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post #1 of (permalink) Old 12-02-1999, 08:57 PM
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Mikuni Carb Selection

I have a 1987 Polaris 400 SKS. The previous owner installed a 488 overbore kit in it and it was ported to the max. I also found out that he had honed out the 38mm Mikunis to 39.5mm. It ran fine like this until one of the piston skirts broke apart. While inspecting my engine parts, I noticed that the web between two of the ports in the bad cylinder had a crack in it; not sure if the piston failure caused it, or if it was weakened from the heavy porting job. Either way I considered it scrap and purchased new 500 cylinder, pistons, and head.

I now have about 1500 miles on this "new" engine and it has yet to run right. It is fine once the clutch engages and the track starts spinning; however, it severely hesitates in trying to do so, especially when it is warmed up. It makes it real difficult to get out of holes and tight spots. I installed the lowest possible gears in the chaincase and a heavier spring in the clutch. This helped some, but only hides the problem.

Starting the engine is also difficult when it is warmed up. It often requires multiple quick pulls with the throttle held at about 1/4. Once it does start spinning on her own, it takes about 5-10 seconds (while maintaining the 1/4 throtle) before the RPM's get high enough for the clutch to grab the belt. I have had it to the dealer for "pre-season tuneups", and I have adjusted the pilot screws, but nothing has changed. I tore apart both carbs and found nothing wrong, no clogged ports or passages.

I am now thinking that the carbs; having been honed-out to 39.5mm may have been fine for the old ported-out engine, but too much for the new stock 500. Not enough airflow velocity at low RPM's to pull the fuel through the carb jets?

If I need to buy new 38mm carbs, should I consider the more expensive "flat valve" style or stay with the "spigot" design? Are they easily interchangeable or do they require different cables and mounting flanges? Do they require the same size jets for a given altitude and temperature? (3000-6000 ft./ 10-30 degrees) What is the difference between the TM and TMX? Both are listed under the "flat valve" category, and both are 38mm. I have a Dennis Kirk catalog in front of me, but I have seen better prices on Mikuni advertised somewhere in SnoWest magazine; who else sells these carbs at good prices?

Thanks for any help you can give me.


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post #2 of (permalink) Old 12-03-1999, 10:31 PM
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Re: Mikuni Carb Selection

You still have some options to check before you junk those carbs. Inside the carbs there are needle jets as well as jet needles, pilot jets, and main jets. These are all changeable. The needle jets have a number such as P-0 or Q-4 etc. Jet needles have numbers such as 6dh4 etc. There are also numbers on the pilot jets and main jets to indicate their size as well. To add another factor to the situation there is also a number on the throttle valve which indicates the cutaway.The higher the number the leaner the fuel mix will be. What you need to do is to find out all these variables from the dealer for that particular motor and then match those components in your carbs. As far as the boring out of your carbs it won't affect those settings too much except you may have to go richer on your main jets by one size because you will have about 4 percent more airflow through the bigger carbs at wide open throttle. Total cost of parts will be a lot cheaper than new carbs for sure.

post #3 of (permalink) Old 12-04-1999, 06:54 PM
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Re: Mikuni Carb Selection

OK I tore apart both carbs and took note of all the jetting parts and compared them with what my Clymer shop manual says they should be.

240 main jet >>> Fine for the temperature and altitude I run at.

6F9 jet needle >>> Same as the shop manual.

P-8 needle jet >>> Clymer calls for Q-0 Are these two sizes close?

40 pilot jet >>> Same as shop manual.

2.5 Throttle valve >>> Clymer calls for 3.0 What effect would this have?

I also noticed that there is a small washer missing on one of the carbs. It goes between the main jet and the needle jet. I will get a new one and try it out, but I doubt this will have any effect. The washer is only .031in. thick, and raising the jet needle up one notch would seem to have the same effect.

I am thinking that if anything, I need to go richer on my pilot jet, since this is the circut that effects the RPM I am having problems in.

One other thing that may be causing my problem is that I can see light between both sides of the throttle valve and the main body due to the oversized bore. Could this be causing a lean mixture at idle? It is not much light, .003in gap per side max. Could a larger pilot jet compensate for this? If so, what size should I start with?



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post #4 of (permalink) Old 12-04-1999, 09:51 PM
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Re: Mikuni Carb Selection

I assume your Clymer manual is for a 500 engine not the 400 you used to have. The P-8 is one size leaner than the Q-0. It appears to affect the fuel flow from just off idle to approximately 50 percent throttle opening. The throttle cutaway is one size richer and affects the fuel delivery from idle to 75 percent throttle opening. According to the book I have they should offset each other somewhat and you should be close. I would play with the air screws at idle to see what effect they have when the motor is warmed up. Turn air screws in or out using 1/4 turn increments to obtain the maximum rpm. Readjust idle screw to get the proper rpm. Retune the air screws again to get maximum rpm. When the air screws are peaked shut off the motor and see how many turns out from full seat you are. If it is less than one turn the pilot jets are too small. The other end of the range is greater than 2.5 turns the jets are too big. If your dealer will let you play with his stock then go up one size at a time until you find the proper size. If not then maybe jump a couple of sizes at first. After you have determined the proper pilot jet then check the throttle valve- operate the engine at low throttle accelerating from idle to 1/4 throttle. If engine hesitates turn in air screw 1/4 turn at a time. If acceleration is improved after adjusting then you need to decrease the cutaway. If you have to turn out the air screw to see an improvement then you need to increase the cutaway. I was wondering if you have tried flipping the choke momentarily when you are trying to get the machine to engage the clutches. If you are running too lean it should make it rev up quicker because in effect you are giving it a richer mixture.As far as the daylight on the sides of the throttle slides I don't know if this is your problem. I just looked at a mikuni carb here and I definitely can't see any light at the sides. In fact the throttle slide actually runs in a rounded out area on the sides of the throttle body so you can't see from front to back at all along the side. If you lift up the slide you can see the channel it runs in. The only thing is you said these carbs ran great with the old motor so maybe it's not that big of deal.

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