Running rough - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**Stephen717** · 12-31-2018, 05:17 PM

So my truck is running a bit rough. I have had the fuel mix screw all the way in and took it out a turn and a quarter until I had it running somewhat ok. But now it won't run unlessitsat half or three quarter choke. Could this be a timing issue or am I just doing something wrong with the set screw

**Wrenchbender Ret** · 12-31-2018, 05:36 PM

Sounds like a carb. problem. It could be a bad vacuum leak or water in the fuel. What engine is in it?

George

Quikshft · 12-31-2018, 08:08 PM

I recently fixed rough runner with a new carb gasket. Just something to check, it had me mystified for a while.

**Stephen717** · 01-01-2019, 02:50 AM

It's a 250, I need to redo the exhaust on it and the gas in it is fresh. Just rebuilt the carb before gas was put through it. Could it be that it's getting too much air from the exhaust leak at the manifold?

Quikshft · 01-01-2019, 01:49 PM

I don't believe an exhaust leak is going to cause a problem with maintaining an idle, though it is something you'll need to work on eventually. If you need a lot of choke applied to keep it running I would look for a vacuum leak of some kind. Broken vacuum hoses, carb gasket, brake booster etc. Something is letting too much air in and foiling the carburetor mix.

**special-K** · 01-01-2019, 02:00 PM

Yep, getting to much air from somewhere. Try spraying a little something on areas to make it rev if it's coming in.

davepl · 01-01-2019, 02:08 PM

If the choke improves and richening up the mixture improves it, you've got a vacuum leak somewhere methinks!

Check hoses, base gaskets, etc... sure sounds like it though.

When you say you just "rebuild the carb", isn't that kind of the smoking gun?

Greasey Harley · 01-01-2019, 02:50 PM

How is the ignition system? Plugs, wires, cap rotor.
Points and condensor (if so equipped)
Are you 100% positive that there are no vacuum leaks?

I bring this up because you can't twist the screws on the carb and rectify these problems. Most "carburetor problems" are actually ignition problems.

I'm not suggesting that you don't have a carburetor problem, just pointing out that you should NEVER try to adjust the carb without looking at ignition and vacuum systems FIRST.

RichardJ · 01-01-2019, 02:59 PM

The exhaust gasket and intake gasket are a one piece item, usually.

If the exhaust manifold-to-head gasket is leaking it should be a no-brainer for you to see that the intake portion of the gasket is also leaking.

**Wrenchbender Ret** · 01-01-2019, 03:33 PM

I take it is a Monojet carb. You may have the float level too low. Or the gasket between the throttle body & fuel bowl is the wrong one or in backwards.

George

toolboxchev · 01-01-2019, 11:20 PM

Do all your ignition diagnosis first. A free DVOM from harbor freight with the correct specs can save you a ton of money and aggravation.

12-31-2018, 05:17 PM	#1
Stephen717 Senior Member Join Date: Aug 2016 Location: Loris South Carolina Posts: 428	Running rough So my truck is running a bit rough. I have had the fuel mix screw all the way in and took it out a turn and a quarter until I had it running somewhat ok. But now it won't run unlessitsat half or three quarter choke. Could this be a timing issue or am I just doing something wrong with the set screw

12-31-2018, 05:36 PM	#2
Wrenchbender Ret Registered User Join Date: Oct 2005 Location: Overland Park, Ks. Posts: 5,190	Re: Running rough Sounds like a carb. problem. It could be a bad vacuum leak or water in the fuel. What engine is in it? George

12-31-2018, 08:08 PM	#3
Quikshft Registered User Join Date: Oct 2010 Location: Kingsburg, Ca. Posts: 92	Re: Running rough I recently fixed rough runner with a new carb gasket. Just something to check, it had me mystified for a while.

01-01-2019, 02:50 AM	#4
Stephen717 Senior Member Join Date: Aug 2016 Location: Loris South Carolina Posts: 428	Re: Running rough It's a 250, I need to redo the exhaust on it and the gas in it is fresh. Just rebuilt the carb before gas was put through it. Could it be that it's getting too much air from the exhaust leak at the manifold?

01-01-2019, 01:49 PM	#5
Quikshft Registered User Join Date: Oct 2010 Location: Kingsburg, Ca. Posts: 92	Re: Running rough I don't believe an exhaust leak is going to cause a problem with maintaining an idle, though it is something you'll need to work on eventually. If you need a lot of choke applied to keep it running I would look for a vacuum leak of some kind. Broken vacuum hoses, carb gasket, brake booster etc. Something is letting too much air in and foiling the carburetor mix.

01-01-2019, 02:00 PM	#6
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,862	Re: Running rough Yep, getting to much air from somewhere. Try spraying a little something on areas to make it rev if it's coming in. __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~

01-01-2019, 02:08 PM	#7
davepl Registered User Join Date: Aug 2006 Location: Redmond, WA Posts: 6,332	Re: Running rough If the choke improves and richening up the mixture improves it, you've got a vacuum leak somewhere methinks! Check hoses, base gaskets, etc... sure sounds like it though. When you say you just "rebuild the carb", isn't that kind of the smoking gun? __________________ 1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible

01-01-2019, 02:50 PM	#8
Greasey Harley Registered User Join Date: May 2013 Location: Post Falls, ID Posts: 884	Re: Running rough How is the ignition system? Plugs, wires, cap rotor. Points and condensor (if so equipped) Are you 100% positive that there are no vacuum leaks? I bring this up because you can't twist the screws on the carb and rectify these problems. Most "carburetor problems" are actually ignition problems. I'm not suggesting that you don't have a carburetor problem, just pointing out that you should NEVER try to adjust the carb without looking at ignition and vacuum systems FIRST.

01-01-2019, 02:59 PM	#9
RichardJ Registered User Join Date: Apr 2010 Location: So Cal Posts: 1,409	Re: Running rough The exhaust gasket and intake gasket are a one piece item, usually. If the exhaust manifold-to-head gasket is leaking it should be a no-brainer for you to see that the intake portion of the gasket is also leaking. __________________ '67 GMC 2500, 292, 4spd, AC

01-01-2019, 03:33 PM	#10
Wrenchbender Ret Registered User Join Date: Oct 2005 Location: Overland Park, Ks. Posts: 5,190	Re: Running rough I take it is a Monojet carb. You may have the float level too low. Or the gasket between the throttle body & fuel bowl is the wrong one or in backwards. George

01-01-2019, 11:20 PM	#11
toolboxchev Registered User Join Date: Jul 2015 Location: 2nd left past the stump on a dirt road. Posts: 2,629	Re: Running rough Do all your ignition diagnosis first. A free DVOM from harbor freight with the correct specs can save you a ton of money and aggravation.



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