How many Pistons do I have to ruin

[loganm]

Before I get this right. It's frust. Broke this one in strict, 3 heat cycles up to 200F and back down, warm up, first ride 1/3 throttle, 2nd 2/3, 3rd did a plug chop and it came up black.

Refreshing your memory; bike didn't run when I bought it. eBay top end, k&n filter on back of stock carb, no silencer, stock header.

Orig 152 main, wouldn't rev unless needle was all the way down. Verdict was bore is too tight, or didn't break in properly/warm up.

Put top end on last night, broke in. 145 main jet. Plug chop is black/ very dark tan.

Pulled the piston off, carbon on top of piston, head is black and oily. I was just going home from a short 15 min ride, the beginning of which I let it idle for 15~min, until cylinder was ~155 and not rising anymore.

Now, 2/3 times I've seized the top end its been at 1/3 throttle. At the end of my first break in ride today, I rode over to my house never going over 1/3 throttle. 3/4 mile ish. At the end of my ride cylinder temp was 280F! Which shouldn't be alarming EXCEPT it was under very minimal load the whole time, not riding it hard with no airflow.

I think it's an issue with my needle. Idling during break in the plug was still white. Running rich enough to deposit carbon on top of the piston really only makes sense if the main is too rich, since the needle doesn't come into play after ~3/4 throttle.

I put the top end back together as is. Not too keen on ruining another piston while figuring out jetting. Swapped the main out with a 140

[loganm]

[just ben]

The needle is actually 1/4-1/2 throttle it's on the main after that.

[Jmoozy27]

Check the float height.

[just ben]

The piston seems to be in backwards as well

[cr480r]

The piston seems to be in backwards as well

Ring pins are facing the intake. Looks right to me

[loganm]

The piston seems to be in backwards as well

Directions that come with the piston state arrow goes toward exhaust side. I double checked when I installed that it is in correctly. You cannot see the arrow in the pic though.

Edit: I am going to try and take it to a shop to get the engine pressure tested tomorrow.

[Dirtcrasher]

You need to check your clearances. How many thousanths clearances are between the jug and that piston.

is your premix being a little rich? Seems likely to me.

[cr480r]

Running rich enough to deposit carbon on top of the piston really only makes sense if the main is too rich, since the needle doesn't come into play after ~3/4 throttle.

pilot jet/air screw up to 1/4.. Needle diameter is 1/4-1/3. .. 1/2-3/4 is needle taper/ clip position territory.. 3/4-pinned is main jet. Most fouling issues are from incorrect pilot circuit.. Often the inexperienced foolishly will drop mains or raise clips trying to clean it up.. Until it seizes out in the street.. Later selling it off blowed up.. Next guy throws on a top-end... Same carb.. Burns it down a time or two.. Sells it.. Repeat... Same deal with crankcase leaks. This is how smokers get a bad rep.. Failures are almost always due to lean jetting, or air leaks. Don't bother trying to jet a bike with an air leak.. Test it. Then start out rich and clean up the circuits one at a time from the bottom up.

[loganm]

You need to check your clearances. How many thousanths clearances are between the jug and that piston.

is your premix being a little rich? Seems likely to me.

You mean oil rich correct? Manual recommends 20:1, should be 32:1 in the tank now. I was told by the person that did the work that 32:1 is minimum.

I haven't checked the clearances, but it is the same size piston as last time, cylinder just got a hone job. I can check it, but I am POSITIVE the engine overheated. It was HOT. Temp gun read 300F on the top of the cylinder when I got back to the shop after it siezed. This was under light load. I'm pretty sure I'm chasing a jetting or air leak issue.

pilot jet/air screw up to 1/4.. Needle diameter is 1/4-1/3. .. 1/2-3/4 is needle taper/ clip position territory.. 3/4-pinned is main jet. Most fouling issues are from incorrect pilot circuit.. Often the inexperienced foolishly will drop mains or raise clips trying to clean it up.. Until it seizes out in the street.. Sells it off blowed up.. Next guy throws on a top-end... Same carb.. Burns it down a time or two.. Sells it.. Repeat... Same deal with crankcase leaks. This is how smokers get a bad rep.. Failures are almost always due to jetting, or air leaks. Don't bother trying to jet a bike with an air leak.. Test it. Then start out rich and clean up the circuits one at a time from the bottom up.

Everywhere I look and everyone I talk to has something a little different to say about this. It depends on the carburetor to a point too I'm sure. I'm not trying to blow off your or anyone elses information but when everything is contradictory I almost have to learn everything myself.

[cr480r]

I have never found tuning info to differ very much from source to source. The throttle positions are approximate.. And the circuits do have some overlap. Just be sure to have a clear understanding before you start making changes.. It's possible to have calibrations that will cause spooge and fouled plugs.. But also seize up every ride..

[loganm]

I have never found tuning info to differ very much from source to source. The throttle positions are approximate.. And the circuits do have some overlap. Just be sure to have a clear understanding before you start making changes.. It's possible to have calibrations that will cause spooge and fouled plugs.. But also seize up every ride..

Due to a lot (a lot) of trial and error in the past I know my way around a motorcycle carburetor pretty well. But I think you may be right I may be dealing with someone else's grenade tuning.

[Dirtcrasher]

Double even triple less oil is used now, but it's each manufacturer that advises the oil ratio.

Some of these guys run with a fraction of that oil from 1986.

[Maico]

I wouldn't run it again until I did a pressure check. Bet the ign seal is toast. No amount of oil or jetting is gonna fix a blown seal. Better check ign timing too.

[John Tice]

I’m only a cylinder person but; If you haven’t done this keep on reading. Is the cylinder round & straight? I’m not a mechanic, many of the other readers know the tuning aspect. Heat cycles? If the cylinder is round & straight, 2 tanks of fuel & a head re torque is enough for a break in.
First pic, the piston is starting to scuff.




The cylinder may need a light cut taken on the base; this will square the cylinder with the crankshaft.


Does your mechanic use a dial bore gauge?


Was the cylinder honed with a hone or an Egg beater?

These are simple suggestions but what kind of tools does the mechanic use when they go to the back room to measure the cylinder?