Finally found the cause of the off idle stumble
08-11-2003, 09:38 AM
#1
Pro
Thread Starter
Finally found the cause of the off idle stumble
Finally found the cause of the off idle stumble
On my 1980 qjet I have had this off idle stumble occurring at low RPM. Here is what I did. Thanks to Lars tuning paper on Q jets and Doug Roe’s book on quadrajets.
I started to track the problem down by this method.
Ignition – replaced the spark plugs, rotor ,cap, noticed a cracked plug wire which can also give you the same symptoms of off idle stumble – replaced all the plug wires as a result
Emissions – checked all the emissions, emission hoses, and vacuum and this was up to spec.
Lars paper – disconnected the accelerator pump with the car running – noticed the first clue. With the accelerator pump linkage disconnected, As I reved the car their was no change at all.
Lars paper – removed the top of the carb off, then things started to fall into place. I have been going to the junkyard to practice taking carbs apart and also source parts from these old carbs. I noticed when I took apart these olds carbs the accelerator pump would essential come out with no effort. On my carb I almost needed a pair of pliers to remove the accelerator pump. I then rechecked Lars tuning paper and he did mention that today’s fuels and additives in them may cause the rubber cup in the accelerator pump to expand, once this happens it will cause the accelerator pump to seize in the bore or bind. Another thing I noticed over the past months, was the accelerator pedal in the car would seem to get stuck at about a quarter of the way down when trying to start the car – this would have been the result of the accelerator pump stuck in the bore. I am ordering a new accelerator pump for my 1980 q jet and should be back up in running. Thanks to Lars for the tuning tips paper and also to fellow members for there suggestions.
:) :)
On my 1980 qjet I have had this off idle stumble occurring at low RPM. Here is what I did. Thanks to Lars tuning paper on Q jets and Doug Roe’s book on quadrajets.
I started to track the problem down by this method.
Ignition – replaced the spark plugs, rotor ,cap, noticed a cracked plug wire which can also give you the same symptoms of off idle stumble – replaced all the plug wires as a result
Emissions – checked all the emissions, emission hoses, and vacuum and this was up to spec.
Lars paper – disconnected the accelerator pump with the car running – noticed the first clue. With the accelerator pump linkage disconnected, As I reved the car their was no change at all.
Lars paper – removed the top of the carb off, then things started to fall into place. I have been going to the junkyard to practice taking carbs apart and also source parts from these old carbs. I noticed when I took apart these olds carbs the accelerator pump would essential come out with no effort. On my carb I almost needed a pair of pliers to remove the accelerator pump. I then rechecked Lars tuning paper and he did mention that today’s fuels and additives in them may cause the rubber cup in the accelerator pump to expand, once this happens it will cause the accelerator pump to seize in the bore or bind. Another thing I noticed over the past months, was the accelerator pedal in the car would seem to get stuck at about a quarter of the way down when trying to start the car – this would have been the result of the accelerator pump stuck in the bore. I am ordering a new accelerator pump for my 1980 q jet and should be back up in running. Thanks to Lars for the tuning tips paper and also to fellow members for there suggestions.
:) :)
08-11-2003, 10:53 AM
#2
Safety Car
Re: Finally found the cause of the off idle stumble (RATT7)
Had the same problem with my Q-Jet. Not all accel. pumps are created equal. I'd get Lars to give you the proper Eshlin(sp?) part number.
08-11-2003, 01:13 PM
#3
Pro
Thread Starter
Re: Finally found the cause of the off idle stumble (Stewart's74)
Thanks - I checked with my NAPA auto parts dealer and they are ordering the accel pump for me - Echlin part #2 - 4056 - I would have not suspected the accelarator pump other for the fact the it was stuck really good in the bore and I had to really pull on it to remove it - Napa should have the part for me in 2 days and I will give it a try and see what happens - Will let all of you know of the results - Thanks :)
08-11-2003, 02:25 PM
#4
Tech Contributor
Member Since: Aug 1999
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Re: Finally found the cause of the off idle stumble (RATT7)
RATT -
Good job tracking down the issue - that accel pump problem is a very common issue, and I keep seeing accel pumps jammed in the bores from the incompatible fuels. Glad the paper came in handy - good job doing the troubleshooting procedure!
Good job tracking down the issue - that accel pump problem is a very common issue, and I keep seeing accel pumps jammed in the bores from the incompatible fuels. Glad the paper came in handy - good job doing the troubleshooting procedure!
