Slight miss need advice
Thread Starter
Instructor
Joined: Jul 2015
Posts: 121
Likes: 3
From: Maryland
Slight miss need advice
1973 coupe with newly rebuilt 350. Have a slight miss at idle. With idle set at 800rpm it will surge up to about 850 rpm randomly. When this happens the timing moves about 2 degrees when checking it. Vacuum gauge has 16" fairly steady with a very slight 1/2" random change at idle. I think it also does it cruising around but it is so slight I am not totally sure. Seems to run well, just would like to figure out the issue.
Things I have done so far:
Checked distributor, set end play. Advance is fine and is all in at 2600 rpm. Using 1 black and 1 silver spring from kit.
Rebuilt carb, new float, gaskets etc.
Adjusted valves while hot.
Have changed plugs, wires.
Have not changed points or cap or rotor. They look fairly new.
Any ideas?
Things I have done so far:
Checked distributor, set end play. Advance is fine and is all in at 2600 rpm. Using 1 black and 1 silver spring from kit.
Rebuilt carb, new float, gaskets etc.
Adjusted valves while hot.
Have changed plugs, wires.
Have not changed points or cap or rotor. They look fairly new.
Any ideas?
Thread Starter
Instructor
Joined: Jul 2015
Posts: 121
Likes: 3
From: Maryland
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
I suspect a problem with the throttle cable. It may not be letting the carb linkage come fully to the [mechanical] stop at idle. Remove the cable from the carb, then 'flick' the carb throttle plates a few times to see if the carb comes back to same RPM at idle. If it does, you have a cable/accelerator pedal problem.
Safety Car
Joined: Jan 2009
Posts: 3,736
Likes: 56
From: anchorage ak
Use starting fluid and lightly spray the edges of the intake, base, sides of carb, anywhere their could be potential of a vacuum leak. I say lightly and I mean lightly, a lot if people use propane or carb cleaner but I've found ether gives a more indicative tale tale and whe used lightly works better by showing even the smallest leak, but any open spark will ignite the ether.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
When this happens the timing moves about 2 degrees when checking it.
Too light of a spring may be allowing your timing to advance too early and this will cause it to idle up.
Other thought is that the pivots on the mech advance weights may be worn out allowing some movement early in the rpm range.
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Thread Starter
Instructor
Joined: Jul 2015
Posts: 121
Likes: 3
From: Maryland
Sprayed starting fluid around intake. No noticeable change.
It has a new performer 2101 intake. I installed it when the engine was rebuilt.
Before I recurved the distributor I put rubber bands on the weights. No change in idle.
Throttle cable has 2 return springs. No problem there.
Carb tuning could be a possibility. The idle mixture screws don't seem to have as much effect on tuning as I thought they should. I can turn the passenger side all the way in and car still idles. The drivers side will stall it out. I can set the idle as low as 400-450 rpm just fine. It acted this way before the carb was rebuilt. I ran a small wire through the idle tubes to make sure they were not blocked.
I am thinking it is either a valve problem or carb problem. The heads were rebuilt with new valves. I am not sure what kind of vacuum reading I would get with a leaky valve??
I have spent days searching the forum for similar problems. I have tried most all the advice I have found. I am at the point now where I might have to take it to someone. Problem is not to many people are very familiar with these older engines.
Thanks for the reply's so far..............
It has a new performer 2101 intake. I installed it when the engine was rebuilt.
Before I recurved the distributor I put rubber bands on the weights. No change in idle.
Throttle cable has 2 return springs. No problem there.
Carb tuning could be a possibility. The idle mixture screws don't seem to have as much effect on tuning as I thought they should. I can turn the passenger side all the way in and car still idles. The drivers side will stall it out. I can set the idle as low as 400-450 rpm just fine. It acted this way before the carb was rebuilt. I ran a small wire through the idle tubes to make sure they were not blocked.
I am thinking it is either a valve problem or carb problem. The heads were rebuilt with new valves. I am not sure what kind of vacuum reading I would get with a leaky valve??
I have spent days searching the forum for similar problems. I have tried most all the advice I have found. I am at the point now where I might have to take it to someone. Problem is not to many people are very familiar with these older engines.
Thanks for the reply's so far..............
Safety Car
Joined: Jan 2009
Posts: 3,736
Likes: 56
From: anchorage ak
Sprayed starting fluid around intake. No noticeable change.
It has a new performer 2101 intake. I installed it when the engine was rebuilt.
Before I recurved the distributor I put rubber bands on the weights. No change in idle.
Throttle cable has 2 return springs. No problem there.
Carb tuning could be a possibility. The idle mixture screws don't seem to have as much effect on tuning as I thought they should. I can turn the passenger side all the way in and car still idles. The drivers side will stall it out. I can set the idle as low as 400-450 rpm just fine. It acted this way before the carb was rebuilt. I ran a small wire through the idle tubes to make sure they were not blocked.
