Hi all, this is my first post here but I've been scouring these forums for a few months now.
First a little background...
1. I'm somewhat mechanically inclined but I'm not up on all of the terms.
2. I purchased the quad back in February to help haul firewood and carry a sprayer. When I purchased the quad the guy who sold i to me said that it was sitting for awhile and it ran rough. Thought the carb was dirty.
3. Since then I've rebuilt the carb with a sportsman 500 kit I bought off ebay (kept the main jet that was in it which was a 137.5 not 140 like spec). I've replace the spark plug, oil, oil filter, air filter, flushed the radiator and replaced a faulty temp sensor for the fan.
OK...now after I first put everything back together it worked better but still not what I thought. I took it out this past weekend after having it sit for awhile and I notice it was a little sluggish and was popping/backfiring. It starts up no problem, and idles great, but when I put it in neutral and rev it up it bogs at around 1/2 - 3/4 throttle. Actually what it does is revs up until 1/2 throttle and then bogs down, two seconds later it speeds up just a bit, then dies down again, back and forth until I release the throttle where it resumes a nice idle.
Here's what I tried...
1. Loosened the air filter to see if it was restricting the air flow - same.
2. Adjusted the fuel/air mix needle both in and out (starting from 2 1/2 turns out) - same.
3. Removed the spark plug (a bit carbon fouled considering its run less than an hour since I replaced it), cleaned and re-gapped - same again.
I've read about the camshafts getting worn down on the exhaust side and bogging. I haven't checked the valve clearance yet, and I'm far from knowledgeable when it come to these machines, but I just have this hunch that it's a carb issue. Float bowl maybe? Clogged jets? I'm lost.
Any advice would be greatly appreciated. Thanks for your time!

 

Cam wear or a simple valve adjustment would be my best guess if you haven't checked it out yet.Backfiring,not able to take throttle were a couple symptoms. Soft cams and wear was a common problem on the early 425 and 500 engines.. http://forums.atvconnection.com/pola...d-cam-pic.html

 

Shoot! That's what I was afraid of. I'm going to check the valves this afternoon after work. Will I need to remove the engine from the quad to replace the cam? Is this something I can do in a weekend?
Thanks for your time and the pic OPT!

 

No and yes. No you don't have to remove the engine and yes it can be done fairly quickly.Took my old hands about 3 hours total including bleeding the cooling system.Drain the engine oil first along with the sump.You have to have the engine at tdc (cam sprocket index pin straight up) remove the cam chain tensioner,remove the cam support blocks,remove the water pump cover to get to the small cam removal cover.Remove the three bolts on the cam sprocket,DON'T allow the crank to turn!!Just let the sprocket and chain lower down into the chain area of the head,then slip the new cam in along with a new exhaust rocker arm(don't forget to order this too!) align the sprocket back where you had it(you can move the chain and sprocket around without moving the crank),install the cam blocks and three sprocket screws,small cover,water pump cover and thermostat. Push the tensioner plunger back into the tensioner body,bolt it to the cylinder,replace the spring and tensioner bolt. The tensioner will then take up the chain slack.Adjust the valves.Then you need to clean the oil tank screen.New oil and filter,leave the radiator cap off to purge any air,plus clamp off the oil tank vent line to prime the oil pump quicker. ALL this if you need a new cam! You might get lucky and just need a valve adjustment....Just look closely at the cam exhaust lobe.

 

Wow, thanks so much for the detailed breakdown of the process. I really appreciate your time. As you said, hopefully it just needs an adjustment.
Wish me luck!

 

So I removed the valve cover yesterday and everything looks good. The exhaust lobe doesn't appear to be worn down, although I didn't remove the rockers. (Is that completely necessary?) The rockers on the intake and exhaust side are moving about the same distance. How much are they supposed to move? It seems sufficient to me. I measured the valve clearance at TDC on the exhaust side and it was near perfect (.006 in), but the intake was tight. Would this cause the bogging and surging cycle I'm experiencing?

