Thoughts on getting 951 fueling dialed in? Typical 165/95 jets no go

[Lt1350]

I have been working on the jetting and trying to get the top speed up on my 1998 951 Xp and have a few questions for group veterans.

The details are: fresh motor with +1.5mm pistons, .040 decked off top of cylinder, light case and cylinder porting, .8mm base gasket, stock head (I recut the squish band to 13.5 degrees and lapped in the gasket surface for flatness) chambers measure 51cc's, squish is .060" and both cylinders pump exactly 160 psi, carb chokes removed and a nice radius machined at air entry under proks. Running 50/50 93 pump gas and 112, so its just over 100 octane, new fuel lines. Solas Concord 15/20 with new wear ring. The hull and all components are all stock and in good condition.
I should mention this is with no accelerator pump, new 2.0 needle and seats, and the carbs have been pressure tested and are sealed up good. Using a new set of BR8ES plugs every time out.

Started out with 165 mains and 95 pilot with pop off @21. Machine seemed very lean off bottom with a bad hesitation and very rich on top. Would only turn 6400 and gps 58. Plugs were black.

Next I dropped to 160 mains and lowered pop off to 17 with 95 low jets. Was a little better off bottom and alot better up top. Turned 6800 and gps 62. Plugs looked alot better and were dark brown.

Currently tried 160 mains and 100 low jets @17 pop off. It's starting to get alot better off the bottom and top end was the same. Turned 6800 and gps 62 and the Plugs look almost black again. Overall this is definitely the best it has ran. Still feels like it should pull a little harder and turn more rpm up top.

With the current set up If I open the low adjusters 1.5 or more turns, the bottom end hesitation is gone and it's getting powerful and responsive. But after a 15 minute no wake zone it gets loaded up pretty bad. With the low adjusters open 1 turn it idles better and just seems cleaner and don't seem loaded up after the no wake zone, but then it has a lean hesitation and is not as responsive when riding in open water again.

I feel like it still needs larger low jets and smaller mains. It seems to REALLY like the fuel off the bottom.

Here are my thoughts and questions/concerns. If I go up a size or 2 on the low jets I should be able to close the low speed adjuster more and still not have that lean hesitation. Will that make it better in the no wake zone since the adjusters are closed more or will it load up worse since the jets are bigger?
Does dropping to a 157.5 main jet seem like a bad idea? That is alot leaner than most people seem to run on here! But the plugs look like it's still really rich and I am thinking that's why it won't turn more than 6800.
Lastly, how do I know if I need a pop off adjustment? It ran better when lowered to 17 but at that point the jetting was so far off I am now wondering if it is a little too low?

What are you guys' thoughts? Tia.

 

[etemplet]

You are doing a lot of things at once which can be confusing. The pop-off also affects high speed to a degree. For me your Pop-Off should be around 19. Low speed jets may be a bit on the large side already so I wouldn't increase those. Your 17psi pop-off is gonna make the low speed rich and may be the root cause of your loading up after a long idle.

I don't remember reading that anyone went as large as you are on the low speeds. I thought 85 was a lot when removing the accelerator pump. I was told to go up 5 on the low speed jet when removing the accelerator pump but if it works it works. You don't want to mess with too many circuits at once anyway.

I'd try going to 157.5 on the high speed jets. The high speed adjusters can give you a bit more fuel if you need to tweak it. Remember also that temperatures are cooling off so the skis are gonna be rich as opposed to hot summer days.

1. make sure the carb doesn't have any leaks.
2. adjust pop-off to 19psi.
3 drop the main jet size as you suggest but simply raising the pop-off could help you lean that out some.
4 I don't like that 95 low speed jet but not being around the ski it is difficult to make recommendations.

I'm not a guru but even they have trouble sometimes. Ha ha Good Luck !!

 

[POPPS]

The mikuni manual wants you to run with the highest popoff possible

 

[etemplet]

When I said 19psi pop-off I was merely relating what a GURU told me about the 951 set up. Matt Braley used to be a racer years ago. Told me he had the fastest RX in the country. I won't argue that point but he knows his stuff. I wish he'd post here again. He also recommended going up 5 on the low speed when removing the accelerator pump but I see people generally go up 10.

My experience on the 951 carb set up with the correct needle seats and spring the pop-off ends up being around 23psi which is in range. Takes some finagling to get it to 19. 21 is not too troublesome.