There she blows again..

[fallenmikethebike]

I did think about a holed piston, or conrod even (really hope not)

I'm sure the mixture should be fine, I did a plug chop a few days before and it seemed ok wide open
I fitted one size larger jet to the main. Wish I had some tools handy to have a look.

If it is a holed piston or rings, I am hoping that I can fish out any bits with a magnet.

Hhhmmm alloy piston !!! + it may have melted in crank , sorry for the downer

[kernow krusty]

If it wasnt for bad luck scrubs, youd have no luck at all mate....
The bit about backing the throttle off sounds interesting. My mate,who has rebuilt more strokers than Ive had hot dinners (and Im no salad fan!), always despairs of injected 2 strokes, coz his theory is that, when you back off the throttle, you also close the oil pump..so, running in top, with the throttle part closed, you are pushing quite a bit of petrol in,(probably on the main jet), but the pump isnt at full stroke...so I guess you are not getting a full whack of oil and its like running at 60 or 70 to 1 ratio (or higher). Hence the poorly engine.
Ironically, your full throttle plug chop was possibly the best way to run it in!
I read somewhere that the best way to bed a stroker in after new piston and rings, was to start it up, let it idle and with some small throttle openings, for about 15 mins. Then let the engine go stone cold. Fire it up again, with a little more throttle used, then again after 15 mins, let it go stone cold. After that, check mixture and settings, and it should be hot to trot.

Was there a chance that the oil level got so low that a few air bubbles got into the pump and it missed a 'full squirt' of oil at a critical moment?
IMO, modern 2 stroke oils are all pretty damned good, and lets face facts, CRM's arent race tuned, hi compression power monsters. I agree with Mike in that I dont think the oil was to blame. You are gonna have to take a peek - look at all the reputations that have been staked on the cause!!
Regards KK

[knackeredMk1]

I had Husky & KTM 125s that did almost exactly the same thing, though premix. It turned over easily enough but virtually no compression. I suspect you will find small amounts of piston skirt smeared up and down the barrel. With a bit of luck you caught it on the clutch and only a new piston is required. You might even find a bit of polishing with some v. fine wet & dry and picking the bits out of the piston rings that it will run

It's the throttling off that kills .

[scrubs]

Cheers lads.

I managed to borrow a spanner set today and decided to strip her down in the car park - as i couldent wait to organise a van
to take it home. Good news, It was the rings as suspected. Big score down the piston (exhaust side) any idea what that indicates?
The barrel is relativley unscathed thankfully, apart from a slight nick - looks like almost a tiny bead of weld - melted piston stuck to the side
but should be ok after a sand. It was a bit of a bugger to get the barrel off this time as the sleeves from the barrel threads
dropped down so I dident have the clearance to lift the barrel over the studs - because with the sleeves it added another centimeter
managed it eventually after much faffing, my only tools were the spanners and my house keys which doubled as a screw driver.

So I need to order a new piston and rings. When I got my barrel back from Stan Stephens he put the pro-x (TRX) one
in and as i remember it was still the standard AR bore width. But I can't remember the Diameter - 66mm or 67mm?

So as far as I am aware - I still have a standard bore.
Any idea where is best to order the piston and rings from? I remember the rings were a different brand to the piston.

[knackeredMk1]

Standard AR bore is 66.4mm. Other CRMs start at 66.0mm. Stan could have made your bore to any size but I suspect 66.5mm You will have to check with him or measure the bore v. accurately.

[scrubs]

Yeah I'm almost sure it was 66.5 as I remeausured when it came back and remember thinking he must have just honed it.
I will double check with them tommorrow though - hopefully they keep records.

[knackeredMk1]

I think you will find he bored it to take out the plating, replated it to less than 66.5mm and then took it out to 66.5mm.

[sparky]

isn't stan stevens away for a while??

[scrubs]

This is interesting.... Got this reply in another forum regarding the piston diagnosis.....

Scrubs, has anyone attempted to port the exhaust port? Sounds like the piston got severely overheated, sticking the rings. Then the flame front escaped down the side of the piston, scorching it. Check your piston to cylinder clearance when you put it back together. If there is too much clearance, the piston will have trouble transferring heat to the cylinder wall, thus overheating the piston. If someone has tried to open the exhaust port, the rings may be jutting out to far as the piston passes the exhaust port and can't "get back" in time before hitting the edge of the exhaust port. Usually this results in broken rings, though, not overheated rings. I'd bet you had too much piston/cylinder clearance.

Knackered, when Stan does the enduro tune, does he do any work on the exhaust port? I'm assuming he does...does any of this sound feasible?

[knackeredMk1]

Stan is THE top 2 stroke tuner in UK & Europe I don't think he will have done anything to sacrifice reliability in 'enduro tune' mode. Whilst your other forum advice is correct in that enlarging the exhaust port can lead to piston ring wear and breakage I don't think this is the case here. (If you have ever looked at the exhaust port on a 250 or above MX bike you will see why I say that )

As for the 'piston is too loose' theory I really can't say. My KTM 125 seized in much the same way as yours with too tight clearances and Stan solved it after others fitted 3 pistons and a replate by making the fit looser

My suspicion is that you were running lean and when you chopped the throttle that was the end. I'd put a new piston in, go up one or 2 on the pilot and raise the needle one notch. You might even want to go up on the main; this would be the safest way to proceed and then work your way down by doing plugs chops. Because my project involves so many changes I'm going to start rich, get it run in and take it to a rolling road tuner to get it finalised.

I can't remember if you made any other changes to your intake and exhaust when you rebuilt Do check your intake for any leaks, but that is probably not the cause. Double check the alignment of the oil pump as per manual.

[scrubs]

Thanks mate. I tried ringing stan today and he is still on holiday.

I did however find the box for my piston - and it was marked down a being a 66.00 mm
So I am assuming I can just by another 66mm and drop it in.

Model TRX 250'87'89
Part No 01.1300.STD

[scrubs]

any one know the best place to pick one up?

[knackeredMk1]

Sorry I don't think you can rely on the box Have you got a really accurate set of calipers or even better a bore guage

When I contacted him about doing my engine he was due to come back early July but he employs others. Has the whole place closed?

[scrubs]

Yeah it seems the whole place has closed, he just left an answering machine message saying he was back on the first of
July. I have got some digital calipers that I can use. It's just a pain in the arse I don't have them with me, they are in leeds
I will have to travel over at the weekend to get them.

Whats the rule of the thumb regarding matching pistons to bores. Do you buy the exact same size piston as the barrel measurements?
or allow a little space tollerance?

[fallenmikethebike]

Yeah it seems the whole place has closed, he just left an answering machine message saying he was back on the first of
July. I have got some digital calipers that I can use. It's just a pain in the arse I don't have them with me, they are in leeds
I will have to travel over at the weekend to get them.

Whats the rule of the thumb regarding matching pistons to bores. Do you buy the exact same size piston as the barrel measurements?
or allow a little space tollerance?

The tolerance is generally allowed for in the recommendation of piston to use for a given bore size.
Me being an old [fashioned] sort of guy , i would recommend a longer and more mixed running in procedure ,before giving it a severe blat down the M anything .Lots of gradually increasing runs is probably the way to go to reap that legendary Honda longevity IMH Opinion ,and the checking of items already mentioned ie mixture, oil pump,it may also be worth putting a half mix of super unleaded in for the first run , that being of a higher octane rating that should help it to run a bit cooler.
Speaking of cooling, check rads for blockages to impede airflow.