Help with a 750 running away?

[ShreddinSD]

Ski wants to run away…750 sp single sbn44. I built the engine, broke it in now when out of the water its running away. If it’s cold I warm just above idle for 5 seconds then on the third hard BRAP it starts wanting to run away. It will on the fourth. Not sure if it’s heat related but seems like it could be. It was pressure tested at 7# for 10 min with no leaks. I thought it was the base gasket on the carb but it isn’t. Not the pulse line either. It’s getting torn down right now to pressure test again. Any suggestions on methods of finding leaks? Or likely spots? It’s a fresh rebuild with new crank seals

 

[Roseand]

Ski could want to run away simply because the low speed adjusters are turned in too far.

 

[fastgtfairlane]

When you do a leak down, spray the engine down with soapy water and look for air bubbles. If the engine has been sitting for a while before starting, it's possible to have air in the fuel line causing a temporary high idle. That will settle down after the lines and carbs are full of fuel again. Also make sure the one way valve on the tank works properly. This will cause random fuel delivery issues

 

[ShreddinSD]

I will try spraying it down with windex this time. I think I hit the crank seals last time for no bubbles. It was idling well for a little while before I shoot a rev and it starts to want to take off. I’ve got to really give it more than a blip for it to start sucking air.

 

[kevbo]

First, start with checking the fuel line from the bottom of the tank to the carb. Then open the bottom carb adjuster until idle rpm slows down, make sure idle speed is low. At this point it should be impossible to get a diesel runaway. If it still runs away you likely have a leak below the carb.

 

[ShreddinSD]

Does the leak need to be near the carb in order for it to pull fuel? I would think so. I wish I had tried that. I had to stop making noise and tear into it. Im not sure my neighbors like jet skis as much as I do, especially when they’re running away…been holding 8# for 8 min now…just like before. I wanted to test with the carb on by capping it off but was afraid of damaging diaphragms. And I think it would leak at the butterfly shaft. Do I need to test at 10-12 psi? I had to shoot a high rev for it to go

 

[bird]

bro, read what others are telling you. It's not a leak. It's probably your FUEL SUPPLY.

Start at the tank and work to the carb.

 

[ShreddinSD]

Ok. If I have a tiny leak in my fuel pickup it could draw enough fuel to run and rev as it should until it’s revved high, then it will draw enough air through the jets on the carb to run away?

 

[kevbo]

Ok. If I have a tiny leak in my fuel pickup it could draw enough fuel to run and rev as it should until it’s revved high, then it will draw enough air through the jets on the carb to run away?

Air in the feed line will first reduce pump pressure then some will pass through the jets. I use a separate pump and high mounted tank to make sure the carb gets no air. I have burned a piston running low on gas.

 

[ShreddinSD]

You were right, Ive never had the jet turned out this far but it’s not running away. Alright, so I got a length of clear hose and went straight from in the tank to the carb so I’m getting no bubbles at all. I couldn’t get my old lines to leak by blowing or sucking on them but I’m sure I was getting a little fizz in there. I had to open my low jet 4 turns to get it to not want to run away. I think it’s been 1 1/2- 2 turns out for years… Kinda feels like the jet is loose and going to lose seal on the o ring at 4 turns out. Would finger porting have had any effect on my jetting? Do I need to go up a jet size?


135 high
100 low
2.3 n/s
Black spring for 17# popoff

 

[kevbo]

Pull that carb apart and make sure everything is clean and working perfectly. Single carbs need larger jets than duals. Ive never been able to use a pilot smaller than 120 and mains smaller than 150 in a single SBN44. An SBN46 would need larger jets than that on the same engine. I also keep my popoff at least 25psi. Idling at 4 turns out with low popoff is what would happen if the pulse line was clogged.

 

[ShreddinSD]

Huh. I will have to get some bigger jets and see if I’m missing anything. So if I go up from a 135 to a 145 should that put my ls screw at 1 or 2 turns out rather than 4? So I have tension on the spring and seal. Pulse line has been swapped. It pumps fuel well

 

[kevbo]

The bottom screw feeds the lowest hole so it sets low idle mixture, set that out of the water { rich of peak rpm }. The pilot jet feeds the holes above. Size the pilot so it fourcycles around 1/4 throttle in the water.