I am thinking it is either a valve problem or carb problem. The heads were rebuilt with new valves. I am not sure what kind of vacuum reading I would get with a leaky valve??
I have spent days searching the forum for similar problems. I have tried most all the advice I have found. I am at the point now where I might have to take it to someone. Problem is not to many people are very familiar with these older engines.
Thanks for the reply's so far..............
It has a new performer 2101 intake. I installed it when the engine was rebuilt.
Before I recurved the distributor I put rubber bands on the weights. No change in idle.
Throttle cable has 2 return springs. No problem there.
Carb tuning could be a possibility. The idle mixture screws don't seem to have as much effect on tuning as I thought they should. I can turn the passenger side all the way in and car still idles. The drivers side will stall it out. I can set the idle as low as 400-450 rpm just fine. It acted this way before the carb was rebuilt. I ran a small wire through the idle tubes to make sure they were not blocked.
I am thinking it is either a valve problem or carb problem. The heads were rebuilt with new valves. I am not sure what kind of vacuum reading I would get with a leaky valve??
I have spent days searching the forum for similar problems. I have tried most all the advice I have found. I am at the point now where I might have to take it to someone. Problem is not to many people are very familiar with these older engines.
Thanks for the reply's so far..............
Thread Starter
Instructor
Joined: Jul 2015
Posts: 121
Likes: 3
From: Maryland
Running vacuum advance, manifold vacuum.
Stock quadrajet carb.
I have a dial back timing light and the timing does not start advancing (mechanical) at idle speed. I changed the vacuum canister with one that is all in at 14" of vacuum.
I'm stumped...........
Stock quadrajet carb.
I have a dial back timing light and the timing does not start advancing (mechanical) at idle speed. I changed the vacuum canister with one that is all in at 14" of vacuum.
I'm stumped...........
The vaccum would be erratic with a leaking valve plus there be other indications such as a back fire through the carb. What springs do you have on the mechanical advance. I use the light springs and with a 950 rpm idle it advances at idle which worked out fine for my situation. But if the timing changes with rpm increase then the question would be is it the rpm change that raises the advance or is it the advance that changes the rpm. What kind of carb do you have and are you running a vacuum advance, if so we're is it plumbed, ported or non ported.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
With idle set at 800rpm it will surge up to about 850 rpm randomly. When this happens the timing moves about 2 degrees when checking it.
Safety Car
Joined: Jan 2009
Posts: 3,736
Likes: 56
From: anchorage ak
Manual or automatic transmission?
Melting Slicks
Joined: Jul 2006
Posts: 2,288
Likes: 488
From: Salmon Arm, BC
Le Mans Master
Joined: May 2007
Posts: 5,020
Likes: 1,120
From: North tonawanda NY
Sprayed starting fluid around intake. No noticeable change.
It has a new performer 2101 intake. I installed it when the engine was rebuilt.
Before I recurved the distributor I put rubber bands on the weights. No change in idle.
Throttle cable has 2 return springs. No problem there.
Carb tuning could be a possibility. The idle mixture screws don't seem to have as much effect on tuning as I thought they should. I can turn the passenger side all the way in and car still idles. The drivers side will stall it out. I can set the idle as low as 400-450 rpm just fine. It acted this way before the carb was rebuilt. I ran a small wire through the idle tubes to make sure they were not blocked.
I am thinking it is either a valve problem or carb problem. The heads were rebuilt with new valves. I am not sure what kind of vacuum reading I would get with a leaky valve??
I have spent days searching the forum for similar problems. I have tried most all the advice I have found. I am at the point now where I might have to take it to someone. Problem is not to many people are very familiar with these older engines.
Thanks for the reply's so far..............
It has a new performer 2101 intake. I installed it when the engine was rebuilt.
Before I recurved the distributor I put rubber bands on the weights. No change in idle.
Throttle cable has 2 return springs. No problem there.
Carb tuning could be a possibility. The idle mixture screws don't seem to have as much effect on tuning as I thought they should. I can turn the passenger side all the way in and car still idles. The drivers side will stall it out. I can set the idle as low as 400-450 rpm just fine. It acted this way before the carb was rebuilt. I ran a small wire through the idle tubes to make sure they were not blocked.
I am thinking it is either a valve problem or carb problem. The heads were rebuilt with new valves. I am not sure what kind of vacuum reading I would get with a leaky valve??
I have spent days searching the forum for similar problems. I have tried most all the advice I have found. I am at the point now where I might have to take it to someone. Problem is not to many people are very familiar with these older engines.
Thanks for the reply's so far..............
Last edited by augiedoggy; Jun 21, 2016 at 07:47 AM.
Le Mans Master
Joined: May 2007
Posts: 5,020
Likes: 1,120
From: North tonawanda NY
I agree and am surprised no one else caught this... Although I'll admit I'm not familiar with Qjet carbs.
Last edited by augiedoggy; Jun 21, 2016 at 07:49 AM.