Burning rich??

[Todd B]

My semi has started to smell like unbuned fuel in the cab. I notice more when I back off. My dad would of said it is burning rich, he passed away a few years ago so I can't call him for advice. By memory I believe I should adjust a screw on the carb.. Any help would be appreciated.

Todd B

[NiftyFifty]

If it just all of a sudden happen it may be time for a rebuild. Are you having and noticeable running or idling issues? Could be a float issue, or you could get lucky and just screw in the mix screw a 1/4- 1/2 turn at a time and try it.

That's the screw near the base of the carb

[MBF]

Check for a fuel leak under the hood to see if something started leaking. Could be simple as a leaky sediment bowl gasket, or a broken bowl.

If you smell it's when it is idling you may get by with an idle mixture adjustment-but that shouldn't change by itself. I'd suspect the needle valve, float/seat asm may be the problem. I believe that is an updraft carb on the cabovers correct? If so, that will be a different carb and probably require a different kit to do the overhaul. Mike

[Todd B]

Check for a fuel leak under the hood to see if something started leaking. Could be simple as a leaky sediment bowl gasket, or a broken bowl.

If you smell it's when it is idling you may get by with an idle mixture adjustment-but that shouldn't change by itself. I'd suspect the needle valve, float/seat asm may be the problem. I believe that is an updraft carb on the cabovers correct? If so, that will be a different carb and probably require a different kit to do the overhaul. Mike

I have a conventional cab (1953) 251. I will check for leaks.

Thank you

[greg g]

adjusting the idle screw has no effect on anything except the mixture at idle. If you are indeed running rich, (did you pull a spark plug or two to see how they are burning?) Do you see any black smoke at the tail pipe while driving?

There are a couple of things that can contribute. Rich means not enough air for the gas being pulled through the carb, is there a simple restriction in the flow of air like the choke not opening all the way, an airfilter needing servicing, or other blockage? Or too much gas for the air flowing in, take the top off the carb and assure the step up valev is not sticking in the open position. Also make sure your float level is proper and your needle and seat in the carb inlet are closing tightly.