piston chewed

M

mattysmith440

New member
Joined
Nov 7, 2011
Messages
161
Age
40
Location
Labrador City, NL
so i hauled my motor apart. the mag side, which i knew was shot, was chewed to pieces, with half of the top ring not even there, and the cylinder mangled. took off the middle cylinder and everything seems fine. so i take off the pto side cylinder and to my surprise the top of the piston is burned down on the exhaust side, almost to the top ring. so what could cause this? the only thing thats changed is my pilots. they were 45 and i changed them to 42.5, and i took my bender can off and put a mbrp race can. it has fresh 91 octane, with a new filter and carbs thoughrouly cleaned numerous times. im running yamalube. i havent had a temp light come on.... i need to figure out what cause it before i spend money to put it all back together, stock or big bore.
 

that is not healthy at all.Something not right there for sure.Check for air leaks from seals or boots...that would piss me off if that was my SRX...I would take my gun and finish mine off it that happened to me..put a hole thru the center piston.If that happened to me I would basically not rebuild and part the SRX out and keep my eye open for a newer sled at that point.
 
all the trouble i have beenthrough is crazy! i feel like blowing it up lol . i think i may keep that motor for parts, and keep an eye out for something else, its just he chassis, hood, suspension, track, everything else on this srx is in TOP shape, looking like it just came off the showroom floor. i have some pics on my blackberry, you got a email i can send them to?
 
Errosion of the piston crown on the exhaust side is an indication of the mixture being too lean rising the combustion chamber and piston temps to a too high of temperature.

This can be caused by a number of things.[I am assuming sled is stock]

Some Possible causes.................................................Solution
-incorrect jetting...........................................return all jets to stock
-dirty carbs/jets............................................clean carbs
-plugged "top Hat filters".................................clean these when cleaning carbs!
-intake air leaks.............................................check reed cages and carb boots
-gutted or incorrectly installed air box................correct
-low fuel in tank causing starvation....................keep above 1/4 tank when run hard
-plugged fuel tank filter...................................replace filter
-water/ice in fuel...........................................drain all fuel and put fresh 91 octane
-crank shaft seals sucking air............................replace both mag and pto seals
-low octane fuel.............................................run 91 octane
-too much ignition advance..............................ensure CDI is stock
-too much compression....................................ensure stock heads


Just some ideas

Bob
 
jetting is stock, but had 45 pilots. returned to stock
carbs were cleaned 3 times in 300 miles, including top hat filters
brand new reed petals, and carb boots
airbox isnt gutted, and is installed correctly
i rarely let my tank go below 1/2 as there is a gas station right next to my house, and when the engine blew, there was between 3/4 and full.
i drained my tank and installed a new fuel filter 300 miles ago.
always run fresh 91 withno ethanol
im assuming the heads are stock, as i wasnt told any different when ibought the sled. and as for seals, they look decent and have no sign of oil leaking out of either of them but i am going to replace both before it all goes back together.

i had no problems in the first 800 miles i put on this year, then i put on my mbrp and changed to pilots back to stock and this happens!!
 
make sure no one play with the needle setting ...................if everything is stock something make that engine running lean for a reason ....crank seal ........u need to find the reason why cause it will happend again on the next rebuilt ....but definitly running to lean
 
havent split the base yet, but the bearings on the top of the rod that the wrist pins go through definalty have seen better days.

i switched to 42.5 because i was told that 45 was too big for my 1700ft and -30 to -50 temps.

the thing is i bought this sled this past fall, without chance to really try it. the guy i bought it off said he only owned it for the summer and claimed to know very little about it. so god only knows if it is stock, if its been ported, etc. because i have found a lot of fishy little things on it
 
its gonna be easy to see if the cylinder are ported now that u got them in your hands...but if everything is clean in carb and those are correct jetting look like u have seal problem on eatch side ..... 1-150, 2/3-148.8 and 42.5 should be out of problem with a stock sled
 
I wonder if there is a problem with backpressure from the other can.I have seen a sled have a seizure from weak springs and bad dounuts mixed in with a very bumpy trail.



.
 
bluewho said:
I wonder if there is a problem with backpressure from the other can.I have seen a sled have a seizure from weak springs and bad dounuts mixed in with a very bumpy trail.



.[/QUOT^^^^^^^^^^^^^^
that would be something to check for sure ....bad seal doesnt work good for 2 stroke ;)!
Click to expand...
 
I had the same thing happen to my bone stock 2000 SRX with the middle cylinder/piston. The sled is absolutely bone stock and the carbs were spotless. The carbs were cleaned less than 20 days and 700 miles before the burn down. The center cylinder burned down right after a 25 mile cruise at 40-50 MPH, no wide open runs. The engine had 13100 miles of perfect running. I examined the carbs right after the burndown and they are spotless, no debris nor plugged jets. I have to believe it was the gas but how to prevent this again is a mystery.

The center connecting rod got overheated and there was a lot of metal debris everywhere in theh crankcase so I sent the crankshft to Jeff at Midwest Crankshafts in WI. He replaced the 2 main bearings, the connecting rod and the connecting rod bearing along with welding the center pin on the crank.

I am thinking of either using an octane booster and/or assembling the engine with the 8DN-00 heads to lower the compression to help reduce the potential for detonation. I had the domes on the new pistons costed with a thermal barrier by Swain to give them a bit of protection.

The sensitivity of these engines to lean burndowns/detonation will only get worse with higher volumes of ethanol in the gas.
 
to be honest 2 of my exhaust donuts are pretty bad, as well as the springs holding the #1 pipe were pretty easy to come off. to add to that, when iput my new can on i never could get it to sit as good as my bender one did, but i eventually did get it to seal so no exhaust was leaking. funny you mention a bumpy trail, i had just got off about 30 km of ungroomed really rough trail,running pretty hard, then when i reached the lake she died. hmmmmm. i wonder.
 
i put a can on my srx and nothing was perfect on it ...i had to heat the tube and fix them in correct position
 
Springs get weak after being removed a few times and over the years..i have found if you can lift up or shake a pipe and it moves the dounuts take a pounding.Double up if you can or buy some new springs .




.
 
Last edited:
brand new reed petals, and carb boots

Why were they changed and was it just before the final ride?
 
You must log in or register to reply here.


Back
Top