My 94 Sportsman has a Mikuni (VM 38?) carb that I am trying to fine-tune.

The machine runs really well the majority of the time, but it does have some issues I'd like to clear up.

The main issue is that it bogs when run up to full throttle. It runs great at 75% throttle, but then acts like it's running with the choke on above that point. It also smells of gas when running and hardly ever needs to be choked/enriched, even when it's been sitting out all night in 25°F temps.

Also, pulling the choke/enrichment lever will kill the engine/cause it to bog. It is my understanding that the engine should be able to run, though at a higher rpm, with the choke on, even if the engine is warm. If this assumption is correct, then, obviously, something is not adjusted correctly.

These two symptoms make me think the carb is set too rich, but I am not quite sure what adjustments to make.

I cleaned the carb previously, and if my memory serves me, the jet needle is in the middle (3) slot, the needle jet is a "30", and the pilot air screw is 1.75 turns out. I am using the factory oil injection system, and it is set/adjusted correctly. The vent lines were all replaced as part of the restoration I did to it.

 

Jetrus can solve your problems no matter if it's a Keihin or Mikuni carb. Does sound rich and probably the main jet at least needs to be dropped.Plug chopping is the best way to tell. Kill it at high speed and check the color of the plug.If dark and carboned up,the main jet is obviously too large. You want a tan to medium brown. MIKUNI TM36 TM40 HS40 PUMPER CARB EXPLODED VIEW Id go at least 10% lower than what you have,have a couple or three jets on hand. Pick the carb you have on this chart.https://jetsrus.com/a_jets_by_carbur..._selection.htm You may have to tinker with the needle clip position also. Plus this might help you understand jetting a little better.http://www.iwt.com.au/mikunicarb.htm

 

I think I listed some bad info; it's a round-slide carb, which Google tells me is a "VM" series.

Aside from simply swapping jets and making a run, are there any others tests I can do to narrow down the problem?

 

Yes,you can pull the black wire on the reverse limiter as they can limit in forward same as reverse when they short out Item#41. 1994 Polaris SPORTSMAN 4X4 (W948039) Electrical/Taillight Assembly Sportsman 4X4 W948040 | Cycle Parts Warehouse If this doesn't help and the VM carb is larger than the VM 34 that was stock then you are most certainly going to have to rejet it as they usually come richer.Stock jetting was a 200 main,30 pilot on the VM34 Sportsman. If it's still running rich on top end then Jetrus can still help. Just see what the jetting is on the new carb,order a couple smaller ones(if needed) and plug chop.May take several trials on this to get it right. Final settings can be with the air screw,in richens fuel mixture,out leans the mixture mainly for the idle. The standard setting is 1.5-2 turns out.Plus the needle clip position up or down affects idle to about 1/4 throttle or so. If you get it to where it screams and runs fantastic,be alarmed. Always do a final plug chop.Mikuni VM series carb jets

 

That sounds simple enough to rule out. Also, it is my understanding that, if everything is working correctly, the 400 Sportsman models CDI boxes do not have a high-speed rev-limiter? Is that correct?

I do not believe that the carb, nor any of the jets, are not the original equipment. If that means it's a VM 34, not a 38, then it likely is. I distinctly remember seeing "30" on a jet, though that may have been the pilot jet.

Would you suggest ordering only smaller size jets? Also, would you suggest ordering both pilot and main jets?

The basic principles of these machines and their functions are not foreign to me, but their I have yet to learn their idiosyncrasies. Given that, I assume that you're warning of running too lean? Is the plug-chop the appropriate way to determine if you're in the right range?

(Also, I assume that I should shift to neutral before killing the engine when doing these tests?)

 

Limiter is for reverse but can short out and limit in forward range.Just pull it's black wire and leave it that way as they all fail sooner or later You don't shift while moving or you can damage the gear case. Just at a high speed run hit the emergency kill or ignition switch and coast to a stop.If you have the VM34 then just check the jetting. If you have the 30 pilot then just check the main. Down here on the old Sportsman 400 I had lower the main down to a 190 for a lot of farmers or they'd foul plugs.A 200 main imo was a tad on the rich side down in our area around 650 feet above seal level. Plug chopping at high speed is the best indicator on jetting.

 

You're in North Texas, I see? That's a pretty good area. I'm in East Texas, myself - the Longview/Tyler area... Small world.

Given your statement, sounds like I need to order a 190 jet, at least.

Thanks for clearing up the plug chopping technique. I thought that coasting would alter the results. I will give it a shot and see what I can learn.

 

Pull the limiter black wire first. Have had people report back that solved the problem a lot of times. If it doesn't help then on with the jetting.Actually you're right on the coasting. Kill it a high speed and stop as quick as you can. Excess fuel can load up and foul up a proper plug reading.

 

Been a minute...

No change with the black wire disconnected. The override circuit does appear to be working as it should, as well, so that's good...

I fiddled around with the carb a bit more after reading various carb-related pieces and based upon that turned the idle air mix out a bit and readjusted the idle. (No othet chnages made.) I have a noticeable change around 1/4 throttle, from what seems to be rich to what seems to be just about perfect. The high-end bogging seems mostly to have cleared up, actually. When it does occur, it seems to be after extended periods of lower RPM use. I still hardly have to choke/enrich it also. I'm wondering if the pilot is too large? Or perhaps the floats or shut-off needle need to be addressed?

I have yet to do a plug chop yet so I'm not postive that it's right anywhere, but it's running better now than before. I plan on spending more time on this when deer season ends in a few weeks.

And that leads me to another question: Based on what I've read of your (OPT's) posts here, it seems that the 200 main jet was too much for farmers and ranchers who putter around. Lately, since it's deer season, I've been doing a lot of that myself, which makes the 190 jet you'e suggested sound like a good fix, but, this won't be a one-trick pony. I want to be able to take this machine out and have some fun and I don't wanna melt it down. Would I be better to stick with the 200 jet and just shake out the cob webs a few times during the season so that I don't risk or give up anything during the rest of the year?

 

A 190 main down here certainly won't melt it down.Have had others than farmers and ranchers drop the main down to a 190 especially during hot summers. What's happening,which is kinda normal to this engine,is that you can load up the bottom end with gas and oil just idling and can bog a tad when you give it throttle until you clean this excess out. With the weather drop that's happening right now,the engine is going to run a tad leaner as all do in cold weather. The 190 would still be fine in most cases,and there may not be as many "cob webs" to shake out in this colder weather if you're doing a lot of riding. Main thing to remember from the old 2 strokes mechanics of years past is if an engine is running better than it ever did and just screams,beware..Always do another plug chop,raise the main jet if the plug is a little on the whitish color instead of a tan or medium brown.