I helped a friend with rebuilding his '81 with stock carbs, pod filters, and aftermarket short megaphone mufflers stock crossover removed) after he'd holed the right piston. No carb adjustments had been done after the filter and exhaust mods, and the bike runs the stock electronic ignition. After he replaced the bad parts in the motor I cleaned his stock carbs well and installed the carb kit from 650Central.com (http://www.650central.com/carb_oem.htm, top of the page), which I believe came from Heiden Tuning. I selected the main and pilot jets from the kit that the directions suggested for freer exhaust and intakes. The bike has been running fine like that (better than before, really) for six months or so since. Since the ignition system was unchanged it would seem that the advance was probably not the fault in this case.
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Just developed a hole in piston
- Thread starter meanstreakmark
- Start date
Mark, I don't question your mechanical competence. There's a strange tendency to blame every engine glitch and misfire on the carbies (it must be hardwired into the human brain), and the posts seemed to be running a little too hard in that direction.
One issue with the ATU is wear, which can cause the advance curve to spread to the point that with advanced timing in the safe zone (=/<40* BTDC), timing at idle will be so far retarded that the carbs blow out of the boots at startup. The other issue is securement, which is what I suggest you check for. If the cogged nut that secures the ATU loosens, timing goes so far advanced that a holed piston results unless the motor is shut down fast.
One issue with the ATU is wear, which can cause the advance curve to spread to the point that with advanced timing in the safe zone (=/<40* BTDC), timing at idle will be so far retarded that the carbs blow out of the boots at startup. The other issue is securement, which is what I suggest you check for. If the cogged nut that secures the ATU loosens, timing goes so far advanced that a holed piston results unless the motor is shut down fast.
Agree with this - had this happen on a friends newly rebuilt by him, Heiden 750 kitted motor I was tuning. Nut came loose on a shakedown run - if you know much about engines/bikes you would know somethings up straight away...luckily - did an immediate shutdown & saved his engine.
I must admit I did cycle it through with the kicker after I shut it down to make sure compression was still good & it hadn't seized - however - in my haste to check the engines vital signs I neglected to kill the ignition.....BIG mistake.
If you've done this, you know what came next..........
In an earlier thread you stated the timing was between the marks at idle. What timing marks are you using and how did you set the marks. Me is thinking that maybe the timing wasn't marked properly when the PMA was installed??????????
Mikes ATU's have had some problems with the arms pulling out of the unit where the weights swing from, If one or both got loose without pulling out it would mess up the timing.
Mikes ATU's have had some problems with the arms pulling out of the unit where the weights swing from, If one or both got loose without pulling out it would mess up the timing.