I still have a feeling its the carb, but again I'm far from an expert. I know the e-clip should be at the 3rd position on the needle. Should I try moving it up or down? If its backfiring out the exhaust does that mean its rich? Smells a little rich. I'd move the e-clip down to lean it out - right? The diaphragm doesn't look cracked, but how would I know if its working properly? Also, should I try lowering the float level. It was level with the bowl when I reassembled it.
Thank you very much for your help!

 

As long as the exhaust lobe is elliptical just like the two intake lobe on each side then you're ok!! Tight valves can cause problems. Usually intakes are the ones to tighten because they aren't as strong as exhaust valves and do "mushroom" a little over time along with seat wear which is normal. Just make sure all valves are set at .006 at TDC. Now that you've hopefully eliminated the valves,IF it still doesn't run well,pull the fuel line from the carb,crank the engine over and you should have a good pulse of fuel. If you don't you could have a fuel pump,clogged filter problem. If fuel flow is ok and you're sure the fuel is clean and fresh,then yes it still could be a carb problem. If you go DOWN with the e-clip on the needle it richens the mixture,UP with the e-clip leans it out. Just leave it at the 3rd position from the top and make sure the float arm is parallel with the carb body when held upside down!. If the float arm feels wobbly and worn,replace it and the float arm pin.Most people overlook this as it doesn't come with the kits. If after all this and you still have problems with surging,you could be down to having an erratic electrical problem. First thing to do on this is to pull the black wire from the reverse limiter module under the front panel and see how it does.. Keep us updated...

 

Thanks OPT for your thorough and clear instructions. I already tried pulling the wire from the limiter, but nothing changed. I still have few questions if you have the time.
1) Do I need to use two feeler gauges when adjusting the exhaust valves as is mentioned in the Clymer manual (since they're on the same rocker), or can I adjust them one at a time?
2) I remember reading that due to the placement of the fuel tank the magnum 425 hardly needs a fuel pump. Does this seems like a reasonable claim to you?
3) The main jet I have in there right now has "137.5" stamped on it. I know stock is 140. I'm guessing the previous owner swapped it out at some time. Would you suspect this was due to it running too rich? Are there other sizes I can test to see an improvement?
4) I attempted to pull full choke at 1/2-3/4 throttle and it didn't make a bit of difference. This doesn't seem right at all. What affect should the choke have at the top end of the throttle? I've looked into adjusting it, but I'm not clear exactly how I'd know it's adjusted properly. I'm just not used to this type of choke on a carb.
Thanks again for all your help!

 

You had the carb off right? Sounds like a vacuum leak to me. I would spray some carb cleaner around the carb as its running to see if it sucks it in revs up.

 

Yes it's best to have two feeler gauges on the exhaust rocker arm. This way you set one valve clearance then go to the next one with the feeler gauge still between the adjuster and and valve stem. This keeps the rocker arm from "cocking" which has been known on some machines to throw valve spring "keepers" which can cause a mess of an engine. If you want to try to bypass the fuel pump,give it a try. Just block off the vacuum line from the carb and see if gravity is enough. 425 magnums seemed to me that they always ran a little rich as the years went by and a 137.5 main shouldn't hurt. The choke should affect how it runs when you open it unless the brass end is partially seized in the opening below the diaphragm. You can make sure by removing the diaphragm and pull the choke lever. The brass end should move back and forth with the choke lever.Another area is to pull the block off plug covering the fuel mixture screw and see how many turns out,but this mainly affects the idle. Just adjust the choke at the carb. It has a 10mm lock nut on the cable,loosen it and adjust the 8 mm cable end to where you have about 1/4" play at the lever.Dirt Cheap Yamaha, Honda, Arctic Cat & Polaris OEM Parts & Accessories – Cycle Parts Warehouse
Plus check simple things like the air filter and prefilter.