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1996 Polaris Magnum 425 4x4
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Discussion Starter #1
Hi Everyone,

This will be a bit long but I would like to provide as much info as I can right off the bat.

I recently bought a '96 Magnum 425 4x4 with just over 600 miles. It was previously owned by a state gov agency and was in rough shape. It had not been maintained well, but was not missing any parts. I've so far rebuilt the gearbox (hole worn thru case by rear drive chain), gear selector, carb, all eccentrics, chain drives, hubs, brakes, and cooling fan following my Clymer manual.

Everything is in pretty good order now except for a stumble when I roll the throttle on quickly from idle to 1/2 throttle engine warm. It will blip a couple of times then take off normally with good power. Occasionally the blip will continue if I hold at 1/4 throttle under medium load.

All intake and exhaust components are connected, the spark plug is new and properly gapped, the carb is clean with original jets, etc is adjusted, choke is not stuck, idle mix is adjusted.

The spark plug was sooty, but dry indicating a rich condition which I tried to tune out by lowering the jet needle. Tiny improvement but the stumble is still there. I know these motors suffer from worn exhaust cam lobes, but I am wondering if there are any other things to try first like a hotter plug. Or if I missed anything. I plan to do a vacuum test and "black wire test" today since I have the plastics off.

Thanks for reading, any thoughts?
 

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Changing the heat range of the spark plug does not affect performance. The spark is the same intensity. The heat range simply controls the temperature of the tip of the spark plug when the engine is at normal operating temperature. Too hot of a plug can cause pre-ignition and too cold of a plug will foul more frequently. The hesitation is a carburation issue caused by a vacuum drop in the venturi upon sudden opening of the butterfly air control valve. Changing the slide needle setting will not solve the issue as the slide needle has a greater effect on the mixture between 1/4 and 3/4 steady (constant) throttle settings. Best way to try to reduce the vacuum drop lag is to raise the fuel level in the float bow/, enrich the idle mixture and increase the idle speed.
 

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1996 Polaris Magnum 425 4x4
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Discussion Starter #3
Changing the heat range of the spark plug does not affect performance. The spark is the same intensity. The heat range simply controls the temperature of the tip of the spark plug when the engine is at normal operating temperature. Too hot of a plug can cause pre-ignition and too cold of a plug will foul more frequently. The hesitation is a carburation issue caused by a vacuum drop in the venturi upon sudden opening of the butterfly air control valve. Changing the slide needle setting will not solve the issue as the slide needle has a greater effect on the mixture between 1/4 and 3/4 steady (constant) throttle settings. Best way to try to reduce the vacuum drop lag is to raise the fuel level in the float bow/, enrich the idle mixture and increase the idle speed.
Thanks for the response Latebird. I have not raised the float height yet, however I did raise my idle enrichment and idle speed a bit. The hesitation is nearly gone now and seems to only happen when going up steeper hills. Perhaps with a too low float height the little fuel in the bowl runs away from the pilot jet and lifts the float? I will raise it up a bit this afternoon and see how it goes.
 
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