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1996 polaris sport

4K views 35 replies 5 participants last post by  Timothy Rogers 
#1 ·
Was driving 1996 polaris sport at top end speed all sudden made loud pop and lost all power smoking a lil to i try starting it and sounds like starter just spinning it self not the engine and when pull start it way to easy to pull no compression after bout 4 pulls it does weird noise f out exhaust like weak back fire
 
#2 · (Edited)
2 stroke or 4 stroke? Sounds like something bad happened. I think the Sport was a liquid cooled 400cc 2 stroke. So with that in mind something failed in the engine and you lost compression. Could have broken a piston or something. Probably need to pull the cylinder head and find out. Maybe pulling the spark plug will tell you something.
 
#4 ·
Sorry, but if it spins over too easily, you have a major failure in the cylinder. Could be it burned a hole in the piston or the piston broke....either way probably gonna need a top end


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#5 ·
The bane of topping out a two stroke - pre-ignition that can't be heard - burns a hole right in the center of the piston

If you find out that this is what happened, it either needs to have a larger main jet in the carb, a colder heat range spark plug, higher octane fuel or a combination of those - it's also possible a crank seal, intake leak or too lean a oil/fuel ratio could have contributed to the problem. Get the crankcase pressure tested before starting repairs. You can inspect the piston by putting the piston a BDC and looking in the spark plug hole. If the crankcase pressure test is positive, and there is no hole in the piston, tear the top end down to see if the piston seized to the cylinder - if it did, then it was a simple case of overheating - topped out too long and the exhaust side of the piston overheated and melted. Melted on the intake side indicates a coolant flow problem.

Good luck
 
#8 ·
The bane of topping out a two stroke - pre-ignition that can't be heard - burns a hole right in the center of the piston

If you find out that this is what happened, it either needs to have a larger main jet in the carb, a colder heat range spark plug, higher octane fuel or a combination of those - it's also possible a crank seal, intake leak or too lean a oil/fuel ratio could have contributed to the problem. Get the crankcase pressure tested before starting repairs. You can inspect the piston by putting the piston a BDC and looking in the spark plug hole. If the crankcase pressure test is positive, and there is no hole in the piston, tear the top end down to see if the piston seized to the cylinder - if it did, then it was a simple case of overheating - topped out too long and the exhaust side of the piston overheated and melted. Melted on the intake side indicates a coolant flow problem.

Good l
 
#10 ·
Caused by pre-ignition - too low an octane fuel, too hot of heat range on the spark plug or fuel/air mixture too lean at full throttle - has little to do with oil or lack of oil, but type of oil could be a contributor - what kind of oil are you running?

If the cylinder is not damaged (and it's probably not) find out what size the piston is, get a new piston/ring kit and put it back together - then check the jets in the carb - also, what kind of air filter do you have? Is the air box stock?

Last - remember it is not designed for road use - don't hold the engine at top end for more than about 100 yards without slowing down to about 1/2 throttle before going top end again.

Let me know what oil you are using, injector or pre mix, kind of fuel (brand and octane), number on the spark plug, size of jets installed in the carb, all the details and I'll advise you as to what you might do to prevent it from happening again.
 
#17 ·
Last - remember it is not designed for road use - don't hold the engine at top end for more than about 100 yards without slowing down to about 1/2 throttle before going top end again.
Sorry for hijacking the thread, but...

Latebird could you please expand on only driving for 100 yards at top speed? I'm curious why the 400L engine is only designed for top speed for very short distances.

Thanks!
 
#12 ·
If you only premix your fuel and do not use the injection pump, I think 50:1 is light on the oil. If I remember right, it should be closer to 32:1. The K&N filter could be part of the problem there. High flow means less filter less restriction and with the stock jetting will: 1 allow dirt into the engine and 2 result in a leaner mix. I think the K&N is great for the street but not for anything that sees the dirt. I stick with the stock paper filters for all my machines. As for jet sizes you will have to remove them and get them under magnification. The number will be stamped into the body of the jet. Could be on the end of the main jet in the photo. The pilot jet is in the round tube in the same line as the needle valve and main jet. Again, the stamping is really small so I have to have magnification to read it.
 
