Jerry,
I suspect that your rear SU is suffering from that most common of all
maladies which affect SUs, namely that its float needle is leaking.
When this occurs, the idling gives all the signs of a too rich mixture
so the tuner leans it out by screwing up the jet. This solves only the
idling problem - and that only if the float needle is leaking only
slightly.
Unfortunately it most certainly will cause the carb to give too weak a
mixture under power, resulting in less power immediately and probably
burnt exhaust valves later. (I burnt out an exhaust valve on a year-old
1956 TR3 in the hills of New York State as a result of my amateur
"tuning".)
Altho you may be able to fix it temporarily by grinding the float needle
and seat with fine grinding paste the only permanent cure is to fit a
new pair - I have found Grose Jets work very well. I put a pair on my
TR3A in 1995 when I rebuilt the SUs; they have performed flawlessly over
18,000 miles and I've never even been tempted to "tune" the carbs since
then.
Most of "our" suppliers stock Grose Jets. Moss's retailed at $5.95 each
in their 2000 April 10 catalog. Buy one for each carb!
jgoulet@finning.ca wrote:
>
> I have been tuning the engine of my TR3A over the last few
> weekends and have run into a puzzle.
> The engine runs rough during idle and also there seems to be
> less power, so I tried the test of pulling a spark plug wire one at a time
> while the engine was idling (1200rpm).
> Cylinders 1 & 2 both caused speed loss as expected but
> cylinders 3 & 4 both caused no change when their leads were pulled out.
> I have swapped cap and plug wires with a spare set with the
> same result.
> Wires are connected 1-3-4-2 going counterclockwise on the
> cap from the first pointed a plug one.
> I have swapped the plugs 1 - 4 and 2 - 3. Yet cylinders 3 &
> 4 do not seem to have fire even though they have spark and fuel.
> I cleaned and gapped the plugs at 35 thou....there is a
> sport coil (40000 volt)
> The plugs were sooty black, worse on cylinders 3 & 4.
> Also, I have balanced the SUs with Unisyn and leaned them
> out as far as they will go, but the rear SU (for cylinder 3 & 4) still
> causes the rpm to rise and stay up when the lift pin is pushed.
> The valves have been set to 10 thou.
> Compression test shows about 140 psi in each hole.
>
> Could the rear SU cause this?
> Can you suggest any more tests?
>
> Jerry Goulet
> 1960 TR3A
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