Carb Theory & Practice

[sjetrider]

I bolted up the full spec 48's to this 701 setup last night and tried it.
117.5P
135M
1.8 N/S with 80g spring (popped low at 22psi)
tried 75 return and a 60 return

The Full Specs definitely have a harder hit at mid. I believe they could hit harder on bottom as well with some tweaking. They seemed a little rich on bottom even at 1/2 turn out (no lean areas at all)
They liked the 60 return better than the 75 and took a little more throttle to get to pipe so lower popoff would be my next step. I would play with the return jet with the lower popoff as well (I am running external fuel pump so fuel pressure may be higher than on board pumps).
I think I would also drop the pilot to 115 at same time.

But I have a guy wanting to try these carbs so I will not get the chance to dial them in.

Kinda regretting posting them right now LOL but these 44's are doing well now.

I personally think there is power to be had going from the 44's to the full spec 48's, NOT huge but noticeable.

Is it because they are bigger or because of the modifications made?????????????????????????????????????????????

 

[fastgtfairlane]

Absolutely blows my mind how different your jetting is, WAY richer and most I see are running near this 115/120 up to 120/125 and 25 popoff. I started there and she just wanted less and less fuel on bottom but lower pop off

You're running a bun pipe correct? Is your cylinder and cases ported? I'm also running a 760 zeel with a slightly modified wax curve. I also tried running this jetting combination given to me by Paul who did my engine work.. it's a reverse jetting setup i tried when I was playing with carb tuning on my engine. 115p, 105m 2.3n/s 65g spring. I didn't care for how it ran like that though so I didn't spend too much time with it. It was basically all or none on the throttle. I primarily chase boats and ride surf and for that I want a smooth power curve that is linear and has a strong pull from idle to max rpm. I'll sacrifice a little peak power for a more usable throttle sweep. For flatwater freestyle, I would agree that reverse jetting would be a better setup for that type of riding.

 

[sjetrider]

Yes, currently has a Bun pipe w/ rrp waterbox. To be honest I expected more bottom end response from it over the PFP i had on there but really no change.
I did try reverse jetting but never got that low on pop off, that seems crazy low.
But my 105 pilot seems crazy low to me as well so who knows. My GF is like you, she wants linear power, I would prefer a POP as I do more flatwater than she does.
Honestly, the 48's where they are right now (lacking a huge bottom hit but hitting HARD at mid a little later) is a better fit for me.
I know I can get them closer but they freaking sold before I had the chance.

I think sadly this 701 with the 44's is still over propped in this OEM 155 pump. More tweaking to be done there.

 

[sjetrider]

The Bun on my 900 definitely hit harder at bottom and pulled nice throughout. Took me a minute to get flatwater down vrs the PFP but for boat wakes the BUN is superior for sure. The PFP is still better for straight flatwater on my setup.
ironically the BUN wanted less fuel on bottom than the PFP, dropped pilot from 135 to 130.

 

[waxhead]

You're running a bun pipe correct? Is your cylinder and cases ported? I'm also running a 760 zeel with a slightly modified wax curve. I also tried running this jetting combination given to me by Paul who did my engine work.. it's a reverse jetting setup i tried when I was playing with carb tuning on my engine. 115p, 105m 2.3n/s 65g spring. I didn't care for how it ran like that though so I didn't spend too much time with it. It was basically all or none on the throttle. I primarily chase boats and ride surf and for that I want a smooth power curve that is linear and has a strong pull from idle to max rpm. I'll sacrifice a little peak power for a more usable throttle sweep. For flatwater freestyle, I would agree that reverse jetting would be a better setup for that type of riding.

NO im running one of our pipes but this setup works well on other brands as well. That 115p and 105 high is way lean im surprised it even got on the plane.
MY go to 44 carb setting is
130 low 110 high. 2.3 and a 95 gram
Your total figure of jetting you put up before on your own ski was 145 in total so i know it would run close.
My general rule to look at is if some one gives a spec of about 240 on 44s or 260 on 46s and about 270 on 48s you know they are close. You can not use this to tune and chase that figure but if they are way off its not usually the jetting its something else causing it

 

[Storbeck]

Should those numbers start with a 2?

