Carb Adjustment Screw Backed Out of 787

[SeaDooDoo90]

Just took my '97 SPX out for the first time this season. It ran like a bat out of hell for 30 seconds, then all of a sudden bogged down and wouldn't rev past 1500 RPM.

I unbolted the carbs to examine and came to learn the low-speed adjuster screw on my PTO carb had come completely unscrewed and backed out of the carb body. It and the associated hardware is now lost in the abyss of my hull so I've got a new one on order.

Anybody know of any tips to prevent this going forward? The only thing I can think of is a higher tension spring on the t-handle. The ski had a grand total of only 4-5 hours since I had completed the fuel system overhaul last year, which included rebuilding the carbs and turning to factory settings.

 

[SeaDooDoo90]

Upon closer examination, it seems my issue is the low speed adjuster screws are binding up when being screwed into the carb body. I can only get about 2 complete revolutions on each one before they become extremely difficult to continue screwing and I'm force to use brute force with a screwdriver. I can get them another ~2 turns before they completely stop, but they still don't appear fully seated to me.

This explains why I lost my adjuster screw earlier, it only had about 1 to 1.5 worth of twists on it.

It almost feels like they're cross-threaded, but I'm unclear how that's possible. The correct adjuster screws were installed when I bought the ski last year.

 

[burtshaver2021]

Upon closer examination, it seems my issue is the low speed adjuster screws are binding up when being screwed into the carb body. I can only get about 2 complete revolutions on each one before they become extremely difficult to continue screwing and I'm force to use brute force with a screwdriver. I can get them another ~2 turns before they completely stop, but they still don't appear fully seated to me.

This explains why I lost my adjuster screw earlier, it only had about 1 to 1.5 worth of twists on it.

It almost feels like they're cross-threaded, but I'm unclear how that's possible. The correct adjuster screws were installed when I bought the ski last year.

I would get a new carb at this point, shouldn’t have to use any tools to screw them in, and I’ve never had one back out, I think you may be right, that they are cross threaded

 

[mikidymac]

Yep. Carbs are trash if you have to force the screws in.

 

[SeaDooDoo90]

I'm going to remove them today and determine if both need replacement or just the PTO side.

I'm also first going to attempt to salvage with a thread tap. Does anybody know the size/pitch on the low speed?

I recall on Mikuni, one adjuster is fine thread and the other coarse.

 

[POPPS]

Take out the screws,, carefully clean the threads on a wire brush,
Take carb cleaner and of course an air pistol, spray the inside and spray repeatedly,
This usually works for me,
I put grease on the threads as my final move

 

[SeaDooDoo90]

Take out the screws,, carefully clean the threads on a wire brush,
Take carb cleaner and of course an air pistol, spray the inside and spray repeatedly,
This usually works for me,
I put grease on the threads as my final move

My low-speed adjuster screws were brand new (ouch, they might already be ruined) so I know they were not dirty and the cause of binding.

A previous "mechanic" of this ski was comatose. I've found several instances of shoddy or downright idiotic workmanship, so it tracks he managed to fart up something as simple as an adjuster screw too.

Like I said, I'm going to attempt to chase the threads in the carb and thoroughly blast with compressed air. If that doesn't do it, it's on to fleabay or Westside powersports for a used set.

 

[SeaDooDoo90]

More progress on the carb revival and an interesting discovery. The PO of this ski changed the jetting from a 142.5 to 132.5 main. This makes sense since I bought it locally in the Reno area and he rode out at Pyramid Lake which is 3800 ft elevation. But, I typically ride my ski at around 1600 ft elevation so I want to go back to the stock setup.

He also changed the needle/seat from 1.5 to 2.0 which brings me to my first question: why would you lower the main jet, but increase the needle/seat size?

Second: I am unclear on how needle/seat sizing works. Are the needle sizes universal, or do you need to match it to the seat size? My understanding with Keihin is they're universal, is Mikuni different?

Third: I have some spare Mikuni 1.5 needle/seats I removed from the carbs on a 2018 SuperJet. The ski had under 20 hours, they still look new. Any reason I shouldn't use those in my SPX's carbs to revert back to the stock configuration?

 

[SeaDooDoo90]

Anybody?? These are softballs for the vets!

 

[mikidymac]

If running the stock airbox you need to go back to all stock settings. I would just do new needles and seats to be safe.

We run our skis at 6,000’ on stock jetting.

 

[SeaDooDoo90]

I ordered the new 1.5 needles (already had some new 1.5 seats on hand). For anyone wondering, the thread size/pitch for the adjusters:

Low Speed: M6 x 0.75 (fine)
High Speed: M5 x 0.80 (coarse)

I chased the threads and the low-speeds are turning night and day better. I think they were crudded up with mineral deposits/salt.

Back to my original question - does the needle size correspond to seat size or is the needle universal?