750sx tuning problems

[TeamGoFast]

I have a 93 750sx I've been working on last summer and trying to finish for this summer, had a blown head gasket last summer which I have fixed now. The main problem I am having is it just has no bottom end punch.
I went out riding today with it, and if you carve it sharp and it kinda cavitates which makes it rev up a bit its like a 4barrel opens up on a car and its a whole new beast and it will just rip if you keep it above 1/2 throttle but then you come to a stop and it just barely moves

Mods:
R&D exhaust
aluminium waterbox (sxi pro)
R&D intake manifold
Ported/smoothed intake & exhaust ports
Mikuni 44 (115 hi 88 low jets)
Primer kit
MSD enhancer
155 compression

 

[tor*p*do]

those jets sound really lean, but I run a Keihin
how do your plugs and piston wash look?

 

[Waternut]

Typically incredibly poor low end performance and then amazingly good performance after the engine reaches around 3-4000 rpms is a perfect indicator that you have an air leak in the engine.

The low jet looks good for a keihin carb but even the high jet is super low. For Mikuni, I'm suprised that thing runs at all. However, that is pure guessing because I haven't spent much time with Mikuni's. They are just usually bigger jets though.

 

[TeamGoFast]

I just looked off of the factory pipe site for the jetting reccomendations to start off with, heres a picture of the plugs. Not to sure what piston wash is though is it like what the cyclinder walls look like...glazed or something?

 

[TeamGoFast]

I just tore into the carb and it has a 110 low and 155 hi for jetting. It has a 2.5 needle valve with the dull silver spring which says 17 psi. Any suggestions on what to do next?

 

[Waternut]

Does adjusting the low speed adjuster do anything?

 

[TeamGoFast]

It did a little but nothing to drastic, it was best at about 2 turns out. I know it should be ALOT better..have a stock sxi and rn superjet to compare to. Just read your sx build thread and saw you blew apart a fiberglass waterbox too, hah just went through that 2 days ago.

 

[Waternut]

Yeah I've blown up a waterbox in my 750 and got a metal one. Then I blew up two in the SJ and got a metal one. I seem to backfire my ski's quite often when trying to start them for the first time. The metal boxes put up with that kind of abuse though.

Sounds like you may need to step up to a 115 low jet. After about 2-2.5 turns out the adjustment screw is out as far as it will go so it may explain why you're getting the best performance at 2 turns out. If a larger jet makes it run even worse than just 2 turns out on the 110 jet, you definitely have an air leak.

 

[TeamGoFast]

Well I rigged up a pressure tester, and it seemed to hold up real well. It held 10psi for just over 15 minutes. I'm going to take it out tomorrow and mess with it a bit more on the lake, and if I have no luck I will order up a jetting kit on Monday.