Thought I'd ask since there are quite a few who have done the conversion, what carburetor setup are you using and jetting.

Please include your Engine size
Carb type
jetting and adjustments
comments

I think this thread may help many people, those who have a setup which isn't dialed in just right, and those who may be thinking of doing a conversion.
I am dropping in a 670 with a FARR mount and pipe, putting in a civic radiator and a spun aluminum gas tank. one step at a time but I have a feeling long travel will be needed next.

583 TM 38s, I have to check on the jetting specs. I modified a ski Doo 538 pipe and run dual Honda 450R air filters with thick oil, no air box.

I will be putting a 583 in one of mine soon. Hopefully this thread has a lot of reply’s.

What did you do for a manifold to get the TM's to clear the trans? pictures?

I mounted the Engine high enough to clear. I know everyone has an opinion about raising the Engine and COG but I've had no issues, never rolled, drive it very hard in eastern USA aka rock land and she runs great.

I was expecting more to chime in, anyone running WC carbs with good results?

Ive been running my 98 670HO in the pilot with either Mikuni BN38I's or BN40I's. I can post my jetting once i look up my notes. Im fairly happy with the watercraft carbs. Once you get them dialed in they perform great, but can be a little more temperamental to get dialed in. Ive had 4 issues with them over the last 4 years or so.

The biggest one is that the intake manifolds will crack if you don't brace them. On the Ski, they had a brace that ran from the head down to the top of the carbs, there is no prevision for this on the sled engines so you need to come up with your own solution. I bent a thin piece of aluminum and ran it from the top of the carbs to the water neck outlet. I have not had a broken intake manifold since i have done this.

The throttle shafts on these BN38i's runs back to front with a small linkage connecting both carbs. The linkage has cheap plastic ends that will eventually fall off the stud and you will constantly have to jump out and snap it back on. They were never made to be in the dirt so they will wear really fast. I built a small linkage with hiem joints and have had better luck, but the vibration still kills them fast. Im still trying to come up with a better solution for this, but might abandon the 670 before i come up with something else.

The stock seadoo choke works poor at best. Remove it and get a primer kit. works 100x better. But there is a thin line between flooded and running, it takes some getting used to know how much your Engine wants, but onces you know it will take right off.

And finally i would recommend throwing the oil injection in the garbage as fast as you can. I was never able to figure out what exactly the difference was between the Skidoo and Seadoo oil pumps, but one had a larger drive gear than the other. I tried running the seadoo pump and carbs and ended up blowing up the bottom end due to lack of oil. So i tried the Skidoo pump with the gear from the seadoo and it pumped so much oil it barely ran. The seadoo pump must not supply as much oil and i am not sure why. For the rotary valve oil you can just loop the top and bottom oil port making sure you keep it completely full of 2 stroke oil, or Skidoo sells a nice small oil bottle meant just for this. I can get the part number if you are interested.

Very different carbs - the BN vs TM Mikuni. TM has a throttle slide with needle as well as a float in the carburetor, while BN have a butterfly with pop-off regulator without a bowl/float. TM run either gravity or pumped with a vented fuel tank for fuel feed, and BN run closed circuit (Ventless) pump feed only. BN pumps are right on the carbs, while all dirt carbs are either external pump or gravity feed. Most BN38 pairs only have a single pump feeding both carbs where-as larger BN and SBN (Super-BN) carbs will each have their own pump. Jetting specs between BN and TM will be completely different as well. The pilot and main jets are completely different series, and then of course you have your pop-off which is dissimilar to slide/needle on the dirt carbs but act similarly fueling wise. BN carbs can run up-side down (Which is why they are used in JetSkis), without leaking fuel out, but TM/VM need to run right-side up and have overflow-vents. The BN carbs also have a LOW and a HIGH FUEL screw, whereas the TM only have a LOW AIR screw. Very different. I don't see why they would not work except as noted - not the greatest protection from dirt.

Tuning BN/SBN carbs, you want to tune your pilot jet and pop-off with a TARGET of 1~1.25 turns out on the LOW FUEL screws else you're not at the right spot. High screw needs to end up at about 0.75 turns where at ~1.25 turns the Engine 4-strokes from being too rich - so start at 1.25 turns and increase main jetting until it 4-strokes, then close high screw down 1/2 a turn to the 0.75 target.
Screws in more than .5~.75 turn means you need to DECREASE jetting, and screws more than 1.5 turns out means you need to INCREASE jetting.

If you are going to run a set of dual BN38+ carbs, and each carb has a fuel pump - I STONGLY suggest drilling out the restrictive returns within the pump side of the carbs with a 3/16" drill bit, and then run a single restrictor (Mikuni BN style size 60 Main jet) in the return-feed collector (1/4" Y fitting), else you could have an imbalance of pressure and trouble pinning the correct pop-off and ultimately carburetion between the pairs - (Meaning carbs will be 'off' sync fueling-wise from each other.)

