piston pins keep failing

[ben4221]

well good news the new pin has held up fine and very surprised the spacers havent fallen out yet you can hold it flat out all day now. i ended up finding a larger jet for the carby and am running 18:1 oil and the mixture needle is on the richest setting. it has insane power now with no head or base gasket but does suffer at very high rpm and i had to retard the timing as it was detonating like crazy, only problem now is its kind of 4stroking at high rpm not sure of what to call it but it feels like the plug isn't indexed properly have tried so swapped out the b5hs that was in it for a bp7hs and indexed it but still has a bit of 4 stroking at high rpm so im thinking it might be a bit too rich or too much oil haha 18:1 is probable too much

 

[Street Ryderz]

well good news the new pin has held up fine and very surprised the spacers havent fallen out yet you can hold it flat out all day now. i ended up finding a larger jet for the carby and am running 18:1 oil and the mixture needle is on the richest setting. it has insane power now with no head or base gasket but does suffer at very high rpm and i had to retard the timing as it was detonating like crazy, only problem now is its kind of 4stroking at high rpm not sure of what to call it but it feels like the plug isn't indexed properly have tried so swapped out the b5hs that was in it for a bp7hs and indexed it but still has a bit of 4 stroking at high rpm so im thinking it might be a bit too rich or too much oil haha 18:1 is probable too much

If you use good quality synthetic 2 stroke oil 32:1 or even 40:1 will help clean up the burn and still be plenty of oil for lubracation.

 

[ben4221]

yeah i got the most expensive pre mix race oil they had ill mix up some 32:1 and see how it runs was thinking i might have to make it slighly leaner to compensate for the more fuel going through the main jet cause jetted it at 18:1 . i also indexed my plug the complete wrong way yesterday so i think thats what caused the 4stroking at top seed. this time i spun the head round 180 so the open part or the spark plug is facing the intake ports and the J part of the earth is facing the middle of exhaust port. it would top out at 41mph but when you pull the clutch in it would still rev much higher so going to raise the exhaust port if correctly indexed plug doesnt fix it

 

[Street Ryderz]

yeah i got the most expensive pre mix race oil they had ill mix up some 32:1 and see how it runs was thinking i might have to make it slighly leaner to compensate for the more fuel going through the main jet cause jetted it at 18:1 . i also indexed my plug the complete wrong way yesterday so i think thats what caused the 4stroking at top seed. this time i spun the head round 180 so the open part or the spark plug is facing the intake ports and the J part of the earth is facing the middle of exhaust port. it would top out at 41mph but when you pull the clutch in it would still rev much higher so going to raise the exhaust port if correctly indexed plug doesnt fix it

It's got no choice but to rev higher when you take the load off by pulling in the clutch,it will free spin up over 12k just with the stock port timming!Are you using the stock #70 jet?

 

[ben4221]

It's got no choice but to rev higher when you take the load off by pulling in the clutch,it will free spin up over 12k just with the stock port timming!Are you using the stock #70 jet?

that makes sense, its got an 80 size in it at the moment i just got back from tuning it and with 80 jet and i have found it to have the most top end is with the mixture needle set to leanest, it tops out at 45mph now but it revs high at idle and when you back the idle screw out it just leans out and revs even higher so im going to put a 75 jet in it and try bringing the mixture needle back to half way im also contemplating raising the exhaust port cause its got a little too much bottom end and not enough at top end. i mixed the oil at 35:1 and it sprays way less out which is great the top quality oil makes such a difference too

 

[ben4221]

heres some pics of the head and port hights at bdc

 

[Frogslayer]

I would guess it was detonation wrecking your pin bearings the whole time. Was knocking like someone dropped a bolt in the cylinder when you went over 70km/h before?

 

[ben4221]

yeah the detonation wasnt helping the cause at all, cant believe how much long its lasted with good oil but it has slight little end noise now so ill know when i get home from work if ifs done another pin, ive got a new crankshaft at work that ill use if the pins gone cause i suspect the rod in the motor to be screwed