Blown Piston on a CB350 - What caused it?

A few days ago I blew the piston on a cb350 and I want to get a better understanding of what could have caused it. Dropping about $500 into getting it fixed, and want to prevent it from happening again of course.

Here's a bit of background to the incident. This was the first time the bike was on the highway since the rebuild. Everything on the bike was redone except for the motor. Motor previously had great compression, so did not tear it apart. Was riding for about 15 minutes, 80 mph at the time when the left side went out. At the exact moment of power loss, first there was about 10 seconds of minor loss of power (20%), then suddenly left side went out completely and blowing smoke.

I'm not sure exactly what went wrong. One suspicion was that it was just too hot for it since it was near full throttle and maybe running lean. Main jets were reduced to 45 mains (from 50 mains). A different suspicion was the the exhaust might have been too restrictive as they are essentially straight pipes with a baffle inside. The baffle has holes in it, but restricts down to 1/2. Lastly after tearing the engine apart, the timing tensioner is all tore to shreds, but the valves show no signs of damage from hitting the piston.

So anyways, again, my guess is it's one of these:

-Overheated. Piston sized, and blew out piston
-Worn Tensioner. Threw off timing, and valve knocked out piston.
-Restrictive Exhaust. Pressure blew hole in piston.

New wiseco 66mm 10.5:1 compression pistons, gasket kit, and KA tensioner are already ordered. Plan to rebuild this next week.

Actually, I'll add to this.

What I really want to know is, does a restrictive exhaust cause this? Is a 1/2" custom baffle for a cb350 too restrictive?

I want to use the same exhausts if possible. But if it is too small, then I'll redo the exhausts altogether.

It's not valve contact. The carbon build up on the piston would be worn and you'd see bare metal.

Burned pistons come from heat. Heat comes from a few places. Usually poor ignition timing, too much compression, or mixture is too lean. I'd definitely investigate the last option as it's extremely uncommon to need to down jet. First is easy to rule out though.

I am betting too lean as well, it would be nice to see the plugs from all four cylinders and the color of the exhaust headers just at the first bends.

it doesn't really matter what caused it because if you have it tuned correctly and no sillyness in the exhaust like a 1/2" restrictor it will run fine
but to foresnick it, is a good thing so what did you have the timing set to ?
and i can't even imagine it running full throttle with a 1/2'' restriction,that is not for very long before it bloes a piston....oh wait ...... ;D
get smart and copy a known good running exhaust design and a long one,sghort pipes are a losing propisition

a cb350 flogged down the road at 80 miles an hour for twenty minutes?

at full throttle

you reduced the jets and did not do a plug chop

you did not go thru the motor ??



i know whats wrong and i am sorry to say it cant be fixed

you cant fix stupid !
'

The outlet of the stock exhaust is only about 1/2 in ID....

That piston looks like it was very hot.....pretty much everyone has covered the possibilities...lean, pre-ignition from the high temp, blows a hole in the piston....Aluminum softens at about 900 deg. F. Combustion runs in the 1200-1500 degree range. The piston is cooled by oil on the bottom, and the incoming fuel/air charge. A lean condition has less fuel to vaporize and cool.....That is why high performance engines run rich, for the extra cooling. High RPM and lean conditions develop heat faster in the piston than the fuel air charge can cool it. The aluminum gets hot , softens, and boom...hole in one.

Air cools the cylinders, but pistons have very little contact with the cylinder, just the rings and very little skirt .... So only fuel air mixture and a little oil splashing on the underside of the piston cool it.

mydlyfkryzis said:

The outlet of the stock exhaust is only about 1/2 in ID....

That piston looks like it was very hot.....pretty much everyone has covered the possibilities...lean, pre-ignition from the high temp, blows a hole in the piston....Aluminum softens at about 900 deg. F. Combustion runs in the 1200-1500 degree range. The piston is cooled by oil on the bottom, and the incoming fuel/air charge. A lean condition has less fuel to vaporize and cool.....That is why high performance engines run rich, for the extra cooling. High RPM and lean conditions develop heat faster in the piston than the fuel air charge can cool it. The aluminum gets hot , softens, and boom...hole in one.

Air cools the cylinders, but pistons have very little contact with the cylinder, just the rings and very little skirt .... So only fuel air mixture and a little oil splashing on the underside of the piston cool it.

Click to expand...

much of the piston cooling from the heat on top is thru the top ring
you would have to prove that it is 1/2, more like 3/4
and of course the longer the small restriction is the more it restricts

Maybe your exhaust hole is a lot larger than mine....

Let's work this backwards. The final failure was detonation. That's what ate the crown and sprayed molten aluminum over the rest of the crown and probably the con rod small end too.

And the power loss was most likely the piston seizing from excess heat and as it seizes, it generates even more heat and that cycle of destruction doesn't take long to peak.

So the next question is why did it overheat. Jetting is probably in that mix along with ignition timing, and excessive back pressure in the exhaust is most likely a factor too.

You must flush out the crank before it goes back in, now that the motor is out.

When the motor goes back together, start a couple of main jet sizes up from stock and with a decent exhaust and make 200% sure that ignition timing is spot on.

Why downsizing mains in the first place?