I just got this PM from Harv1

He is a person who swaped out his blown 05 SP400 (425cc) motor with a 500 H.O he found on ebay without the 40mm carb but he used his old 34mm and intakes from his 425 motor.

Sounds like you may not have to rejet your 34mm carb for the new cam.

Here is his swap out a 425 thread.

http://forums.atvconnection.com/pola...gine-swap.html

 

Awesome! That's great news. I'll definitely keep an eye on that thread.

Now I guess I'll just have to order up a HO cam, and an exhaust rocker tomorrow. Just need to compare prices from the local shop and what I can get online. I had high hopes for an ebay auction that ended tonight, but the price for the used cam and rocker set almost reached the price of a new cam and exhaust rocker. No thanks. My intakes are fine. lol

Hopefully in about a week I'll be able to give an update on how she comes back to life again.

 

Yes I did re-jet the 34 and it ran fairly well, but when I did the 40 it really woke it up, now I have a 500 ho motor with some porting and a 11.5 to one wiseco piston in the old 425, that machine runs very well.

 

That sounds like the route I'm probably wanting to take with mine. Kinda like having a sleeper of an old rough looking four wheeler. Not sure about the high compression though... Then again the extra cost at the pump isn't nearly as bad when you don't use as much fuel like in a car, so who knows.

Do you happen to remember what jet you put in your 34mm? Would give me a good bench mark to work from if I feel the HO cam is making it starve for fuel. I'm gonna stick with the small carb for a little while at least.

 

Cant remember but think it came with a 135 and I went to a 145 main, didnt change the idle jet. Makes a difference what elevation your at, I live at 5000 ft and ride to 10,000

 

I have the old 425 motor with a near new 500 ho cam if your interested. Never used any oil always ran well.

 

I'm around approximately 500-1000ft in this area of WV. So, whatever jet I have, I might try bumping it up +10 to see if it helps. Couldn't hurt to try.

Would have loved to have bought it from you before I started this project, or the cam and exhaust rocker from you more recently. However it doesn't matter now anyways, because I just ordered a new HO cam and rocker last night... I paid less for them brand new than an auction I watched end on monday for a used rocker set with a used HO cam on eBay. People are crazy.

The exhaust lobe on this one was worn down to the point that it was causing way to much back pressure in the system and caused it to break. My mom and sister were riding on it and were attempting to go up a hill when it suddenly stopped and "made a noise like a chain broke." So there it sat for 4+ years untouched after Dad bought a new Sportsman 500 instead of fixing this Magnum. I assumed that the timing chain had broken and I could just pop the side open, swap it out, and be riding later that night. No such luck. It ended up that the piston had split in half horizontally from all the stress, one valve broke, and two valves got bent in the process.

I already have the piston, circlips, and rings replaced with a brand new ones. (Before I knew the 500 jug/piston would bolt right on. Grr) Replaced the head with a used one, but it didn't come with a cam or exhaust rocker. At the time I bought the used head I didn't realize my cam and rocker were worn, so I still needed those. Luckily I found out the HO cam would work before I paid the same amount for a new one that would be standard duration.

Here is the proof:

 

The 5000 jug will bolt on but the stroke is different which would cause the piston to hit the head or bottom , so its a good thing you did'nt try that.

 

You would need the 500s piston rod for the its jug to work. A 425 with a 500 rod ,jug and piston is a 500 non H.O .

 

Well I suppose its a good thing I didn't try that then, because I would have been mad about having to buy a new rod too. lol I would have hand cranked it slowly to make sure everything was working properly before trying and smashing the head though. I check everything in triplicate before I do something that could potentially damage an engine. Still would be something to consider at a later date though.