Holed piston - advice?

[Shadow24v]

well had an unfortunate incident with my 78SE. lost the right piston on the highway on tuesday night. (see main thread here) In a nutshell, holed right side piston on the highway, suspect over advanced ignition timing for the issue. I'd like to get some advice as to the best and least expensive way to fix a holed piston. I'll make sure the left side is OK, but it had 140psi when i did a compression test on it after the incident so here's hoping that that side is ok.

I was thinking of doing a quick and dirty piston replacement using a used or NOS piston and the rings off of the old piston. for the cost of gaskets and a piston (around $100) i could potentially be back up and running. Is there anything that would logically make this an ill-advised repair method?

As much as i'd like to do the Mikes piston/jug kit, $500 is a bit beyond the budget...

 

[retiredgentleman]

How many miles/kms since the last top end overhaul? If the engine has a lot of wear it would be false economy to just do a quick and dirty job.

With the work to remove the engine and open up the engine, the wise thing would be to use new rings, lap the valves, new cam chain, new front cam chain guide, new valve stem seals, gaskets, oil seals etc. Piston/cylinder clearance shouild be checked.

Yes you could do it quick and dirty, but the results may be good or they may be bad

 

[Shadow24v]

good point, there are ~28,400 miles on the bike. no idea when the last overhaul was, if ever.

Is it even possible to do top end work like a piston swap with the motor IN the frame?

 

[TeeCat]

Shadow, I'm curious.... have you been able to determine why the timing was over-advanced? That's scarey! No warning at all? Bike had been running okay, seemingly?

TC

 

[jay760]

I'd check the manifolds as well, more likely to be an air leak.

 

[Shadow24v]

I'm pretty sure the over advance was probably an ID10T error. I have upgraded to a GM DIS car coil with a transistor circuit using the points as a trigger. I adjusted the timing via the bottom plate to get it set correctly at base timing. I did verify it got to the full advance mark, but i don't thing that it was at 3500rpm, probably more like 2000-3000. My thought is that the points themselves were off causing an over advance at >3500 rpms

Bike was running like a champ before it popped though. Best its run since i got it running.

I did have to set the left side mixture screw at around 3+ turns out (with a 47.5 pilot) to get it to a strong idle on both cylinders. Pilot jets, left side mix adjust and new plugs were done the day before it blew.

 

[Tomterrific]

Cheap rebuild if time is cheap.
Pull the engine.
lap the valves.
Use sand paper to get a crosshatch on the cylinder walls or you can try a real hone. Don't go nuts with the hone, just get a cross hatch finish on the cylinder walls.
Rering using Honda rings, or stock rings if you can find them, on stock pistons.
This seals up the combustion chamber and should give power equal to a new engine, maybe not quite as reliable. :^(

I'd put in a new cam chain and guide while the engine is apart but that makes it more expensive.

Run premium gas for detonation safety. You never know when you are going to need the octane until it's too late.

Tom

 

[Shadow24v]

Honda Rings? what honda used similar rings?

I was running 93 octane when she blew...

 

[5twins]

Points don't affect the advance ..... ever ..... period. It's solely the advance unit that controls that aspect of the ignition. If you tuned the carbs the way you said you did, that may be why it blew up. I've never heard of a '78 carb set being set up that way. It's so far off base it's not even in the ball park, lol. Left side mix screw only tunes left carb and cylinder, not both.

 

[Shadow24v]

Points don't affect the advance ..... ever ..... period. It's solely the advance unit that controls that aspect of the ignition. If you tuned the carbs the way you said you did, that may be why it blew up. I've never heard of a '78 carb set being set up that way. It's so far off base it's not even in the ball park, lol. Left side mix screw only tunes left carb and cylinder, not both.

the right side hardly gave me any issues so I spent most of my time getting the left side to work better. All the indicators i had pointed to the right cylinder being in a decent ball park. vacuum gauge readings for balance, plug color, mix screw (stock 2 1/4 turns out) all seemed to indicate everything was OK. Unless i'm missing something that pointed to the tuning on the right being FUBAR....

EDIT:
I read something RG just posted about the ATU and the ears fitting the advance shaft. I know mine have slop between the weights and the shaft. I thought that slop on the ATU was normal.... (first vehicle with points or mechanical advance, used to working with programmable ECUs...)

 

[Tomterrific]

Grizld One has a clever fix for the advace weights wearing in the advancer mechanism.

It's true, way too much advance will detonate like crazy even with high test. Always check timing with a timing light at full advance.

If the carbs are vacuum balamced but you are having a problem with idle then one carb may have a spec of dirt in the idle jet. This is a very common problem with any old bike. You can pull the mixture screw and use spray carb cleaner to blast the dirt back into the float bowl.

Tom

 

[Shadow24v]

Funny thing is i just rebuilt the carbs a few weeks ago. soaked in carb cleaner and all new jets, idle screws' enricher valve, the works. I could about eat off the carbs when i was done...

I guess these motors are not as tolerant of ignition advance/detonation as i'm used to with my car...