Should I go Backwards or Forwards???

[93SJ760]

I just swapped out my dual 38's for dual 44's and I am now being told by an ex racer to run backwards jetting as the 44's run best that way. Can anyone back this up or have other advice?

Here's my set-up for reference:

760 bored to 83.5mm I believe
Freestyle Porting (Cylinder Porting Only)
Dual SBN 44mm carbs (Stock 44mm flame arrestors)
Carbon fiber reeds
reed spacer
Factory B-Pipe with Blaster Mod Chamber that has been shortened 1.5"
MSD Ignition with timing advanced a couple more deg. on the stator
175 - 190 psi Depending on which day of the week it is. (0.045 squish)

So I don't have the jets to do the backwards jetting yet so on the wknd I tried to go as close to even as I could with the jets I have.

I tried 140 mains with 100 pilots - way too rich
130 mains with 100pilots screws at 1 turn out May be too lean?
The machine took off on me dieseling off the rev limiter on the dock with no plug caps, fuel lines disconnected and my hands over the carbs trying to choke it for what seemed like an eternity. I figure the only reason it stopped is because it started to seize and the load bogged it out. Since the mag side plug is white and the PTO side is tan I figure it may just be an air leak due to repetitive carb removal and installation.

So...
I will be doing a leak down test before running it again.

And...
I have recommended backwards jetting on order which is 130 pilots and 105 mains + additional in that range.

BTW I have 2.3 N+S with shiny silver springs and fuel keeps filling the carbs as it sits there not even running so I believe pop-off is too low and I will be changing to the dull silver springs. For some reason I got the two mixed up and thought the shiny ones were the 95 gram springs when I set up the carbs.

Any help is appreciated as I want to get this thing on the water and ride.

Thanks,

Tyler

 

[Matt_E]

Popoff is way too low if it's doing that.
PM Waxhead for some more info on that jetting.

If I was you, I'd try 2.0 N&S/95g spring (25psi popoff), 115 pilots, 125 mains. See how that works.
Oh, and if the cylinder is a 760, the bore is at least 84 mm.:cheer:

 

[SuperJETT]

Stock jetting for Yamaha 760 44's (they are slightly different than Mikuni 44's you buy individually) is 115/135 and 115/137.5 I'd say you're just off and need to go through the whole tuning process.

Paul's normal recommendation to start with is 120P/130M.

 

[93SJ760]

Cylinder bore mistake

I guess I made a mistake on the bore which I imagine could make a difference.

I know I have 1 bore left on a set of 760 sleeves so I guess I'm around 85? I'm not too sure and don't remember the piston sizes I purchased.

I'm assuming from the advice you guys are giving me that I should stick with forward jetting? Does anyone know why some would recommend reverse jetting? Is this mainly for racing?

Thanks again,

Tyler

 

[Matt_E]

Both work. I have found at least three completely different jetting setups for my engine. All worked.

 

[matty180]

I like running a bigger pilot than mains. I run 130 pilots with 110 mains on mikuni 44"s with a pop off of 20psi. I ruun this type of jetting on both of my ski's. But that what I like.

Matt

 

[Matt_E]

Matty, you run drypipes, correct?

 

[93SJ760]

Interesting

From what I've been reading on here and the "other" site I take it that when I put my speedwerx dry pipe on it may make a huge difference in jetting due to pressure changes in the crank case? I had figured I may need a little larger on the high for the extra HP on the top end but that's it. It may actully need to go smaller up top if the pipe is pumping the air through faster and causing a stronger signal and sucking more fuel into the venturi.

Tonight I'll pressure test, put some 95 g springs in, target between 22psi and 25psi for pop-off and hopefully get back out to test the jetting.

