4/6 EGT temps?

[Checkmarks]

Entire grounding strap is burned with 370's, wow.  410's would be the next jump I would make. But not before tearing down the carburetors again for hardware inspections. 

370's that sounds mighty suspicious like there must be some other factor playing a problem.  The problem is mighty clear, not enough fuel. 

I would slap on a different set of carburetors.  Four cans of carburetor cleaner?  Check that needle jet is clean and your jet needle is the correct length.  I am thinking the needle is not getting raised enough to allow free flow of the main.  370's cannot be correct.  The inlet needle and seat could also be slowing the flow.  Something is wrong.  How big are the carburetors?  A person could put 600 jets in a 32mm carburetor and the limiting ticket is likely the needle jet and jet needle. 

You mentioned replacing a piston? (new piston on one side other old piston) I could be certain the motor is lean and the previous owner knows there is a problem, hence the monster jet.  The 550 of mine uses a 250 ± jet 38mm carburetors.

When you finely get to see the burn line on the corner you'll say, "no darn way". 

[gixxer6]

The carbs were nasty, but I took my time and got them spotless.  Every passage is free and clear.  I meant to record all of the jet/needle sizes while they were apart, but I forgot.  I did notice that they had 40 pilot jets, not that they have any impact on WOT.  I am quite certain that they are 36mm carbs. 

If the needle is too long, raising it a notch or two should make a noticeable difference.  But I will start by pulling the carbs and verifying that they are clean, then record all of the jet/needle sizes.

[kawhead]

i have a set of 38 mic's on the bench, bored to 39.5, bells cut...[i got them from billy? from ill. off this site over a year ago]..set up for a stock 550...370 mains,42.5 pilot... set up for no airbox,screens on intakes....410s at 9000 rpm on your ltd doesn't sound that far off now imo....

[Checkmarks]

Should 410's get you to proper calibration your spread would look like this:

@ -40°F 455 main  @-20°F 439 main @0°F 429 mains @20°F 413 mains @40°F 407 mains

This is assuming at the same altitude.  More importantly you must infer from the numbers given they are the calculations.  I was aimed for your testing done at 20°F.  It is better understand the spread of jets you need in your pocket.  I use a piece of lexan with the assortment of jets needed for my sled and they stay with that sled. 

Kawhead may have some insight.  Are you trying to run the airbox?  Then the airbox would make Kawhead's 550 very rich because extra vacuum caused from the box sucks more fuel.  I have first hand experience how clean your parts are

On a bonus side if you carbs do adjust the main 90pts you can run all summer (watercross) and winter with the turn of the screw when you are dialed on.  90pts should make the spread even when it is 110°. 

[gixxer6]

The spark plug test was done in my garage at about 40°F.  About 600ft above sea level.  No air box installed, just the freshly cleaned K&N pod filters. 

I should have some time tomorrow night to get the carbs off to record the jet sizes. 

[Checkmarks]

Still some though on your motor.

The snow twister 440 motor put out 75hp.  36mm carburetors with power jet.

You are working on a motor ported, Aaen piped potential to 100hp starting from a very similar 77hp stock 4/6 motor.  When you take the twister carbs and put them on your stock 4/6, I say sure it works, but use the twister air box. 

When you look at 100hp motors, what typical sized carburetor do they run?   Ski doo 521 motor at 95hp out of the box uses 40mm VM Miks. 

Sproket runs the 4/6 motor in his clone shark.  He had his motor dialed in on a dyno @ about 100hp.  I would bet he would share the exact information you need for your sled to include hardware in which Mik.

Sure a person can put a two barrel on a 454, then you cannot expect to put out 500 ponies.  There is some relationship with Hp and Cfm but I have no reins of that horse for two stroke snowmachines. 

I would waste the money and get a hold of some 440 or better mains.  Run the burn procedure with new plug.  If it dulls the entire grounding strap again, dump the carburetors and flush up some 38s.  40mm carbs require a longer throwing throttle but you can adjust your cables to manage the 38's.  Then I would consult Sproket for his final setup.
 