240 for 44's, 260 for 46's, 270 48's?

 

[waxhead]

Should those numbers start with a 2?

240 for 44's, 260 for 46's, 270 48's?

Thank you. I edited it. It australia here and I had not had my first coffee lol

 

[Storbeck]

Has anybody noticed a trend for jetting vs pump load?

Seems like more pump load usually means a larger high speed jet. I haven't done enough testing of different setups to really say it's a trend but it kind of anecdotally seems to go that way from what I've seen, which is also the reason why I take jetting recommendations from the internet with a grain of salt because they could be way off due to pipe water/temp setup and pump load. Well also ignition.

 

[waxhead]

Has anybody noticed a trend for jetting vs pump load?

Seems like more pump load usually means a larger high speed jet. I haven't done enough testing of different setups to really say it's a trend but it kind of anecdotally seems to go that way from what I've seen, which is also the reason why I take jetting recommendations from the internet with a grain of salt because they could be way off due to pipe water/temp setup and pump load. Well also ignition.

I have noticed higher load means bigger pistons till you run out of overbores mainly caused by meltdowns. You need bigger jets in your ski with higher load due to the excessive heat generated by the load. It helps cool the engine and also means that the engine doesn't make the heat energy it should make. This is why people saying you can dyno tune a two stroke jet ski is wrong. They have massive ramp rates compared to what they see in the real world and this means you have to make them rich so they can deal with it, I am a great believer in pitching your ski for the rpm the engine makes it power at. I would rather over rev the engine than overload it

 

[mikesx]

Has anybody noticed a trend for jetting vs pump load?

Seems like more pump load usually means a larger high speed jet. I haven't done enough testing of different setups to really say it's a trend but it kind of anecdotally seems to go that way from what I've seen, which is also the reason why I take jetting recommendations from the internet with a grain of salt because they could be way off due to pipe water/temp setup and pump load. Well also ignition.

I have noticed higher load means bigger pistons till you run out of overbores mainly caused by meltdowns. You need bigger jets in your ski with higher load due to the excessive heat generated by the load. It helps cool the engine and also means that the engine doesn't make the heat energy it should make. This is why people saying you can dyno tune a two stroke jet ski is wrong. They have massive ramp rates compared to what they see in the real world and this means you have to make them rich so they can deal with it, I am a great believer in pitching your ski for the rpm the engine makes it power at. I would rather over rev the engine than overload it

So would it be advisable to start tuning an unknown/new setup with a blatantly low prop, achieving ideal jetting, and then adding more prop?

Would adding more prop require rejetting?
Like a zero and span type of tuning ?

 

[kevbo]

Should those numbers start with a 2?

240 for 44's, 260 for 46's, 270 48's?

Thats a dangerous prediction. Much safer to make the main/top screw rich of peak rpm and the pilot just rich enough for good response. I have a 701 that needed 140 mains and a 750 that is still too rich with 115s. Both have SBN44s. I tried to explain to the engine what it needed, it ignored me.

 

[waxhead]

Thats a dangerous prediction. Much safer to make the main/top screw rich of peak rpm and the pilot just rich enough for good response. I have a 701 that needed 140 mains and a 750 that is still too rich with 115s. Both have SBN44s. I tried to explain to the engine what it needed, it ignored me.

I do say its not a hard and fast rule, If i see something along way from those figures then I start looking for other issues. I mean if you run outerwears your jetting will be less as they restrict flow, If someone says to me I have a 130 low and a 130 high on my 44s and it runs well then I know there is something wrong thats making this jetting needed. Like an air leak

 

[Storbeck]

So would it be advisable to start tuning an unknown/new setup with a blatantly low prop, achieving ideal jetting, and then adding more prop?

Would adding more prop require rejetting?
Like a zero and span type of tuning

I would say yes based on (a little) personal experience and also what smarter people than me have said.

 

[waxhead]

So would it be advisable to start tuning an unknown/new setup with a blatantly low prop, achieving ideal jetting, and then adding more prop?

Would adding more prop require rejetting?
Like a zero and span type of tuning ?

