Jetting help needed, 750, dual 38, bpipe

[ProjectSki]

I have a 750 small pin out of a 1994 xir with the stock dual 38 keihin’s, a bpipe, 185psi head and 9/17 skat.

having trouble dialing in the bottom end. It states and idles, but stalls if grab half to full throttle. If I baby it on the way up I can get it going.

the 38’s have 3 jets. low/mid/high. I’ll pull them to get specs tonight. They should be correct, the xir it came out of ran like a clock when it was taken out of service. Carbs were rebuilt a few weeks ago after they were sitting for a few years.

 

[Lugnut]

By chance would you know the pop off on both carbs?

 

[ProjectSki]

I rebuilt both carbs and the pump, but I did not go with genuine Keihin kits..... because they were $200. Probably screwed myself.

 

[kevbo]

Its possible the kit came with stiffer needle springs. If so, the popoff will be higher and make the idle mixture and pilot jets noticeably leaner. You may be able to clip the springs and be back in business. Or install the next size pilot and open the lower screw. The top adjustment screw should be power tuned at full throttle anyway. The pipe will make everything leaner as well so that could be most of the problem. These engines can be very easy to burn up so stay on the rich side of peak power.

 

[ProjectSki]

Thanks, Kevbo. I kept the original springs when doing the rebuild. My popoff guage was broken when i went to check so have another on the way to check. What should pop off be? If its out of spec which springs would i need to purchase?

 

[sturat]

stalling on WOT mash = lean

Turn lows out 1/2 and highs out 1 (from where they are now) and try again. The triple jet keihins have a decent amount of play in them and are actually pretty good carbs.

 

[fastgtfairlane]

i had those carbs on a 750 out of a ssxi with a pipe that i put in a 550. with the stock flame arrestor with 1 spring per side, and the rubber grommets on the sides removed. i ended up at 21psi (lightest spring in kit) and 78 pilots, 50 mids, 132 highs and 1-1.5 turns on each screws.

 

[ProjectSki]

Thanks for the specs and suggestions. Waiting on pop off gauge and I'll pull carbs apart today if I can get to it to find out what jets are in them.

@fastgtfairlane when you say one sprig per side and grommets removed, did you mean you modified the flame arrestor screens for a single screen and no rubber surround? Or are you talking about something else?

 

[fastgtfairlane]

Yes the screens inside the flame arrestor and the rubber trumpets on the sides.

 

[sturat]

What he jetted them for on a 550 is in no way comparable to how you need it for the 750. You're lean. Turn the screws out and try again. Built a similar setup in an sxi pro and we used the 3 jet keihins, 2 years later not a drop in compression and it runs all day without issues. All we did was dial the screws out, no rejetting needed.

 

[fastgtfairlane]

The stock jetting for the sitdown was 72p, 45mid and 130 main with popoff in the 30s. My settings were stock engine with a pipe. Will be in no way lean for an x2.