Low compression two stroke

[A100man]

My (bodgers) course of action would be as follows.

Remove piston from con rod.

Measure piston clearance, if OK (in spec) rub off any high spots on the piston with wt and dry

Buy new rings and gap them correctly

Fit new rings and button it up

Run carefully for a couple of hundred miles before I tries to wring the neck of the blighter.
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Now: A100, GT250A, XJ598, FZ750

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600

[MCN]

[jeffyjeff]

[A100man]

[stinkwheel]

As I remember there was an issue just after you built this engine too with an exhaust leak, shitty fuel in the tank and a leak at the inlet manifold where it was popping, backfiring and stuttering after a cold start.

Bet that's when this happened.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[PotatoHead202...]

Someone has been over today to work on it. The points plate had slipped retarding the ignition by about 1mm out. He is adamant the bore and piston are OK enough to run. The carb is blocking again as after a dozen kicks the plug is bone dry. I've already cleaned it several times since the weekend including an ultrasonic cleaner. I'll try an auxiliary tank and new fuel tomorrow.
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Honda VFR1200F, Honda CBF500 ABS, CZ125 now CZ175

[PotatoHead202...]

[Nobby the Bastard]

[PotatoHead202...]

[PotatoHead202...]

[Nobby the Bastard]

As it's a 2 stroke, the timing being out would either limit revs from normal (retarded) or make it incredibly difficult to start (advanced.)

It's unlikely to cause seizure.

It's clearly seized.
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trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

[stinkwheel]

If it has low compression, it's not going to suck fuel in properly, or transfer that fuel into the combustion chamber. If it wont blow your thumb off the sparkplug hole, it wont suck petrol in.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[PotatoHead202...]

[Robby]

oh, it's another thread where the full story gets drip-fed.

If you ask for help diagnosing a thing, people are going to concentrate on the thing and assume that everything else works. In this case, I would assume that the fuelling and timing is OK, so the problem has come from the rebuild.

From your new information, the only thing to say for sure is that your engine got hot. It could be a problem inside the carb, it could be an air leak, it could be parts that don't fit, it could be timing going all over the place.

Also, 25:1 sounds like too much oil to me. Best to look around some model specific forums to find out what people are using with modern oil. Too much 2-stroke oil means less petrol, which can lean it off a touch. Not enough to cause the damage here, but simply adding more oil for peace of mind is not a great tactic.

[PotatoHead202...]

[Easy-X]

You'd probably go for a higher oil to petrol ratio breaking in a new piston/barrel like wot this was.
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[Bhud]

I'm not trying to be a dick; just trying to be helpful.

I've got a similar problem at the moment, except with a 4-stroke engine.

If you've got a way out, take it, is all I'm saying. It's just a bike and you've got to rationalise it. I worked out the costs of repairing it versus what I could get for it on the market in its blown state, before deciding to take the plunge and dive into it. This reduced my sense of frustration and annoyance.

[A100man]

[stinkwheel]

Does it not have an oil measure cup in the fuel cap with the ratios stamped on?

As above, I think the piston is saveable, the rings seem shot. Bore probably just needs a hone.

Although what it almost certainly actually needs is the barrel and piston taking to someone who does rebores and the bore checking to make sure it's a) Round. b) Paralell and c) The correct diameter all the way along. Prior experience tells me it could be oval and/or tapered and/or the wrong diameter.

You can kind of do a lot of this with feeler guages and measuring ring end gaps at different points but it's much better done with a bore-guage.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[stinkwheel]

[lingeringstin...]

Looks like it got hot and damn near seized to me, which I'm VERY familiar with having destroyed a few MZ engines in exotic manner.

Also I have always found that everybody including the manufacturers make the pistons too tight in two strokes. I used to hone the buggery out of my barrels before putting new rings on. Even a lot of engineers who should know better bore the barrels of two strokes to the "right" tolerance for a four stroke piston and that's too tight in my opinion.

I've successfully used a lot worse barrels than in that picture. I'd just give it a damn good hone and pop in another piston and rings of the same size as before. Two stroke isn't rocket science but it is quirky.

All kinds of shit can make a two stroke run hot but main seals are often overlooked as the culprit because they're hidden away and you just don't think about them because it SEEMS like a carb issue, so you spend all your time tearing your hair out rebuilding and replacing carbs that were perfectly fine in the first place.

On the two strokes I've had, if you think it's the carb playing up, it isn't. It's one of the main seals sucking air. When the seals begin to do that a lot of people think it's the carb and end up faffing about with jets and all sorts of bodges that DO seem to work temporarily and reinforce the belief that it was a carb problem, but eventually the bike just won't start at all and they can't figure out why. The carb is rebuilt, there's good spark, everything looks fine but it just won't start and all your mates swear it still seems like a carb problem. No. It's the seals. It's ALWAYS the bloody damn seals! Bastards. I hate them. They make your life miserable.

Seals can also cause the compression to do stupid things.

Also, if you have an automatic oiler you should check that it's actually oiling. I hated the things. They were just another link in the chain of shit that will probably go wrong. I always did away with mine and put the lube in the petrol tank.

[Easy-X]

[stinkwheel]

[PotatoHead202...]

I gave up with this over summer due to other things getting in the way and sent it to a mechanic to look at - after having 3 other people round to check it over in person.

His initial findings are that the piston and barrel (purchased as a set) didnt quite fit together properly and that the rings were of poor quality. That the bore had some tight spots which were catching the piston and causing it to stick slightly, resulting in it overheating and ruining the rings. He thinks the last 50/60 mile run I did on it on a hot day were what finally pushed it over the edge.

He is going to hone or rebore it (I can't remember what he said) to get rid of the tight spots, clean the piston up and fit new rings.

He also mentioned about how he used to have them as a kid back in Bulgaria and how they'd even run them on 4 stroke oil when nothing else was available.
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Honda VFR1200F, Honda CBF500 ABS, CZ125 now CZ175