1989 gpx 250 blown piston gasket - major edit- pics added

[Martin]

hey folks,
a while ago i pulled off at the lights with a mate of mine, tried to drag him off (he beats me every time, 650 vs 250) so just for fun.
i pull off and hear a sort of soft whomp noise, and the bike looses a shitload of power, spits out white/grey smoke and runs like shit.

SO i changed the electrics, coils, leads, spark plugs, ignition controller, no avail

SO i pulled the engine out. thinking blown piston rings. i've pulled the engine apart and the piston heads are pretty much black, the inside of the pistons are brown and the valves seem to be chewed yup, but that may just be carbon build up. also coolant drained out when i pulled off the big end
the head gasket seems to be ok, but the piston gasket (the one between the big end and the...base of the engine) is blackened.
so its pretty obvious what's wrong.
question is, where do i go from there? do i give everything an acid bath? do i replace everything? is it just the piston gasket? im pretty sure coolant was in the pistons (there are a lot of fluids going around because it was a messy job) that didn't get there from general mess.

from what i know of cars, if there is coolant or oil in your pistons, its a head gasket. my bikes head gasket looks ok, but the piston one has scoring. what else could it be? have i found the problem?

also, while i have the bike apart, are there any performance upgrades i could do?

thanks

[Nomad]

be good to know what i could do to my gpx 250 in terms of performance :p

Nomad

[timmyrocks]

get your heads polished and ported and put a big bore kit through it
oh also put zzr600 carbs on it to allow more fuel for the performance work done
cheers Tim

[Nomad]

more fuel does what? lol i thought it was bad?

Nomad

[timmyrocks]

more fuel and air, more power.
bigger carbs aloow more air and fuel,,,,,,, more power

[Martin]

so a port n polish and new carbs
here's hoping it's just a blown gasket and not something even more sinister

[Martin]

tried to get a close-up of the piston rings

valves, both cylinders are in pretty much the same condition

piston head

piston gasket

reverse side

the piston gasket is obviously blackened, which on a car means blown gasket so i assume its the same on a bike. one spark plug was burnt white and the other was jet black. is this a case of new parts or acid bath? (obviously i'll need a new gasket)

[mike-s]

the amount of oil means either theres a load of oil in your airbox (check it) coming from the one way valve from the air recycling.or oil came in via the oil galleries. I would put my money on the latter though. lastly how was the oil or coolant? any indication of cross contamination?

[Martin]

oil was black (newish oil too), coolant clear green, but the stuff that came out of the bottom of the radiator was naturally browner, normal i guess for an old radiator

[mike-s]

well sounds like time to flush the radiator at the least as well as clean the barrel and head faces and check the runout with a set of feeler guages and a steel ruler.

[Martin]

whats the easiest way to clean them, acid bath? and whats the run out?

[mike-s]

acid bath? oh no, degreaser and a brass brush.and the runout = warp. So put the edge of a ruler across the head diagonally and measure any gap between the ruler and the head, thats the runout/warp and anything more than 0.05 - 0.08mm should really be treated by getting the head and deck (to of the barrels) machined flat.

[Martin]

i wouldn't have though a wire brush would be the best thing to use on the inside of an engine...might end up with metal shards in the engine...?

[Strika]

i wouldn't have though a wire brush would be the best thing to use on the inside of an engine...might end up with metal shards in the engine...?

Aahhh...it might pay to remove the pistons from the conrods then!

You need to get yourself some measuring devices. A micrometer and a set of vernier calipers to start...... and you will need a dial gauge to check if the pistons and barrells are out of round and some feeler gauges for the piston and ring gaps. Depending on how old the motor is, if you have the top taken off it already, it might pay to pull the bottom end apart and check the crank too while your at it. You'll need some plasti gauge to check the crank journal clearances.

Personally, I would hunt up a 2nd hand motor from the wreckers....I bet the cost factor wouldn't be much different!

[Martin]

damn unless i can borrow those tools off people (a mate of mine is a mechanic) then i'll just clean it up and re-gasket it all and put it back together