> I have built an supercharged engine with a one three quarter inch
>SU carb. The engine was set up OK, and drove for several months.
> At some point, it became necessary to adjust the mixture again, to
>richen it. It soon became apparent that the mixture was
>progressively leaning out over time as I had to 'chase' it with the
>mixture adjustment nut. At times, the spark plugs burnt white (lean
>mixt.) and the engine would only run with the choke and
>considerable overheating (air cooled motor).
> I suspect the throttle spindle is worn, and ask - Does any body
>have any suggestions, or better ideas, as to what is wrong? It is an
>old carb, from a Austin Allegro 1750, so wear is not unlikely.
>
>Thanks for any info,
>
>Matthew Roberts
Though this embarrasses me greatly, I will relate a similar problem I had.
When I first rebuilt my SUs in the Sprite, I replaced both jets etc. What
I didn't notice was that I left one of the jet O-rings in the fuel bowl.
When I installed the new jet, it pushed the O-ring back into the bowl,
where it caused no trouble at first. With time and exposure to petroleum
products, it swelled slowly, reducing the fuel flow to the carb, causing a
progressively leaner mixture. I eventually stripped down the carb, and
found the extra O-ring, but not after many frustrating sessions with the
car.
A leak at the throttle spindle (or any vacuum leak) should manifest itself
mostly at closed throttle conditions, such as idle and constant speeds,
whereas fuel flow problems are more likely to cause acceleration problems.
Pulling the choke out will improve the former, but not the latter.
Good hunting! sf
Stan Fickes British Cars
Wireless Communication Tech. 1951 MGTD (under restoration)
Advanced Technology Group 1965 Austin-Healey Sprite (daily driver)
Apple Computer, Inc.
408 974-7653
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