[sprocket]

A lot of good information read so far. Without knowing the correct heights that Aaen did for you is pretty tough to Jet. 36mm sound too small of a carb. I can tell you what set up I had but the conditions are different. 42mm 35 pilots 390 mains bbo 2.5 cut away 7dhs 2nd clip everything matched meaning intake boot to cylinder glued down with 1104 yamabond, my Compression was 180 lbs per cylinder I ran 116 (c-16) race fuel with Klotz R50 @ 50:1 open carbs (no air cleaner or pods). My engine made 109hp @ 9800rpms so it is possible to reach over a 100hp but more than likely Aaen doesn't give you the maximum according to HP he told me years ago he is more conservative then full out race engines I am Not knocking him he is very talented but there is other people that will push the envelope a little more remember I'm racing your trail riding big difference I never once hooked up a probe to my engines I always started hi on my jetting and read my spark plugs and jetted it accordingly. I know for a fact on Dyno specs a stock engine is way less than 77 or 76hp stock more like mid 60s with a mild porting mod pipes 40mm compression @220 per cylinder my engine dyno'ed out at 86hp with 48 foot pounds of torque @ 9350rpms it doesn't sound like much these runs were on 2 different dyno's, I also ran stock timing, coils and Ngkbr9es plugs

[sprocket]

Oh say your estimated horsepower is 90hp by the time it gets to the track it's more like 40-45 hp and it can be more or less depending how things are set up. Now take your carb specs write them down and start testing keep your notes. The cut away tells you how much air will be allowed through the carb. Champ engines only can run a 34mm carb and make stupid hp but now these engines are way different then say the stock vintage engine

[gixxer6]

Yes, there is ALOT of good info here.  I really appreciate the help! 

At this point it may be helpful to point out my intentions for this sled.  I'm not going to racing it at events.  I'm not going to be doing 100 mile per day trail rides either.  I just want a fairly reliable, fairly fast, fun, vintage sled that will give my sons 10yr old MXZ600 fits from 0 to 70. 

That being said, I do have a set of 38's, and a set of 39.5 carbs that I could run.  At this point I just want to make it run without fear of melt down.  I'm willing to give up a few HP that I'd gain from running larger carbs to have the convenience of dial-a-jets. 

[kawhead]

you may be able to put your dail a jets on the 38s, same body from merc fa to invader,[major body casting]    fa twister used adjustable jets, not power, air box was a very large square tube like that sucked air from behind the headlite,its baffle had large holes the size of your finger to stop the big chunks    i'd run some 410s or so just to see what the change is.... and this is a great thread.....

[gixxer6]

Carbs are apart.  They are still spotless. 

Slide: 2.5
Needle: 6DH3 (clip in 2nd from bottom)
Needle Jet:  166 Q-0
Main:  370
Adjustable Main:  1.5 turns out (full rich)
Pilot:  40
Seat:  1.5

I didn't know that there are different sizes needle and seats.  It looks like that is my problem.  The hole in the 1.5 seat is smaller than the hole in the 370 main jet...

[Checkmarks]

Yes,

I am glad you found the issue.

 Q-0 needle jet may end up too rich in the mid range but you will not be lean.  Follows something like this:  When the clip is in position 5 (as deep as the needle can go) you can put in P-8 and move the clip back to the 3rd position (middle slot).  Should it go the other way needing more fuel in the mid range clip position 1 then you can go to Q-2 moving the clip back to the middle position (#3).  See the madness?

After you get that inlet seat changed, I would bet 370 puts you in the very rich range.  Maybe 320 jets, and but it will be rich.  You will get to see the burn line move down grounding electrode and hit home the idea always start rich.

WOT main jetting first then adjust the mid throttle. 

[gixxer6]

What size inlet seat should I get?

[kawhead]

as memory recalls vaders started out with a 1.5 and went to a 1.2 to stop high speed rough surface flooding....

[gixxer6]

I picked up some 1.8's at my local sled shop.  When I told the guy what I wanted and why, he looked at me like I was crazy...