On Mon, 5 Jun 1995, Peter Barrance wrote:
>
> My TR6 seems to be running lean based on the white deposits I'm seeing on
> the plugs. Also, it tends to stall when I crack the throttle momentarily
> and back off. The power seems to be there but it runs hot in traffic. I've
> tuned up the ignition and set the timing to where it seems to run best (max
> advanced w/o pinging)
>
> After disconnecting the throttle shafts and setting the throttle stops 1.5
> turns out, the 'lift the piston with a screwdriver' test sharply decreases
> the revs on both carbs - again pointing to a lean mixture.
3 Things come to mind. Worn throttle shaft bushings $$, Incorrect needle
mixture adjustment, Incorrect adjustment of by-pass valve or bad by-pass
diaphragm. I'd bet on a combination of all 3.
> Unfortunately I seem unable to do anything about it. Turning the needle
> adjustment using the allen-wrench carb adjusting tool seems to do nothing
> for me. In fact, I pulled the piston out of one of the carbs and tried
> turning the wrench and watching the needle - it doesn't seem to move in or
> out relative to the piston. Am I misunderstanding something?
Do ensure that the needle holdown screw (on the side of the piston,
pointed normal into the needle) is loosened slightly before adjusting the
needle position, and ensure that the vertical slot in the needle carrier
is lined up with the holdown screw. Does this make sense? Perhaps not
until you pull the whole thing apart. From what you describe, your
holdown screw may be missing/damaged.
> Also, a question regarding the direction in which I should turn to richen
> the mixture. I would have thought I should be unscrewing (turning
> anti-clockwise) the needle, thereby increasing the gap between the jet and
> the needle, allowing more fuel in. This seems contradictory to what
> Bentley's says.
Turning the adjusting wrench in a clockwise direction (when viewed from
above) will result in the needle being retracted into the piston. This
has the effect of richening the mixture, since the tapered needle pulls
away from the jet and allows more fuel to flow over the bridge.
The throttle by-pass valves are something to check out while you are
tinkering with the carbs. They're on the right (forward) side of the
carb bodies, all the way toward the manifold flange. Inside is a
diaphragm and a spring loaded plunger. It's not uncommon for the
diaphragm to be cracked, and this can result in an over-lean condition as
the purpose of the valve is to allow incoming air to by-pass the throttle
butterfly. New diaphragms are supplied with rebuild kits, and adjustment
of the valve is pretty straightforward.
> Thanks for any help you guys can give! I'm still enjoying the list.
I suppose there's always the possiblity of a general vacuum leak. Spray
starting fluid, carb cleaner, or an unlit propane torch around the
sealing surfaces of the head and carburettors while the engine is idling
and listen for changes in engine speed. There shouldn't be any.
Have fun and good luck,
Greg
Greg Meboe meboe@wsunix.wsu.edu
Dept. of Mechanical and Materials Engineering
Washington State University, Pullman, Wa.
'85 XJ-12 H.E. (daily) '67 Spit-6 '74 TR-6
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