Stock 750sx stumble/hesistation/dies from idle to wot

[roman]

Hey guys.

I have a 750sx, all stock. Runs great other than its a bitch to cold start, as well as it'll bog and usually die if I idle for a bit and then go WOT. For it to stall, I have to idle first for a bit. If I do a very short idle (less than 1-2sec), and go straight to wot, or if I roll into it WOT, for the most part runs acceptable/fine. Everywhere else in the rpm range it runs fine.
Carb is the keihin CDK2, with verified stock jetting.
Engine has 150/150 in each hole and engine holds 7psi for over 10 min.
High speed screw is 1 screw out. Low speed is like 1 1/4 (where ever I got the highest idle speed on a warm engine). I rebuild the carb.
Reeds are in amazing shape.
All fresh intake gaskets.
New stator last season as my last one wouldn't work when it got hot.

Thanks!

 

[SXIPro]

What are you running for flame arrestors?

 

[roman]

What are you running for flame arrestors?

Factory flame arrestor. Its a rectangular screen that sits inside the airbox on top of the carb. The airbox/water trap that is attached to that airbox though is missing. If you don't understand I can take a photo.

Also I guess i should mention, I run it 40:1 on premium 93 as my Superjet requires 93 and I only want to mix one batch at a time.

 

[roman]

Alright thanks guys. Ill get a photo of the engine bay and intake and build a pop off tester and report back with what the pop off pressure is. Ill report back in a couple days.
Thanks!

 

[SXIPro]

You need an airbox. On my SXI Pro, all I did was remove the 'trumpets' to get more airflow and it ran like crap as you describe, so with the whole thing open like yours....fuggedaboutit.

 

[roman]

Ok so this is what my engine bay looks like. Online it looks like most people have the box I’m missing also deleted. Even if this is causing my stumble, which it very well might be, there has to be a way to tune it out right?

 

[kevbo]

The stock jets are 140 and 75. The main is a little small but can be tuned with the top screw. The pilot is too small and will give hesitation unless the weather and ski are really warm. I would install a 80 pilot jet and adjust the bottom screw so the engine will spool up after idling for one minute. You stock flame arrestor is the best for keeping water out of the engine.

 

[roman]

You seem very knowledgeable. What are your thoughts on the dual carb keihin setup from the Green 750 engine that came stock on 750 sxi (non pro)? I have a set of those from my parts ski. It has reed stuffers too. I have all the parts/brackets to make it work. Would it improve my bottom end performance? That is what I am after with this ski. Over the next winter I'd like to send the jug out to get freestyle ported.

 

[SXIPro]

You seem very knowledgeable. What are your thoughts on the dual carb keihin setup from the Green 750 engine that came stock on 750 sxi (non pro)? I have a set of those from my parts ski. It has reed stuffers too. I have all the parts/brackets to make it work. Would it improve my bottom end performance? That is what I am after with this ski. Over the next winter I'd like to send the jug out to get freestyle ported.

If you are considering trying to change your ski into a freestyle ski, I'd advise against it. Been there, done that, spent a ton of money and a lot of time for limited results. You'd be better off selling it and buying a SuperJet, or keeping it for recreational riding and running buoys and buying an SJ for freestyle.

 

[roman]

If you are considering trying to change your ski into a freestyle ski, I'd advise against it. Been there, done that, spent a ton of money and a lot of time for limited results. You'd be better off selling it and buying a SuperJet, or keeping it for recreational riding and running buoys and buying an SJ for freestyle.

The Kawi is kind of my girlfriends ski. Our other ski is a 09 SJ with all the bolt ons and tray work for freestyle. They are both sort of recreational use I guess. We both like riding both and switching through the day. The Kawi is more fun to slide around I find. It needs more bottom end though. We don't do much in terms of extended high speed riding, and I dont know of any buoys in the part of Michigan we are in. We just go around, rip donuts, do submarines and try other tricks. I just figured if i rebuild the top end, I might aswell get a little work done for more power while its apart. The dual carb setup I have sitting around right now. If it works better Id switch to that.
For now though I want to make this run perfect before I do anything else, including swapping over my SXI cdi.

 

[kevbo]

You seem very knowledgeable. What are your thoughts on the dual carb keihin setup from the Green 750 engine that came stock on 750 sxi (non pro)? I have a set of those from my parts ski. It has reed stuffers too. I have all the parts/brackets to make it work. Would it improve my bottom end performance? That is what I am after with this ski. Over the next winter I'd like to send the jug out to get freestyle ported.

Those Mikuni 40 dual carbs with some port work will wake it up a little with the stock pipe. It can go plenty fast with a good aftermarket pipe regardless. But ,,, the old 750 is not any kind of a freestyle ski. I like them for chasing big boat wakes because they land smooth. They are also great for running the old style slalom course on smooth water. A 3738 CDI has a better ignition curve. I recommend concentrating on the maintenance items first, it probably needs a lot.

 

[roman]

So I never did update this but I will now.
Since I posted this thread, I turned out the extra 1/4 on the low side like you said. Runs so much better now. I might try another 1/8th out and see if anything improves.
Also since then, I bought a 1994 750 ss xi. I think the plan over the winter is going to be to refresh the top-end, and swap the head and carbs from the xi over to the sx. Dont care about what power the sit down makes as its just for friends to ride.