Engine builders --Piston reading---help

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2/11/2018 1:33 PM

Engine background--78 RM125C. EG full race porting, 6 petal Mossbarger reed, Ceramic bearings, PVL ignition, Lectron 34mm carb, . Bike runs on C12 with 32:1 Klotz Super Techniplate. Jetting seems spot on,no flat spots,or blubbering. ,timing is set to PVL specs. Bike runs ridiculously well, its actually dead even with a stock 2016 YZ125 in a flat drag race, which is incredible for a 40 yo 125.
Raced this bike at MidOhio and pulled 2 holeshots against guys 50lbs lighter than me ,lol.

Now the problem-- I just popped it apart to refresh it with a couple of my other vintage rides and found this dilemma. Ring is stuck on the exhaust side, underside of piston is burnt, top is grey/black with grey specks around outside edge . This engine has only 2 hours on it at the most,but obviously overheated drastically and looked to be on the verge of disaster. What the hell is up ?? Ignition too far advanced ?? running lean ??air leak? squish too tight/ too loose?? I really thought everything was done perfectly and set up and tuned correcly----- So Paw Paw or any other guru please chime in..... Photo

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2/11/2018 5:35 PM
Edited Date/Time: 2/11/2018 5:38 PM

The underside of the piston shows a small amount of trans oil in making it into the combustion, thus a very small leak in the crank seal is possible.
The piston spots show some very light detonation most likely due to the squish being just a bit tight.
The ring being stuck is from too much heat due either from the squish issue or running just a little bit lean.
Remember that an air cooled engine is less tolerant to these issues than a water cooled engine.

Paw Paw

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2/11/2018 6:43 PM

A leak down test before disassembling can reveal any possible air leaks. Piston shows no real scuffing on the exhaust side. Carbon stuck ring, or did the ring land collapse on the piston ? As Paw said, heat may be a factor there.

I tend to be careful with the timing. Just looking at the dial gauge from a bit different angle can make as much as .01-.02mm from where I want to be.

I can recall spark plug reps who could not see any detonation in reading plugs, yet when engines were pulled down, aluminum sprinkling / erosion on the piston crowns and heads was revealed.

Leak down test it. Squish and timing , as well as jetting, all figure into the heat factor. May just have to back off the sharpness of tune to get it to live. May be as simple as taking just a bit of timing away from it.

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2/12/2018 7:22 AM

Thanks guys.I was leaning towards "a little bit of everything" as well. Maybe richen it up a sconche, retard the timing slightly and double check the squish. In your opinions ,where should the squish be set on an air-cooled 125 engine? I've read wild variations. Man, I hate to de-tune it because it runs so good, but I definitely would like it to last longer than a couple of hours ,too.

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2/12/2018 8:41 AM
Edited Date/Time: 2/12/2018 8:42 AM

Do the jetting and the timing first and then see where you are at. The squish is just maybe a bit tight for the timing and jetting you had, but may work with those slight changes. Be sure to address that crank seal as well.
As far as the squish....Look up the two stroke tuners hand book on line and follow those recommendation for your ride.

Paw Paw

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2/12/2018 3:17 PM

Make one change at a time. Pull away a bit of timing first. As little as .1 or .2 mm can make a difference .

Gordon Jennings, Kevin Cameron have great insight to all things two stroke. Google them. Better like to read, though.

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