#14 ·
OK - the plug is right, oil is thin - 32:1 is recommended - 24:1 is even better - has the oil pump been removed and blocked off? K&N flows more air leaning out the mixture especially at full throttle. Look at the jet very closely - there should be a number on the end or one of the flats.

There were two carbs used in 96 - one called for a 250 main jet and the other for a 130 - the difference is in the model number of the frame - is your a W968540 or W967827 - I believe it will be the W968540 and the main jet should be a 250 or larger.

Here's the number on the pilot jet
137311



Main jet should be numbered on the end, but could be on the side depending on the manufacturer of the jet
137312

Here's the picture from the service manual showing the location of the jets
137313
 
#19 ·
Neither is the main jet. Top one is pilot jet, middle is the main jet and the bottom one is needle valve.
 
#25 ·
A lean mixture is too much air, not enough fuel to go with it. The air/fuel mixture is done inside the carb with the jetting. The oil/fuel ratio is done either at the injection pump or by adding oil to the fuel. In your case it appears that with the K&N filter which allows more air into the engine, the jetting needs to be changed so the carb lets more fuel into the engine. Main Jet and Pilot Jet sizes will probably need to be bigger along with the main jet needle style or size changed. I am not an expert when it comes to tuning carbs. I tend to stay with the stock stuff to avoid that. Latebird is an expert so he can get you started in the right direction.
 
#26 ·
LOL... a mile @ WOT on a 2 stroke is a LOOONG time. Even more so if running somewhat lean anyway. After all, you DID melt a hole in the top of the piston?
A 2 stroke will run very strong when lean but they also run MUCH hotter so you have to be careful with jetting. I used to do a lot of radical hill climbing on a big bore 2 stroke dirt bike in my younger days. You always turned the fuel off when taking a break to prevent the possibility of fuel leaking while sitting around. If you forgot to turn the fuel back on and started up a long climb the bike would run like gangbusters as it leaned out just before the fuel in the carb bowl ran out and it quit right in the middle of the hill lol! Very strong but also very hot. The engine can't stand that for long.
 
#32 ·
Yes it was really quick on take off and would have to turn fuel off or would run like shit for lil bit but the weird thing is that when it happened( burning the hole n piston) it wasn't that hot and i been watching u tube video's have better understanding between u tube and all the help from you guys
Its not all that bad OP. As latebird said earlier, the cylinder probably isn't hurt. Get the carb jetted right, a stock air filter, the oil injector working or properly blocked off and a new piston and rings and you should be good to go. Even the 4 stroke manuals tell you not to run WOT for long periods. These are trail machines and not meant to run WOT on the road for miles at a time. WOT runs on the trail are fine and won't hurt a thing because you can't hold it there for long without letting up.
I have been watching u tube videos all day and got better understanding of jetting rich and lean mixture all that good stuff and i ordered carb rebuild kit and piston and rings and found air box on eBay that stock n same year as mine so wish me luck lol and appericate the help i will let ya knw how things turn out
 
#28 ·
Latebird,
Appreciate your response on why WOT on a 2 cycle engine is bad. Makes perfect sense.

Also appreciate the detailed insight in the post above. Especially on what to look for when purchasing an ATV.

Stave and Polman,
You are also very helpful contributors as well. So grateful to have you all on this site.

Thank you
 
#29 ·
Its not all that bad OP. As latebird said earlier, the cylinder probably isn't hurt. Get the carb jetted right, a stock air filter, the oil injector working or properly blocked off and a new piston and rings and you should be good to go. Even the 4 stroke manuals tell you not to run WOT for long periods. These are trail machines and not meant to run WOT on the road for miles at a time. WOT runs on the trail are fine and won't hurt a thing because you can't hold it there for long without letting up.
 
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