OOOOps. Smoked Piston. Need Some Jetting advice

Apr 3, 2017
20
0
32
Niagara
Hey guys i think i know what i did wrong here but i would like some input. Replaced Crank seals and bearings. used yamabond to seal cases, and used a little bit on the reed cage. i dont have a air leak tester but i did the oldschool can o carb cleaner around everything and didnt see any change. this motor ran beatuiful and idled nicely too.

First time out after breakin i did a few plug chops and the plug had a nice cofee tan, but i think my jetting was off. It soft siezed after a few quick blasts at wot. it unstuck and then kept running. i brought it home and then did a compression test. i still had 130lbs, but exhaust side of piston was smeared and so was intake side. Cylinder is savable since no gouges just scuffs. piston was destroyed.

im running FMF Fatty pipe. Stock Silencer. No lid with a unifilter. TDR Reeds. Cylinder Kit. I got a 290 jet in there now. is it possible that i need to go to like a 350 or so? i think this is where i screwed up. i live in ny. and also the needle was on stock clip setting.

it also had a strange knock jut off idle when u would get on throttle and get off. its NOTHING internal everything was to spec. its more of alomst like a decel detonation. Should i go back and check my stator timing?

Its been a while since i messed with 2 strokes. Im a 4 stroke guy now but missed having one. just want some input. thanks
 
300 should suit your mods, do another plug chop to confirm.

A proper leak test is required to test for air leaks, spraying stuff at it cannot be relied upon.

Did you seal the collar to the shaft that the gear side oil seal runs on.

Do you warm the engine up to operating temperature before you ride?

Where is the clip on the needle it should be in the middle.

Adjust your idle as per specs.

Idle speed adjusting.
Set float level, make sure that all jets are clean and fuel flows freely from the float bowl drain
when the screw is cracked open.
Start with air screw 1.5 turns out.
Set idle screw so that it has some sort of idle. With TORS it is the big screw under seat on top of
unit. With no TORS is is the brass screw midway down carb body.
Warm up motor and then set idle screw for a faster idle, 2000rpm+
Adjust air screw either way to get the fastest idle.
Adjust air screw a little at a time leaving 10 secs or until the motor responds.
Re adjust idle screw for desired idle .
if the air screw ends up being more than 2 1/2 turns out the pi;]lot is too big.
if the air screw is less than 1/2 a turn out the pilot is too small
 
No I did not seal the collar to the shaft I dont think. Well there was an oring behind the but that i did put in when I reassembled it. The idle and low speed was set. I replaced the pilot for a bigger one. Is there another way I should seal that collar? I'd hate to take it apart and that not be the problem. It defiantly was not burning tranny fluid.
 
Is that oring the way to seal it? It went in between the gear and the shaft underneath the but. I'm wondering If I should have used differrg oil or broke it in longer. It pulled hard and shifted great but when I shifted into 4th it had s slight bog. Maybe go richer on the main? I have done many 2 stroke motors and this is honestly the first I had soft sieze. Wonder of thrbclearnces were still to tight. It was an eBay cyl kit but it was a very tight fit.
 
Yes I did buy the kit off eBay I bought 2 of them.the cylinder that came with the machine is at the upper wear limits. It is probably useable with a new piston. I know these aren't the greatest kits but I did chamfer ports and ran a ball hone down and cleaned cleaned cleaned everything before assembly
 
Why did you need a bigger pilot jet?

Did you do heat cycles and re torque the head and cylinder nuts before you rode it?
 
so Blaaster can u tell me if i have the crank seal collar sealed properly? i picked up another parts blaster yesterday. it has a fmf powercore 2 silencer and its missing the carburetor seat and gas tank but has a good bottom end. good extra wheels. it has a broken reed. so while im waiting on parts to come in, i want to chevk a few other things. im going to give the cheap kit one more shot, but im also going to pick up a standard bore namura forged piston. after measuring the factory cylinders after honing i found that the are still within tolerance according to yamaha book. so either way, ill be up and running soon
 
ok so i found one issue with my motor. when i pulled apart my new parts motor i found that it contained cylinder locating dowels on the 2 pto side stuyds..... these were missing on my motor that i just smoked the piston in. this is the first blaster i have owned bnut not the first engine i rebuilt. they were not on the motor when i diassembled and rebuilt it. could this have anything to do with my issue?
 
Hey guys i think i know what i did wrong here but i would like some input.

What was your ring end gap?

What was piston to cylinder wall clearance?

Cast or forged piston?

Were the port windows chamfered?

Did you do any piston prep before installing top end?

Post a pic of the piston. The black marks on the piston would show you didn't check some of the above.

Otherwise, I vote air leak.

Post some pics of the damage.
 
I deburred the piston chamfered the ports on the cylinder. Cast piston. I should have checked clearances but I didnt because the kit was new. I'm getting another one free under warranty so I will check this one. Where do I buy a leakdown tester for the blaster? And I replaced both crank seals. Only thing I'm unsure of is the collar. There was an oring under the but recessed into the gear.I put that back in but my other motor didnt even have one. I can post pics later
 
so i had a hard time finding things for the leak tester but i did get the motor back together with new cylinder kit. seems to run better now that aligment dowels are in there and i put the powercore 2 silencer on it. the knocking noise on decel is gone. and it runs alot smoother. i started with a 340 jet. waiting to get it out to my property to plug chop after breakin. another thing noticed was it runs much cooler. i cant open it up out 4th in my nieghboorhood. has a slight bog when hits 4th its probably rich with the 340 but on the safe side for breakin. i got my other bottom end almost ready for disassembly and i have parts on order at work to make leak tester
 
Wow i have the same problem i just rebuilt whole motor broke in and now it wont start for plug chops it was running like 2 hours ago ..... I dont get it
 
What kind of piston was used for rebuild?

Tried pull starting it?

In your initial post back on 4-16, you said you had a 290 main in and it was showing a good color on the plug.

Might I ask why you changed to a 340?
How's the plug look?