Motor Mystery....why me???

DoktorC

DoktorC

Member
Joined
May 11, 2003
Messages
431
Age
49
Location
Parry Sound
Here we go again. Doing my annual inspection and found my mag and centre pistons missing some material on the exhaust side. Now, it looks like I have good wash and 74 thou squish so...WTF?? Partial throttle lean maybe?? I've got some pictures...let me know what you think.

I've had this happen before, but to a greater extent (it blew up). I completely blew the exhaust side off the center piston but I always thought is was bad northern gas..I had a lot less squish then 54 thou....
 

It Looks Like Timeing!! Has Your Crank Been Trued And Welded??
 
Cylinders are perfect (thank God!!). The crank has been trued and welded...but I have never checked the trueness...That was sort of in the back of my mind but its run along time between failures (like 3000kms).
 
Check It You Might Be Suprised!! I Have Seen This Before.
Who Did Your Crank??
 
The guy that did my crank is a local guy (he also did my light porting) who builds carts and other cool stuff.

My needles could definetly be the issue...I cannot remember what the needles are set at. I'm hoping that it is a part throttle issue...that's easier to fix...
 
But I'm an expert lol!! That's why I keep all my broken parts...to remind me to quit being stupid...but the pile seems to keep growing.

Now..I started remembering things..I only drove the sled for about 100kms last year and I may have ran the first run on old/regular gas...that can't be good/smart. I'm not sure how critical having premium would be to an engine with that much squish but I don't imagine it would have helped the situation.
 
I was thinking more and more about that on the way home...truely proving I'm an idiot...the sled did actually quit on me on the first ride. I thought it was running low on fuel so of course I don't have any premium so I throw some regular that my dad had in the garage. My brother and I where just screwing around in the field so I didn't give it a second thought..clearly I should have!

Anyways, I was also thinking that bad fuel could/would mimick advanced timing because basically that's what is happening...am I on the right track there??
 
DoktorC said:
But I'm an expert lol!! That's why I keep all my broken parts...to remind me to quit being stupid...but the pile seems to keep growing.

Now..I started remembering things..I only drove the sled for about 100kms last year



DOC!!! Are you sure that was 100 km's last year?? Was that before or after the huge wheely that destroyed the track?? If you are keeping all of the pieces that are wrecked... your going to need another garage... P.S you have no luck... or maybee all bad luck.

I am no expert but I think these guys have a point with the bad gas. We have had some bad gas before (gotta love smooth rock falls). Hopefully you figured out who this is and your sled is back together. I'll try to keep it in mind to ask you if you have good new gas in the tank this year....
Click to expand...
 
Bad fuel would do them all ,look for some thing on that cylinder ,like a craked or loose boot . Maybe a vacuum leak .I would definitely look at needle height .
 
I Would Go With Needle Height. Do You Have Egt's? I Couldn't Tell You How Many Time I Played With Them (watching My Egt's) To Get It On The Money. Gas Could Definatly Be A Factor , But Exhaust Going Away Usaully In My Experiences Is A Good Indication Of A Lean Condition. I've Seen Some Funky $hit Happen When The Shelfs Removed From The Airbox Too
 
Well the shelf is in the airbox, boots are good and I rode 2000+ kms with the exact same set up...and it broke within 100kms of my first ride which I had 1 year old regular gas....One answer seems obvious. That doesn't mean its right..but you can see why I'm leaning that way. I'm just doing an EGT now so at least I'll have something to judge.
 
sounds like a tuning problem, especially because it's the mag and center. You must have leaned trhe mains or gave it some compression and didn't tune the mid range. The mag and center cylinders have Q-4's and the PTO has a Q-8 thats why you didn't burn that one.
 
jetting was 146,147,147...but it hasn't changed in 2-3000kms..if it were lean you would think it would show up right away..or at least show in the wash. I think I'll raise the needles a clip just to be safe..but I'm still not satisfied..

Its also at 74 thou squish...which is definitly not tight..
 
its a nozzle problem thats why you show wash and still burn the edge. You're hurting it at 1/4 throttle
 
1/4 throttle...whats that???? lol. But again..why so light and why did it take 3000kms?? I run on the trail so I do spend alot of time on the needle..
 
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