well i got my 77 honda xr75 back from the shop a couple of days ago, and it runs great! when you get the rpms high it crackles a bit, but it doesnt matter because i dont have any business getting it that high, but why is it doing that? and i think there is crap in the tank, so does anyone have a suggestion to get that stubborn dirt out of there? i couldnt get it all out last time. One more question, how will higher octane gas affect my performance and overall how it runs? i know for a fact too high octane hurt your performance cause it is too hard to burn, so someone please answer a few of my concerns, thanks TJ
Back when I had 70s vintage dirt bikes (in the 70s), high rpm missing was usually due to the spark plug starting to get fouled.
brand new ngk spark plug though
If there’s crud in the tank then you need to remove the tank and clean it out - thoroughly.
Cutting out at high rpms could be a fuel flow problem (tank strainer?), carb problem (clogged jet, low fuel level, etc), improperly adjusted contact points, weak condenser or coil, etc.
High octane gas should not be defined as “hard to burn”. It burns slower than low octane once ignited.
It’s unlikely that octane rating has anything to do with your problems.