blown plug and a hole in piston

[CadillacBanshee]

my friend has a bike thats kind of built. ported, coolheads (18cc), tors elimination, tiger built crank and gears, so on and so forth. He has rebuilt his engine twice in one month, both times the spark plug in the left cyl was toasted and there was a hole blown in the piston. Any one have an idea what the problem could be? The first time the bike went out the crank went out and the plug was blown, and the last 2 times only the plug has blown.

[dawarriorman]

Detonation maybe? What elevation are you at, and what octane gas is he using.

[THEREALBUDLIGHTBANSHEE]

sounds like a lean condition on the left side ,check left carb

[ogre03]

Start with what dawarriorman and cadillacbanshee said... Definatly find out what octain fuel he is running with those 18cc domes, also Look for air leaks especially in the rubber boots connecting the carbs to the cylinders.

[ssanddemon]

Super lean! That hole is there because the plug was glowing white hot until it melted thru the piston.

[CadillacBanshee]

sorry, it was 91 octane with yamalube racing oil. Dumont dunes is where it happened the first time, dont know elevation.

[ogre03]

I'm no expert on this, and I'm not sure how much compression your porting would offset. But I'd say he needs to change out his domes for some 20's or maybe 21's or start running race gas. I still think you need to look for an air leak too. Does the piston look melted or all chipped up?

[fastbanshee8]

Did he replace the crank seals, when he replaced the crank?

[ssanddemon]

Well, here's the wakeup call for him- You CAN'T run 92 octane in 18cc domes! The extended WOT running of Dumont didn't help either, since I still think he was running lean on that cylinder as well. Your buddy needs to do some homework, or he's just gonna keep burning up motor after motor.

[fiveOnick]

Did he replace the crank seals, when he replaced the crank?

447037[/snapback]

 

bingo. You should be running higher octane fuel but detonation doesnt normally burn a hole in a piston. Thats a lean condition, often caused by the clutch side crank seal.

[Wicked98Shee]

definately a air leak ... look at ur carb boots and psooibly the left side crank seal .. do a leakdown test to check the condition of the main seals

[Wallrat]

Dumont is, for jetting purposes, at sea level. Sounds like you had a combination of problems on this one. Both air leak and not running race gas when you need to. But it may be just detonation...I've seen extreme cases where a piston had a hole blown through it from detonation alone.

Edited December 10, 2005 by Wallrat

[dawarriorman]

What does the plug/cylinder/head look like for the other cylinder. My bet is they should be pitted somewhat. It could have an airleak, but damn people thats all anyone ever says anymore. "Your engine blew, oh it must be an airleak". Granted it is a common reason for a motor to go, its not the only one. I would think he would have to have his exhaust port cut extremely high to be able to run pump gas with 18cc domes. And yes detonation can easily put a hole in the piston. The engine not only gets hot when its detonating, there is extreme pressure in there, and it can blow a hole right through the pison. One side will go before the other, thats why it only happened on one side (generally one side will be a tad leaner).

[BigRed350x]

It just pisses me off when people have bikes like the banshee and don't know how to take care of them. Especially if you are putting mods on it or buying one with mods... make sure you know what those mods do to the engine. Tell your friend to give his banshee to me and go buy a 400ex that he will never have to touch. If you don't know what would make a huge hole in your piston, you shouldn't have the thing to begin with.

[FIRST BANSHEE]

bingo.  You should be running higher octane fuel but detonation doesnt normally burn a hole in a piston.  Thats a lean condition, often caused by the clutch side crank seal.

447043[/snapback]

detonation usually will burn a hole in a piston, with 18cc domes you need race fuel. a air leak on the left side could be crank seal, bad or installed wrong, leaking base gasket, carb boot or gaskets. you definitly know one problem is the fuel, find the others before you ride again!!!