Ted,
I had a similar experience when I first installed the 493 in my
autocross car. Fortunately for me it cleared up as quickly as it
appeared and I have no idea what caused it. Last year at the first event
I ran with the new motor, I started it up and it was obvious that only
one cylinder (mag side) was operating. We switched spark plugs, leads,
confirmed fuel was getting to the cylinder but we could not get the PTO
side to fire. Next morning we put everything back together and it ran
fine. It has run fine ever since. Ah, ya gotta love the mysteries of racing.
-Jeff
Ted Rudolph wrote:
Anyone else having trouble tuning a 493? Can anyone give any insight into
properly tuning one?
My engine (new from Tony Murphy in 2004) was running pretty well last summer,
although at the last race of the season (the Fun One at the Glen) the PTO
cylinder did not come up to temperature as quickly as the Magneto side. If we
let the engine run for a few minutes, did a lap around the paddock & then
changed the PTO spark plug the engine would run fine. Once we finished out
warm-up procedure it would run great - in fact, it was very fast all weekend.
We didn’t do anything over the winter except remove the balance tube and plug
the holes. This year at the Pocono Double the engine was a bear. Same
problems getting the PTO cylinder to come up to temperature, although changing
the plug after warming didn’t help things. The PTO plug would be wet when it
came out, while the Mag plug was a nice milk-chocolate brown color.
During the first on-track session, the Magneto Cylinder temp would spike high
(1500) at full throttle, while the PTO side would drop to nothing. If I
breathed the throttle a little, the cylinders would drop back to normal
(although the PTO was still low – 800-900 degrees vs. 1200 on the Mag side).
Between sessions we went to richer jets – the only ones available were 340s
(thanks again to Steve Moore!) instead of the normal 290s. It ran ‘better’ in
the afternoon on the bigger jets, although a bit cool (EGT) and it certainly
lacked top-end power.
Next morning we moved the needles (stock needles) to lean the engine back a
touch and it ran better at the start of the race, but after five laps behind
the pace car it was loaded up and took the last two green laps to clear out –
but boy did it feel fast on the cool-down lap.
At Pocono we tried swapping the Carbs, the plug wires from the coil, swapping
the reed valves and swapping the rubber carb flanges/reed covers (all from the
PTO to the Mag side & vice versa). None of them fixed the cold PTO problem.
Likewise, we also did a compression test – both cylinders showed 120-125, so
they weren't totally out of whack.
Anyone have any ideas? All that seems left is the actual cylinders or the oil
circuit (too much oil fouling the plug? Can it become unbalanced on one side?
I’ve read the Mikuni manual and it hasn’t helped any. Is there a better
2-stroke tuning/performance book available?
Thanks,
Ted
#98 Raptor, NE Div
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