Diagnosing my no-spark runaway 951

[L. Dean Ficarro]

Here is everything i can think to share about the build and event conditions:

EVENT
1. After idling under load for tuning purposes, i took the ski (98 GSXL) home to flush. Experienced a high 4500 rpm idle on the trailer so i backed off the idle screw. ~3000 rpm
2. Idled on the hose to flush, pulled hose, short-stabbed throttle to push water out and the engine engaged “high-speed jetting range”
3. The revs bounced 4500> 5k> 7k runaway
4. I hit start/stop> checked for a stuck throttle> pulled the plug boots off> it was already in a “hot fire” condition
5. Remembered the fuel shutoff, hit it, causing a shutdown all of that was maybe ~30 seconds

INFO BELOW
BUILD
engine- 951 with a fresh hone and new WSM molykote pistons, well within spec on all “thous” just under 170 psi on both cylinders

carbs- Prok filters w/ outer wear, choke delete and primer install
jetting set to “Ben’s specs” (85 lows and 167 mains with 2.3)
lows at 1 1/2 turn out
highs at almost close to 1/4 turn out
(Idle screw set to 1650 rpm in the water)
New full Mikuni rebuilds, pop-off tested to 10psi

Plugs BR8ES, gaps measured to spec, msd wire and boot

Oil pump calibrated
91 gas + 16.1 oil in tank for break-in period

WHAT HAPPENED and WHERE DO I START?

Bummed out, thanks for anything yall can add

 

[mikidymac]

First off, your idle is too high. The maximum you should ever have a 951 is 2800 out of the water.

Removing the choke plates prevents you from just pulling the choke to kill a runaway ski. Runaway is not uncommon on a 951.

Removing the choke plates and installing aftermarket Proks reduces the vacuum signal to the carbs and makes the dangerously lean 951 even worse.

Your only option is to reduce the idle, if that doesn't work you are going to have to make it richer on the low speed to cool the combustion chamber but this might lead to hesitation.

Here you go from the legend....

"Bill's top 12 causes of runaway 947 engines.

1. Most common reason is out of the water and the idle set just 100 to 200 rpm too high. The confusing part to a 947 newby is that all other 2 stroke Sea doo engine models are set to 3000 rpm out of the water, but a 947 idles perfectly in the water when you set the idle speed at 27-2800 out of the water.

2. Allowing the head to get too hot becuasue it is running without water going through the motor when on the trailer.

3. And some of these next reasons could be a toss up in order, Replacing the carbs onto the engine for whatever reason and setting up the throttle cable adjustment too tight at the carb cable bracket, leaving the throttle plates slightly cracked open.

4. Clogged or partially clogged low speed jets.

5. Low speed adjusters are set too far in, not allowing enough fuel to pass at idle speed.

6. Water in the gas tank that ends up inside the carbs blocking fuel flow through the jets.

7. A poor job of keeping the carb base gaskets aligned when replacing the carbs onto the manifolds equals an air leak.

8. A pin hole or split in the plastic tubing that connects the engine case pressure fitting to the check valve in the rave valve pressure supply line, or a broken check valve in that same line. This allows air to go into the engine during up strokes, leaning out the mixture.

9. One stuck needle valve that will not open.

10. Rust inside the carbs, see #6.

11. A pin hole in the fuel pump pluse line.

12. A rod hanging out the side of the cases ( or just any crack or hole in the cases ). Do not laugh, I have had customers come in saying they needed a "tune up because the motor runs away on them " only to point to that rod hanging half way out of the motor and the lower case half nearly sawed in two.

Bill O'Neal
WCM"

 

[L. Dean Ficarro]

Im on it, gonna go through all of my vacuum and carb stuff and poke around elsewhere if necessary. Im wondering how the engine was drawing enough air to run over 7k with the throttle shut and didnt slow down after i backed the idle screw off to close carbs. That was a wild experience, especially after having it in the water with no idling issues. It was idling around 1700rpm at the time but i took that to be part of the modified 14/19 i put in the pump. I will back the carbs way off and check for leaks before i turn it over again.

Mik, thanks for keeping an eye out for everyone on here. This is my third rebuilt engine ive done purely for the fun of it and first runaway so it was a fairly eye-opening experience. I just so happened to buy a burned up engine with no manifold present so i thought id try to introduce aftermarket air and got a boot in the arse.

Cheers

 

[mikidymac]

I runaway will scare the crap out of anyone.

It will run like a diesel with no spark and almost no fuel when it is running away. The only way to stop it is to make the mixture richer to cool the combustion chamber to stop the auto ignition and the easiest way is with a choke.

Even when a 951 is perfect they can runaway.

If there is one ski I don't recommend modifying it is a 951 as they are so stressed and temperamental bone stock.

 

[L. Dean Ficarro]

I have taken your words to heart so many times before but i was a dummy and had bought the parts to do “Ben’s Mods” three years ago and just so happens i didnt want to buy and wrestle that damn box off. (Again)

Lesson learned!

Would you happen to have a spare black box sitting around? Think ill jet it back down to stock and leave it alone so i can enjoy it.

 

[mikidymac]

Sorry, I actually threw one away last year.

Once you get used to it you can have it off in minutes.

If you need one get one from a DI ski as the have a fitting on top to fog the engine without messing with the box. That’s what I switched to and threw my carb one away.

 

[L. Dean Ficarro]

Sorry, I actually threw one away last year.

Once you get used to it you can have it off in minutes.

If you need one get one from a DI ski as the have a fitting on top to fog the engine without messing with the box. That’s what I switched to and threw my carb one away.

Thanks for everything, Mik

 

[Boyd12]

I may have a Di air box with all of the wonderful plumbing laying around somewhere!

 

[L. Dean Ficarro]

I may have a Di air box with all of the wonderful plumbing laying around somewhere!

You have my attention “0_0”

 

[Boyd12]

You have my attention “0_0”

I will look for it tomorrow and let you know!