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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: mcintyrederek on July 15, 2015, 07:02:18 pm

Title: 350 w/Qjet rough idle, stinks, runs good on highway.
Post by: mcintyrederek on July 15, 2015, 07:02:18 pm

1986 C10, 350 crate motor with 30K miles, Quadrajet carb, no emissions.

Has been running excellent for several years and started to notice a slight rough idle when in gear (TH400 transmission) but would otherwise run great on the highway.

Several days later, the idle as turned into a violent shake and I almost lose it altogether at a stop light, unless I step on the gas a little and brake at the same time.  The exhaust sounds like I'm running on 6 cylinders, but I'm near positive that ignition is not an issue (unless I'm overlooking something again).

No spark knock, timing set about 6 to 8 degrees advance initial, secondaries open fine and HP is good for a stock build engine.

Checked all plugs and wires (but like I said, runs great at highway speeds).

Removed the carb.  Filter looks good, no vacuum leaks, intake to carb seal is fine.  Idle adjustment screws removed and the bore is clean with no trash.  The engine idles just fine when I partially close the choke plate (I have converted to manual choke).  This tells me the engine is running lean at idle speed.  I attempted to adjust both idle mixture screws and backed them nearly all the way out with no real effect. 

It's strange this just happened all of the sudden.  Is there something else in the carb that can "wear out" that I might have to rebuild, or am I overlooking something.  Thanks for the help.

Mack.

Title: Re: 350 w/Qjet rough idle, stinks, runs good on highway.
Post by: enaberif on July 15, 2015, 10:10:59 pm

most quads need a rebuild as the parts in them are old and brittle.

Title: Re: 350 w/Qjet rough idle, stinks, runs good on highway.
Post by: Irish_Alley on July 16, 2015, 01:09:17 pm

http://rmcavoy.freeshell.org/Q_Jet_bushings.html

Title: Re: 350 w/Qjet rough idle, stinks, runs good on highway.
Post by: mcintyrederek on July 16, 2015, 04:52:14 pm

Thanks for the link, that's a cool tip.

Title: Re: 350 w/Qjet rough idle, stinks, runs good on highway.
Post by: mcintyrederek on July 16, 2015, 04:53:31 pm

I may have an ignition problem, I bought one of those $39 red 65K distributors off Ebay last year and am starting to suspect perhaps a problem with it.

Title: Re: 350 w/Qjet rough idle, stinks, runs good on highway.
Post by: blazer74 on July 18, 2015, 01:33:43 am

I'm going to say it's carb rebuild time, partially blocked idle tube or channel causing lean shift. Runs better highway when on main circuit.

Do a good visual on vac lines to pvc, brake booster etc., pvc valve stuck open or broken pvc  line causing massive vac leak.

Throttle shaft bushings will cause lean condition as in the mentioned in link but not all of a sudden.

Brake booster could be a big vac leak also if bad but you would feel it in your brakes.

Rule out basics plug wires, cap, rotor first .





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Title: Re: 350 w/Qjet rough idle, stinks, runs good on highway.
Post by: Don5 on July 18, 2015, 01:58:52 am

Quote from: Irish_Alley on July 16, 2015, 01:09:17 pm

http://rmcavoy.freeshell.org/Q_Jet_bushings.html

Thanks for the link. That was really interesting. :)

Title: Re: 350 w/Qjet rough idle, stinks, runs good on highway.
Post by: mcintyrederek on July 18, 2015, 08:06:40 am

The problem was a crappy distributor, as mentioned in another thread started.

Thanks for the info, this is all interesting and good information, which helps me to understand the entire fuel system better.