Update for July 25.2010
If you want to make this ski a lot more fun and have much more bottom end, get a piece of schedule 10 stainless tubing which measures 4" I.D. x 1/16" thick wall x 4.5" long (any place that sells metal will cut you a peice, I just happened to have some at work so it was free for me :cool2
and add it between the stinger and the rubber coupler. It makes it into a poor man's B-pipe but totally changes the attitude of the ski. I did this just a few weeks ago and there is absolutely no bottom end bog anymore and no struggling to get up out of the hole. As for the coupler between the stainless pipe and the stinger, I just used a 4" rubber coupler found in the plumbing dept. at places like Rona or Home Depot for about $14.00
I tried using a blue Keihin 42mm carb with this set-up but it sucked horribly. Sucked tons of gas and was crappy on performance. I used the stock jetting, then upped only the main jet to the 160 (the stock pilot being 78) with very minimal improvement. Stick to the stock 38mm Keihin, it has good top end but works at it's best through bottom to mid range. One thing I have found with the engine mods and all the bore out work is that the stock 38mm Keihin does need a slight high speed mixture screw adjustment. I found that the ski runs great but seems to have a lean surge on wide open throttle. This is because of a lack of fuel charge to compensate for the new CFM demands of the engine mods. The carb is not sending a rich enough fuel charge through the high speed adjuster (at stock settings) which will give these side effects. You will just have to try adjusting it a little at a time and running it until the ski sounds like there is no surging. Before you do the adjustments, take it out and listen/feel for the surge, you will feel it happening as the ski will feel like it picks up and holds good top end, but for a split second will drop off just a touch then pick up again, almost as if you just ran over a sand patch. I noticed that with my ski, it happened all the time. I tried a bigger main jet which cured that but made bottom end tuning a pain and used more fuel due to the mixture screw adjustment.
Inside the carb and adjustments: I originally started out with stock jetting and mixture screw settings. I found that slight occasional surge so eventually I tried using a larger pilot and main jets going from the stock size of 75 pilot and 140 main to 85 pilot and 152 main. The results were that the slight surge was cured but the ski became noticeably harder on fuel, was running a bit too rich and was hard to tune in. I put the stock jets back in, cured the bottom end bog while staying with stock mixture screw adjustments at 5/8 out for the high speed adjuster, and 1 1/4 turns out for the low speed adjuster. For the top end surge, going from 5/8 to 3/4 of a turn out on the high speed adjuster should cure it unless your metering diaphragm is dried out and crispy sounding. That will require a bit more tuning until you get a new carb kit in there. When I re-jetted, I also tried a range of different needle valve springs for different pop-off pressures hoping to cure the bottom end bog but found that the stock spring was the best for the stock airbox and flame arrestor set-up even with the over-sized intake and exhaust manifold bores. The only time pop-off really needs to be changed is if you start playing with aftermarket F/A's. This is due to the larger airflow they offer and when more air comes in, the pressures inside the intake manifold are lowered which in turn requires lowering the pop-off pressure to balance the carburetion requirements. But I wanted to try it anyway for the experience points.
I think of all the things I've done to my ski, this is by far the best improvement yet. One thing I forgot to mention, when you put this set-up together, the new stainless pipe will slip over the first little bit of the stinger and the first little bit of the main pipe. To help insulate the new rubber coupler, I pushed the new pipe as far in to the first coupler as I could and locked it down with a gear clamp (you will feel and hear the new pipe butt up against the stock main pipe), then pushed the stinger as far into the new pipe as I could being able to hear and feel the stinger butt up against the new pipe and locked it down with a gear clamp. How this helps to insulate the coupler is by preventing it from having any contact with the exhaust gases (which reach temps of 1200*) and only dealing with the transferred heat through the stainless surfaces of both stock and added in pipes (which only range between 110 and 230* depending on the cooling set-up). I have run this set-up several times now running non-stop through a full tank each time (about 2.5 hours per tank) and the coupler hasn't melted or shown any signs of melting. The next thing to come will be a Mikuni SBN44 carb and intake, I'll keep an update of that when it happens.
August 08.2010
An update is required as I just found out that the stainless tubing I used apparently is not an overly common item. But I still think a steel supply company should have some kicking around somewhere since they are there to supply as many machine shops and outside sources as possible with as much diversity as possible. One alternative which might be more abundant in stock than the stainless is schedule 10 aluminum tubing. From what I understand it is supposed to be the same in dimensions and will save you a small amount of weight. If this material is not available where ever you go looking, or is just ridiculously expensive, the next best and probably cheapest route would be to find someone with an old stock exhaust they want to sell. All you will need is the straight portion that has been welded onto the stinger. Cut that off and add it to your exhaust the same way as mentioned above. The only thing you will need to worry about is butting the extra pipe portion up as tightly as you can to the leading expansion chamber and the same for the stinger to the extra pipe portion. This is because your heat barrier won't be there as it would be with the schedule 10 tubing. You want as much metal on metal contact as possible to prevent those exhaust gases from coming into contact with that new rubber coupler or it might melt down. There is also a chance it will hold out reasonably well it just depends on the internal cooling.
