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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: Cowman85 on March 16, 2009, 08:32:13 pm

Title: q jet problems or timing?
Post by: Cowman85 on March 16, 2009, 08:32:13 pm

i have an 86 350 3/4ton silverado i've done a tune up (plugs, wires, oil, filters, carb cleaner, fuel pump) i seem to keep having a problem stalling at very inopportune moments. i will give it gas and it will just die, with a vacuum sound from the carb.

parked or neutral there is no problem, only on quick steps of the pedal the engine seems to lope and choke a little.
when youre in drive with the brakes on and you step quick on and off the pedal again the engine sometimes stalls and knocks, but if you apply the throttle gently there is no stumble or hesitation.

-Anyone have any ideas of what could be wrong in the carb or otherwise?

i have used lots of carb cleaner and my next step is rebuild, i am thinking of a new 500cfm carb just to stick on there for a few months while i strip that q jet.

-Would it be a simple bolt on for the new carb? (this is my first time having to tinker with the carb)

Title: Re: q jet problems or timing?
Post by: TexasRed on March 16, 2009, 08:37:40 pm

Sounds like maybe an accelerator pump problem.

Title: Re: q jet problems or timing?
Post by: Lt.Del on March 16, 2009, 09:15:57 pm

i agree.  Also, make sure your choke is set correctly.

Title: Re: q jet problems or timing?
Post by: Cowman85 on March 18, 2009, 06:29:52 pm

i checked the fuel spray and there is none at all, but there is sufficient flow from the pump and a brand new filter.

Title: Re: q jet problems or timing?
Post by: gold84gmc on March 21, 2009, 08:51:31 pm

i had a similar problem with my truck and i was going to say check the fuel filter but you already replaced it....i would try rebuilding the carb and see what that does

Title: Re: q jet problems or timing?
Post by: TexasRed on March 21, 2009, 09:37:58 pm

You checked the fuel spray of the accelerator pump? Try pushing the accelerator pump piston down while the vehicle is idling in park. If it stumbles a good deal, such as almost stalling, the accelerator pump is fine.

Title: Re: q jet problems or timing?
Post by: Cowman85 on March 22, 2009, 12:46:53 pm

i figured thats where i would start, since i isolated the tank, lines, and pump.
i finally took it into my mechanic on the west end, it stalled and would not start again, no fuel spray into the carb.
i told him what happened and what i replaced etc, took him 2 days to find this:
The cam lobe for the fuel pump was worn down. He said that he doesnt think he has ever seen that go before, the other lobes all the time but not that one.
so his recommendation is to put in a stand alone electric fuel pump, wait to replace the cam because if i do then it will stress the other parts of the engine, just wait for the rebuild and do it then, sounds about right to me.