Old Polaris Tech
Bought a 1996 Polaris Magnum 425 4x4 that needed repaired. It had gotten hot due to a bad fan and a valve guide slipped down and the valve broke it.
After a new used head I had it back together. First start up it ran rough and had a pecking in the head and the kill switches wouldn’t work. I checked the compression with the compression release in the cam removed and it was 150 lbs. Did some looking and discovered the CDI was bad as I could push on it and the kill switches worked and it ran better. I’m a long way from rich so I bought a aftermarket CDI and all was good. The pecking in the head was the compression release I had that I wound the spring a full turn and had to tight. So now it sounds great. Took it out for a ride and as soon as it starts to pick up speed the engine RPM bogs down slowly. The engine also pops either through the exhaust or carb . The more I push the throttle the more it bogs, pops and won’t pick up speed. Let off the throttle and it idles and then the same again. It’s like it is running out of gas or the parking brake is on. Here is a list of things I have checked and replaced and tried so far with no luck.
Cleaned carb and checked float level, Checked vacuum diaphragm slide for holes ect, checked the jets, New air filter, ran without air filter, ran with choke on, new plug, new fuel filter, new fuel lines, new gas, checked cam lobes, adjusted valves, disabled rpm limiter, ran with gas cap off, New battery, checked cam timing.
Is there anything you can think of that I check? Could this be the aftermarket CDI?

 

make sure the etc is set correctly with the 1/8" gap. if not right it will sometimes act like your hitting the rev limiter as well. just a though to rule it out.

 

Might not be a bad idea to pull the recoil cover and clean the pick up coil and flywheel. Any sort of mud/dirt can cause problems.
Plus you can check the stator wires that go to the cdi and also the coil. All resistance values +/- 20%
Exciter: red to green 3.2 ohms resistance.
black/red to green 446 ohms
Pick up: white/red to white 97 ohms
Coil primary(coil tab to ground) .3 ohms
Coil secondary(coil tab to plug wire,cap removed) 6.3k ohms
Plug cap 5.0k ohms
Timing(must have timing light and rpm meter) 30 degrees @3500 rpm
As far as the aftermarket cdis,never been a fan of them and a lot don't work right,but pull the black wire from the cdi and see if it helps. If it does could be a short in the ignition switch or wiring or the left control switch on/off wiring leading down under the panel. Sometimes the left control switch itself or the connectors can be a problem if dirty or not making good contact.

 

Old Polaris tech
Checked all ohms and all good. Rechecked the cam just to be sure as i see that causes similar problems. The exhaust lobe is 1.287 ths so this is good. Did not check the ignition timing yet but found something interesting. If i remove the air box lid and filter there is no changes it still bogs and pops but if i remove the rubber inlet tube going from the air box to carb it runs like a raped ape good power and no bog. Any Ideas?????

 

A lot of times just cleaning a carb,setting float level isn't good enough. Still sounds like it's running too rich(worn float needle) or something still stopped up/installed wrong if you have to pull the air box lid,filter and air boot for it to run right. I'd always install a carb kit,not worth having to go back into a carb for the second time,plus eliminates the carb as the problem if it still doesn't run right.http://www.ebay.com/itm/NEW-SHINDY-CARBURETOR-CARB-REBUILD-REPAIR-KIT-FOR-POLARIS-425-MAGNUM-1995-98-/311242775486?pt=Motors_ATV_Parts_Accessories&hash=item487783dbbe&vxp=mtr Plus here's the fuel section from the manual that may help .https://www.google.com/url?sa=t&rct=...,d.aWw&cad=rja

 

This will be the 4th time ive been in the carb...lol. I ordered that exact kit today. The kit comes with a 155 main insted if the oem 140 wont this be a problem as im already too rich? Also im missing the part labeled #4 in the pic below. It says its a ring and i can see how it would raise the needle up which would make it carb richer. Do you think that has anything to do with it? Also the carb overflows fuel from the bowl over flow like the float needle and seat
are bad. I checked the needle and seat by applying 5 lbs psi to the fuel inlet pipe with a mighty vac and it held pressure. Do you think there could be a crack in the carb body causing this?

 

You NEED part #4 which is a plastic spacer that holds the jet needle straight and yes also raises the needle. Just use your 140 main jet and check that the replacement pilot is a #40 or use the old one also. The kit is still the best way to go as the oem needle and seat assy is the price of the complete kit.Carb overflowing is probably the main thing wrong as this is usually caused by a worn needle and seat plus possibly a worn float pin(#16) if the float arm is wobbly!! The worn seat,float problem just causes a rich problem in itself by dumping too much gas into the intake no matter what the jetting is. This will make 5 times which is 4 times too many on having to go through a carb!

 

Thanks for the info. And yes the float is wobbly so i better order that pin. Thanks for your help and will follow up after kit arrives. one more question for ya.
The brakes work great on this atv but i can not set the parking brake because the handle has no travel at all. As soon as you squeeze it the brakes are on. No free play at all.
so i cant get the handle to move enough to set the thumb hold. Is there any adjustment for this? been through the manual but saw nothing.

 

These old red handled master cylinders were kinda crappy on this as some wold do as yours,not enough travel to lock the lever or the teeth on the lever would strip and not able to catch the brake lock. Parts on this are almost NLA. Dirt Cheap Yamaha & Polaris OEM Parts & Accessories – Cycle Parts Warehouse You can always swap the complete thing out for a later model one on ebay or Cycle Parts Warehouse as the brake line will fit,BUT a little pricey.Dirt Cheap Yamaha & Polaris OEM Parts & Accessories – Cycle Parts Warehouse

 

^^^This

My first new Polaris was a '95 Magnum 425 and needed to adjust this once.