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Old 01-11-2009, 07:45 PM   #1
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cleaning and ajusting a tillotson carb is fairly simple. Diasassemble carb, makeing note of where each part came from.Special attention to how the gaskets are removed as well as the fuel pump diaphiram.

soak all metal parts in a carb cleaner, denatured alcohol, or lacquer thinner. DO NOT soak any fiber washers, or O rings, as they will soften and not be reuseable.

Often times, foreign matter can become loged in under what is called a " Welch" plug. These are round plugs that are dimpled on the main body of the carb, located near the needle and seat. To remove these, drill a small 1/8th inch hole in them and pry them out. To reinstal, your carb repair kit should have them and they are put back into place by inserting them, CONVEX side up and taping them down lightly in the center with a 5/16th flat end punch and a hammer.

the fuel inlet control lever ( needle and seat conntrol) should be set so that the lever is flush with the metering chamber wall.

Reassemble carb in reverse order.

hi speed needle adjustment ( needle on left hand side faceing engine) is 1 to 1 1/2 turns out from lightly seated position.

Low speed setting ( needle on right hand side faceing engine) 1 to 1/2 turns out from lightly seated position.

Start engine and let it warm up to operating temp. Adjust the low speed screw in or out to obtain a smooth idle.

To adjust the hi speed needle, block the back of the sled up and rev to wide open throttle. adjust screw to obtain MAX engine speed. Then back the screw off approx 1/8th turn.

Now go for a ride and run the sled up to full speed. Shut the engine off and let it coast to a stop and pull the spark plug (s) out and check color.The plug ( s) should be tan in color to slight black. if they are white or grey, richen the HI SPEED needle ( turn out) and retest. If they are black and sooty looking, adjust the HI SPEED needle again, this time turning it in to lean the engine and re test.

NOTE 1... adjustments to either needle will affect the other needle. So be patient.

If the carburetor settings are too rich ( too much fuel ) the engine will falter at that section of the track corresponding to that particular range of the carburetor settings. A spluttering or irregular firing sound will be heard.

This sound is also called 4 stroking.

If this sound occurs at low speed while at full throttle, such as coming out of a tight turn, it is usually due to the Low Speed adjusting needle being turned out too far. If the sound occurs at high speed while at full throttle, such as on a straightaway section of the track, it is due to the High Speed adjusting needle being out too far. In either case, needle adjustments must be made inwards gradually, in approximately 1/8 turn increments. It is best to adjust the Low Speed needle first for best overall low speed performance and then follow with the final high speed adjustment.

If the engine does not 4 stroke or give the too rich firing sound, any place on the track, it could be because the engine is running too lean ( not enough fuel ). This is easily checked by opening the needles out in 1/8 turn increments until the engine 4 strokes. In general, the optimum performance and engine reliability will occur at carburetor settings just slightly leaner than the point where the engine will begin to 4 stroke.


NOTE 2. Tillotson carbs can be a lil more finicky to temp and weather changes than a mikuni carb. A 5 degree temp change, either warmer or colder can make the engine run poor. More so with the hi speed needle than the low speed. Good news is, a slight adjustment either rich or lean will fix the problem, and is why most tillotson carbs have a finger barb on the hi speed needle. ( no screwdriver needed to turn it)

If the temp drops, turn the HI speed needle out to richen. Colder air requires more fuel.

If the temp climbs, then richen the HI speed needle by turning it in.

BUT!!!!! only make small adjustments!!!!... 1/8th of a turn should be all that is required.AND... be patient. Sometimes takes the engine a min or two to fully respong to the change in settings.

There is another issue for tuneing problems on Tillotson carbs. This is called the "pop off pressure"

This pressure is regulated by the spring tensoin on the metering lever or fuel inlet control.

If you find that the engine will not chop off clean or is slow to come back from high rpm's when you lift the throttle, this is a sign that the pop-off pressure in the carb is too high. The cure for this is to lower the pop-off pressure.

If it is too high, we need to use a weaker spring under the metering lever. We can shorten the spring by cutting off one turn at a time, with nail clippers or small side cutters.This is a very delicate operation and as the operator will see, removing only one turn can make a big difference.

NOTE 3. be sure that ur carb kit came with a new metering spring.

I would like to emphasize one more time that ideally we should have different tensioned springs. However if done carefully the above method will be satisfactory.

If you experience loading-up problems, this is a sign that the pop-off pressure may be too low. The cure for this is to raise the pop-off pressure.

B) In order to increase the pressure we need to use a stronger spring or very carefully expand the existing one. This operation needs to be done in very small increments. Otherwise we will end up with a very high pressure

NOTE 4..Expanding and then cutting the spring a few times will weaken it and we will end up with an erratically working carburetor. If possible, different springs should be used.


Hopes this helps
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Old 01-20-2010, 12:40 PM   #2
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Old 01-20-2010, 08:30 PM   #3
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Here's a link to what he said, only w/pics for dummies like me. Tilly and Walbro fixin' These guys fly with these carbs, so I guess we're safe enuff on the snow! Thanks for the writeup 'Juice, always good to go over the basics.
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Old 01-21-2010, 03:10 AM   #4
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Thanks 88... Allways seems we go through this with ppl every yr, not that its a big deal to help out.I did have pics attached, but I am guessing the admins deleted them cuz they wern't mine. So I think what I am gonna do is, go to my fav wrecker, and see if I can get a cheap, beat up, nasty grungy dirty old tilly carb, and a rebuild kit, and do a step by step, with my own pics on how to service, clean adjust etc and maybe see if it can be pinned.

Thinking do the same with a Mikuni carb as well as both the walbro and mikuni fuel pumps.Though I did see one either this site or maybe it was dootalk that someone had allready done the mikuni carbs.
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Old 03-06-2013, 07:32 PM   #5
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I am having issues with the carb starving for fuel it seems. I am able to get it to idle fine but as soon as I increase throttle it boggs down. Any help would be greatly appreciated.

This is for my 1973 Silver Bullet TNT with a Tillotson HD carb.

Thanks,
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Old 03-14-2013, 05:39 PM   #6
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 Quote: Originally Posted by 73SilverBullet View Post
I am having issues with the carb starving for fuel it seems. I am able to get it to idle fine but as soon as I increase throttle it boggs down. Any help would be greatly appreciated.

This is for my 1973 Silver Bullet TNT with a Tillotson HD carb.

Thanks,
Welcome to the cat house. I'd venture that you've got some blocked passages in your HD yet.

Before the Cat guys give you too much grief about riding a banana (even if it's a silver banana), stop in over at Dootalk and check out their vintage forum. Lots of guys familiar with the old Doo sleds that can help ya out. Hope to see ya over there.
Vintage Ski Doo's - DOOTalk Forums

I've got a '73 SB TNT 340 too. Neat looking sleds even if they are kinda heavy pigs for the era. Currently putting my '72 TNT 340 back together this year...but that's enough Doo talk from me here.
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