one or the other would have solved the problem, when they didn't I wanted to find out why.. I'm not saying it's carb issue, I'm trying to make sure I have all individual pieces working properly to get it running the best I can. As I've said before, I do not know enough to know what is normal and what is not.
When I disconnected the vacuum advance canister to set the timing, the RPMs still have the 50-70 RPM fluctuation, but when I plugged it back in it really started jumping, which was new. I've checked for vacuum leaks and didn't find any. I made sure the carb is secure to the manifold and that the bolts are not over-tightened. If I loosen the front right bolt a little a leak does form but goes away when tightened back down.
I tested the vacuum advance canister and it starts moving at 5"/Hg and peaks at 13", I've got about 20"/Hg at idle in neutral. It sounds like the vacuum advance should be increased to 18"/Hg, correct?. Is the only way to test how many degrees it advances to test it while the engine is running with the timing light?
I started the idle mixture screws at 2-1/2 turns out and turned them in about 3/8 of a turn. I will hopefully get the chance to start it up tomorrow and play with the idle mixture screws some more to see if increasing the mixture smooths out the idle.
The throttle arm sticking slightly open was something else I mentioned that is separate from the idle fluctuations. I don't know if you remember, but when you shipped the carb to me there was a thicker, rubber gasket in the package (along with all the spare/old parts) that's about 1/4" thick, is that an appropriate gasket to use with this carb and the 2101 intake? I've read about different gaskets causing the throttle plates to hang on it., do you think it could be causing this problem?
Thanks for the help!
Originally Posted by lars
If the throttle does not want to close completely, you have an installation problem. You have had installation errors with your carb as related to your divorced choke, as we have discussed. Also, your carb had multiple problems and issues with regards to altered & damaged throttle linkages, all of which were corrected and repaired. It sounds like you now also have a timing problem. Your carb, when tested, held rpm within +/- 10 rpm - that's the acceptance criteria that I use when testing the carbs, which is likely tighter than any factory setting. According to the test data sheet on your carb, your carb, when tested, was operating at 845 - 855 rpm, which is well within a normal stable range. Most carbureted engines don't idle within 30-50 rpm. If you are fluctuating significantly more than that, it's likely that you have a vacuum advance control unit that does not meet the "2-inch rule" requirement, so your timing is fluctuating, causing an rpm fluctuation. You might want to try unplugging the vacuum advance and seeing if the idle rpm stabilizes without the vacuum advance hooked up. The lack of a solenoid will not cause idle rpm instability. Have you adjusted your idle mixture screws to see if you have a mixture issue? Slight variations in engine vacuum at idle will cause changes to the idle mixture setting, and there is no assurance that your engine produces the same idle vacuum as my test engine. Try backing each screw out 1/2 turn and see if it has any effect on idle rpm stability. If not, return to original setting, and then screw them IN 1/2 turn and see if the issue changes. This is outlined in the instructions with the carb that I sent you. If you feel there is a carb problem, just send it back and I'll re-test it to see if I can duplicate the issue.
Sorry for the delay! I had a full response all typed up and then my computer decided to update and restart two nights ago and lost it all. Ugh!
I had it idling good on Monday. RPM swings were only about 15-20, which sounds like that is good.
Other specs:
Idle Vacuum = 21"/Hg (I forgot to check this in drive)
Mechanical timing = 36° @ 2,700 rpm
Initial timing = 20°
Vacuum advance ~ 20° @ 15"/Hg (starts at 5"/Hg)
Idle mixture screws = 3.5 turns out
Idle RPMs = 950 (1025 with vacuum advance hooked up)
Since it has good vacuum, can I turn the idle mixture screws in some to lower the RPMs? The Idle adjustment screw is not touching at all, except when I pull the throttle closed.
(Once I get acknowledgement of the above mixture setting, I will adjust the vacuum advance canister so that it's closer to within 2" of the vacuum then.)
I don't know if you remember or not, but I used the 1/4" thick gasket that was in the box you shipped to me. Is that an appropriate gasket to use with this carb and the 2101 intake? I was wondering if that was causing the throttle to stick open that little bit. It doesn't feel like it sticks at all when giving it gas from that point, so maybe not? That's why I was wondering about the solenoid, if it also serves to pull the throttle shut that last bit.
I think that is all I wrote before. Thanks for the help!
