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Discussion Starter #1
I know it's not a car but this is the only place I can think of to ask about re-jetting snowmobile carbs. I just picked up a '79 SRX with a rotax 503 and dual carbs. I want more power out of it, the carbs have already been replaced and the main jets are 120's. What size jets would you guys suggest I use? I've got dual tuned pipes, and i think the carbs are mikuni 38mm. I know that they are Mikuni just not sure what size exactly. Anyway, What size do you suggest?
 

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It would be extremely hard to recommend a jetting for that snowmobile since they are best calibrated at the altitude they will be run at.

Purchase some NGK Gold Palladium plugs and when the first 1/3rd of the center electrode is silver you have hit pay dirt. Any more then 1/3rd and you are running lean.

EDIT: I forgot to mention that this would be assuming a full throttle run with a quick shutdown.

You can also read the piston crown to see if you are running fat or lean. Look for a clean ring about 1/4 ~ 3/8 of an inch from the outer diameter of the piston. You will also see a little more clean area on the piston at the transfer ports.
 

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Discussion Starter #3
I'll do that!
While I'm at work tommorow I'll pick up the plugs and try some different jettings. someone I know suggested 190's or 200's but I thought it might be running to rich like that, just wanted to be sure. I figured if no one could help me out directly with jet sizes I would grab two of each up to the size suggested and give em all a try tommorow evening. By the way did you possibly mean platinum plugs or did you mean exactly what you typed? I just want to be sure so I know I'm looking for something that does exist.
Thanx! :thumbup:
 

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Exactly what I typed.

Make sure they are the correct heat range.

Good Luck and remember they run best right before they burn down.
 

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what jets?

After you check your plugs etc to see if you are running too lean, I would go to your friendly dealer and get 2 of each size of the next 3 larger sizes and do some experimenting with them. It doesnt take much to richen jets. You failed to mention whether you have modified the air intake on it or not, but I am thinking that if all you did was to put on after market exhaust, that just going up one size in jets will be sufficient.

Ive been through this with my 79 XS1100 Yamaha motorcycle. It has aftermarket headers and a UNI air filter. Surprise of all surprises, when I went just one step larger in jets, it got too rich. Wound up going back and putting the stock jets in and setting the jet needle one step higher. The needle sticks down through your jet like on a Carter or Edlebrock carburetor. Mine have 5 rings around the top where a snap ring holds it to its diaphram. The factory setting was the middle ring. Putting the snap ring on the next lower groove richens the carb up and the more you twist the throttle the higher the needle goes, richening it even more. That was all it took with mine. You may even want to try that before buying other jets. BTW, my bike has Mikunis on it from the factory, and I am sure they are the same design as on your Snow Mobile.
 

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Discussion Starter #6
Re: what jets?

Max Keith said:
You failed to mention whether you have modified the air intake on it or not, but I am thinking that if all you did was to put on after market exhaust, that just going up one size in jets will be sufficient.
I just have the carbs without a breather box or air filter and like I had said the carbs were replaced with slightly larger ones and a slightly higher flow pump. I believe they are 38mm carbs and the main jets are 120's. Thanx for the help guys!
 

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Discussion Starter #7
PAY DIRT!!!!!!!!!!!!!!!!!!!!
Hit the jackpot! the jets were 110's and were underated for the carbs and engine itself. 200's Are the absolute jackpot, crisp throttle response, tons of botom end pull, and ran the speedo needle all the way around to bottom, vertical. 1/3rd of the electrode is silver, nice clean ring ,and boy when I say bottom end pull I mean it! Thanx for all your advice and by the way, it ran better with just typical NGK plugs (BR8ES)
 

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Reply to REKRAP,

Glad you got it sorted out.

I only used the Gold Palladium for tuning and once it is dialed in I went back to the BR9ES.
 

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Rotax engine in a Yamaha SRX? Must be a swap, they were 440's stock and were known to be quite the grenade engine. The Rotax is certainly a much better engine, got any pics? I would like to see the shoehorn.:)
 

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Discussion Starter #10
I have some distanced pics but it was no shoehorn. Fits better than the original with the exception of cutting a hole in the cowling for the fan. And light, don't even go there!!
 
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