seizure?

[ukcr5hundie]

hi folks

Im hoping someone can give me some advise and tell me where im going wrong.

Ive just finished building myslef an afc with a 89 motor. The problem ive got is ive only done about 10 miles on the bike and it has seized twice.

the first time i put it down to the guy i bought the engine off supplying me the wrong size piston, so i had the barrel rebored and fitted a new piston the correct size, bought some new oil (castrol a747) and mixed it at 30ml per litre.

2 miles up the road same thing happened?????

i check carb (39mm pwk) and all is fine, check jetting and thats fine too.

i dont know why it keeps seizing and cant afford to put a piston in it every 10 miles

could it be that i didnt drill holes in the piston for lubrication of the exhaust port bridge?

thanks in advance

[Tharrell]

Did you relieve the bridge when it was bored?

[NightBiker07]

Too lean of jetting probably. overheating and seizing is my guess. My 3 wheeler used to do that.

[Gmbond]

Air leak????

[Kuma]

Sorry, I've got to ask,
Did you check your coolant level

[ukcr5hundie]

ok, so had a chance to spend time time looking at the bike this evening.

checked the jetting again and in my pwk im running a 55 pilot 178 main and 1.5 turns out on the air screw. Its set a bit rich at the mo until its run in.

checked the coolant thats fine too, so im thinking its got an air leak somwhere.

im guessing the leak would need to be around the crankcases rather than carb or reeds? is that right.

the motor has some damaged to the cases but i didnt think much of it as thought it was on the gearbox side.




there is also a small hole in the join in the cases at the bottom not far from the drain plug, it looks like its supposed to be there tho

[Roostius_Maximus]

you're right about the small hole. its for draining the ignition side case.
That bolt too long?
i'd pressureize things and see if the case is also leaking to the trans

[Kuma]

could have a crank seal leaking on the flywheel side too.

[NightBiker07]

start the bike, and while it is idling, start spraying any gasketed areas with starting fluid. if it is leaking, the starting fluid being introduced into the leak will change the engine's RPM. easy, cheap, dirty way of air-leak hunting.

[ukcr5hundie]

Thanks for all the help lads, ill try the things you mentioned and report beck with what i have found

[jbsleddin]

The best thing you could do right now is invest in a leak down testing kit, or if you're super ambitious (and a cheap ass like me ) you could opt to make one yourself. I, myself, don't have the ambition or time to dick around trying to put a kit together myself. Bought the Motion Pro kit several months ago. I have to say, I don't know how I got by as long as I did without it! It's already paid off with many hours saved from chasing my tail unneccessarily!

[ukcr5hundie]

cheers for the advise j, i should be able to knock something up, i cant see anywhere in the uk that sells the motion pro kit.

[ukcr5hundie]

this is what i came up with to leak test my motor.



and this





i got to about 20psi on the gauge and found that it had a leak from where the rubber boot joins the reed block. I have a spare and tried that but it was leaking in the same place,

The gaskets were new when the motor was built and the metal plate on the boot looks like its in good condition and not distorted.

Ive put some silicone gasket sealer between the joints but not sure if its a good idea leaving it like that, just in case the silicon blows out and it seizes again.

[AlisoBob]

... got to about 20psi

Carefull Hotrod.... blow out ther center gasket and you'll be splitting the cases. There is a narrow, poorly supported section right between the crankcase, and gear box.



Dont push your luck.

I wouldnt go over 10 psi.

Do you have photos of the piston?

[ukcr5hundie]

cheers bob, maybe a bit too much pressure.

but hopefully thats causing the problem.

Not got a photo of the piston as its back together now, it was scored in 1 place mainly right on the inlet side closest to the lefthand side of the bike

[AlisoBob]

The first piston...

[jbsleddin]

Yeah, 20 psi is enough to push seals out too, The kit I have recommends 8psi max, allowable to lose 1 psi per minute, more than that and start looking for problem areas. I'd go out on a limb and say you've already found your problem though, unless more leaks are apparent too.

[ukcr5hundie]

[AlisoBob]

I dont think jetting had much to do with it.... Rings butt and gall up the intake side, shoves the piston hard over to the exhaust side... game over

[Roostius_Maximus]

did the bridge have relief to spec?
what was your ring endgap?
actually, it was probably right, by looking at the pin we can tell it has seen some heat also.
so...
it didnt sink the dome in from lean wfo
it didnt start to blow chunks out of the piston from lean or too hot of a plug
it didnt 4 corner sieze from running hard cold

so you didnt ride it wfo, and it probably idled fine, it will have got to big rpm, then closed the throttle a bit and squeaked it right?

It did close the gap on the ring
It did stick at the bridge

bridge clearence isnt there or enough
the ring took the heat, got long and butted the endgap

it likely needs more fuel on the needle, more bridge relief and more end gap on the ring

[Roostius_Maximus]

sorry, its been hella busy, took me like 2 hours to compose that post

[Tharrell]

I don't see any lube holes in the piston?
IDK but, no holes and IF the bridge wasn't relieved.....
Those 2 together would seem to be a perfect storm.
Since I'm not an engineer by any stretch, I try to take the best advice I can get.
On the 500, it's here and everyone preaches about those 2 things

Got any pics of the cylinder?

[AlisoBob]

Its a cast piston, no holes are needed on the skirt.

[ukcr5hundie]

ill take some photos of the barrel/bridge when i get home. When the barrel was bored there was still some relief on the bridge? should i relief it some more?

so i dont need any holes drilled in the piston, is this right?

i not sure im following all the lingo your chatting, wfo? wide f+{}king open?

how do i measure the end ring gap?

both times when the bike seized it was in the same spot on the same road which is strange, anyway I was being careful with it and as soon as i opened the throttle to about a 3rd it seized. Exactly the same both times

ive checked the jetting 3 times, so cant be that, should i raise the needle?

thank for all the help folks, really appreiciate it

[ukcr5hundie]

actually, it was probably right, by looking at the pin we can tell it has seen some heat also.

sorry the picture is a bit misleading, that pin is from my 250 ktm, that had seen some heat tho as it overheated last time out and has just had a top end rebuild