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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: 76GreenBeast on June 29, 2006, 09:27:00 am

Title: Rough Idle
Post by: 76GreenBeast on June 29, 2006, 09:27:00 am

My 76 K10 has started having a "rough" idle.  Its most noticeable when it is idling in gear (TH350).  I first thought it was mis-firing, so I did a tune-up (cap,rotor,plugs,wires) yesterday and its still doing it.  When I start driving the engine runs great, so I don't think the timing is off.  Could it be the carb or fuel filter clogging up enough to make it studder at idle?  I've never changed a fuel filter on a carb set-up before are there any special fitting tools required?

Thanks.  

BTW it has a stock Quadrajet (thats been re-built twice) with the stock fuel filter location.

Edited by: 76GreenBeast at: 6/29/06 8:29 am

Title: Re: Rough Idle
Post by: Lt.Del on June 29, 2006, 10:18:00 am

The fuel filter on a qjet is right in front.  you see where the fuel line goes in.  There is nothing hard about it.  just loosen the nut..be careful of a spring action in it, the filter will go flying.  go to any auto parts store and get the paper cylinder style filter for a couple of bucks.

Did you check your vacuum?  Seems to me like a vacuum leak.  Carefully spray starter fluid or wd40 around you hoses, carb gaskets, etc.  while your engine is idling.  If your rpms go up, you found a vacuum leak.

SgtDel
aka "Andy"

www.delbridge.net

1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4

Title: Re: Rough Idle
Post by: 76GreenBeast on June 29, 2006, 05:22:00 pm

I don't have very many vacuum lines on this truck.  One goes to the THERMAC temp sensor, one goes to the vacuum booster, and one goes to the Trans modulator.  I don't have any of the heavy emissions junk on lighter and later models.  Would a leak in any of these lines cause a studder at idle?  I don't know when these lines are "energized" so to speak such that there would be a parasitic leak from the line.

It started doing this when the weather started getting hot here in SoMD.  When I start it up in the morning it doesn't do it, only when the engine is up to operating temp.  Could the change in outside temp combined with the engine heat cause the line to expand and leak???

Thanks for the help....

Title: Re: Rough Idle
Post by: 1976Scottsdale on June 29, 2006, 05:47:00 pm

Spray the WD-40 around the base of the carburetor and make sure that the gasket is not leaking.  Mine is doing the same, so I have to figure it out when I have time.

Title: Re: Rough Idle
Post by: 76GreenBeast on June 29, 2006, 07:23:00 pm

I checked some of the vacuum lines, and they look OK, When I was running it just a few minutes ago, It wasn't doing it.  I sprayed WD-40 on the vacuum advance line going to the distributor and nothing happened.  The line was kind of loose at the connection to the metal line running around to the front of the carb, so I don't know if thats the culprit.  Maybe the heat from the engine with the hot day is making the line expand around that fitting and make it leak slightly.  There was also a gash in the line that went from the carb to the THERMAC sensor (on the sensor end) but I'm not sure if a leak there would cause the problem......

It has something to do with the hot weather, it wasn't doing this last fall/winter......

Title: Re: Rough Idle
Post by: Lt.Del on June 30, 2006, 01:12:00 am

Do you have hot air choke????
may need adjustment.

SgtDel
aka "Andy"

www.delbridge.net

1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4

Title: Re: Rough Idle
Post by: Rickster on July 04, 2006, 04:04:00 pm

If by "gash" you mean the vacuum line is split open, it needs to be replaced.  Besides the damper in the snorkle may not close on start up.  On top of that to much air is being drawn in below the trottle valve which can cause problems - like the fuel mixture is then to lean.  Do not want any stray particles to be pulled into the intake either.