Dual 44 Mikuni observations

[#ZERO]

I have a stock Yamaha 760 intake set up and I'm trying to get them dialed in and it has the three hole low speed circuit carbs with a fresh rebuild on them.

The jet-ski runs great from a third throttle up to full throttle and will jerk you arms lose but anything below a quarter throttle gives me problems. The engine sputters and pops slightly like its running too rich. I tried a lower pilot jet but it makes the mid range suffer and I installed a 115 gram spring which made it better but the problem is still there. The problem seems to get worse the more I open up the low speed adjuster screws. I think the engine wants more pop off pressure but to get any more I would have to go back to the original 1.5 needle and seat. I'm just curious if anyone has any ideas?

Right now I'm running;

117.5 Pilots
125.0 Mains
2.0 Needle & Seat
95 Gram Spring
¼ Low Speed Screw
¾ Turn High Speed Screw

Here are some of the engine mods.

• Riva Girdle Head 35cc
• Factory B-Pipe with Mod Chamber
• Wiseco 84mm Pistons
• Ported 61x Cylinder
• Ported 62T Case & Epoxy Filled
• 760 Intake Epoxy Filled
• 62T Lightened Flywheel
• MSD Enhancer Ignition
• MSD Ignition Coil
• Prok Air Filters
• Brand New Stock Reeds
• Boysen Reed Stuffers

I did mount the carbs backwards for easier jetting, shorter pulse lines to the fuel pumps and because I didn't have a longer throttle cable at the time. I just wonder if I'm getting too much signal to the carbs at quarter throttle with them in this direction.

 

[Watty]

I'm no expert, but I'd be aiming for about 22psi popoff with your setup. It looks like it has some good flow for starters, so it should be just right.

Tie it up to a dock or something where you can load up the pump and start from scratch. Get it snappy off idle, if you can't get it to snap no matter what you do, go up and down with your popoff an note the difference.

The problem as I see it is that you are trying to tune the wrong way around! Forget that the top/mid is good, and concentrate on the bottom end. Get that right and you can work your way up.

Remember, you'll never get a good of idle snap to hold WOT (in one movement) if your popoff is too high, or too low.

 

[Matt_E]

As mentioned in my PM, perhaps try lowering popoff to low twenties, drop the pilot. Then tune for bottom and mid, then move on to the top.

 

[SUPERTUNE]

Hey Chris, What throttle plate # come with the stock 760 carbs?

 

[#ZERO]

Hey Chuck...It says 160 stamped on both the 44mm plates.

 

[nikad58]

What is... has the three hole low speed circuit carbs ? Are they stockk 760 44's ?

 

[#ZERO]

Yes...the stock oem 44's Yamaha 760 carbs have the three hole low speed circuit.

One is for the low speed adjustment and the other two are the bypass holes.

 

[nikad58]

could it be your mains are loading up your low end ? Seems like you have too big of mains.. I heard that when you fill your intake it takes less (smaller jets)

blocking off the crossover in the stock inlet manifold setup has much lower inlet manifold volume resulting in much stronger signal and more responsiveness to adjustment..

 

[SkiDaddy]

My setup is very similar to yours, except the carbs are mounted w/the throttle drum facing the rear of the ski (stock orientation). I started off w/25 PSI pop, 120 pilots, 125 mains (safeurr: , now running 110s) & worked the pop off down to 15 trying to get it clean right off the bottom. 1/3 & up was great. As I dropped pop the crispy midrnge softened. Then ?I went back to 25 PSI pop & reduced the water in the pipe w/the top screw. Viola!:woot: :woot: :woot: :woot: Now strong right off the bottom. So I highly recommend you close off all screws except the top & work w/it. Much easier than rejetting.:Banane01:

Now I'm wondering if a little higher pop might strengthen the midrange hit?

 

[#ZERO]

The mid range and top end were fine it would pull 7250rpm on flat water testing when tuned with the larger 122.5 & 125 mains at one turn and lost power with the 120's.

The popping and sputtering at quarter throttle was almost gone with the 34 lbs higher pop off pressure and with the low speed screws turned almost all the way in.

I have my pipes screws all turned in except for the bottom screw which is at a half turn and I run a 1.5mm restrictor at the stinger tip.

I'm really surprised some of you guys are only running 110 mains and having it pull strong at the top end. :surprised:

 

[Matt_E]

I played around with different popoff pressures and corresponding low speed jets for quite a while
I went from 14 psi all the way to 32. I made sure the low speed jet was sized accordingly.
I found 24psi to be the overall best in my setup.

 

[waxhead]

try 120 low 105 high
18psi pop off
1 turn on top and bottom
let me know what you get

 

[norcal ex]

i have some 44s i want to run. i was inspecting them today and i noticed that the low speed adjuster can only be turned a half turn all togehter. it has stoppers that wont let it turn any more.... hmmm this normal?

 

[SkiDaddy]

i have some 44s i want to run. i was inspecting them today and i noticed that the low speed adjuster can only be turned a half turn all togehter. it has stoppers that wont let it turn any more.... hmmm this normal?

Mine come all the way out (cleaned everything).

 

[waxhead]

Flick the plastic covers off
there are screws under them

 

[SUPERTUNE]

Chris, AND the rest of you that are trying to use the stock OEM carbs are very hard to tune for FREESTYLE throttle hit! (remember Yamaha made them for fuel ECONOMY and some added performance) Just sell them stock 760 carbs and buy some new 46mm's, freestyle motors require FUEL to have throttle response and stock 760 carbs WON'T cut it without a total circuit rework!!
Do you see any of the aftermarket carb guys building race carbs out of stockers, not much. Most Novi's, All of Full Spectrums and I think all of the Jetworks Blackjacks also use the aftermarket mikuni's for a base carb. How much is it worth to you to pay for an update re-circuit work on your OEM yamaha carbs, Just need to know if it would be worth my time to develop 760 carbs for this...

P.S. This is MY opinion only...I do NOT claim to know it all, I just know what work's for my engines...

 

[douglee25]

Chris, AND the rest of you that are trying to use the stock OEM carbs are very hard to tune for FREESTYLE throttle hit! (remember Yamaha made them for fuel ECONOMY and some added performance) Just sell them stock 760 carbs and buy some new 46mm's, freestyle motors require FUEL to have throttle response and stock 760 carbs WON'T cut it without a total circuit rework!!
Do you see any of the aftermarket carb guys building race carbs out of stockers, not much. Most Novi's, All of Full Spectrums and I think all of the Jetworks Blackjacks also use the aftermarket mikuni's for a base carb. How much is it worth to you to pay for an update re-circuit work on your OEM yamaha carbs, Just need to know if it would be worth my time to develop 760 carbs for this...

P.S. This is MY opinion only...I do NOT claim to know it all, I just know what work's for my engines...

I got a question Chuck. How about the 46mm carb that originally came on the wetjet? Is that closer to or the same as an OEM 46mm? Thanks.

Doug

 

[SUPERTUNE]

Doug, I just don't remember what they had for throttle plate #'s, Are you talking about using 2 of them for duals or for a single only use?

 

[douglee25]

Doug, I just don't remember what they had for throttle plate #'s, Are you talking about using 2 of them for duals or for a single only use?

Just a single. I've been contemplating going to a bigger single shortly. I don't know if I should go 46mm, 48mm or what? I may still be able to pick up a wetjet manifold/46mm combo for my 61x (if it's not sold already). I'm unsure what the best move would be.

Doug

 

[SUPERTUNE]

If you NOT using 62T cases and using 61x cases, Use the Wetjet 46mm carb complete w/manifold! That Wetjet set-up will work very well when you can find them.