Compression Confusion

[Tau]

701 stock bore, girdled ada head. Rear Carb plugged high speed jet lead to rear piston failure. Rebuild both carbs. Cleaned crank, put engine back together with new rear piston. Now I see it was very stupid to not replace front piston as my compression is 150 front and 182 rear. Ski idles perfect and runs like new at all throttle positions. My question is what damage am I doing if I run it this way? Crank? Only a few weeks of riding here in south Puget sound and wondering if I can run another dozen hours and then replace front piston as fall project? Thanks for help. Tod

 

[Harbor]

Run it..

keep any eye on your pisser off that cylinder, watch for spitting water.. this would indicate the dome oring is leaking compression

 

[Tau]

Run it..

keep any eye on your pisser off that cylinder, watch for spitting water.. this would indicate the dome oring is leaking compression

Will do. I pulled the head so orings are new. But you think running it is fine? Everything Ive read about uneven compression is in regards to finding out why one is off, in this case the reason is known, the dummie didn’t replace it when he had a chance haha.

 

[Harbor]

Sometimes the orings come out of the grove when installing the head and reduce compression…

on my ski once I see the pisser spitting I replace the orings, don’t want water in my cylinder…

if yours are good I’d run it… that’s all I was trying to say.. fix it over the winter

 

[Tau]

Sometimes the orings come out of the grove when installing the head and reduce compression…

on my ski once I see the pisser spitting I replace the orings, don’t want water in my cylinder…

if yours are good I’d run it… that’s all I was trying to say.. fix it over the winter

Got it, thanks for reply I appreciate it. Ill watch for that, solid spray so far

 

[Tau]

So the Rear Cylinder was Honed and a same size original Bore new Piston and new Rings were installed?

Deep Grooves in the Rear Cylinder Wall could also account for leakage and low Compression. Also excessive Clearance due to wear.

Worst case scenarios are Exhaust Gas Blow-by and Piston Skirt Slap.

Blow-by will glaze the Cylinder Walls and Pistons reducing Lubrication, and Hot Exhaust Gases will travel into the Crankcase and maybe into the Carb Pump via the Pulse Line. Damage occurs over a relatively long time.

Piston Skirt Slap can lead to the Piston cracking and breaking apart trashing the whole Engine. How likely this can happen is dependent on how much the Clearance is out of Tolerance. And how high the Engine is revving.

Let us know how it holds up and post Photos of the Rear Cylinder and Piston upon disassembly.

Original cylinder was undamaged. The ada head was chewed up a bit so I just smoothed it out, no damage to or near the slots for the head orings. One new piston and rings only. The new piston has the 180psi. The front piston was reading 150 pre problem and still reads 150. Ill let you know what happens, next month I’ll tear it down and replace the front piston

 

[Tau]

Pic of piston

 

[fastgtfairlane]

I might run that in a stock backup ski but in a stand up where you're riding it hard, running a pipe, more compression etc. That will not last long at all with 30 psi different compression. General rule of thumb is if one cylinder fails, always do both cylinders. Keep everything even. Just think about your crank how long it will hold up running 30psi difference in compression.

 

[Tau]

I might run that in a stock backup ski but in a stand up where you're riding it hard, running a pipe, more compression etc. That will not last long at all with 30 psi different compression. General rule of thumb is if one cylinder fails, always do both cylinders. Keep everything even. Just think about your crank how long it will hold up running 30psi difference in compression.

Thanks for the input, lot to learn on engine work, i appreciate the advice