Surging at full throttle Yamaha SJ650 superjet 6m6

[FIREotter]

Hi, I have a SN650 superjet that I water tested over the weekend with a fuelling issue (all testing has been in the water).

I've been playing around with the carb, set the high and low dials both to 1,1/4 turns out to start which I believe are stock.

It's starting well, and I can ramp up the throttle until it's reving high, but it I pull it quickly it bogs once I open the throttle fully. If the throttle is pulled wide open from idle it will surge rapidly (rev off rev off rev off under a second appart.) while the throttle is held open.

I believe it's a fuel issue as if I add a squirt of fuel from the primer it'll stop surging and get up to full throttle.

I tried adjusting first the high speed way out to richen it up (all they way out another 2 turns eventually) and it made no difference other than changing the frequency of the surging. I also turned up the low speed until it was difficult to start, neither fixed the issue.

My next step will be giving the carb a really good clean, the fuel lines all look good, though I'll probably swap over the fuel filter too.

Has anyone else come across this issue. It's a new one for me. Anything else I should be doing on the next test.

Thanks.

 

[WFO Speedracer]

I would pull the carb apart and give it a once over , it could be as simple as a plugged inner fuel filter screen or in the case of a Yamaha , a hardened main diaphragm. Just make sure you use OEM Mikuni parts when you put it back together

 

[Big Kahuna]

Also check to be sure you are not sucking air into the fuel lines. At lower RPM's it will run fine.. higher RRM will loose pressure and your sputtering...

 

[brapperdoodle]

I always mention this to folks who might not know, but it is important to make sure your fuel system is holding pressure and does not have any leaks. That pressure is an important part of the fuel delivery system. It’s also very important that your engine does not have any air leaks, both of those things can effect the way the ski performs and rideability. Double check that your 1-way valve is working on the tank also. I don’t necessarily think this is causing your issue, but it is worth noting!

 

[joeschuit]

It's surprising how clogged that inner filter screen can get before it starts choking the ski. I've also heard of the main fuel filters getting clogged as well. Just takes a pinch of fuel to idle and cruise around.

 

[smoofers]

Every response above is great advice and I recommend checking it all. I had a similar issue once and it turned out to be a creased fuel pump check valve diaphragm in the carb.

 

[subnsxr]

happened to me, it was the tiny internal fuel filter, would clog up every few rides, hasn't happened in many years