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Hello all, new to this forum hopefully I can find some help because I'm running out of steam. I took a 95 scrambler 400 on trade and cant get the thing to run. I did all the usual things carb clean new gas fuel filter etc. Then realized I had no spark, found a broken ign. Coil and bad cdi so swapped them out and still nothing, I did replace the coil with one I already had from a blaster 200 so not sure if that matters. Started tracing voltage and found reverse limiter switch was faulty and was killing my spark. Jumped that out and wah lah spark. I have a decent spark coming out of it, and still cant get it to fire off. I tried starting fluid and still nothing. Did a comp test and got about 65psi so I'm making something here. So I have spark a clean carb, free flowing fuel, and compression but i still cant get the thing to fire off even on spray. Any ideas???
 

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You don't have any compression - normal compression with the throttle wide open is about 120 to 150 - anything under 100 and you are lucky if it starts and when it gets down below 90 - if it starts using starter fluid, it will not develop enough power to move - at 65 psi with an open throttle, it does not have enough compression to start.
 

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Discussion Starter #3
I didnt do it with w.o.t. that was throttle closed.. but either way I guess that means in headed up s**ts creek huh... but even still wouldnt the spray be able to light it at least?
 

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If the compression is too low or the spark timing is off or the spark is too weak to jump the gap in the compression and turbulence of the combustion chamber, then no - it will not light.. Recheck the compression with the throttle wide open to know whether it has a chance of running. I'm not sure how you "traced voltage" as the voltage is AC and intermittent - you need a peak voltage tester or oscilloscope to measure the voltage to the components. I think you did pretty good so far considering you do have a clue as to what you are doing and BTW, the coil might be the problem. The coil has a resistance that is matched to the output of the CDI module. A coil off a blaster may not provide an adequate spark to light the mixture
 

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Discussion Starter #5
Thanks for the help!.. yeah I just started checking around voltage with a dmm and everything seemed to check out so I hopped on the old internet and did some digging amd found that a lot of people were having issues with the override so I bypassed it. I guess it's probably a good idea to get a matching coil just so I can definitly eliminate that from the equation, but if I'm only makinGBhalf the comp I'm supposed to be then I dont have a chance. I'll do another test tomorrow with it wide open and see what happens.. I guess if it all checks out maybe I'll go to ignition timing?
 

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Not gonna matter that much if the throttle is open or not. The piston, rings, and cylinder are shot.

I'd venture to say the piston is melted or has a hole in it from detonation.

The last owner, or one of the previous ones either

A. Ran it outa injection oil and burnt it up

B. Ran cheap 87 octane gas and detonated it to death. Use only 91-93 octane, preferably ethanol free too.

C. Just plain beat the shit out of it holding it wide open down the road and melted the exhaust side of the piston

D. All of the above.



Sent from my SM-G930R4 using Tapatalk
 

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Discussion Starter #7
Yeahh that seems to be about how my luck works out..that seems to be a common issue with these no? Any recommendations on which piston to get.. can I get away with a namura or should I go wiseco?
 

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Nothing wrong with Namura - I have used them in many different brand engines and have had no problems. Nice thing about Namura, their top end kit comes with gaskets and a top end bearing. Wiseco is good for racing, but the motor will not know the difference a Wiseco and a Namura - only the mechanic and the owner will know.
 

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Discussion Starter #9
Yeahh I've never had an issue with namura, it's been in every one of my Yamaha's.. figured I'd throw it out there maybe polaris dont like em.. well appreciate the help guys I'll order it up and hopefully have her runnin by next week👍👍
 

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I dint have a service manual, I should probably get one eeventually, do you guys know offhand the ring gap spec, tightening pattern and torque spec of head? In sure I'll find it with enough digging figured I'd try to cut out the middleman
 

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Ring end gap .007-.015 is standard - in a used cylinder anything under .020 is acceptable and it will run fine with .030 gap, but at .030 gap with a new ring, it's worn to a point that a bore job will make it run better and last longer.

Cylinder torque is (10mm stud w/14mm nut) - 25 ft lb
Head (8mm stud w/12 or 13 mm nut) - 20 ft lb

Torque sequence is 3 steps (10, 15 then 20) in a crisscross pattern.
 

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I am guessing that this 400 is a water cooled 2 stroke. So pulling the cylinder is not going to be a simple as it would be with an air cooled 250 engine. As long as you are in there anyway, do the whole job. With the compression that low the cylinder walls are probably messed up and a new piston and rings will not be enough to fix it. Take the cylinder to a shop and have it measured. Let them decide how much oversize to go. They can bore the cylinder and fit the piston so all you have to do is put it back together.
 

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Discussion Starter #13
Ring end gap .007-.015 is standard - in a used cylinder anything under .020 is acceptable and it will run fine with .030 gap, but at .030 gap with a new ring, it's worn to a point that a bore job will make it run better and last longer.

Cylinder torque is (10mm stud w/14mm nut) - 25 ft lb
Head (8mm stud w/12 or 13 mm nut) - 20 ft lb

Torque sequence is 3 steps (10, 15 then 20) in a crisscross pattern.
Awesome thankyou!
 

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Discussion Starter #14
I am guessing that this 400 is a water cooled 2 stroke. So pulling the cylinder is not going to be a simple as it would be with an air cooled 250 engine. As long as you are in there anyway, do the whole job. With the compression that low the cylinder walls are probably messed up and a new piston and rings will not be enough to fix it. Take the cylinder to a shop and have it measured. Let them decide how much oversize to go. They can bore the cylinder and fit the piston so all you have to do is put it back together.
Yeah I know what you mean, in a perfect world that's what i would do, but I dont want to sink any more money into this thing than I have to so I'll go standard bore first and see what my ring gap is and then decide from there, I already pulled the cylinder and it's not bad no scoring or anything, so I'm hoping a quick dingle ball hone job will be enough to get my cross hatching back and I'll be good to go, if not it's easy enough to do again at least its not a four stroke lol so no timing or anything 👍👍👍👍
 

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Yeah I know what you mean, in a perfect world that's what i would do, but I dont want to sink any more money into this thing than I have to so I'll go standard bore first and see what my ring gap is and then decide from there, I already pulled the cylinder and it's not bad no scoring or anything, so I'm hoping a quick dingle ball hone job will be enough to get my cross hatching back and I'll be good to go, if not it's easy enough to do again at least its not a four stroke lol so no timing or anything 👍👍👍👍
Std bore? You are optimistic or unrealistic. A 95 two stroke has a 99.9% probability of having been bored out aty least on time and if used regularly, an 80% probability it has been bored at least twice. Measure the bore, check the top of the piston and/or the ends of the rings for a possible oversize marking.
 
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