Re: [MV] M38 Carburetor acting up

Alan Bowes (
Mon, 08 Dec 1997 13:09:01 -0700 wrote:
> I am not very familiar with the inner workings of
> *any* carburetor, and this one (Carter YS) should
> be simple, I guess, but, sorry, I'm stuck...
> About half the time the carb works well, with the
> mixture adjustment behaving normally. The rest of
> the time the engine goes into a strange mode -
> the mixture seems to have jumped way richer,
> with gas smell in the exhaust, a bit of smoke,
> rough idle, etc. When it is in this "mode", the
> mixture adjustment screw has absolutely no effect!
> Crank it all the way in, crank it all the
> way out, no difference.



Here are some observations, not in any particular order of precedence:

1) You probably won't actually know for sure what is causing the problem
until you tear down the carburetor. There are a few external tests, but
don't waste your time. The carb can probably benefit from a good
cleaning anyway, especially if you've recently replaced other components
in your fuel system, dislodging internal debris in the process.

2) The mixture screw only affects idle mixture, not the mixture through
the power circuit.

3) Even though these might be considered "simple" carburetors, they
still have tiny orifices inside that can easily become clogged. For
example, the emulsion tube (air well) on many carburetors may have holes
as small as a human hair. If one or more of these holes are clogged,
insufficient air will be mixed with the fuel prior to its entry into the
discharge nozzle in the venturi. Besides making the fuel more easy to
atomize, this air also has a leaning tendency, as a greater proportion
of air is introduced with increasing air velocity through the venturi.

4) There are also parts with close tolerances. For example, consider the
relatively close clearance between a step-up piston and cylinder. If the
piston gets stuck in a position where it is holding the metering rod up,
it will allow an overly rich mixture under anything but the heaviest
load conditions.

5) A leaking or stuck-open float valve can certainly cause an overly
rich mixture as the bowl fills with fuel and overflows through the
venting circuit into the throat. On some carbs, the higher fuel level
may also prevent the air emulsion tube from introducing air when it
needs to do so.

6) A loose jet can create an overly rich mixture.

7) I assume you've checked this, but is your choke opening all the way?

Ordering an overhaul kit is a good idea. There are a few 'gotchas'
involved in good quality carb overhauls. It would help if you have the
manual for the carb. Some tips: Cleanliness is imperative at all stages
of an overhaul. Don't take apart throttle or choke plate assemblies
unless you know your stuff. Don't bead blast or pressure-wash anything
(there may be some rare exceptions to this, but it's best just not to do
it). Take precautions with the many flammable and toxic liquids, mists,
and vapors that you will encounter during the overhaul.


(Salt Lake City, Utah)

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