650/X-2 Tuning 650sx

[dbattersby20]

I've been having trouble tuning my 650sx. The only upgrade I have on it is a mikuni 44sbn. I just replaced the reeds (which I thought might've been the problem). It's very unresponsive. Also, I can go out on it and it will run ok (not great though) and then 10 minutes later it will barely go over 5mph. It just seems to have a mind of its own. I need to check the compression again (I checked at the beginning of the season and it was about 147 in both), but does anyone have the same setup and is able to give me a starting point for pop-off pressure, jet sizes, and where your needles are? I just pulled the plugs and the rear plug is a solid brown color (similar in color to rust) and the front plug is just a little darker brown and a little wet.

 

[H1guy]

I am fairly new to the sport but have been doing a lot of reading and I am an automotive technician by trade which helps but these little motors can be a real pain even being so simple. I have just recently went through some tough times with two ski's I just had thrown my way.
What I would check first is that your have good water flow coming out of the pisser. If it is plugged it could get hot in the short time your riding it and overheat. This could be some of the running issue. If your compression was at 147 at the start of the season and nothing has really changed other than the carb upgrade then it should still be around 147. What was the 44mm off of and was it modified?
With the carb make sure it is good and clean. use compressed air if available when it is apart to blow out the passages and the jets. When setting up the carb it is best to do it once it is at operating temperature, but a good starting point is 1.5 turns out on the high speed and 1.25 on the low speed needles. Then trom there the ski should run.
It might not run great but once it is at operating temp then you can adjust the needles accordingly.

 

[dbattersby20]

Thanks,
I have good water flow coming off the pisser when I'm running it on the hose, And I'm fairly certain when riding and the nose is up I can see a good flow coming out, but i suppose the temp good still be high. I'm not sure what the 44 was off but it isn't modified, and I rebuilt the carb when I put the new jets in. I've been trying to tune it using the idea that if it falls on its face when you pull the throttle it's lean and if it stutters it's rich, but i find a lot of the time it's not really doing either. It feels more like when your in a manual car going up hill in fifth and you hit the gas but nothing happens cause there's not enough power.

 

[WFO Speedracer]

Starting point is 140 main, 115-120 low speed, 2.0 needle and seat with the black spring, shoot for around 20lbs popoff

 

[dbattersby20]

ok, I have a 120 low and 150 main in at the moment and the high speed is pretty much all the way out so ill definitely get a smaller high. I just got my popoff pump in but could only hook it up to the fuel inlet valve and i put my finger over the return valve, but i really need to take off the pump body and test it the real way, i just need to get a tip for the pump.

 

[WFO Speedracer]

Wait the high speed is almost all the way out with a 150 main, that has to be way too rich

 

[dbattersby20]

oops, I meant the opposite of what I said... high speed was almost all the way in, but i think that was also partly cause low-speed was rich (at least I think it was)

 

[H1guy]

From my experience when it feels like it is struggling like you said it is usually rich. if it is lean and running it would have plenty of power almost seem like it is running too good, but then you melt down a piston if your too lean. I just had the same issue with my 650 sx after the carb rebuild and found that a buddy that was helping had turned the needles out way too much and I was running rich. It was very boggy and wouldn't get out of the water. After I found this I screwed the needles all the way in. not too tight and turned the low out 1.5 and the high 1.5 and tuned from there. this is also on a ski with just over 105 psi in each cylinder. It is low but still works very well. It pulls my buddy out like nothing and rips pretty good and he is 220lbs or more. it seems like he is bigger every time we hang out.

 

[WFO Speedracer]

You should be able to adjust to out with the needles around 1 turn out, any farther in than tha and you need a smaller jet, any farther out than that you need a larger jet.It is very important to start with the correct needle and seat and spring first as popoff pressure affects the jetting greatly.

 

[dbattersby20]

Thanks, the piece of fuel line coming from the pop-off gauge needs to be connected directly to where the filter is right?

 

[WFO Speedracer]

No assemble the pump side of the carb, put the popoff gauge on the fuel inlet, block off the return, check it with the diaphragm cover removed and then again with it on.