Originally Posted by lars
If the throttle does not want to close completely, you have an installation problem. You have had installation errors with your carb as related to your divorced choke, as we have discussed. Also, your carb had multiple problems and issues with regards to altered & damaged throttle linkages, all of which were corrected and repaired. It sounds like you now also have a timing problem. Your carb, when tested, held rpm within +/- 10 rpm - that's the acceptance criteria that I use when testing the carbs, which is likely tighter than any factory setting. According to the test data sheet on your carb, your carb, when tested, was operating at 845 - 855 rpm, which is well within a normal stable range. Most carbureted engines don't idle within 30-50 rpm. If you are fluctuating significantly more than that, it's likely that you have a vacuum advance control unit that does not meet the "2-inch rule" requirement, so your timing is fluctuating, causing an rpm fluctuation. You might want to try unplugging the vacuum advance and seeing if the idle rpm stabilizes without the vacuum advance hooked up. The lack of a solenoid will not cause idle rpm instability. Have you adjusted your idle mixture screws to see if you have a mixture issue? Slight variations in engine vacuum at idle will cause changes to the idle mixture setting, and there is no assurance that your engine produces the same idle vacuum as my test engine. Try backing each screw out 1/2 turn and see if it has any effect on idle rpm stability. If not, return to original setting, and then screw them IN 1/2 turn and see if the issue changes. This is outlined in the instructions with the carb that I sent you. If you feel there is a carb problem, just send it back and I'll re-test it to see if I can duplicate the issue.
That vacuum advance has far too much, especially for the amount of initial you have to run to achieve 36 total. You’ll want to have no more than 10-12.
The 1/4” gasket works just fine on the 2101. If you believe the AC idle solenoid is preventing the throttle from closing all the way you can remove it with only 2 screws. I would get your idle in check (~850 in park) and controllable with the idle speed screw before worrying about the mixture.
I would try revving the engine above 3,000 RPM with the vacuum advance unplugged and see if it goes higher than 36. If so I would dial back your initial and make sure it’s all in where you want it.
Last edited by Piersonpie; Aug 22, 2025 at 01:26 PM.
That vacuum advance has far too much, especially for the amount of initial you have to run to achieve 36 total. You’ll want to have no more than 10-12.
The 1/4” gasket works just fine on the 2101. If you believe the AC idle solenoid is preventing the throttle from closing all the way you can remove it with only 2 screws. I would get your idle in check (~850 in park) and controllable with the idle speed screw before worrying about the mixture.
I would try revving the engine above 3,000 RPM with the vacuum advance unplugged and see if it goes higher than 36. If so I would dial back your initial and make sure it’s all in where you want it.
I was thinking I needed to get the idle speed down first, then adjust the vacuum can, but yes, it will need to be adjusted. Since the vacuum is pretty good, can I turn the mixtures screws in to lower the RPMs? Or is there another way to lower it?
I don't have a solenoid, I was wondering if it pulls on the throttle, as well as pushes it, to see that would help close the throttle plate.
No, the timing does not advance any more after maxing out at 2,700 RPM
Thanks!
Your idle speed generally won’t be changed that much by the idle mixture screws, unless you’re on the far extremes of rich or lean. The factory would actually tune them by leaning them out until the idle would drop 50 RPMs, but this is far from ideal. Whatever Lars had them set to on his running stand should be pretty close to on the money for your engine.
The solenoid only pushes on the throttle lever, it doesn’t pull. I would try looking for a vacuum leak. Also reducing your timing at idle with a correctly calibrated vacuum advance should help too.
Here’s another thought. Are you sure your choke is opening all the way and you’re coming down off your fast idle?
From: At my Bar drinking and wrenching in Lafayette Colorado
You cannot adjust idle speed with idle mixture. Bad idea.
When I tested your carb, part of the test procedure is that the engine can be "killed" by backing out the idle speed screw. Your carb passed the "kill test" and has full idle speed adjustability down to "0". If you have the idle speed screw fully backed out, the lever is not contacting the idle speed screw, and the engine is still idling, you have a vacuum leak, or you have a divorced choke problem that is not allowing the choke to fully open and is not dropping the fast idle cam down all the way. Also, if you can lower the idle speed by pushing on the throttle lever, you have a binding problem in your accel linkage. You need to fix both problems. The idle speed screw should be able to kill the engine regardless of timing setting - do not use timing to set idle speed. You have a carb installation problem with air leaks and linkage problems.
The gasket I provided to you is the correct gasket, and has nothing to do with idle speed.
As noted earlier, if you suspect any problem with the carb, or want it re-tested to see if the problem is with your installation or if it's with the carb, just send it back and I'll immediately set it up on the test engine and provide you with results.
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How steady should RPMs be at idle?
