I picked up a set of tm38-133 flat slide carbs for cheap. I have read short bits on using flat slide here but nothing of any detail.
Has anyone used these carbs and if so what jets did you get to work. Looking for a starting point.
I have chambers j&R I plan on using, along with pods The engine was recently rebuilt.
The carbs were off a rotax 780 triple snow machine.
Those are big carbs, if you don't have pretty hard porting. Bigger carbs are used mainly for much higher RPM. You will find, temp changes have a very big effect, and you will need to learn finesse rolling on the throttle not to bog out the motor......
TM38's are pretty large but should work well if it's ported and you plan on pulling big revs.
What's in them at the moment?. I'd start with what you have because you have it, but other than that i'd say
Main Jet : #350 could end up as small as 280 or as large as 450. stock are 370 and 380
Pilot: stock #45 to start with
Float valves: Must be 3.3 or 3.5 unless you run a pump. Sleds use 1.5 with pumps
Needle: start in the second slot down. You will probably drop the needle though
Needle jet: 389 series Q-4. They typically come with Q-8 and slim (rich needle)
Reed valve sleds are typically set up richer at the bottom end than a piston ported motor mainly because they don't have the reversion that richens up a piston ported motor.
780 was rated at 156HP and made about 4mpg as I understand it.
Yeah I figure i will have to go along way down on the jet sizes. I will be opening them up this weekend to take a look and see whats in there.
Thanks for the responses. i will keep you informed as i go along.
The float valves are a first on the list "Float valves: Must be 3.3 or 3.5"
That's a trap for young players. We think about main jets and needle jets and we rarely think to check float valves. Most snowmobiles use a simple pump operated by crankcase pulses to pressure feed fuel, so they come with 1.5mm diameter needle and seat assembly. On a bike with only a few inches of gravity to move fuel, we need much larger float valve seats.
Snowmobiles are jetted for cold conditions so they tend to have large jets. If that one was ridden at high altitudes the jets may be relatively small. The other thing to keep in mind is that gas velocity through the carb on a full throttle 7800cc sled turning 8500 - 900 rpm making 156hp is far higher than part throttle conditions on one of our fat old ladies.
It is quite possible that the carbs will need surprisingly large jets. You will only know when you fire it up and get it jetted. It will also probably be a little susceptible to change in atmospheric conditions.
Good luck and please let us know what you find.
Olav Aaen in WI is the sled carb expert and has alternate needles and slides.