Also a known fact in the twin jet-ski world is - a big ass single carb usually works out much better than dual smaller carbs on smaller displacement engines (<700cc) - that is if you can get the single carb manifold. Dual BN38s won't always perform better than a Single SBN46 or SBN48, and then a single carb is 5x easier to tune than duel carbs. Single eliminates throttle linkage issues, synchronization issues, 3x the fuel lines, twice the problems, space problems, intake hook-ups... single carb may make our swap more achievable given 'Frankenstein' installations. Single bigger carb usually hits harder torque-wise in the low to mid range making it ideal for trail and short-track. Maybe go DASA/Full-Spec SBN48, or Full-Spec SBN49/50, or duals for a desert runner I guess.

I suggest running a single SBN46 or larger actually; and I can help with tuning if you need. Given the goal is about a 600cc Engine, there should be no need to run bigger than a single SBN46. You can get a single SBN46mm carb brand new for about $300 from http://www.blowsion.com or if you can find a used mid-90s Mastercraft WetJet Kraze jetski - the whole skis can be found on a trailer for about $500. Always use Genuine Mikuni rebuild kits for BN and SBN carbs.

thanks ZC i'll look into the single carb setup currently it has a pair of WC carbs, are there any markings that tell you exactly what model they are? just popped them open, the pump looks good, jetting is 130 and 60, I'd still like to hear what others are using hopping to be pretty close so it can be just dialed in with the screws

I expected more activity here, these conversions I see are coming more often as pilot and odyssey parts become harder to find.

something to watch for if you buy a watercraft intake system, speaking for the intake/rotary valve cover, you will need to do some clearance work on the WC cover as it hits the crankcase casting at least on the 670.

I don't have anything else at this time, I decided to put in a re-manufactured crank in, ordered today, received gaskets so it's just waiting to go back together. I am concerned with the main bearing oiling on the 670, I was contemplating drilling some new oil passages for the center bearings but it seems this has not been a mod that many have done and most say that it is a solid Engine so I guess I'll trust the engineers at Rotax, I don't understand why they went a different route than the rest of the 2E world.

Where did you buy the crank from? Hope it wasn't SBT.

Got it from MCB Performance, they said it was a remanufactured crank from Ski doo.
I take it you had a bad experience from SBT?

So Nitro, what did you determine to be the proper jetting for the 670 with dual 38's?

Zero, can you elaborate on the carb modification, also I've seen several jetting setups where the PTO side has larger jets than the Mag side, is this something you are aware of?

thought I'd through up a post as the Spambot seems to have taken over the site like a cancer, Hoser where are you??

For whatever reason, jetski manufacturers tended to observe the rear cylinder blowing much more often than the front. Yamaha even took further precaution and increased the combustion chamber 1cc to reduce compression in the rear by 10psi with their 62t 760 Engine. From the factory many ran 1 step larger main jet to help combat a blown rear.

I’ve blown two pto cranks (one main bearing, one bottom bearing) in my skis, but only blew one front piston. Never have I ever blown a mag crank.

I’d say both carbs should be tuned with similar jets but I have seen some engines want more fuel in the rear for whatever reason. If jetting is way off between two carbs I’d suspect popoff pressure need to be equalized.

For a collective return read through these. Use 60 main jet in the collective return line for dual carbs.

http://www.greenhulk.net/forums/showthread.php?t=177671

http://www.novi-tec.com/images/installa ... XRF-SS.pdf

While I cant confirm for a Rotax 670 Engine, I'd say you should start with the following approximate baseline jetting. This should get it running and should only be within 1~2 jetting sizes and pop off for what you need. This jetting spec is close to a 'stock' Yamaha 701 jetski Engine. If you have Engine porting and open intake / exhaust jetting will change obviously.

Low Screws - 1.25 turn (See note above on screws)
Pilot jets - 70 or 72
N/S - 2.0mm
Popoff spring - 95g (Dull silver)
Popoff pressure - 25psi
Main jet - 135 to 140
High screws - 1.5 turns (See note above on screws)

If she loads up and blurlers off the line, raise pop off pressure to a 115g (Gold/Platinum) spring to add throttle response crispness. You can also try decreasing pilot jet to 65 or 68 if that's what the Engine needs. If she dies off the line, raise the pilot jet to a 75 to richen it up.

Call me at 678-778-5533 if you have any tune-in-progress questions.

Thanks for the advise,
I'm pretty much a newbie to rotary valve engines but one thing that to me is an obvious difference between the mag side and the PTO side is the way the rotary valve opens and closes each cylinder, mag side starts opening the port from the bottom and then the top finally closes while the PTO side opens from the top and then the bottom closes, there has got to be some difference in the performance between the 2 cylinders even if all else is the same. At least that is how I would see it, this would be one reason that porting could be different from side to side, another could have to do with the watercraft carbs as one has the pump, it seems that the carb with the pump could have more pressure to work with than the other?