I've been working on a 04 foreman 450 for a while now and have a problem I can't figure out. It's been bored with a new piston. The rings are new and have the proper ring gap and have been stagered properly. The valves are set as well. It has a wiseco piston that is a dome top for higher compression. On the first attempt to start it choked down on the compression stroke then started spinning over pretty easily after that. It wouldn't start so I checked the compression. There is only 60 lbs of compression. I checked the crankcase vent for excessive blowby but it blows and sucks like it should with the piston going up and down in the cylinder. So my compression is going somewhere but where? Then I noticed that the exhaust was sucking in. I mean with tremendous force. It will pull a suction on your hand that hurts. There is fuel blowing out of the airbox too. So, why is this happening? It's a pushrod motor and I haven't messed with the timing. When this bike originally broke down it just suddenly shut off on the guy while he was riding and wouldn't start back up. It is firing. Please give me some info.

 

Sure sounds like valve timing. Even though its a pushrod motor, it does have a cam driven by a chain, in the mid block. Id check the timing of the crank to cam.

 

I'll do that this afternoon. Maybe it just jumped time when it shut off with him.

 

The easiest way to check the valve timing deal is to take an air blower with a rubber tip and bring the engine up to TDC and inject about 80psi through the plug hole.
If you hear a lot of air coming out the pipe you have a bad valve or bad timing. Same way with the intake.

 

You sure the valves weren't set 180* out ..

 

I didn't mess with the mechanical timing on the cam. Is it possible to have the timing 180 out just by removing the rocker assembly?

 

<div class="FTQUOTE"><begin quote>It has a wiseco piston that is a dome top for higher compression. On the first attempt to start it choked down on the compression stroke then started spinning over pretty easily after that.</end quote></div>
With a comment like that, I would say you smacked the valve with the piston and bent it. Easy to check with air in the plug hole on the compression stroke.

 

If it was the right Wiseco piston for the motor using a stock camshaft then it shouldn't have bent a valve ..?.. What did the piston instructions say ..?.. anything about camshaft lift ..?

 

Sounds to me like your way out of time or you broke some valves. Also if I remember rite some of those Honda motors have a compression relief valve set up and if you turn the motor the wrong way when installing the cams it will not let it releese until you take it back apart and reset it.

 

The piston is not hitting the valves. I've checked that already. I read something about that valve as well. It didn't say anything about reseting it though. It said there is a third lobe on the cam that works that and if the lobe wears out it'll cause a problem and the cam has to be changed.