Generally, backfiring through the carb
is a carb problem. The Mikuni tuning manual (available on this site) for the HSR
recommends tweaking the accelerator pump to come in sooner for correction, if it's "off-throttle" popping; a most common situation.
In my experience with stock and HSR carbs, that seems to work.
But, I can envision a situation where stock jetting (lean) combined with too much timing advance could worsen --maybe even cause--the "cough". Essentially, something is causing rapid combustion "peaking" while the intake valve(s) are open. Assuming you have only added a Dyna
and not re-jetted??
Try backing off the Dyna to 34A. I think 36A is a little aggressive for most applications
And, I know the following statement controverts the old hot-rodding lore of running a lot of advance, but I believe Yamaha's twin plug, 4 valve and intake design was intended to lessen the amount of advance our motors need, by achieving more efficient combustion
Since timing advance starts combustion as the piston ascends, "negative workforce" results. If combustion can be made faster through different designs, less advance can be run, minimizing the negative workforce. I think that was a design objective by Yamaha (along with emissions considerations).
Thus, the range of timing advance on our motors is less than a V-Twin without such differences. I recall older Harleys ran around 40 degrees. The difference is a significant reduction in negative workforce.
And, if that's not enough controversy, here's a little more...
Once departed from stock tuning parameters, I can see where running higher octane can benefit combustion altered by jetting and timing changes, despite prevailing opinion that combustion ratio alone dictates it octane requirements.
Jim<br><br>Post edited by: Bucardo, at: 2005/05/07 00:19