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Those of us with TM 40 Miks on our sleds have had to put up with elevator idle speeds and an off throttle on throttle burble which helps trench the sled in deep snow. The sled seems fine when new and then the issue starts gradually with high idle rpm hot most noticeable after a good run and you come to a road crossing or stop to talk. Your idle is pegged at 3500 rpm and whacking the throttle flipper doesn't bring it down or only temporarily. Then you start having a hesitation or poor throttle modulation during slow speed maneuvers. The high idle comes down if you turn on the choke (enricher) which leads us to believe the idle mixture is too lean. True lean mixture = clean elevated idle speed and reducing the idle speed screw will only go so far then the next cold start the sled idles too slow. What can the problem be:

1) Vacuum leak ?
2) Poor quality fuel ?
3) Bad carburation ?
4) Ignition timing ?

[attachment=36996:worn_out_slides.jpg]

My vote goes to poor carburation. Worn slides and carb bodies to be exact. Yes installing larger pilot jet will help but the condition will get worse leaving a bigger hole in the throttle when you launch. Thunder products and others sell an air foil with a venturi modifier to cheat the carb into thinking it's smaller. Smaller carbs atomize fuel better and fuel vapors burn cleaner than large droplets that puddle in the intake tract and on the reed petals. The TM 40s BRP chose to use on most mainstream sleds are easier on the thumbs give good response and are equipped with a coolant warmed de-icer feature. The older VMs were quite the opposite. The main issue with TM 40s is the throttle pistons tend to wear out quickly (something the chrome plated brass slides in the VMs rarely did). Once the aluminum TM throttle slides wore through the green Teflon coating, things get nasty. Traditionally aluminum riding on aluminum doesn't play nice. It needs a barrier to provide a buffer between moving parts otherwise galling and metal wear is accelerated. The pistons should fit loosely to a point but depend on engine vacuum to hold the tight against the engine side of the carburetor. If they don't fit properly air finds it's way by them and there's your problems.


[attachment=36997:worn_out_pins2.jpg][attachment=36998:eek:nly_one_hole_worn.jpg]

The other problem is the linkage which opens and closes the throttles is prone to wear. The links (much like chain links) are fabricated from brass. Now one would think brass links running on hardened steel pins would be a no brainer. The sacrificial part would be the softer brass link which incidentally only costs $8.10 each. But noo the pins take the brunt of the wear and their not available separately from the $81.47 slides (Valve Piston 2.0). After that shock wears off were stuck with the dilemma of whats the most cost efficient repair.

[attachment=36999:Brass_links.jpg]

Of the many many TMs I've dissected, I have found similar wear patterns stemming from the fact that the stock linkage doesn't pull the throttle up straight and the proof is always shown in the links being more worn on the RH side and the throttle slide and carb body has accelerated wear in the same places. This wear BTW allows air entering the carb to by pass the sides of the throttle valve and not get the benefit of passing the idle circuit or needle jet. This air dilutes the mixture at idle (high idle speed) and delays the signal to the needle jet when the throttle is edged open (burble). I have tested this theory two ways.

1) Using a feeler gage found suspect carbs with more than 12 thou (.012") clearance between the throttle valve and carb body. This allows the throttle piston to rattle around quite a bit on engine with marginal reed valves or delaminated boots. The push pull signal from the engine just turns the slides into aluminum Maracas.
2) Loose fitting closed throttle valves (resting on idle screws) allow between 20 ~ 45 cfm air flow past them while tight fitting ones allow only 12 ~ 18 on the flow bench.

All is not lost here as you have some options. If the carbs are loose but the throttles are in good condition (not worn right through the barrier) you can adjust the fit in the carb body. If the sled has ingested way too much powder / water and the slides are scored (grooved). Seek some new or low mileage replacements and massage them. Worst case, live with the drive ability woes.

[attachment=37001:Wear_spots.jpg]

These arrows indicate wear spots to check when deciding whether carb is serviceable or not. Use a straight edge and check for flatness in these areas. Note the bumps on the slide locating rails on the carb body. These will need to be smoothed out to prevent further throttle slide wear. We'll get into that later.

