Problems With Deto?

[fireburns99]

A couple of months back at the dunes i was having problems with the spark plug unscrewing itself. Some people thought it was a sign of detonation and others thought other things. Well i finally got around to pulling the head off because the cylinder was still making good compression. I posted some pics below, but it looks like something weird was going on by the weird burn mark on the head.

Also there was a lot of carbon in the cylinder, which seems unusual. I'm looking for any opinions you guys have. Also at the same time this happened i think my radiator cap was bad, it wasn't holding pressure, so maybe i overheathed it?

Thanks for taking the time to look.
http://members.dslextreme.com/users/fireburns/Pics/Picture 001.jpg
http://members.dslextreme.com/users/fireburns/Pics/Picture 002.jpg
http://members.dslextreme.com/users/fireburns/Pics/Picture 003.jpg

[zedicus00]

im gunna stick with my originall answer. DETO/ on the bright side it looks like you were running it pretty rich, which helped keep it from seizing.

[fireburns99]

Ok thanks zed. I just wanted to double check again, now that i have pics to help describe what it looks like. So i need to replace the piston and rings then right? And do you think a hone job should be fine?

[zedicus00]

have it honde an see what it looks like. definatly do the piston an rings, take some fine sandpaper to the head and get rid of any high spots, try to not alter the head dimensions too much.

[fireburns99]

How many times can you hone a cylinder and replace the pistons before you need to bore it to the next size? I know its been honed twice at .40 over. Do i need to just step up to .60 over?

[fireburns99]

btw thanks for all the help zedicus!

[TheFontMaster]

I would say bore up to .060 over. Homing takes off about .005 inches of material.

[zedicus00]

check piston to wall clearance. i think max is like 50 thousandths? dont remember for sure off hand... got a manuall?

[fireburns99]

yea i'll check the manual for the clearance specs. Thanks for the help.

[86 Quad R]

the piston to wall clearance is dependant on the piston manufacturer. different pistons require different tolerances. i would check to see what the max is for the piston that you have.

btw just so you know. the piston should be measured at the skirt(perpendicular to the wrist pin) not at the top where the rings are.

[fireburns99]

thanks for the tip 86 quad r.

[wilkin250r]

Detonation is pretty violent. When it happens, it often will actually blast metal off the surface of the piston.

I don't see any evidence of detonation on your piston. Deto will make one edge of your piston look like it was sandblasted. Your piston crown, although dark with carbon, looks in perfect shape.

What are all the grains and gunk, though? That stuff obviously should be there. Did it fall in when you dissassembled, or was it already in there?

[fireburns99]

Thats the thing, i really don't know when that got there. It seems to just be carbon. I'm inclined to say that it fell in during disassembly because the cylinder walls look clean and smooth as could be, meaning no scratches or gouges, which would have been apparent if the engine had run with that crap in there.

If it doesn't seem to be deto to you wilkin, what would cause that weird burn mark on the head. Shouldn't the head be clean? I ordered a new piston, to be on the safe side.

One more cabeat is that i actually had a problem with the spark plug unscrewing itself when riding hard, which is a sympton of deto i was told. I tried several plugs and they all did it. I had never had a problem with this before. Also in the middle of my dunes trip it starting running lean, and i had to bump up anther jet size. I don't know if any of this matters, but i figured all the facts may help.

One last thing, does it look like the head gasket gave to anyone else but me? You can see where it looks like it blew out.

Thanks for the help wilkin.

[dober250R]

when u run at the dunes, always bump up 1 main jet, because your always at full throttle. At least, I am at the dunes :devil:

[norcalduner]

Originally posted by dober250R
when u run at the dunes, always bump up 1 main jet, because your always at full throttle. At least, I am at the dunes :devil:

i definitely agree

[fireburns99]

i always jet it slightly rich. It seems weird that it just did it this trip. I've been tot he dunes before and sanddragged, and never had this problem.

[86 Quad R]

it appears to me that the head gasket could have been leaking.

[fireburns99]

yea thats what i thought too. And that would lean it out. I think that may have been the problem. Because i have run that setup for a year, with the same kind of gas and never a problem with deto. If the head gasket leaked that would cause a leak that would lean it out.

[86 Quad R]

this could explain why the rad wasnt holding pressure. the compression getting into the cooling system will cause it to over flow.

[fireburns99]

i agree. I thought it looked funny, but never though anything of it till i looked at the picture again. It would make sense, since it would cause all the symptons i had.
so if that is indeed the problem should i replace the piston, or just change the head gasket?

[86 Quad R]

imo

i would pull it down, check the piston to cylinder clearence, inspect the piston and cylinder. to be on the safe side i'd lap the head and the top of the cylinder to ensure things are flat.

Lapping (http://macdizzy.com/1989bhopup.htm)

if it turns out that the piston needs replacing then i'd replace the wrist bearing aswell.

[zedicus00]

so wait, in the pictures is that just little chunks of carbon or something then? i thought it looked like pitting all over in the outter areas?

[fireburns99]

yea its just carbon. It must have gotten knocked loose when i disassembled.