What's causing this carbon buildup...

If this is hwy mileage you should get around 19.You carb may be running rich.I checked the jets in my carb and they were all at of spec on the rich side. Some times the repro jets are not even close to spec.
 
Yeah, that's what I think too, Duffy. But my plugs are the right color and I have tested all the jets and I'm on spec. I don't understand why I can't get better mileage, except I notice when I can burn non-ethanol I can get up to about 18 and 19 mpg. So I'm gonna blame the corn.
 
Maybe too obvious, but perhaps a bit too much MMO or other Fuel additive? I would suspect that for a short trip under 15 or so miles, the cylinder temps may not be getting hot enough to burn out any oily residue from a fuel treatment.

Hope it helps :)

- Dave
 
Dave in Boise said:
Maybe too obvious, but perhaps a bit too much MMO or other Fuel additive? I would suspect that for a short trip under 15 or so miles, the cylinder temps may not be getting hot enough to burn out any oily residue from a fuel treatment.

Hope it helps :)

- Dave
Click to expand...

I don't use any additives.
 
Hi I have a simular problem as you and while took me ages to find it turned out to be inlet manifold leak on just 1 flange causing just 2 cylinders to run lead and hot and 2 cylinders to run rich
this may not be your problem I had the same issues
Colin
 
I get much better mileage whenever I can get non-polluted fuel for any of my cars, including my '09 Silverado...and the engines run smoother....

lesse...more miles per gallon means less pollutants per mile means the EPA should be happy...arg, what was the purpose of polluted fuel again ???
 
tbirdtbird said:
I get much better mileage whenever I can get non-polluted fuel for any of my cars, including my '09 Silverado...and the engines run smoother....

lesse...more miles per gallon means less pollutants per mile means the EPA should be happy...arg, what was the purpose of polluted fuel again ???
Click to expand...

Makes the corn corporations richer.
 
colin1928 said:
Hi I have a simular problem as you and while took me ages to find it turned out to be inlet manifold leak on just 1 flange causing just 2 cylinders to run lead and hot and 2 cylinders to run rich
this may not be your problem I had the same issues
Colin
Click to expand...

This is an interesting theory, because I had some trouble seating my manifold gaskets last March. I have new gaskets now, and I may just put them on and see if that helps.
 
If you can adjust the air mixture screw on the carburator per spec (1 1/2 turns out ? ) then the intake manifold is probably not leaking unless it is a very very minor leak. If the air screw can only be turned out a small amount then you probably have a leak at the intake manifold . Spray some oil around the intake manifold while the engine is running and see if the rpms change .
 
Duffy1 said:
If you can adjust the air mixture screw on the carburator per spec (1 1/2 turns out ? ) then the intake manifold is probably not leaking unless it is a very very minor leak. If the air screw can only be turned out a small amount then you probably have a leak at the intake manifold . Spray some oil around the intake manifold while the engine is running and see if the rpms change .
Click to expand...

Air mixture screw seems correct. No change in the rpms when tested as you suggest. Hmmmm...:confused:
 
Duffy1 said:
If you can adjust the air mixture screw on the carburator per spec (1 1/2 turns out ? ) then the intake manifold is probably not leaking unless it is a very very minor leak. If the air screw can only be turned out a small amount then you probably have a leak at the intake manifold . Spray some oil around the intake manifold while the engine is running and see if the rpms change .
Click to expand...

play in the carb throttle shaft can also cause this
 
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