burned down motor again.... suggestions?

[snogoer36]

my neighbor has burnt down '03 z570 3 times in 250 miles. first time dealer said oil pump was to blame. dealer rebuilt and 150 miles in next season burn again. top end rebuilt and 15 miles in burnt down again. 3rd rebuild was total rebuilt with complete gasket kit and oil seals, clean carbs, and complete once over. rode 85 miles taking it easy. running oil in tank next morning got on it and within 5 miles burnt down again.

VM-34 carbs with 480 P-4 jet, needle clipped on 3, 250 main jet in one and found 260 in the other. air screw out 1.5 turns, idle was great not pinging and very strong until she just died, with hole in piston. Out of options!!! dont know what to do.....

 

[lewisb13]

Get a new sled? Just kidding. That looks like you're running SUPER lean. Can you get any more fuel in there? See how it looks like a really light gray? Should have BLACK carbon deposits on the piston tops. Notice how the hole is where the plug is at? That tells me you arent removing any heat from the combustion chamber. What plugs are you running? Maybe trying going one range colder as well...

 

[snogoer36]

thanks for the post....running on BR9EYA plugs and also double oil. Oil from pump plus oil mixed into tank. definitly light grey but just don't understand how it is lean. if i am not mistaken, timing on 570 is fixed/non-adjustable by timing sensor module. 18deg BTDC @ 2000 not sure if i am advanced or retarded or spot on, but doesn't adjust? manual says timing sensor module is to be 185 ohm +-20%. i metered it at 186.4

 

[artic570]

i wonder if it could be pinched fuel line? maybe fuel tank vent line pinched or plugged? fuel pump problem, are the vacuum plugs on carbs cracked?

 

[snogoer36]

thanks....i'll check the fuel lines tomorrow. i'll take any suggestions at this point. Not sure what the vaccuum plugs on the carbs are though.

 

[lewisb13]

I know youre going to check, but you definately have fuel issues. I mean think about it, what causes SUPER hot cylinder temps? Your AFR is way high (lean). Do you have a wideband? Your air/fuel screw only controls the mixture at idle and just off idle so that explains why you arent having this problem at idle. Once you get above idle is when your pilot and main jet controls the mixutre, and at that point, its all mechanical, basically the size of the opening the main jet (or pilot jet depending on where you are at in the RPM range) gives your fuel mixture. Sounds like you have the wrong size jets or a plugged fuel line, or worse, you have some vapor lock going on. Its interesting that BOTH pistons basically look the same, obviously theres a hole in one, but it looks like both cylinders are suffering from the same condition, which leads me to believe the problem is in the fuel system BEFORE the carbs OR this thing is jetted wrong. What elevation are you at? A wideband would be key here...

 

[artic570]

1 vacuum plug on each carb, they are on the same side of carb as the air screws toward the front of the carb.

 

[snogoer36]

don't have a wideband unfortunately. 0-1000ft shows 260 to a 230 main jet varying in temp ranges on the chart. we are at 800ft and the sled has a 260 in one and a 250 in the other, bought the sled that way.

 

[lewisb13]

So yeah Id take a look at fuel supply up to the carbs. Either obstruction, vapor lock, etc...

Might not hurt to even rig a milk jug with some fuel line, just to make sure.

Let us know what you find.

 

[jimfsr]

If you can get a dial indicator that fits the spark plug hole, make sure the pistons are at TDC when the flywheel marks are lined up. It does appear to be running lean, but an out of phase crank shaft can cause the hole in the plug(timing way too advanced), also causing a lot of heat, which can make the other cyl. run hotter than normal, causing a lean type condition. You didnt indicate which piston had the hole, but it really doesnt matter.On a fan cooled engine, if the first cyl. in line runs hot, the second will too, because the air flowing over it is way hotter than is should be. Just some thoughts...

 

[snowboss]

You may be over looking one thing. Maybe it is getting so hot in the cylinder because the burned gasses can't get out!
I had my neighbors 600 ZL do something very simular looking and it ended up being that the exhaust can was pluged. We borrored one from a buddy and put it on and that was it. Also make sure the exhaust end is not bent from hitting a rock or something.
Just my .02
Dave

 

[G]

You can rule oil out. That is either fuel starvation or a timing issue.

 

[Duffy]

This does not look like a fuel or oil issue, if it's running lean it will burn the exhaust side of the piston. Check the timing when ever you have a hole in the center of a piston. Do not go by the marks on the fly wheel and stator plate, use a timing light or a dial indicator. TIMING TIMING TIMING

 

[salmonbum]

maybe dumb, but how are the carb boots?

 

[snogoer36]

the pictures of the pistons are with the exhaust side facing the camera. hope that helps. i am in the process of getting a dial indicator and will check as soon as the new pistons come in. Carb boots are in perfect shape, not cracked dried out or anything. exhaust can and outlet look perfect, btw. again thanks for everyone's help it is greatly appreciated.

 

[ubee]

super lean condition, just like taking a oxy acth torch and holding flame on top of piston. use unlit propane torch to check for air leaks

 

[doomsman]

Sleds have ((advance boxs)) they actually retard (back out some of the advance) the timing. Get the specs and check the timing per the specks. It is scary sticking your head down in there while reving the engine but without a scope it has to be done.
You may have an issue with the retard box. GOOD LUCK.

 

[snootbiscuit]

running ethonol? using iso-heet? check e-clip setting!!

 

[indy_500]

maybe dumb, but how are the carb boots?

x2 on the carb boots

 

[snogoer36]

The shop manual says the following and that's it no more instruction on timing:
NOTE: Ignition timing can be checked and/or
verified on all snowmobile engines; however,
ignition timing can be adjusted only on certain
models.
Ignition Timing - Engine Warm
BTDC - 18°/2000 RPM - 2.049 mm (0.081 in.)
NOTE: For timing purposes on the 570 cc, use a dial
indicator and scribe a mark on the flywheel for 18°
BTDC

i know that the timing sensor module retards the timing at higher rpm which is controlled by the module itself and the CDI box. unfortunately, both pass the resistance test charted out in the shop manual. i really want to find something majorly wrong or obvious but nothing is. i am going gray stressing about what it could be.

Running 87 octane from 2 different fill-ups in 2 different states, no additives, needles clipped on 3.