I just rebuilt the carb on my 2004 SP700 and installed the DynoJet kit for this model at the same time. Part of my motivation was performance, and part of my motivation was rough idling at lower temperatures.

The carb was and is spotless, all the old and new jets are clean and clear, etc. With everything back together the engine fires right up, has great throttle response, etc. I haven't been able to take it on a road test because it's completely snowed in.

The idle is set to the specified 1200 +/- 50 RPM. But as the engine idles for a while, the idle RPM's slowly drop... and drop... and drop until they're down in the 800's. Eventually the engine just stops running. Blipping the throttle brings it right up, and the idle repeats its "decline from 1200" routine again. Setting the idle screw up simply increases the starting point but does not eliminate the problem.

Give it throttle and the engine revs right up and will stay at any speed you desire indefinitely, so I don't think it's a float problem.

Pulling on the choke a little makes it worse, which makes me believe the idle is rich, so I turned in the pilot screw about 1/8 turn at a time but it didn't change anything.

What could be wrong here? What should I try next? Thanks!

 

what size pilot jet was in it stock, what size did you go to, you might be to rich still and may have to go down a jet size on the pilot circuit

 

Stock pilot jet. The kit did not include a replacement, so I cleaned the one that it had and put it back in.

The machine is all stock except for the Polaris Plus/White Brothers performance exhaust. I have a few less disks in it to keep the noise down a bit, but I presume it is more free-flowing than the stock muffler. Therefore, if anything, there should be normal-to-MORE air flowing through the carb which means the stock pilot jet would be normal-to-lean, not rich (same amount of fuel, same-to-more air).

Are you saying that a rich pilot circuit would cause this behavior? Remember, it takes *minutes* to slow down enough to die. Example: With idle set to ~1200, it probably doesn't get down to the 800's for 3-4 minutes. Whatever is happening, it takes a long time to have its effect. And blipping the throttle even a little, just to get the RPM's back up to ~1200, resets the whole thing.

If the pilot jet was too large (too rich), wouldn't that mean the idle would just run rich all the time? I can't see how the "extra fuel" would "accumulate" and build up over time, slowly reducing the idle RPM's. The pilot jet injects the fuel into the throat and it goes straight into the engine, so it seems to me that whatever effect it would have would be a consistent one, not something that works SLLLLLOOOOOOOWWWWWWLY over many minutes. But I've been wrong before!

I'm baffled.

 

I think I have it running properly now - but the story is VERY interesting.

First, let me quote two phrases from the DynoJet literature regarding the mixture (pilot) screw:

1) "Mixture screws are set too lean. Turn counter clockwise to richen the mixture."

2) "Mixture screws are set too rich. Turn clockwise to lean out the mixture."

Clockwise means "tighten", so these statements (which are consistent with each other) seem to indicate that to go leaner (i.e. get less fuel) you tighten the pilot screw. In other words, the pilot screw regulates FUEL, not air. Back it off and you get more fuel, tighten it down and you get less.

The Polaris service manual says that the pilot screw should be set at 2.5 turns out. I'm running the stock pilot jet, and my pilot screw was still sealed with that little brass plug, so everything should be factory stock in the pilot circuit. When the OEM muffler was still installed, throttle response was normal and everything worked fine.

When I installed my Pure Polaris / White Brothers muffler a while back, I noticed a brief hesitation when punching the throttle off of idle (no other problems occurred). The new, higher flow muffler should have increased airflow through the engine, so presumably the pilot circuit was now running a bit lean. This was confirmed by the fact that pulling out the choke a bit, and thus richening the mix at idle, fixed the hesitation. No big deal - I'll fix that when I get around to rebuilding the carb by adjusting the pilot screw.

When I got ready to rebuild the carb, I purchased a DynoJet kit for this machine. The kit doesn't change anything having to do with the pilot circuit, so I'm still using the same pilot jet, pilot screw, etc. However, their instructions recommend setting the pilot screw to 1.5 turns out (instead of the 2.5 turns recommended by Polaris, as noted above). That's what I did when I reinstalled the carb.

My first message in this thread details what I found - my idle would slow more and more until finally the engine would die, taking 3-4 minutes to gradually go from 1200 RPM to stall. Starting at 1.5 turns out as recommended by DynoJet, I moved the pilot screw back and forth around that setting by 1/8th turns trying to dial it in, but couldn't find a setting that did anything different.

This morning I was thinking about it and realized, again, that DynoJet didn't change anything in the pilot circuit (they emphasize this in their instructions). So if the pilot screw regulates fuel, as clearly stated by DynoJet, and I was already lean at the factory's 2.5 turns out due to the higher flow muffler, 1.5 was definitely too lean and might be the source of my problem. So I cranked the pilot screw back out another turn, to 2.5, and tried that. No luck, same problem.

Finally, out of frustration, I decided to tighten the pilot screw all the way in and start working out in 1/4 turn increments, just to map what the engine did at each step. I tighted the pilot screw all the way in, started the engine - and the problem was gone!

I backed the screw out 1/2 turn as an experiment. The idle continued to hold. Only when I neared 1 turn out, or farther, did the idle start to die off like before. The idle held great at 0 turns out (fully tightened in). I worried that I might have poor throttle response, but no - punching the throttle caused the front end to buck up like I was headed for a wheelie. I've NEVER had takeoff like that before.

Being a little paranoid, I didn't want to leave the pilot screw all the way in. So I set it to 1/2 turn out, started the engine, and walked away to do other things for a while. The engine idled perfectly for 15+ minutes, never varying a bit, solid as a rock.

So there's my story. The only thing that makes sense is that the pilot screw on a Mikuni BST 34 actually regulates AIR, not fuel. But that contradicts the Polaris manual and the DynoJet instructions. I increased airflow with a higher flow muffler; the resulting hesitation could be cured with a bit of choke, thus confirming a lean condition; yet loosening the pilot screw made things worse and tightening it made them better.

Any theories out there? I'm utterly baffled.

 

WAATV, you seem to have a good grasp on how things work.

But I would not have guessed that a totally closed or only 1/2 turn open idle mixture screw would have been the answer, having agreed with everything you wrote about the symptoms, stock settings, muffler, etc.

Glad you got it worked out though. Your write up was very clear and easy to understand.

 

I still want to understand WHY, though. It just doesn't make sense.

 

Mine does the exact same thing. I have not did anything to mine and mine is a 2004 sportsman 700. Mine will run at around 1200 rpms and then slowly dye. I have pulled the choke and makes it worse. How do I turn it down. I don't see it on the carb.
Thanks

 

<div class="FTQUOTE"><begin quote>Mine does the exact same thing. I have not did anything to mine and mine is a 2004 sportsman 700. Mine will run at around 1200 rpms and then slowly dye. I have pulled the choke and makes it worse. How do I turn it down. I don't see it on the carb.</end quote></div>

The pilot screw, which is what I adjusted as described, is covered by a brass plug on the bottom of the carb. You must remove the carb from the engine to drill out the plug so you can access the screw underneath. Be VERY CAREFUL when doing this because it's easy to drill through the plug and damage the screw.

 

Extra note: I spoke with DynoJet to get their opinion of what's happening here, even though their jet kit has nothing to do with the pilot circuit. They were as mystified as I am. I'm not sure the guy really believed me at first until we talked for a while and I established some credibility with him. I'm not sure *I* would believe me, either....

 

Ever since i have had mine it seems like it is runing rick or like the choke is one. I have ask my dealer to adjust the pilot screw before and they wont. I will give this a try.
Thanks