Carburetor main jet sizing

[wander2039]

Need some schooling in main jets. Replaced the carb in the CV80 since it had the old vacuum choke and was kaput. A carb for a Polaris Sportsman 90 is almost a drop in replacement. Started right up to my amazement and runs good an idle. Falls on its face though beyond that, I adjusted the AF screw and almost gets there but not quite.

The stock Mikuni had a 52 jet and I swapped that in since the one in it wasn’t marked. It must if been slightly larger, 55 or 57. Going to get a kit off Amazon but thinking I need to try maybe a 65? How many steps should I go at once?

 

[sunruh]

why do you think you need to go up on size and not down?
pilot jet -> needle jet clip -> main jet (simple 2stroke carb) --- yes there is more like slide cut away, needle size and taper(s)
and on some carbs (4strk) a whole lot more

if you very very slowly move the throttle more open does it catch up or not?

 

[wander2039]

Good point.. didn’t realize the needle could be adjusted. It was in the middle, I adjusted it so it would be richer. It doesn’t catch up if I slowly open the throttle. It’s better but still bogs out. The kit I was looking at does have smaller jets also. Idles ok so thinking pilot should be close enough?

 

[Cujet]

Like any jetting, experience with your engine, in your application, at your elevation, is going to be what tells you to move up 3 jet sizes.

But that's my first guess, try 3 sizes up. Should help the bog upon throttle opening.

 

[KrisZ]

Since you’re using a different carb, you’re pretty much in an uncharted territory and will have to keep experimenting until the engine behaves normally.
If raising the needle helped a little, you need to keep going with a larger main jet, perhaps even a larger pilot, as there is a transition period under partial throttle between the two. I would put the needle back to the middle position.

 

[wander2039]

Since you’re using a different carb, you’re pretty much in an uncharted territory and will have to keep experimenting until the engine behaves normally.
If raising the needle helped a little, you need to keep going with a larger main jet, perhaps even a larger pilot, as there is a transition period under partial throttle between the two. I would put the needle back to the middle position.

I debated heavily if I should get some pilots when I ordered main jets. I didn't so knowing how things go I'll probably need those also. Based on everything I've read on bore size and carb size should be in the 70-90's. Guess I'll find out this weekend.

 

[AndyB]

If the new carb has a slide, I would be working on the slide cut away. That effects the off idle performance.

 

[JWC86]

One thing is for certain. You will be able to remove/install the carb with your eyes closed by the time you get this thing dial in. Keep us posted please, just curious how it reacts to your changes.

 

[wander2039]

One thing is for certain. You will be able to remove/install the carb with your eyes closed by the time you get this thing dial in. Keep us posted please, just curious how it reacts to your changes.

I've already taken it out 3 times.. I agree with this statement LOL

 

[Jaguar-EC]

Try a full range of main jets on both flat surface WOT and under load (uphill, going thru sand or mud, hard acceleration). That may be 5 different jets. Use any of them depending on if your typical riding is flat road WOT or rough going under load. Then take the piston out to look at its underside. If oil has burned black there then clean it off and retest with the jet you choose. Then take it out and look again. If oil has burned black there then you need an oil with a higher % of synthetic (because its more heat resistant) or yoou need a bigger main jet. Of course with any heat issue you also need to try different timing settings to make sure the timing isn't too advanced.
Hey, you asked and I'm not going to give any less than the best advice.
Here's my page that tells what is in each engine oil: dragonfly75.com/moto/oil.html

 

[sunruh]

why not try moving the clip up (ie needle down) 1 notch.

sometimes going richer is NOT the answer!

 

[wander2039]

Just a update ordered the wrong jets.. not surprised should of took my caliper to it. Takes Mukini jets which I just make sure I ordered this time. According to manual the stock main jet size for the cv80 is 75, don't know why it has 52 on it. The stock main jet size for the Polaris is in the 80's. From what I was reading with a pod filter 120-130 is what some of them is running. I did move the pilot jet from the stock carb to the new one, I can ride around all day at 1/4 throttle. Still runs great at idle and starts right up.

 

[wander2039]

I think I picked the hardest carb for a newbie to tune. Ordered the wrong main jets twice, this carb takes jets for a VM 11. I haven't found where I can order multiples for this carb just individuals. I've got a 95 main jet in it now but pretty sure it needs a bigger pilot jet. If I could hook up the stock air box might work as is. What I find so odd is for as long as the Polaris 90 was built how hard it is to find exact info. Thinking I'll get a 37.5 pilot jet. Sure would be nice to order a pack of sizes!

 

[Graham Piccinini]

I feel your pain... Unfortunately all I can say is - Good luck!

 

[repairman54]

Needle jet affects mid range. I fought with a modified 2 stroke 500cc snowmobile motor that I inherited. VM carbs. Fortunately, it had exhaust temp gauges on it. It ran lean part throttle and did have some bog in it. Going up with the needle jet was my cure. Main jets affect 3/4 and up, pilot circuit is idle only. If lowering needle clip so needle is higher improves it then needle jet needs to be larger.
Go onto Mikuni's site and read up on tuning. Pulling off the stock airbox throws everything off.