Top end break in

Yaaamaaahaaa3

Yaaamaaahaaa3

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Jan 25, 2010
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56
Location
Harrison, Ohio
End of last season lost mag piston on my Viper. Tore it down and rebuilt the top end ith new pistons and rings. Had to replace cylinder on the mag side also. It is all back together and it started right up and sounds great. My question is what is the best way to break it in? I am heading to Michigan next week and didn't want to bring it back home and do it all over again. I trail ride on hard pack. Should I avoid top end for a certain amount of miles? First time on a top end rebuild and couldn't have done it without this forum.

I ran it yesterday and ran fine however sat for about 25 minutes and went to start it and had to pull the heck out of it, I gave it a little nudge of the throttle while pulling and it fired right up. What could be causing that? Cleaned carbs. Compression after rebuild was 90 across the board, don't the rings need time to seat?
 
just ride it, you don't need to break it in, it will do it on its own.
 
1. Why did it blow up...

2. Alot of Vipers require a teency bit of throttle to fire when warm. Mine included. No problem there.

3. You did your compression test wrong. 90 is low it should read closer to 130. Also your viper will have lower compression on the clutch side cylinder this is normal. Try it again maybe with a different tester.

4. You should be running 50:1 premix in the gas tank for one tank with new top end installed. Also, you can do a few heat cycles. Lift the rear off the ground and snap the throttle Over and over not letting rpm get over like 3000-4000. Do this until it reaches operating temp and then kill it. Let it cool right cold and do it all again but this time bring the rpms up to around 6000. Vroom...vrooom...vrooom...until you reach temp. Then let it cool. You can do 2 or 3 heat cycles like this to help break in the new parts. Avoid prolonged WOT. try not to sit at one rpm. Once the premix tank of gas is empty you should be golden. Ride er like you stole er.

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Youllbe2nd said:
1. Why did it blow up...

2. Alot of Vipers require a teency bit of throttle to fire when warm. Mine included. No problem there.

3. You did your compression test wrong. 90 is low it should read closer to 130. Also your viper will have lower compression on the clutch side cylinder this is normal. Try it again maybe with a different tester.

4. You should be running 50:1 premix in the gas tank for one tank with new top end installed. Also, you can do a few heat cycles. Lift the rear off the ground and snap the throttle Over and over not letting rpm get over like 3000-4000. Do this until it reaches operating temp and then kill it. Let it cool right cold and do it all again but this time bring the rpms up to around 6000. Vroom...vrooom...vrooom...until you reach temp. Then let it cool. You can do 2 or 3 heat cycles like this to help break in the new parts. Avoid prolonged WOT. try not to sit at one rpm. Once the premix tank of gas is empty you should be golden. Ride er like you stole er.

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Click to expand...

you should absolutely NOT run 50/50 premix in the gastank, as along with the oil injection adding oil to the gas, it will make the fuel oil mixture LEANER not richer, the oil displaces the gas and the sled will run leaner, that's a old wives tale they used to do on the old snowmobiles.... DO NOT RUN PREMIX and the oilpump on the sled!!!
 
mrviper700 said:
you should absolutely NOT run 50/50 premix in the gastank, as along with the oil injection adding oil to the gas, it will make the fuel oil mixture LEANER not richer, the oil displaces the gas and the sled will run leaner, that's a old wives tale they used to do on the old snowmobiles.... DO NOT RUN PREMIX and the oilpump on the sled!!!
Click to expand...
No shit?!

Hang on...what's 50/50 premix? You mean fifty to one? Some aftermarket piston manufactures recommend running 50:1 for one tank. Why do they do that? I've always done this. Had a mystery melt down on a Polaris last year while still running premix on new top end...what your saying makes sense. Just can't figure out why its recommended.

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Last edited:
It was a really cold evening,below zero, and I was in a 35 mph zone. I knew I was a little lean because it's mean and must have bee in that middle too long. The temp light started to flash and next thing I know bang.

Positive side of things, I now know how to re build a top end.


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no problem, the little bit richer in the hottest running cylinder is a good thing!
 
Hey MrViper- just to get some clarification on the pre-mix after top end question. Your saying it is not a good idea to run a premix, even at 50:1, after a top end rebuild? i wasn't sure if you were objecting to running specifically 50:50, or the 50:1 as well.

I'm in the process of getting mine ready for its first few miles and was about to add a 50:1 mix as well- want to be sure i don't undo all my hard work!
 
you do NOT need to run premix in the gas tank and still use the oilpump on the sled, by doing so your just leaning out the air/fuel ratio, making it run leaner. All that needs to be done is heat cycle the engine and run it as normal after that, theres a moly coating on the rings edge that help them seat with the bore.

I start up a new engine make sure it doesn't have any leaks, go over my adjustments on oilpump cable gap, carbs, etc. let it get good and warm and vary the throttle.Let it cool off and then ride them just as I would if they had 1500 miles on them as you let a liq. cooled sled warm up anyways before taking off.
 
mrviper700 said:
you do NOT need to run premix in the gas tank and still use the oilpump on the sled, by doing so your just leaning out the air/fuel ratio, making it run leaner. All that needs to be done is heat cycle the engine and run it as normal after that, theres a moly coating on the rings edge that help them seat with the bore.

I start up a new engine make sure it doesn't have any leaks, go over my adjustments on oilpump cable gap, carbs, etc. let it get good and warm and vary the throttle.Let it cool off and then ride them just as I would if they had 1500 miles on them as you let a liq. cooled sled warm up anyways before taking off.
Click to expand...
This is great info. All the wasted years!!

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hmm i guess i don't need to smoke out my neighborhood when i rebuild my next top end!

Thanks!
 
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