Tyler

 

[waxhead]

If an engine needs reverse jetting its simply you just jetting as the engine asks
its nothing fancy or a black magic thing
it just giving the engine the fuel its demanding

here is where i would start on that engine
125 low
100 high

around 15-18 psi

i guess that is reverse jetting

If your engine did a runaway its not because of that jetting so you need to look for a reason
you may have an intake gasket not sitting right
You can not jet your ski until you make sure you have no airleaks

i would say its on the mag side as this seems to have the whiter plug

the 130 jet will be fine to try in that setup and should be close to the mney
depending opn the porting and how high things have been lifted

With a mod chamber that been shortend as well as a ported cylinder i would say you will need to be closer to 15psi to help the ski get off the dock

I hope this helps

 

[93SJ760]

Thanks everyone

That helps a lot.

Like I mentioned, I will do a leak down test first.

Then I'll put the jets in that I ordered and give an update on how the new jets work.

Tyler

 

[Mile9c1]

I'd try the stock jetting first. I don't see any way a motor would run with 125 lows and 15psi popoff, it just doesn't make sense to me.

 

[SuperJETT]

I ended up with 1.8/80 popoff.

 

[waxhead]

try it mile it will hit so hard down low you will be suprised and very clean in the surf
with my limited 701 i would run 120 low and 16 psi pop off
100 high it was awesome in the surf and very strong on the race track in its time

 

[Mile9c1]

I really hate taking my carbs off... it runs so well now.... maybe later in the year.

 

[93SJ760]

Pop-off

I borrowed a pop-off gauge and it was reading around 12 - 13 psi pop-off for the existing set-up (65g springs)... so I put some 95g springs in and hit between 17 and 18 psi pop-off. This was with 2.3 N+S.

Rubber gaskets are cut...
Leakdown and hopefully installation of new jets will come tomorrow.

Tyler

 

[waxhead]

sounds like you are on the right track

 

[matty180]

Matt E, I have 2 superjets

1 with R&D dry pipe, 48 Novi's and ported etc, etc..( race gas)
1 with Bpipe Mod chamber, 44 mikuni's ,ported cylinder etc,etc.( this is the freeride pump gas)

 

[93SJ760]

Leak-down... Check

Leak-down seems good which I'm happy about. Didnt' want to tear apart an engine I spent so much time assembling.

blocked off at carb inlets to manifold and blocked off at 3 bolt flange on FPP manifold.

10 psi and both times it dropped to about 9 psi within a few seconds, then held for aprox 1/2 hr at 9psi until I released the pressure.

Didn't get tuning as I'm waiting for my new jets. I guess it's not worth trying to tune with my 130 mains and 100 pilots? Probably just be too lean down on the bottom?

Tyler

 

[Matt_E]

I'd give it a try...it's not that different from what your first post had, though.

 

[93SJ760]

2nd Jetting session

So I went out tonight.

18psi pop-off
2.3/95g/130M/100P, both screws at 1.5 turns
Extremely rich. Water had a sheen on it all around me.

Up against a wall, I played with the low speed until I had the fastest idle (aprox 3/4 turn)
I then played with the high speed until it had what seemed to be the best response. I could turn it in until it had a lean hesitation (aprox 1/2 turn) then out until it was loading up and not revving high (aprox 1.25 - 1.5 turns). Response still isn't instantaneous but it's just as good as any of my old skis.

This is where things became messed up. I was riding testing the throttle response. Going 1/2 throttle then letting off and coasting and then getting back on it. The machine would bog every time and the 3rd time it happened it never came out of the bog. I rode it to the launch with not even enough power to plane. Checked for spark. It seemed like one was intermittent so I replaced the plug with another used one. Started it back up and same thing. I then tried priming it and that worked. Every time I got on the primer it has good throttle response but otherwise it won't even come out of the bog. What could have changed while I was riding? Another air leak? It's strange that it worked one minute and then not the next.

Any ideas? I'm going out now to do another comp test just in case then I'll throw my new jets in tomorrow. They still didn't come in. I'll try tuning with the 125 pilots and 100 mains and see if I can get it running good with that.

It's still puzzling how it all of a sudden lost power. I guess if it still runs like crap tomorrow I'll have to do another leak down.