Nothing to do with LSR anymore, but some interesting stuff I just ran into. I built a "pattern" copy of a 1938 Triumph Speedtwin, using a Chinese V-twin that came by way of a good friend. I needed to move the carb from a "vertical downdraft" position between the cylinders, to a side mount position. While trying to get an interesting look, I made up a manifold that tucked in tight and kept the carb out of my right knee. There should be a pic attached of the bike.
Trying to get it jetted correctly turned into a nightmare. After days of struggle, I reworked the manifold to accept an old Honda SL350K1 carb (very simple round slide, that always ran well). It was giving me similar problems to the trouble with the flat slide Mikuni I started with.
To shorten the story...here is what I finally realized. There were only two RPM ranges (above idle) where the engine would seem to run cleanly. No matter which carb I tried, or what I did with the jetting, those narrow RPM ranges did not change.
Today I cut the manifold as short as possible and the bike runs well. The key lesson here is the fixed RPM points that were useable, when everything else was ugly. I hope this might help someone, someday.
JimL
P.S. I built this (in this style and size) because I came across a 1938 Triump Speedtwin gas tank, and the 1935 BSA M2 forks. It is kind of like building a car around that cool gas cap you find at the garage sale!
Jim,
Just to make sure I am getting this right, you made a forward facing inlet manifold "extension" to move the carb away from the "troublesome right knee location" that is standard on carbed V-twins?
Without some dimensions to confirm, it appears that you might have doubled the intake length, AND, added a 90 degree turn into the mix as well?
My own experience with projects that "extended" inlet manifold length is some what mixed. I would distill it and comment that in situations of normally aspirated carb installations, the throttle response was very poor until high enough rpm was achieved to promote "reasonably high" gas flow speeds. Often, these experiments proved "difficult" to drive, because of the throttle response issue.
On the other hand, supercharged applications fared better, but mostly they were full race, wide open throttle situations where low speed driveability was ignored. For a street driven "pleasure vehicle" however, low speed throttle response can only be ignored at the displeasure of the end user . . . . . . .
Older, and presumably wiser, I now spend a bunch of time with simulation programs such as PipeMax (tm) and others before cutting parts.
JMHO & 2¢, hope you and others find some value in it.
Mathhelpsalotboy