Can a mikuni carb be tuned (high speed) like a chainsaw (richer until 4 stroking then backed off slightly)?

[777funk]

On a chainsaw, the high speed adjustment is easy, just richen to the point sputtering under load then lean it back a bit to where the engine smooths out.

Does this type of high speed tuning work on a PWC?

I have read the Mikuni carb manual and understand it ok enough, but as a new pwc owner, but long time 2 cycle fixer, I want to relate what I know from past experiences. I have yet to see anything on tuning a jet ski in that way. Maybe it's not possible?

 

[GGuillot]

On a chainsaw, the high speed adjustment is easy, just richen to the point sputtering under load then lean it back a bit to where the engine smooths out.

Does this type of high speed tuning work on a PWC?

I have read the Mikuni carb manual and understand it ok enough, but as a new pwc owner, but long time 2 cycle fixer, I want to relate what I know from past experiences. I have yet to see anything on tuning a jet ski in that way. Maybe it's not possible?

What ski? dual carb? I somewhat do this with the HS needles, I start with each carb at 1/2 out from seated, then turn in 1/8th at a time, then make a run (on the water) to WOT, when I don't pick up any higher RPM with the next tweak in, I back off 1/8th.

On both of my 787 carb skis the final setting ends up being about 1/8 to 1/4 out, with usually the PTO tweaked out a little more than the MAG carb, but neither ski on either MAG or PTO carb do I have them shut at 0.

 

[777funk]

This is a single Mikuni carbed Tigershark (my first PWC bought cheap with a bad crank bearing (due to bad PTO side seal) and bad impeller bearings (both sides). I figured the carb tuning process would be similar since they use the same Mikuni square pump carb. I rebuilt the engine and impeller setup and it now runs great... BUT it's making black spark plugs. So it's obviously rich in the low speed jet, low speed adjuster, pop off pressure, high speed jet, or high speed adjuster. Low speed screw at idle is adjusted ok from what I can tell.

I was really hoping I could get WOT so rich that I could 4 stroke it (richen until sputtering starts); then back it off a bit and keep it at that setting. I could not get it to 4 stroke at all. I assume it's lean enough on the high speed adjustment and high speed jet that this is not possible. This type of adjustment has always been my assurance that a Stihl or Husqvarna isn't going to stick a piston due to a lean condition. I was hoping to do something similar on the jet ski as well if it can be done. Obviously with a low speed jet, LS adjuster, Pop off, HS jet, and HS adjuster screw, there's a lot more there that can foul the plugs. But WOT is always my biggest concern since high RPMs can be the biggest of risks on bearings and pistons melting.

 

[racerxxx]

Seadoo up until around 98 I think ran the highs closed completely. And even then they only open the PTO carb high open a little. In 26 years of playing with seadoos I've never had to stray from the factory settings. I set them on the bench, install, and go. I'd like to think you should be able to do the same with the tiger shark. I quick googled and found SBT had listed the settings. Mind you they set these s little far from the factory so guys aren't out having engines every weekend. If there room for improvement, yes. Are you going to get any more that you'd feel, nope. Is set it to stick and roll with it if it were mine.

https://www.shopsbt.com/forum/showthread.php?t=37670

 

[777funk]

Seadoo up until around 98 I think ran the highs closed completely. And even then they only open the PTO carb high open a little. In 26 years of playing with seadoos I've never had to stray from the factory settings. I set them on the bench, install, and go. I'd like to think you should be able to do the same with the tiger shark. I quick googled and found SBT had listed the settings. Mind you they set these s little far from the factory so guys aren't out having engines every weekend. If there room for improvement, yes. Are you going to get any more that you'd feel, nope. Is set it to stick and roll with it if it were mine.

I like it that they mention snipping off coils from the pop off valve spring vs buying another spring. My kind of 'get the job done' mentality! In my case, pop off was at 22psi (too low) which may be part of my black spark plugs. The low mid range was a bit rich and 4 stroking. I'll have to order a tighter spring. I believe the one in there was bright silver.

The instructions they have say there's not a high speed adjustment. There is a screw on mine marked 'H' as well as a low speed screw so maybe they have the instructions based around a particular year. I will say it doesn't seem like the High speed screw does anything on my machine.

 

[racerxxx]

Yeah, I would NEVER shorten or stretch a spring. Buy the correct one.

 

[777funk]

Yeah, I would NEVER shorten or stretch a spring. Buy the correct one.

In a pinch, I might cut a spring as they mention in that bulletin... But much much better to just buy the right size.

I once used an innertube as a fuel pump diaphragm. Got us through a fun day of fishing. It failed the next day and I ended up limping the boat home using the squeeze bulb as a fuel pump (2 man job). The OEM part made it in the next day and all was back to normal. Sometimes you do what you gotta do.

That said, the fuel mix is too risky a part to mess with anything less than the right parts. I definitely agree with you 100% on just buying the right part.

 

[AKnarrowback]

I agree with the closed HS screw. Not sure on tigersharks, but early DOOs were jetted so they could not be run lean with any carb setting errors with the adjustment screw. The screw was seated for stock jetting and could only richen the mixture.

At sea level in warm air conditions the jetting was good to go. Any rise in alttitude would only richen the mixture. On my 94 XP I have ridden at 18F with the HS screws all the way in without any poor running and still with good plug color.

 

[mikidymac]

Absolutely never cut a Mikuni spring. Use the correct one.

This is from the late great Bill Oneal, the man the legend...
"I do not agree with SBT about cutting springs.

As a racer, I know what happens when you start cutting springs.

If you cut a spring or if a spring is rusty, it will never stay the same over time. It will always get weaker.

Mikuni makes plenty of spring sizes and plenty of needle valve sizes to get any pop off that you desire. You are much better off reading the Mikuni Pop Off Chart provided by Mikuni online in their SBN carburetor handbook, then pick the correct N&S and spring combo to achieve the correct PO pressures that you need.

Keeping in mind that a 2.0 N&S is large enough to feed enough fuel to any stock or limited Sea Doo carbureted engine. The only good reason to use a larger N&S is to get the pop off where you want it, or to feed some monster modified engine for racing. "