I've been trying to fine tune my 96 magnum 425 for about a month and something is still just not right. It starts up quickly, idles fine, but seems a little flat within the first 1/3 of the throttle range. Past that it speeds up until it gets to the top end and then it seems to lose power again. There is some infrequent backfiring through the exhaust but really not much. It’s a little more sluggish (symptoms are worse) when it’s cold.

I recently replaced the carb (using the correct jets) but I had to drill out the brass plug covering the air/fuel screw. It was completely screwed all the way in so I backed it out the 2 ½ turns. I’ve checked the cam and it looks great. I adjusted the valves, replaced the plug and air filter. Airbox seems sealed, and yes, I have the snorkel and foam attached.
So does this sound like a timing issue, bad coil, poor compression, improperly adjusted carb, or something else?

Almost forgot to mention, my choke doesn't seem to work at all. It makes no difference at any time. I’ve adjusted it back and forth but I’m apparently not doing something right. Again, it starts up fine without it. Any idea what’s going on?

Thank you very much for your help!

 

You said you replaced the carb,new carb or used? Did you install a carb kit, new needle and seat? Plus a lot of people overlook the float arm and pin. If wobbly and worn it could be some of the problem. Plus it needs to be level with the carb body when held upside down. Around 2 to 2 1/2 turns out on the fuel air screw is about average.This mainly is just for the idle circuit. Pull the top cap off the carb and make sure the choke plunger is moving in and out properly and fully seating. You can adjust the choke cable at the carb to where you have about 1/8"- 1/4" play at the choke lever. If choking it doesn't affect running, then something is wrong somewhere.Backfiring still could be the beginning of a cam problem and just looking at the cam lobe is not an accurate way of telling. Have some one measure the exhaust lobe, can be done with a mic while it's still in the engine. Exhaust lobe should be between 1.2884 to 1.294 inches. Manual says wear limit is 1.2766. If below that, could be the cam is not opening the exhaust valves enough to obtain full power.

 

Thanks replying OPT. The carb is new. I actually bought one through ebay from a link you provided awhile back. Bought a carb kit too, but I had to order a new main jet separately.

This was the reason I bought the new carb. I apparently had broken one of the posts that holds the float when I was rebuilding the old carb.

That's what I adjusted it to, but as I stated in the original post I had to remove a brass plug to gain access to the screw. It was screwed in all the way. Why?

Ok, this is a little frustrating. I keep messing around with this thing but I just can't seem to adjust it correctly. I had the same problem with the original carb. If quad starts right up, why do I need it to work? Don't get me wrong, I want it to work. I'm just confused as to why its not, and why it seems to make no difference.

The cam may be an issue. I'll need to borrow my dad's micrometer to measure it (thanks for the specs by the way), but I might just end up waiting on this. Its amazing to me how a few thousandths of an inch can be such a pain in the rear!

Thanks for your help again OPT. You really should get compensated for all your time on this forum.

 

The main thing is that the choke plunger does slide back and forth and seats properly in the channel at the top of the carb blocking off any fuel from the choke circuit getting into the carb with the lever off. I know also that the Magnums didn't seem to need choking hardly at all as they got older,maybe only on the coldest days! Plus the fuel air mixture screw shouldn't have been screwed in all the way! Someone screwed up from the factory. I've found this to happen on a few early Trail Boss/Magnum 325s that were hard to warm up and idle correctly. You might try experimenting on the fuel air setting and start out at 1 turn and see how it idles and takes throttle and adjust accordingly.. A small screw driver OR a carb adjuster would help.

 

I don't know if its the same but check the rubber intake boot off of carb front they are super prone to splitting that was my sporty 700 problem also try adjusting the throttle adjustment on the cable on your bars just throwing out some stupid maybes better than cam problems

 

My '96 magnum 425 does similar things, starts with no choke, has a little stumble when moving and holding the throttle at about 1/8-1/4 throttle, backfires when moving fast and letting off throttle. Idles good cold, gets worse and wants to die after it runs awhile. Can someone provide all the specs on the adjustments and where to set them at. Such as choke adjustment, throttle cable, air screw, float level. I did find the valve clearance awhile back on a web site and set them.

What went bad on these magnums that wore out the cam? Wasn't gas getting into the oil somehow?

 

Just a bad heat treatment on several years worth of cams was the problem. Seems like most Magnums I got a hold of would start with out the choke,plus were also bad on running rich. If you haven't installed a new carb kit I'd do so. Plus make sure the float arm and float needle in the carb aren't wobbly and worn.These don't come in the carb kits and have to be ordered from a dealer or online.The float just needs to be parallel with the carb body as far as adjustment. Even though you've adjusted the valves,if the cam is wearing down this could be part of your problem. Nothing like a good manual that can give you all the specs,dimensions and trouble shooting checks. Here's a pdf service manual link that can help you a lot.Index of /ORV Manuals/Polaris/1996-1998 - All Models

 

thanks for the help and link. Any suggestions on where to get carb rebuild kits? I also need a fuel shut off valve, one of the little rubber mounts with bolt in it that the carb mounts on, and the carb kit. I just got done taking apart the carb and everything was fairly clean. Some buildup may have come out one of the air openings on the airbox side of the carb through the main body of the carb. there is a large tear in the rubber boot that holds the black and yellow foam behind the gas tank. This is before the main air filter so its not sucking dirt but maybe running lean because not going through the foam? Seems to run better but not great after the carb clean, still backfires letting off the throttle. Plug is a NGK BKR6ES and is black, I have a BKR5E, is the 5E ok to run?

 

over time , weather , heat , moisture degrade the ignition wire coating

use a 1/2 roll of electrical tape , and run the tape around the ignition wire starting at the coil all the way to the spark plug cap then back to the coil

you spark plug wire is probably leaking electricity to the frame

 

I put in the 5E plug and it helped a ton, no more backfire, its like a completely new machine.

Has anyone successfully rebuilt a fuel shut off valve on their own? Its probably just a simple o ring that goes bad but I can't see how it comes apart.

Also, any ideas how to troubleshoot the radiator fan? Seems like it should have come on my last trip out and didn't.