[attachment=37002:three_thou.jpg]

First remove the carbs and take off the mixing chamber cover. Insert a feeler gage (starting small) and measure the clearance between the throttle slide and engine side of the carb body. If there's less than 8 thou (.008") no action is necessary and look elsewhere for vacuum leak. If more, take your time and measure all the way around the slide to be sure the wear is even (correcting crooked fitting slides is more difficult). You will need to mark what the minimum clearance is on a notepad. This is the tightest fit of the throttle slide and the target measurement to calculate amount of material need to be removed to gain a tight (3~5 thou .003~.005") clearance. The maximum amount which can safely removed from the car body or cover is 7 thousands each. After that your squeezing th o-ring rear cover gasket too much and would need to machine the gasket groove deeper.

[attachment=37003:Carb_milling.jpg][attachment=37004:Milling_close_up.jpg]

Working at removing .004" per pass then reassemble and measure clearance. You can perform the material removal using a good quality milling file and lots of patience. I machine mill for dramatics. Removing material here reduces the effective clearance between the throttle cover rails and the throttle piston. Don't forget to remove some metal from the 2 alignment dowels before you test assemble the carb or you will bend the cover perhaps crack it.

[attachment=37008:Sanding_...de_rails.jpg]

Once I have removed enough material to tighten up the throttle piston fit I the do a few passes on 400 wet or dry with the cover. This will give back some clearance if you remove too much and smoothen out the guides. Be sure to test fit throttle piston using the gasket to duplicate the torsional stress played on the cover.

[attachment=37009:good_pin___spacer.jpg][attachment=37011:Throttle...r_spacer.jpg]

Lucky for this guy I had some good pins lying around to replace the worn out originals. I also make and install a nylon shim to better centralize the linkage pull on the throttle piston and get the links out of the previously worn grooves.

[attachment=37012:Out_of_groove.jpg][attachment=37010:Linkage_spacers.jpg]

The upper shim is .060" and the lower shim .030" to augment the shim Mikuni already uses. Use a shim too large on the piston pin and you'll find the c-clip will not go on or bind up.

[attachment=37013:More_central.jpg]

Now you can see that the linkage is pulling the throttle piston from the middle and not side loading the piston or rails plus the links are riding on on a new unused part of the pin.

[attachment=37014:second_groove.jpg]

If you find the throttle piston binding at any point throughout it's excursion and don't see it contacting the slide rails anywhere, check the second groove that the piston rides in, it may need to be widened with a rifling file a couple of thou.

[attachment=37015:Carbs_done_1946.jpg][attachment=37016:Carbs_done.jpg]

Upgrade that corroded Mikuni hardware with good quality stainless and you ready to Rock
 

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Discussion Starter #3
Is that a butterfly throttle or slide?[/b]
It's a slide throttle. Mikuni calls it a slide and BRP calls it a throttle piston.
 

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Great article, doc!

My slides have the coating pretty well scratched off at this point. Could slides with the coating worn off be re-coated with the same material? Is there a better material that might be used in it's place? I've noticed the side thrust on the slides when pulling them up before, but never thought to try and correct it.
 

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Discussion Starter #6
How can you tell if the slides are damaged?[/b]
Look at the first photo... their damaged. The round slide (VM) Mikunis usually have chrome plating peeling off the throttle slides and a very loose fit in their bores.
 

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Ok, yeah i have VM, mine fit pretty tight so they should be good. Ill take a pic anyways to let you see.
 

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Discussion Starter #9
Great article, doc!

My slides have the coating pretty well scratched off at this point. Could slides with the coating worn off be re-coated with the same material? Is there a better material that might be used in it's place? I've noticed the side thrust on the slides when pulling them up before, but never thought to try and correct it.[/b]
I tried Loctite epoxy but it doesn't stay stuck to the slides. Maybe SwainTech has something like skirt coat which would be cost effective. Geomentry and snow/water ingestion seem to be the culprits. As it stands smoothing out the bearing surfaces is all we can do to limit the carnage.
 

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Hi Simon

That wear on the slides is on the air box side, right?

The symptoms sound like the problems I'm having but with 800 miles on her, I doubt that this is my specific problem. I didn't see any scrapes on the air box sides. I couldn't easily remove the carbs enough to inspect and clean all the corrosion that happens inside the intake boots, and also to inspect that side of the slides. The boots didn't have any cracks nor delamination yet as far as I could see.

With all of the wires and hoses on these carbs on the 800 Ptk. they sure don't make it easy for shade tree mechanics.

Jeff
 
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