Low Speed Operation on 951 Engine

[dean thompson]

I took my rebuilt 951 engine out for a second time today. It runs great at high speed operation (>3500 or so), but at idle and low throttle it doesn't seem to run so good (to me). The ski has new pistons, seals, etc. The fuel lines were all changed. Two brand new Mikuni carb kits were put in each carb after they were throughly cleaned. The carbs synced and set at 1/4 and 2. I previously thought a rich mixture was causing this because I was premixed and running the injection pump, however, it did the same thing running straight gas with a new set of plugs.

Here is what it does. At low speed operation the ski idle is rough to me (1500 rpm on water). It almost seems as though it is missing (from a timing standpoint). It feels almost like it is lurching at low speed. Other than that, the ski runs fine. There is no bogging down and it comes off the line very fast with no hesitation.

Is this just a normal 951 operating mode, or can you give me pointers on what to look for? One thing that is different on this ski - my mechanic suggested I run BR9ES plugs versus the BR8ES plugs recommended by Seadoo. I am still gapping them at 0.024 in. Any help is appreciated (as usual).

[ June 07, 2003, 02:18 PM: Message edited by: dean thompson ]

[sea doontcha think]

Dean,
Sorry about the double quote.
My jetski had BR9ES plugs in it instead of BR8ES when I purchased it.
Did your mechanic say as to why he recomended the colder plug instead of factory BR8ES?
Thanks in advance,
Dan

[dean thompson]

Here are some plug pictures (my camera won't do close pics well at all). With only one hour of operating and some idle washing at the end. There is a lot of soot on the electrode and the grounding tip. The outer rim is clean. I did hit quite a few no-wake zones for idle (I know this is not the best way to read plugs). Seems to support too cold of a plug? I think I found out he recommends this because he races professionally and they work better at constant WOT operation. Maybe they are not the best for rec operator though????

http://home.houston.rr.com/thompsdwp...Pics%20004.jpg

[Colin GSXL]

When you rebuilt the carbs were new needles and seats fitted? Could be those leaking making it rich at idle. Plugs do look rich.

[dean thompson]

I inspected them and reused them. I wish I had a pop-off tester now. It is simply an unknown. I am going to follow a Group K carb tuning guide to set my low speed screw tomorrow. Unless Bill or SBT tech can steer me in the right direction. They have never let me down before.

I did remove the second plug and replace both with 8ES's. Interestingly, the mag cylinder appears to be running much better than the PTO. It had some dark (a little too dark) color on the rim. The PTO had no color what so ever (rich, right?).

[Bill O'Neal]

Why do you have the low speed adjusters out 2 turns, when the service manual says to set them at 1.25 turns out? That alone is enough to cuase your problems. Those low speed adjusters are real sensitive to changes......

[dean thompson]

Bill,

My manual for the 98 GSX-L states that the low speed screws should be at 2 turns out. Are you using/recommending 1.25 for the screws on 951's.

I am wondering if I have a leaking needle on the PTO carb because I cannot explain the reason for the plug difference between cylinders (reguardless of how I did the chops).

[ June 09, 2003, 06:49 PM: Message edited by: dean thompson ]

[Technical Support]

I don't know what 'manual' you are reading but it's not an OEM Service Manual.

[Bill O'Neal]

Is it a grey top GSXL? They had tiny primary jets (actually too small) and SD put bigger jets into the 1998 red top and other models with the 951 motors.
If you think you may have leaky needle valves, you probably do.
Is a check valve stuck open in the accellerator nozzels, allowing fuel to be siphoned from them?

[dean thompson]

Bill,

It is an all red GSX-L. It had 82.5 pilots in the ski. Thanks for the tips to check on. My Seadoo Factory manual Section 13, Subsection 2 (Technical Data) for a GSX Limited does specify 2 +- 1/4 turns for the low speed jets and 0 turns for the high speed jets. That is exactly where they were set when I removed them when re-building the carbs. Another symptom is that I cannot seem to keep the rpms stable at most any LSS setting. I haven't tried the 1-1/4 setting yet, though.

Thanks again.

[Bill O'Neal]

My guess is you need some new neeldle valves, AKA, needles and seats. Sounds like yours leak.
Your boat came with the larger LSJ's that are now standard on all 951's.

[ June 09, 2003, 10:24 PM: Message edited by: Bill O'Neal ]

[dean thompson]

Just out of curiosity, what size are they installing now? Is that where the 1-1/4 LSS setting is coming from as well (newer models)?

[Bill O'Neal]

82.5 is the standard size now.
Hey, they used to put 80's in there on Grey tops, they siezed a bunch of motors too. They also had smaller main jets with 160 mag and 162.5 pto mains. 165's are standard now. Get this, the grey tops had a 130 gram spring too. It called for 1.75 turns out on the LS adjusters, zero on the HS adjusters. Then they had a stupid solenoid with hoses connected to the diaphram covers to lean it out further. Then they called for about 3.5-4 thou piston to clyinder wall clearances. No wonder they lost motors left and right.........
In the 1998 regular production run, they dumped all that junk and set them up pretty much the way they are now.

[ June 10, 2003, 09:36 AM: Message edited by: Bill O'Neal ]