Andy from Performance Engineering has sent me the files from the dyno runs I’ve done from the last couple of years and with software downloaded from Winpep I am able to view all of them to compare.
I’ve been looking forward to this because I wanted to see where I could maybe make gains by modifying the needle. It’s very interesting to see the comparisons and to consider the next steps.
These are the most interesting charts, torque and power:
To be clear, the settings are as follows (a bit mixed up on the graph notes)
Run #17: Twin carb set up (avgas), High comp head (32cc)
Ign set at 22degs, main jet 280, #4 needle jet position (5bottom), 1/4t out on pilot air screw
Run #18: Same as 17 but the second run.
Run# 19: Same as above except needle now raised to #3 position (middle)
Run #28: Same as above except ign set 23.5, needle on middle position #3.
You can see how 17 and 18 really want to rev further, and that’s with the needle on position #4. You can also see that they must be too rich at the beginning, as there’s no curve taken. I’m guessing Steve must have felt it flat and simply not sampled it at the beginning.
So it looks to me that the needle could be reduced in diameter in the final section to enable a bit more richness at the top end without compromising the bottom. That should give me an extra five hundred revs according to the chart. That should give me the best of the start of #19 and the end of #17/#18. I can work the amount out simply by checking the difference in the needle within the pitch of a needle adjustment notch.
The timing change seems to have a decent effect in increasing peak torque across the range, so I’ll leave that where it is.
It’s interesting that the needle position change seems to have had an effect within an area that you’d expect to be almost beyond its influence, as I understand it. I need to check the needle jet and needle at that position to see whether it’s restricting the main jet. You’d think that it could be, looking at the graph.
As for the beginning and my lost 400revs, I’m not exactly sure. Part of me thinks that it might be weak because the fuel is too low in the needle jet as I’ve set it to not drip out the pilot air screw assembly. As the carb is at an angle, this obviously reduces the level in the needle jet. This theory is slightly supported by the fact that the very first section (around 2750 revs) was a little stronger with the 300 main jet and needle at the highest position.
However, the ‘technical guy’ from the Amal stand at Stafford said that the float level was set at 0.9″ from the top face of the float banjo. Extrapolating that into the carb means that this works out to be within the thread for the needle jet in the jet block. That’s not very far up the needle jet anyway.
Later carbs made for two strokes had the rear part of the emulsion tube cut away as this created a situation where more of a low pressure area was created to lift the fuel from the jet. Apparently (Amal guy) this was done when the higher performance two strokes came along. It may be that the response time was shortened and at higher revs this became important… I just don’t know. The other ‘known unknown’ is that I think the bellmouths aren’t made with the requirements of gas flow as the primary design factor. I suspected that they wouldn’t be perfect but the way they are made leaves quite a significant ridge around the inside against the edge of the carb inlet. Not ideal for laminar flow as I understand. It may be that this, along with the less than ideal brief radius at the end of the bellmouth is responsible for a meaningful restiction to flow. I didn’t take into account in any mathematical way the extra venturi wall area when I originally planned my conversion to the twin 1″ carbs, but the overall increase in aperture is around 10 to 15%. If I’ve got bad turbulence into the carb then that might cause problems..also since the emulsion tube is right at the bottom of the venturi, it could be that disruption to the flow entering the carburettor is effecting the flow over the emulsion tube.
I think I’m going to try to temporarily effect a radiused entry using silicone sealant and see how it works. The radiused bellmouth I’m going to try and do using epoxy resin, just to test. I’m probably not going to know, but I will have the bike back on the dyno at some point.
Longer term, I’m probably going to file both carbs out to 1 1/16″ (around the top and sides) to increase flow, but that’s not for now… oh and make some decent bellmouths.