1988 XC250 revival 2016

thats the problem here..he really doesnt know what the bike is supposed to run like and is new to two strokes. i have concerns about the intake skirt.

I rode it with the whooped original bore and piston, so no I don't know how it really stacks up against a original piston. Compression figures should be the same, the important measurement, the wrist pin height, is the same between the two pistons.

The intake skirt is roughly 1/8 of an inch longer and exhaust is close to a half inch longer but neither comes close to touching the crank or connecting rod.

I'd like to see a stock piston, mine was broken to begin with, so I'm not sure the exact shape they should be. Are they supposed to be equal length on each side with a half circle cut out?

To be fair I am new to two strokes but an engine is an engine is an engine. I had a local two stroke engine building shop advise me on the pistons and they said it would be easy to cut the piston skirts to match but theres no real reason to as, according to the shop's engine builder/machinist, as the difference shouldn't cause that much of a power difference. In fact he said what little you'd lose up top you'd likely gain on the bottom end. These guys build custom 1500cc two stroke triples for drag racing purposes and I know they know what they are talking about.

The longer intake skirt clears the whole intake port when at TDC, so really what you'd be "losing" is a split second of fuel charge on the down stroke since the slightly larger skirt begins to close the port just a little earlier than the original. Again your still getting the whole port open so I don't think your really losing that much.

Heres my honest estimation of the power; before with the cylinder that measured 160psi with a welded skirt, the bike would pull the front end off the ground on my dirt road in third gear just by rolling into the power band with a slight pull up on the bars and now with the fresh cylinder I can't keep the traction once it hits power band, it will only spin tire.

Thanks Kyle, they are most likely correct, trimming the skirt isn't really an issue, I could time it the same as standard just as long as the piston in this one is intact, I've only had it running for about a minute.
The only reason I ask is that my understanding is that the skirt height on the inlet side is what determines the whole personality of the engine, I remember using a 1979 YZ250F piston in my 1980 IT250G back in the eighties and it made a huge difference to the power delivery. The IT piston had two windows in the intake side of the piston, the YZ had a huge semicircular cutaway.
I would probably trim the skirt anyway, but great to know that this piston works in this motor.
Thanks again, Tony.

yes, good to know the skirt does clear the whole intake at tdc...
i also would trim as well to keep timing...weird that wossner screwed this up.

That makes sense. This bike feels good on the low end putting and the sudden onset into and through the power band to the top of the rpm range is pretty abrupt.

Anyhow I do have a question about reeds and their operation. Being made of fiberglass they must have a useful service life span right? Old fiberglass is brittle and weak, that much I know. Now they exist to block off the rest of the intake on the compression stroke correct? If you had a weak or even broken reed wouldn't you lose some compression out of the carb?

That being said mine are visually ok, however being unsure of the age makes me want to put a fresh set in there ASAP. Being only like 40 bucks for the set I'd say they are also reasonably affordable.

I agree. The engine shop did tell me that if I was unhappy with the results to bring the piston back to them and they would gladly cut the skirts down. I guess I'll have to start a go fund me to fund buying a second piston, having it machined and then having both piston setups dyno'd by the engine shop haha

For me its just fine, either way it will be way more power than I need for the tight "trails" (usually just openings between trees in the woods) I'm currently riding. If I was out on a track where getting more than the first three gears in was standard stuff maybe I'd think differently.

Still curious as to what the stock XC240 piston looks like. Is it similar to the one Wossner sold me or is completely different? If the bikes are so similar why such a drastic difference between pistons?

The engine shop said port timing was dictated off wrist pin to dome height, not the skirts. Am I missing something here?

I will post pics when the top comes off this motor, I wonder if this piston looks like the one you removed? I hope not.

The pin to dome height affects the exhaust timing, the intake skirt determines the intake timing and duration. It may not make a big difference though.

Here are photos of the stock piston I pulled out of my '87 250WR... also with cracked piston skirts.

Someone definitely modified mine then! mine was cut out evenly on both sides. Also the cut out looks much larger than the "original" piston I took out. Though maybe not. Wouldn't take much to match the Wossner intake and the exhaust side is long like the one posted by Mogly. Good to know that the exhaust is supposed to be long. I may be inclined to tear the new piston out and have it cut now that I know what its supposed to look like!

Pictures of the "original" piston.

all you would need is a new base gasket. just leave the head on.

Well I finally got around to doing a compression test on the husky and the results were less than satisfying. (more like terrifying)

I'm reading 155psi

Upon discovering this I tore the top end down and inspected it thoroughly. The piston and ring look absolutely fine, as does the bore. I used all new gaskets throughout and none appear to be leaking. I also checked the head on my glass top coffee table and it did have a slight wobble side to side. Are warped heads common on this bike? Its never over heated on my watch and I do know how to proper torque a head so I don't think I caused it.

Bike runs awesome though, starts in one kick and seems way more powerful than the old piston which on the same gauge read 160. Also of note the gauge wasn't holding pressure and slowly creeping down. Maybe its had it? It does have a obnoxiously long hose which if I understand correctly can indeed cause a false low reading but by that much I'm just not sure.

I've only burned about 3/4s of a tank of fuel, might it just take considerably more time to wear in the ring? The bore was still pretty nice and cross hatched.

Anyone got any clue as to why it would be over 50 psi shy of what it should be?

new old piston to compare to, no rings or i would buy it.

http://www.ebay.com/itm/HUSQVARNA-M...ash=item58e1ddd565:g:Fg0AAOSwZVlXvOKX&vxp=mtr

you either have something wrong with the bike, or a bad gauge.
are you holding WOT kicking until the needle quits climbing?
you wont gain more than a handful of psi by breaking in, you should have really have all right off the bat. something is badly wrong if you really do have 155 psi. all the earlier 250 bikes have about 175-180 with a fresh topend, with 87-88 having quite abit more. mine has always sat at 215-220

hmmm, i do have a nos ring...

But what could cause that low of compression? Literally only rings and head gasket right?

its probably/hopefully a bad gauge...might have to borrow or buy a good one. you could have a crankcase sealing issue or crankseal but that would have to cause some running issues. weird. if the ring seized into the piston it will drop like that but i doubt that happened, especially if its still harder to turn over by hand.
the head wobbling is a bit odd, but since you removed the head im not sure you can reuse that head gasket.

Sounds like the intake cycle doesnt pull the right amount of air to compress???

I suppose that could be possible. I'll have to give the engine shop a ring and pick their brains on it.

Doesn't sound likely if the piston skirt clears the top of the inlet port, and Kyle says it does.

I would be drawn to the wobbly head. First I would Get another gauge and test another cylinder then use your old one and compare to see if it was the compression tester, (process of elimination) Test your lawn mower. If gauge is diff. get a new gasket and reinstall and check it with both gauges and compare again. Maybe rig up and do a leak down test and spray around every thing with soapy water to look for bubbles and watch your new gauge to see if its leaking down. Remember it could leak through the crank seals and you would never see this, only on the gauge. If it is leaking down and you don't see any bubbles remove the ignition side cover and maybe try n spray some wd-40 behind the ignition (look n listen)?