Superjet Bogs

I could use some help troubleshooting from the group.

So I finally got Project Boat Anchor out yesterday https://www.x-h2o.com/threads/project-boat-anchor-superjet.193224/. Holy Crap! 6 years has passed.

So to start, fresh engine with JM crank, OEM seals, OEM gaskets, New Reeds, 0.50 over Pro-X pistons bored by Group K, Group K lightened OEM flywheel and bone stock. Engine held 8 psi for hours.

Carbs rebuild with all genuine Mikuni parts, pop-off checked and held pressure when sealed up. New OEM fuel filter and fuel selector with JM premium fuel hose.

All this was done in 2020 and never run.

Ok, so engine fires up and idles great. When cold a couple pumps of the primer which is plumbed into the return line so no air leaks when it fails on the supply lines. Chokes have been removed.
As soon as you go to give it gas in the water it bogs and tries to die, if you give it more throttle it will die. If you can feather the throttle and get it past that 1/8-1/4 throttle bog and ride it out for a couple hundred feet you can get it to clear out and run good. It will run fine until you shut it off. If you snap the throttle when it is up and going it doesn't seem to have much response but doesn't die either. Still breaking in the engine so taking it easy on it.

After a half tank the plugs looked nice and tan. I am pretty sure it is carb related and will be going through them again but just wanted to check with the group.

At least the Old Man was able to finally get some water time on the Boat Anchor and the Kid loved his SN.

Sounds like the low speed screws can be turned out another 1/4 or 1/2 turn. Are you using the complete original airbox?

joeschuit said:

Sounds like the low speed screws can be turned out another 1/4 or 1/2 turn. Are you using the complete original airbox?

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Stock airbox and screens.
Stock jetting and screw settings.

Come on out on those low speed screws about 1 turn to a turn and one half and see what happens , you will probably have to turn the idle screw in some to get it to idle again .

Yamaha's are notorious for sending out their skis running extremely lean , if it doesn't help it's simple enough to put it back where it was.

I have pretty much seen this across the board on 701-760 Yamahas over the years , a lot of the sitdowns won't even start with the choke they are set up so lean , most go from not starting to starting withouthout the choke after the low speed screws are set properly.

WFO Speedracer said:

Come on out on those low speed screws about 1 turn to a turn and one half and see what happens , you will probably have to turn the idle screw in some to get it to idle again .

Yamaha's are notorious for sending out their skis running extremely lean , if it doesn't help it's simple enough to put it back where it was.

I have pretty much seen this across the board on 701-760 Yamahas over the years , a lot of the sitdowns won't even start with the choke they are set up so lean , most go from not starting to starting withouthout the choke after the low speed screws are set properly.

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Thanks I will give that a try.
Do you have any good base screw settings that have been close? All I have is the old superjet manual.
I already pulled the carbs yesterday and will give them the once over first just because they have been sitting for years now.

mikidymac said:

Thanks I will give that a try.
Do you have any good base screw settings that have been close? All I have is the old superjet manual.
I already pulled the carbs yesterday and will give them the once over first just because they have been sitting for years now.

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It is a very good possibility that the main diaphragms are toast then , they turn stiff like cardboard after a number of years if they are not stuck to the carb bodies and have no pinholes in them you can try soaking them in brake fluid, sometimes they will come back to life after that.

Thanks WFO, I will be pulling them apart tonight. I also have new Mikuni kits in the parts bin.
Double check everything.

Put in 75 lows while you are there

waxhead said:

Put in 75 lows while you are there

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Are they really that lean stock?

mikidymac said:

Are they really that lean stock?

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its an emission thing. This will pick up torque a lot

They really are set up that lean , if you go the pilot jet route then do ont turn the low speed screws out , it does not need both.

Thanks everyone.

Pulled them apart and nothing obviously wrong.

Fuel pump held pressure and not leaking.
MAG and PTO both held pressure so no leaking diaphragms and both were soft and still look new.
The 2 little and one big LS transition holes all flow brake cleaner nice and strong through the pilot jet hole.
MAG:
130 main
70 pilot
LS 1 turn
HS 1-1/4 turn
115 spring
1.5 seat
Clean pop at 58 psi and set an held at 10 psi

PTO:
130 main
70 pilot
LS 1 turn
HS 1-3/8
115 spring
1.5 seat
Dirty pop at 58 and set and held at 50

Definitely replacing both needles and seats for peace of mind but nothing I can see that would make it run so bad. I have ridden superjets with lean stock jetting that were nothing bad like this.

you are right a stock superjet goes ok, but a 75 low makes them go better

waxhead said:

you are right a stock superjet goes ok, but a 75 low makes them go better

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Understood but mine doesn’t really go at all unless you can work past the problem.

Hit the start button and it fires up and idles, get to 1/8-1/4 throttle and it dies, give it more throttle and it dies. Now if you can hold it at 1/8 throttle and work the rpm’s up slowly eventually it will take off and run normally.

Anything electrical that could cause this?

I was chasing a really bad misfire in the beginning. Swapped out coil and CDI with known good and it cleaned up then put originals back and again fine so I don’t know.

Once you get it past the first long bog it actually runs good on and off throttle and at different rpm’s so it’s strange and I expected to find something in the carbs.

Make sure you blow out all the jets and the carb circuits

waxhead said:

Make sure you blow out all the jets and the carb circuits

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Already done with brake cleaner and compressed air. Been rebuilding these carbs for 30 years. Never ran into one running like this.

mikidymac said:

Already done with brake cleaner and compressed air. Been rebuilding these carbs for 30 years. Never ran into one running like this.

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All good, mate — I wasn’t trying to imply you didn’t know.

just making sure the forgotten and often overseen has not been forgotten and overseen .... fuel cap seal ok ? tank vent ok ? .... when it runs u tried feeding mopre fuel with primer ? what happens

How did the diaphragms look, I replace mine every 2 years. Also check to be sure your fuel lines are not sucking air, have a tight connection to the hoses.

waxhead said:

All good, mate — I wasn’t trying to imply you didn’t know.

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No offense at all Wax. Just saying that I think I have checked most things.