Low Speed Stumble

[elvismith]

Hi folks,


I have a stumper I hope y'all can help with. I've searched around and can't find an answer on here, so sorry if this has been asked and answered.


'88 GW, 140k, all stock.


Starts like a dream and is great 35 mph and over. Between 0 and 35 mph, a complete disaster. She stumbles and bucks and seems to choke out only to come back to life. And there is an occasional backfire in the carb. Also has a "tea kettle whistle" (almost like the alternator whine I used to suffer through the stereo on my '74 Ventura) when idling or decelerating, but goes away at acceleration.


I suspected a vacuum leak but can't find anything. I always though that backfire in the carb was a timing problem, but it seems to be otherwise good.


Any ideas?


Thanks!

[inkedmonkey]

Really sounds like vacuum leak.

[MysticRob]

If nothing else has changed (i.e. timing, etc), then that backfiring can also be a symptom of too much or too little fuel delivery, which could mean one or more carb 'circuits' have gone out on you.

Looking at you, accelerator pump circuit.

[elvismith]

Thanks guys. I think you're right - vac leak affecting the acc pump circuit. Even futzing around with the acc pump this morning made a world of difference. Still have the insanely loud whistle (wife from inside the house "what the hell is that racket") and I suspect that may be the vac leak. Will keep looking.

[tgreese]

I have found that the accelerator pump diaphragms on these carbs do not last very long. Maybe it's modern fuels, or decline in quality? You can buy just the pump diaphragm and replace it without removing the carburetor. Dorman sells them - https://www.amazon.com/Dorman-HELP-9.../dp/B000CO7IS6 - cheap from AMazon, or any parts store can get Dorman parts the same or next day.

NB the accelerator pump action is mechaincal, connected to the throttle linkage - no vacuum involved. Typically the engine will run fine at constant speed, but stumles and coughs when the throlle is opend more. These diaphragms often start leaking to the outside too.

Check the TSM. It will have procedures for various operating problems. It also explains each circuit in your carburetor, with helpful diagrams.