important GMC running problem

[TooTallMike]

Dear esteemed team,

 

I am tearing my hair out with a GMC that needs to be at a D-day event at the end of the week. It has just had an overhaul consisting of a partial strip-down and re-paint, also conversion to 12V, alternator and positive earth, clutch change, and some brake work.

 

It starts eventually on 90% choke but then if let in even slightly it dies very quickly. It will rev up on 90% choke although it's a bit lumpy. If the choke is pushed in while revved up it sounds a lot better but very quickly and irrecoverably dies. There is a lot of white smoke when it's revved up. I've tested for air leaks and checked fuel supply. I've stripped and thoroughly blown out the carb. The regulator is missing its butterfly so that's not the problem.

 

When I saw the smoke I wondered about the hydrovac going bad, but it's the same even with it disconnected and the vacuum pipe blanked off. It has been suggested that if that is the problem there may still be residual brake fluid in the inlet manifold thus giving a false negative. This will be my first thing to look at tomorrow unless anyone has any better suggestions?

 

I'm sure I've seen this problem before but I'm stumped.

 

Thanks for any suggestions - MG

[N.O.S.]

Don't know if you have access to a TM, but just had a quick look:

 

Assuming jets set correctly, what about sticking crankcase ventilating valve? Idling circuit in carb clogged?

 

White smoke indicates brake fluid but no idle / running without choke suggests poorly adjusted jets?

[TooTallMike]

Don't know if you have access to a TM, but just had a quick look:

 

Assuming jets set correctly, what about sticking crankcase ventilating valve? Idling circuit in carb clogged?

 

White smoke indicates brake fluid but no idle / running without choke suggests poorly adjusted jets?

 

Thanks Tony,

 

I should have mentioned that the truck ran perfectly about 2 months ago when parked up for work. It hasn't been run at all in that time and the carb was not touched during any of the work.

 

I blew everything through with an air line when the carb was stripped but the only jet removed was the idle. I'll have a look at the others too tomorrow.

 

- MG

[Pete Ashby]

I'm assuming Mike the white smoke only appeared with the current problem.

Have you have checked your coolant level hasn't suddenly gone down ?.

This a really long out field thought but a gallon of diesel hasn't found it's way into your tank?.

I'm also assuming you have gone through your plugs and ignition leads to see if one cylinder isn't firing?

 

Had a bit more of think on this, you say some brake work was undertaken did that include the hydrovac

Have you checked to see if the brake fluid level in the master cylinder has gone down?

 

Pete

Edited June 4, 2014 by Pete Ashby

[eddy8men]

like pete said contaminated fuel seems likely and is easy to clarify with a bit of fresh chucked down the carb

[TooTallMike]

Thanks guys, someone else also suggested contaminated fuel so I'm going to rig up a fresh jerrycan as my first test this afternoon. Also hoping to borrow a known good carb if needed.

 

I'll let you know how it goes.

 

- MG

[TooTallMike]

Well, it was contaminated fuel. We've no idea what the source was but the truck now starts and ticks over on new juice like the proverbial sewing machine.

 

Many thanks to all for thoughts and suggestions.

 

Cheers - MG

[g0ozs]

I had a similar problem after taking my first SUMB to a pressure washer after I first brought it home - I eventually found a leak where the fuel filler pipe joined the tank, and got quite a lot of water out after letting the tank settle and draining it.

 

Iain

[Pete Ashby]

Well, it was contaminated fuel. We've no idea what the source was but the truck now starts and ticks over on new juice like the proverbial sewing machine.

 

Many thanks to all for thoughts and suggestions.

 

Cheers - MG

 

Glad it's sorted Mike

 

regards

 

Pete

[eddy8men]

glad it's sorted and your stress levels have returned to normal