Viper only getting 8000 RPM's

mrviper700 said:
not the compression in my opinion, think about or go back and read this whole post, you got peak rpm then you didnt, then you got it again. If the compression was the problem it wouldnt vary the rpm at differnt times as it would be down on power ALL the time.

I hate to see people waste money on stuff thats not the cause but everyone has there own way of doing things.

while a 100lbs is on the lower end, theres differnt compression guages give differnt readings.

It also could be from the bender head gasket and also if the heads been modded and is matched to the pto cylinder which was larger. The opticool gasket makes the squish larger which reduces compression as does the head mod, BUT, a low compression motor makes rpm easier. Like I said its your $$ but thats not the problem in my opinion, you need to replace the stator first with either a known good unit or brand new yamaha.......
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So I thought I found the problem when I found the low compression, especially when it got so much lower when it ran (idled) for a few minutes on the mag side. Then it seemed to make sense ( in my mind ) about it not starting as good when it was hot and sometimes having it not pull the rpm it should when it was hot. A weak piston ring will make an engine not start as good when its hot, but I didn't know it wouldn't affect the peak rpm, I assumed it would kill it if the compression was too low. Do you think it would be worth tracking down another compression tester to compare the readings ? If I got the same results with a different tester I don't have to worry about a weak ring breaking ? That was my major concern as I've seen some cylinders after a ring broke. Maybe I'm worrying about it too much. Maybe I should take it for a test run and see if what I've done on Sunday has made any difference and if not order a new stator. I agree with you I don't want to throw a bunch of money at it, I'd like to fix it on the first try. Thank you again for your advice.
 
Well I tried my sled out today in the daylight when I can see better. I could hold the throttle a little longer and I seemed to get more uniform results. I could hit 8500 rpm fairly regularly and then the rpm would start falling off every time. It would pull hard until it hit 8500 then it would start to feel flat and the rpm would drop, sometimes below 8000. I tried unplugging the headlights and turning the hand/thumb warmers off and it didn't seem to make any difference. Does this still sound like a bad stator? If so I'll order one on Monday and hope that does it.
 
Well I tried my sled out again today. I noticed my brothers Viper didn't have washers on the secondary spacer and then if that didn't make any difference I was going to try his clutches one at a time to try to narrow down my problem. On my last pull the engine really died down and it sounded like it was running on two cylinders. I figured I fouled a plug so I rode it slowly down to my garage and when I checked the plugs they looked real good for the 8 degree temps, they were a nice golden brown. So for the heck of it I tested the compression again and the pto and center cylinders were 90 psi and the mag side was 40 psi, OUCH !! This is what I found when I tore the engine down. I need a new piston, ring and cylinder. There is a gouge over the exhaust port on the cylinder, its hard to see in the pic, but its there. If anyone has a good cylinder with a matching OEM piston for a good price I'd be very interested. Also I found a broken front motor mount so I need one of those to. I'm going to post an add in the wanted section. I've seen some on ebay, but I'd like to get it from a member hear first. Then I can get a set of new rings and get this thing fixed. Hopefully this is all that's wrong and there aren't anymore surprises waiting for me.
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That sucks. Atleast she didn't leave you stranded out in the middle of nowhere. Post a picture of the center and pto piston's. I don't see any wash on the mag or center. Also blow-by on the center which may explain your low compression readings.
 
I'm experiencing the same trouble. I cant pull much over 8000 rpm. Then as the sled warms up it becomes progressively worse. After reading this and a couple of other posts I ordered an OEM stator on Friday. Im installing then testing on Friday. I will let you guys know the result. ---Mac---
 
Mac said:
I'm experiencing the same trouble. I cant pull much over 8000 rpm. Then as the sled warms up it becomes progressively worse. After reading this and a couple of other posts I ordered an OEM stator on Friday. Im installing then testing on Friday. I will let you guys know the result. ---Mac---
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let uss know mac im in the same boat :o|..................................................superfan it look like the power valve catch the top ring and broke the piston
 
Yeah at least I was just up behind my house trying it out so I was lucky there. I know when I got this sled last year it wasn't running when I got it. I cleaned the carbs and put fresh gas in it and got it going. The carbs weren't in the boots right and the compression wasn't all that good then either. Looking back now I would say the previous owner had most likely started the damage by leaning it out by not installing the carbs right. modsrx you're right it did hit the PV there is a small mark in the PV. The compression was low in that cylinder and my guess is the ring broke and caught it and that's where the damage came from. With some good used parts hopefully it wont cost me too much to get it going again. Here's some pics of the pistons at different angles. I'm going to take the pistons and cylinders to my repair shop tomorrow and see if the pistons are within spec. Hopefully I only need the one bad one.
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those look to be the cheap pistons from ebay, look on the underside, do they say 8EK on the wristpin boss?
 
You don't miss anything MrViper I think you're right that they aren't oem pistons. The numbers I saw ar 8J under the top of the piston and the #'s aren't on the wristpin boss. So I guess this isn't the first time this engine has been apart. Is that an SPI piston ?
 
Well I have a update on my Viper. I finally got my gaskets this Wednesday during a snow storm. I started putting it back together Thursday night and I finished it last night, well sort of. It all of a sudden had a weird electrical problem that wasn't there before. I tracked it down today and somehow I pinched the pulse coil wire and broke it so I soldered in a new piece of wire and it runs really good now. This sled needed new pistons and rings when I bought this sled last year by the looks of it, oh well, live and learn. I had 120 psi in each cylinder right after I got it together, I don't know if it would change after running or not, but that seems like it's about right with the Bender head. I tried it out for about half an hour and it seems to run really good. I only held it wide open for a few seconds since it not broke in yet.It will hit 8500 rpm's pretty quick and the motor feels strong. I'm going for a ride tomorrow and when I get it broke in hopefully it will hit 8500 rpm and stay there. Thank you everyone for all your help, I learned a lot about my machine with all this.
 
Best way to break it in is to heat cycle it once, maybe twice at the most, then wring the guts out of it!!!!
 
I went for a short ride today of around 60 miles and it seemed to go really good. I only whacked it wide open for short bursts and it would hit 8400-8500 rpm. After a little more break-in I'll have a little more courage to hold it and see what it'll really do. I do think the clutch set up feels good (stock primary 53/43 helix green dot spring at 70) it works good for the trails I ride on. I hope it doesn't drop rpm when I really hold it wide open and it ends up needing a stator to. I've spent enough money so far this year. So far though things seem to be good and I'm happy, so I'm going to think happy thoughts. LOL
Oh and just a side note that has nothing to do with this issue, but I have to say that I love the IQ rear skid. It rides really nice and I haven't really messed with the settings yet.
 
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