Got a 425 Magnum, the bike has an unknown amount of miles on it and the engine is getting a little tired. Its got decent low end power and accelerates okay to 25-30 mph but really slows after that. It'll hit about 40 mph. Its got a white primary, k&n and a Supertrapp ids2. Valve clearances are good so I'm thinking the cam lobes are worn. I'm gonna tear into the engine so I'd like to bore it out and change the cam. Anybody know how large I can safely bore it to? Any recommendations on a cam and carb size would be much appreciated. Thanks in advance

 

Any thoughts???????

 

Im not entirely sure on this, but I believe that someone on here has put a 500 HO cam and carb on their 400. As for boring it, only bore it as big as you need. If the cylinders are not damaged at all, a .10 bore should be fine. However, I am not convinced it is your engine. If the cam lobes were worn, the engine would run like crap especially as the RPMs increase. Sounds more like a clutch problem. How old is the belt? Have you cleaned the clutches lately? They may even need a rebuild on them.

 

I have read on this forum in the past that these Magnums are prone to wearing cam lobes. I don't for sure if thats my problem or not. The quad does run quite well its just the acceleration is weak after about 25-30 miles an hour. I don't know if this in normal for the machine or not. I've read on this forum about guys hitting 50-55 mph on there 425's not a hope in hell on mine. Would have to be a hell of a steep hill. My buddies bombardier 330 HO will smoke my ***. I never really thought about the clutch issue. When I changed to the white primary it certainly seemed to help. I think I'm just gonna tear into the whole thing engine/tranny and all and do the rebuild and hope for a decent improvement. Any other thoughts or advice would be much appreciated. Thanks

 

I agree that it's probably clutch related. If the motor runs good I doubt it needs a rebuild.

I'd tear into those clutches. Clean, inspect and repair anything that needs it. Check that you have the appropriate clutch weights in the primary. Once everything is in spec and you put a new belt on I'd bet that will cure the acceleration probs.

 

If your cam is bad you will know it when you adjust the valves.I would bet the secondary clutch has some bad parts along with a worn belt.The 425 motors are great my old one had lots of hard miles and it still didnt use oil.

 

Thanks for the replies guys. Definitely starting to look like a clutch issue. Never really gave it much thought. The belt is new and I blew out the clutch when I changed it but that was it for cleaning. flwflh how did your 425 run. Seems to me I've read on hear that yours went pretty good? How was it compared to your 500? Did you change your cam? Was your acceleration strong up to top speed? Thanks in advance.

 

Mine ran strong but not as fast as a 500HO it would run close to a non HO.I had it set up with the exact same stuff you have with the excetion of a blue Aaen secondary clutch spring.Never had any motor troble other than a waterpump seal.