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998cc said:

On the uphill runs, at WOT in third gear, it lost power. Slightly backing off the throttle, power returns immediately. That was the indicator that the mains are a bit small. Will take another look at float levels.

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Agreed. That sounds like mains.

Float bowls take a little while of sustained WOT before they empty, then they take a little while to refill and resume fueling suites as well, so quite different symptoms.

Back to the Vincent: Pulled out the 290 mains this week and installed 300's. Much better response at WOT and even a bit better mid-throttle.

I usually turn off the fuel just ahead of pulling into my driveway and then running the fuel out idling before putting bike away in the garage. The front cyl was dying out much earlier than the rear which sounded like a low float level. It turned out to a plugged idle air jet that took 90 lbs of air pressure to clear! Took it out for a ride yesterday and was pleasantly surprised with the jet change.

I just came across the settings I have. I have a Rapide with 8:1 pistons, 105 cams, straight through Toga muffler and the round air cleaners (Wassel) that are used on Triumph Bonnevilles. It has 34mm Mikuni VM carbs off a snowmobile. It has manifolds made from some castings I got from Terry Prince. The manifolds have a sudden reduction in size down to 32mm and then taper down to 30 mm which is the size of the opening in the head. I didn't open the head out more because I thought it was not necessary and also I thought that one day I might install a set of 1 3/16" 10TT9 carbs I have on there (1 3/16" is 30mm) Maybe I will never do that. 30mm ports are obviously not to big for a 500cc cylinder.

D6H3 needle in center groove
159 P6 needle jet
#35 pilot jet
2.5 slide
170 main jet

I have put about 12,000 miles on it with this set up. It runs really well. It starts immediately, and also idles well. It is noticeably faster than Vincents that are set up stock. It seems to keep up with the other ones that are supposed to be fast. It can do long runs at wide open throttle on mountain passes two up at 85 MPH so it is not jetted to lean, it is just right. Sometimes on a higher pass a slight richness is noticeable. I think that the air cleaners have some restriction and also the 105 cams, with straight through exhaust, give a very strong vacuum. It is surprising how small the jet is.

The fuel economy is between 50 and 65 mile per gallon (Imperial gallons) Usually it is 50 but sometimes on the highway I get the higher figure. I am sure that if I put in a 300 jet my engine would barely run at full throttle.

It won't accept full throttle until 3000 RPMs but if I am trying to go fast the engine is above that speed anyway. It still runs really nicely at 2000-3000 RPMs for causing on country roads.