Oversize bore problems . . .

[other]

Just sticking a toe in the water here, see if anyone has heard of problems boring the CR over the 1.0 oversize Honda recommend. My mates AF has a 96 engine, bored to 90.5 (1.5 oversize). The f@#king thing detonates and seizes constantly (so far, 4 times, but saved the bore with a light hone each time !). Now, this engine has been gone through THOROUGHLY. It was bored very nicely, with a torque plate, and has good relief on the exhaust bridge. I have lapped all the case joins, all new gaskets and seals, there are no airleaks anywhere. It runs a brand new PWK, with 180 main, 50 pilot and DGH needle in clip 2. Pistons are always Vertex or ProX, sized with .06 clearance, ring end gaps checked around 16 thou. I have milled the squish on the head to provide clearance to the oversize piston, and Plastigauged squish clearance to the piston at 1.9mm. It has been strobe timed to 12deg BTDC at 4000rpm, and checked for advance spikes in the CDI map, all good. It runs on 98 octane fuel, at 40:1 synthetic oil, at between 75 and 110 degrees C, and only ever occasionally loses a drop of coolant past the 1.1 rad cap . . . . . Yet every second ride, it melts a hole in the crown of the piston !
Now, the curious thing is, all of the above is absolutely identical spec to my own AF, and I have never even come close to damaging a piston . . . .
SO
We swapped all my ancilliaries (stator, flywheel, CDI, coil, carburettor etc) onto the trouble bike, flogged the crap out of it, and couldn't seize it . . . so we then slowly swapped the original parts back one at a time, continuing to hammer the thing mercilessly between each component change. No seizure, even after we put ALL the original ancilliaries back on the seizer . . . next ride out, the bastard seized !
The ONLY difference between the 2 engines is that mine is on a standard bore, and the seizer motor is 1.5 oversize. (Oh and he has a Fatty, mine has a Gnarly, but I don't think this is an issue . . .)
Has anyone seen problems before with these bigger O/S pistons, could this be the problem ? Coz I'm at my wits end with this thing !

[Kuma]

what is the comp ratio? with as much experience melting the piston does he know what the circumstances are, full throttle at high rpm, partial throttle coming down a steep grade? maybe drop the compression a little, check for leak paths, seals, carb boot, pipe, I know you said it's a new carb but any chance it's defective leaking around the slide?
good luck,

[other]

Not sure of the ratio, i've not actually done the maths, but compression pressure is 175 psi, cold engine, wide open throttle, which seems about right . . . (again, very similar to my own bike at 165 psi) All leaks were totally eliminated, and I even undertook a VERY dangerous test, to ensure this . . . With the bike running at idle, I went over the entire engine, ignition-side crankshaft seal, intake boot, carb etc with a can of Aerostart (ether based starting fluid). Any leak would cause a dramatic rise in engine revs, and there was nothing . . . . (and yes, I did have a fire extinguisher primed and standing by ! !).
I also checked any possible cause of fuel starvation . . . I set the float height to 15mm not 16, fitted larger diameter fuel pipe without fuel filter, and removed fuel tank vent etc etc.
I wasn't there for the first seizure, but the seizures I know about have occurred under 3 different circumstances. The first was after an hour of running-in, the first time he hit full throttle, POP, big granulated hollow in the piston crown, and molten aluminium stuck up the rings.
The second was after 2.5 hours running in, and occurred at just over half throttle, deccelerating for a corner (opening the throttle and flooding the piston with cold charge stopped the detonation, but it was too late, the piston crown was already damaged, and the plug coated in a spray of molten aluminium.
third was after 10 minutes, this time half throttle and accelerating . . . and again we were luck enough that the molten piston particles coated the plug, cutting the engine out dead and preventing more damage.
The only thing the 3 seizures have in common is thay they have all occurred whist riding heavy, dry sand. ie very heavy loading on the engine.
I take your point about the carb Kuma, I wondered about an airleak around the emulsion tube or similar, so I made up a rudimentary flow tester to simulate a draw through the carb. This crude rig obviously wouldn't give me anything but a comparison to a known good carb, measuring the amount of fuel delivered to an equivalent volume of air. It was very close in performance to mine, so that makes me think it's probably not the root cause. I wish the parts substitution excercise had been more conclusive ie, 'didn't seize with my carb on, detonated like f*ck with his carb on, therefore problem is carb'.
Thanks for the input man, I'm really desperate with this one. I've never had a CR5 beat me before, and I'm not happy about it ! Haha

[2strokeforever]

i would richen it up a little and see how it runs

carefully inspect waterpump and gear

maybe intermittent CDI problem, not likely but possible

plugged off pipe or silencer would make it hot

[other]

Cheers 2stF. We ran it on a chassis dyno earlier this week, to check what was going on. Nothing conclusive to be learned tho. It read a little lean off the bottom, ( but Lamda's are notoriously inaccurate in 2 strokes at low revs,) so we richened up to a 55 pilot. Still a little lean, but better, so I slipped in a 60, better A/F ratio on the screen, but popped and banged on the overrun like a PJ . . . . . Temperature remained fine, and even when I turned the radiator blower fans off, she puked out coolant from the overflow, but radiator and barrel temps never went over 110 C. Water pump is all new, and I installed it very carefully. I'm confident theres no problem there. We've also had several different pipe/silencer combo's on it during the seizure/parts substitution, so I don't think its that . . . .
I am leaning towards the CDI spiking the advance randomly and dramatically (I did find the trigger wire broken inside the insulation, and arcing bad enough to burn through. . . . This was between the second and third seizures. I fixed it, and the bike suddenly started much easier, but still seized again . . . . ) I suspect it may have caused some electronic damage to the CDI unit, maybe. I have several here that ran fine on other bikes, I'll probably chuck one of those on as an additional insurance measure.
We are riding in sand today, so will see if richening the pilot from 50 to 60 stops this tomfoolery !
Thanks for your input.

[iggys-amsoil]

I'll take a stab it this,

At what point does it sound like little rocks hitting the skid plate? Oh!, no skid plate. Thats the sound your looking for. Oh wait that sound puts holes in pistons.

You say the clip is second position, 2 from the top or second from the bottom? Why havent you moved that? A 180 main should be big enough.

And put the 55 pilot back in.

Its possiblle you may have to find an E needle like a EGH. Thats whats in mine but it has mild porting. Bored 90mm. FMF

You see I was in Glamis, major sand, and at the drags if I held the throttle at 3/4 thats the sound I would hear, At the time I had the 175 main in, When I had her pinned no pinging, If I backed it off, she would pinging. Yeah I could run the drags that weekend but not fun knowing she could let go.

[other]

Update. She Didn't Seize ! Its as rich and doughy as hell, and blows black smoke out the tailpipe, but it didn't seize. It's a bit lean feeling on WFO, and plug-fouling rich everywhere else, but its now gone over 100Km without replacing the damn piston ! Hoo-bloody-ray !

[NightBiker07]

Sounds like you have a jetting demon - I had one with my 250R/300R bigbore trike. Jetting was either rich and blubbery, but the very next size main down, she ran clean until it seized. Fat pilot jet so starting was cake. Fooled with needles and clip positions - always the same. too rich straight to seizing. It always was 3/4 throttle seize..........