Many contributors to this newsgroup have offered good advice on how I
might deal with the enrichment problem that I was experiencing with my
1976 GMC MH. To summarize, the exhaust would blow black sooty smoke
whenever I would accelerate hard to pass or use maximum throtle on steep
grades. The problem started after I had the carb "rebuilt" last summer
to cure a hard hot starting problem. It was especially bad at altitudes
like northern New Mexico's on a recent trip . After recieving many
good suggestions and reading Doug Roe's book on Rochester Carburetors
(this gave me the confidence to take mine apart) I think I have found
and corrected most of what was wrong. The following e-mail to Wes
Caughlan of Cinnabar Engineering gives more details of what I found.
Wes and Alex Wong were particularly helpful in giving me the lead to
examine the gaskets in the carb.
Phil Stewart
Date: Tue, 17 Feb 1998 20:57:55 -0800
From: "Philip L. Stewart"
To: GMCMH
Subject: Re: Carburetor
>
> Phil,
>
> Alex Wong and I discussed carburetors yesterday during a routine meeting.
> Alex is a Professional Engineer who is on our Engineering Staff in
> addition to being Western Manager of GM's Service Technology Group. Prior
> to joining GMSTG, Alex was with GM's Rochester Division, and he is
> considered one of the foremost Quadrajet authorities. Alex is
> coordinating our remanufacturing program to make GMC motorhome
> carburetors available.
>
> Alex says there are a number of things that could be causing your
> black-smoke problem. First, we both point to the fact that you feel the
> problem commenced after you had the carburetor overhauled and a kit
> installed. According to Alex, there are a number of kits in the market
> that have air-horn gaskets that plug air bleeds in the motorhome
> carburetor. These gaskets will cause excessive secondary enrichment.
> Secondly, the spring clip on the needle valve is very often installed
> incorrectly, and this will increase float level, which will cause
> excessive enrichment. Third, the (secondary) air valve adjustment may not
> be correct. The adjustment procedure for this valve is on page 6M-3 of
> the X-7525 Maintenance Manual. Fourth, fuel pressure may be excessive.
> (This is generally caused by electric fuel pumps.) You should check fuel
> pressure between the fuel pump and the carburetor, and it should not
> exceed 5 psi.
>
> We could remanufacture your carburetor and return it to you close to RFG
> requirements. We do this for a number of customers every month. However,
> we could not guarantee this would solve your problem, because we cannot
> flow the carburetor without the motorhome.
>
> Wes Caughlan
Wes,
I want to thank you for the suggestions on what to look for as possible
causes to the fuel enrichment problem my Q-jet was experiencing.
When I took the air horn off to check check the float level setting, I
noticed that 2 gaskets had been installed between the air horn and the
float bowl assembly! Not only that, but the gaskets each were somewhat
thicker than the replacement gasket that I bought with the Delco carb
repair kit sold for my model. This increased the effective thickness
well beyond what Rochester had origninally determined as needed. The
result I believe was both the primary and secondary metering rods were
being raised out of their jets too early, changing their fuel metering
characteristics to the rich side of the fuel to air ratio.
I can only guess why there were 2 gaskets installed, but it looks like
that when the gaskets were stamped out by the manufacturer two clung
together ( I had to intentionally pull them apart). Perhaps the
mechanic who rebuilt the carb last summer was in too much of a rush to
realize that only one should be installed.
While I had the air horn off I reset the air valve to the shop manual
spec. (it now takes a little more force to open, which I also believe
helped to lean out the mixture since the secondaries should now open a
litte later). I also made sure the made sure the float was properly set
and installed the rest of Delco kit parts.
After putting the carb back together, I find the engine is running much
better without any hint of black smoke under heavy acceleration or wide
open throtle. I plan to soon see how it works at Tennesses's highest
road pass in the Somkeies (over 5,000 ft.) but I belive that my problem
is now solved well enough that I can wait for my next trip to Michigan
(sometime next summer I hope) when I can visit your shop in Sandusky to
have the carb checked out more thougherly and adjusted as needed.
Again, my thanks go to you and Alex Wong for your help. PS. I intend
to copy this message to the Patrick Flower's GMC Newsgroup.
Phil Stewart
atri63
