I don't think your description sounds like vapor lock: that malady will usually
cause the engine to shut off completely, including at idle. I would first
suspect a small piece of solid contamination (dirt or perhaps a piece of rice)
has found its way into the carburetor's float bowl. When the engine is running,
the dirt is being moved around by the motion of the fuel until it is drawn into
and blocks the main jet, thus shutting off the fuel supply. The idle jet is
independent. The dirt settles to the bottom when the engine is off, and that is,
coincidentally, the perfect opportunity to remove the offending article.
Jim
Steve Conley wrote:
> Mike-
> I don't know much about Mikuni's but let me relate my similar recent
> experience. I had almost the same symptoms. Mine turned out to be a chunk
> of rubber hose in the needle valve chamber of my Z-S carb (freakin chunk
> would plug needle valve at sufficient suction!).
> Here are some of the other suggestions that I received from the list and
> elsewhere:
> 1) Fuel pump - check output ( aprox 1 pt per minute)
> 2) Fuel filter - plugged/restricting flow
> 3) Vapor lock - insulate offending fuel line
> 4) Vacuum build-up in fuel line from tank (vented gas cap? plug in fuel line
> from tank?)
> 5) Vacuum from carbon canister - pull line from carbon canister to carb
> Hope this helps!
>
> Steve Conley
> Marysville, WA USA
> '76 MGB Roadster
> swconley@foxinternet.net
> MGB Online: http://web3.foxinternet.net/swconley
> NW MG Golf Tournament/Rallye : http://web3.foxinternet.net/swconley/golf.htm
>
> ----- Original Message -----
> From: Michael E. Blair <mblair@nfis.com>
> To: <mgs@autox.team.net>
> Sent: Saturday, May 06, 2000 7:22 PM
> Subject: Is this vapor lock?
>
> > My 78B has a Mikuni carb on it. I have finally got it tuned to the point
> > that it idles good but am still going to replace it. My problem is this;
> the
> > car starts and runs great for about 15 to 20 minutes. After this point it
> > seems to lose all power. Still idles great at 800 rpm, but step on the gas
> > and it spits and sputters. It seems to be related to heat and this is what
> > steers me towards vapor lock. The heater hose line that was hooked to the
> > original auto choke on the original carb is now routed directly under the
> > mikuni... could this be heating up the carb enought to vapor lock it?
> >
> > Any thoughts?
> > Mike
> >
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