Woking on an SXi Pro OEM carb fix

It is apparent from what I've heard and seen first hand, that there's a definite problem with throttle shaft cracking on the OEM SXi Pro Mikuni BN40-38 carbs.

In my opinion, the problem is due to the throttle valve clamping screws being poorly 'staked' (screw threads deformed after installation to prevent the screws from backing out) by Mikuni manufacturing. Due to vibration and thermal cycling, the screws tend to loosen over time, allowing throttle plate movement. If the throttle plate loosens enough to bind in the throttle bore, the split brass throttle shaft will eventually crack from fatigue.

In some instances, as has been posted in this group, loose throttle plate screws have been inhaled by the engine, which is obviously a 'bad thing'.

I have recently been looking for a set of SXi Pro carbs for an acquaintance who experienced a broken throttle shaft, but in light of what appears to be a typical flaw, fixing the carbs appears to be a better choice than just bolting on another set of stock carbs that stand a good chance of having the same problem.

In my personal situation, the front carb throttle plate screws were loose, allowing the carb throttle plate to float around in the carb bore, which resulted in the split brass throttle shaft cracking, probably due to the unclamped plate binding in the carb bore.

The throttle plate showed obvious signs of wear and binding, and it was only a matter of time before one of the plate screws ended up going through my engine.

From a performance standpoint, the OEM Mikuni 'I' carbs work extremely well with a few tuning tweaks, but I also feel that a throttle shaft upgrade would be a prudent mod for any SXi Pro owner running the OEM carbs. At the very least, inspect your carbs, and pay very close attention to the throttle plate screws. Make sure that they are tight, and the screws have been staked to prevent loosening.

I am working with Jetinetics to offer a cure for this problem, since Jetinetics already offers replacement stainless steel throttle shafts for most Mikuni carbs, and my intention is to provide Jetinetics with the necessary information to offer a 'kit' to install SS throttle shafts in place of the fracture prone original Mikuni parts.

Current SS Mikuni throttle shafts offered by Jetinetics can be found here: http://www.jetinetics.com/

(View the online product catalog 'Jetinetics Accessories' page)

Dan 'Mikuni gave us the shaft' DePardo

Just fixing the typo in the Subject header...

Dan 'Wok and Woll Typo' DePardo

The shaft is not the problem, the screws are. The shaft you sell is sweet, but are you going trust Joe Public to properly secure the butterfly screws? You have lawyers, right? Good luck.

Hi Ronald;

I think it's both. The OEM SXi Pro front carb throttle shaft is very long (~15 cm), and driving both carb springs significantly flexes the split brass shaft, which helps to work the marginally staked screws loose over time.

Have the failures you've seen only involved the front carb?

Uh, I don't sell the stainless shafts, Jetinetics does. I don't have their parts in my hand yet, so I don't know comes in the kit, or what I'll have to do to make it work with the SXi Pro SBN 40-38s.

However, concerning securing the screws, two words: brass screws

I'm going to recommend that brass throttle plate hardware be supplied, rather than the difficult to stake steel hardware that Mikuni uses.

In my opinion the original Mikuni shafts should have been stainless steel and the hardware brass, rather than the other way around!

Automotive-style brass throttle plate screws are very easy to thoroughly stake so that they will not vibrate loose. Loctite 271 would be good additional insurance.

Dan 'Carburetion Frustration' DePardo

The simple fix is to a) mount the linkage between the carbs rather than on one end or b) reduce the tension on the shaft from the springs.

On my 98 sxi pro i had 2 stock intake manifolds break. They both broke in the same place. The spot where the threads are on the manifold breaks off. One of the intake maniflolds that broke caused my motor to seize. I had a enough of that so time for some new carbs. I got a set of worx intake manifolds and a set of unmodified brand new dual 44sbn carbs. Bolt all of this on my boat and works great. Two months later iam blasting across the lake and the motor stops. Oh shit what was that. Motor could not of seized i know my jetting was a tad on the fat side that day. So i hit the start button and the motor fires up and i head staight back to the trailer. I pulled out the spark plugs and one of the plugs is smashed up from something going through the motor. I pulled the head off and the dome is all pitted up from something going through the motor and of course the pistion was seized. I look at the butterflys in the carbs. All four threads are in the butterflys. So now iam thinking crank has went out. OH SHIT. It cannt be. Motor does not have that much time on it. So i load up and go home. Pissed off course. I get home. Rip off the cylinder and look at the crank. Crank looks fine. Now iam lost. What went through the motor to smash the spark plug and the head. I look at the carbs again. All four threads are through shaft of the butterflys so the bolts did not back out. So i fip the carbs over and look some more. Holly SHIT. On one of the carbs the bolt head of the screw that holds on the butterfly snapped off. The threads were still going through the shaft but the head of the screw was gone. Holly shit. Lucky me. So i rebuilt motor and now have a set of umodified 46sbn carbs on the boat. Two months later iam blasting across the lake again and my boat starts to miss fire. What the shit. Put in new spark plugs. Head back out on the water and the boat still miss fires. What the shit. Load up go home. Check plugs and compression. Both are fine. Must be electrical. So i pull the flywheel cover off. HOLLY SHIT. The hub on my light weight charging jetinetics flywheel is broke in half. Flywheel was barely on crankshaft. I was lucky enough that it did not ruin my startor plate. So i call jetinetics. They are no longer in the flywheel business. You are SOL pal. So i now have a 350.00 paper weight. Now iam pissed. So i bolt on the stock flywheel and hit the water. Iam now blasting across the lake again with the biggest grin on my face and saying to myself this jet skiing thing is AWESOME. later

The SXI PRO Story:

I think you need to keep this thread on track. Some of the issues you described do not pertain to this thread.

You had to turn the carbs upside down to see the screw heads? You can see them from the top so something must be awry on your carbs.

As for the flywheel, how are we to know that you properly main- tained it to prevent any problems?

Hi Mark; Yow. You've had more than your share of 'luck' that's for sure!

Apparently this is not unusual, but looking at the casting, I'm also not surprised. It could be casting flaws, or improper drilling and tapping, but the most likely the cause is over torquing the carb mounting hardware.

While I'm not saying that you were at fault, one thing anyone working on an 'I' carbed SXi Pro needs to keep in mind is that although the 'I' carb through bolts are the same size as 650/750 head and exhaust manifold studs, they should only be torqued to 69 in/lb.

(annoying hardware failures snipped)

(laughing) I gotta admire your attitude man!

Dan 'Better luck to you in 2001' DePardo

I wonder if the fact that your motor stopped suddenly caused enough torsional stress on the flywheel to crack it. Just a thought.