Where' the power?

Just come back from a road test with 280 main jets, acheived 110mph sitting bolt upright! Acceleration is much improved past 5000rpm in top gear. This is certainly a step in the right directing and will continue to experiment with different jets, maybe some 270s and 290s. I still find the midrange rpms in top gear a little soft, I may try advaning the cam timing to see if that helps. I have the 3 slotted cam gear from RGM to allow for some easy exlerimentation.

The carb tuning sequence is

1. Idle (just so you can get it started reliably)

2. Main jet

3 Needle jet and needle position

4 Throttle cutaway

5 return to idle

You are still on 2 and until you get this right there is no point in moving to 3 or trying to improve it with cam timing adjustment as without 2 being right you cannot be certain the wrong main jet is influencing mid range.

MLW said:

Just come back from a road test with 280 main jets, acheived 110mph sitting bolt upright! Acceleration is much improved past 5000rpm in top gear. This is certainly a step in the right directing and will continue to experiment with different jets, maybe some 270s and 290s. I still find the midrange rpms in top gear a little soft, I may try advaning the cam timing to see if that helps. I have the 3 slotted cam gear from RGM to allow for some easy exlerimentation.

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Thought about buying that RGM cam gear about 50 times, but never did.

Ask Jim S if he would go beyond the 3 degrees advance position with that RGM cam gear on a street rod type build. His cams might have some advance already ground into them. Going too far might be a mistake. I don't really know though, since I've never used that cam.

Soft in the midrange is probably the needle shape as often mentioned by good old Al. Advancing the cam timing might help cover it up.

Get to where you know the main jet is too big, then go down in size until correct.

Triton Thrasher said:

Get to where you know the main jet is too big, then go down in size until correct.

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Advancing the cam can compensate for a restrictive exhaust, but increase exhaust noise. The exhaust system can affect the jetting. The timing of the exhaust valve opening changes how hard the pulse from the combustion chamber makes the exhaust resonate. At top dead centre both valves are open - the resonance in the exhaust affects the carburetor. It is a matter of balance and where you want the power band to be. If it comes on at 4000 RPM - the bike usually needs to be ridden faster to remain sensible. You have the low balance factor to consider. I would not lean-off the main jet until you have the mid-range jetting really right. Festina lente - go fast slower.

Change the needle position before change the timing. Lower the clip so the needle is raised to make it richer because that is what you've needed so far. If its too lean you risk siezing a piston. And yes - go even richen on the main jets. You've got an engine that needs more fuel to produce the HP its capable of.

acotrel said:

You have the low balance factor to consider.

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No, it's an Isolastic frame.

jseng1 said:

Change the needle position before change the timing. Lower the clip so the needle is raised to make it richer because that is what you've needed so far. If its too lean you risk siezing a piston. And yes - go even richen on the main jets. You've got an engine that needs more fuel to produce the HP its capable of.

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If the jetting is too lean, the motor will usually give a warning when you ride the bike - if it misses, STOP and raise the needles one notch. If you are seeking performance - as lean as possible without causing damage, is the way to go. I always leave changing main jets to long last. The recommendations usually specify two sizes of jets - start big and go smaller. The only time main jet size can affect the mid-range jetting is when they are too small. If the needle jet and needle are nicely lean, you might need to feed the throttle on, but the bike will accelerate faster, and the rich main jet becomes less relevant. It is acceleration and throttle response that you need. With a road bike, to use the main jets properly, you probably need to be on an Autobahn. If the bike accelerates faster, it usually has a higher top speed. Needle shape is important. Think about what you are doing - carburetors meter fuel, they do not increase its flow.

With my bike, my previous bike had a certain sized main jets, so that is what I used and I have never changed them. Everything has been about idle and md-range jetting. I did not expect the needle shape to make much difference - however, it does. When you wind the throttle on fast with quick-taper needle the acceleration rate is usually slower and top speed is lower. Watch the guys who wobble around corners, get the bike pointed then whack the throttle open. It needs to be wound on from way back in the corners. With my bike, I accelerate from the beginning to the end of corners. I always brake into corners. When the back goes down, the bike steers itself.

Many years ago, I could not believe that my bike was quicker after I had leaned-off the jetting, Steve Oszko was the fastest Victorian when Manx Nortons were the go, so I asked him about it. He said, 'I run the jetting as lean as I can - right down the needle'. Steve is now dead, and his son still has the works 350 Manx. He raced it at Goulburn, and did not know how to stop it from missing.

My theory is that it is more fun if everybody races faster.