After years of testing different props, I've realized that prop pitch doesn't make a huge difference in top speed. A taller prop can help your ski hook up better in rough water, but it also puts more load on your engine. Don't worry about over-revving and blowing up your jet ski engine—these things are built tough with roller bearings and a solid one-piece conrod.

If you're racing a two-stroke, you'll want to pitch the prop to hit your target RPM. But if you're just out having fun with your friends, a smaller prop can actually make your ride feel a lot better, and you'll have more fun spraying your buddies. Plus, it takes some of the load off your engine, which makes it less likely to blow up and burn a hole in your wallet.

If you do decide to load up the prop later, keep in mind that you'll probably need to upsize your jets since the engine will generate more heat. How much you need to adjust depends on how far you go with it. Just watch your tachometer—it’ll give you a clear warning as it starts counting down slowly at first, then quickly as the heat builds up.

 

[fastgtfairlane]

NO im running one of our pipes but this setup works well on other brands as well. That 115p and 105 high is way lean im surprised it even got on the plane.
MY go to 44 carb setting is
130 low 110 high. 2.3 and a 95 gram
Your total figure of jetting you put up before on your own ski was 145 in total so i know it would run close.
My general rule to look at is if some one gives a spec of about 240 on 44s or 260 on 46s and about 270 on 48s you know they are close. You can not use this to tune and chase that figure but if they are way off its not usually the jetting its something else causing it

I was talking to sjetrider about the bun pipe and jetting hes running on his 701 with 44s. The 115pilot and 105 main actually wasn't lean at all because of how low the popoff was. Larger needle and seat makes the jets basically act larger. I've seen a few other setups with similar jetting as well. If anything it was muddy on the bottom. Your jetting is higher because you have a smaller needle and seat. You're running a 2.0 with 95g spring, mine was a 2.3g with 65g spring. Very low popoff. Came on the pipe earlier. It ran strong just didn't care for the way it came on the pipe so I went back to my prior jetting which is 120p 130m not 125m that I mentioned earlier by mistake.

 

[fastgtfairlane]

Yes, currently has a Bun pipe w/ rrp waterbox. To be honest I expected more bottom end response from it over the PFP i had on there but really no change.
I did try reverse jetting but never got that low on pop off, that seems crazy low.
But my 105 pilot seems crazy low to me as well so who knows. My GF is like you, she wants linear power, I would prefer a POP as I do more flatwater than she does.
Honestly, the 48's where they are right now (lacking a huge bottom hit but hitting HARD at mid a little later) is a better fit for me.
I know I can get them closer but they freaking sold before I had the chance.

I think sadly this 701 with the 44's is still over propped in this OEM 155 pump. More tweaking to be done there.

Pump tuning with a 155 and a 701 is crucial to get the engine to perform well. Also what elevation are you? I'm at around 650-700ish. I also wonder if the center bleed chamber I have maybe needs more fuel than say a regular superjet chamber or a powerfactor pipe that has a straight through design..a riding buddy with a similar setup had to go up a jet or two when he put a blaster mod chamber in his v3 tigercraft from a superjet mod chamber but don't recall how much. The center bleed design definitely puts more pressure back to the engine than the straight through style pipe.

 

[waxhead]

I was talking to sjetrider about the bun pipe and jetting hes running on his 701 with 44s. The 115pilot and 105 main actually wasn't lean at all because of how low the popoff was. Larger needle and seat makes the jets basically act larger. I've seen a few other setups with similar jetting as well. If anything it was muddy on the bottom. Your jetting is higher because you have a smaller needle and seat. You're running a 2.0 with 95g spring, mine was a 2.3g with 65g spring. Very low popoff. Came on the pipe earlier. It ran strong just didn't care for the way it came on the pipe so I went back to my prior jetting which is 120p 130m not 125m that I mentioned earlier by mistake.

When the throttle's wide open and the engine is revving, the carbs aren’t too concerned with your pop-off pressure—it doesn’t really impact the mixture at higher RPMs. Mikuni even mentions that the smallest needle and seat (n/s) flows more fuel than the carb can actually use. Personally, I was running a 2.3 with a 95 spring, which gave me around 20-21 psi pop-off pressure on my gauge.

That being said, there are a thousand different ways to tune, and everyone has their own approach. I’ve never seen a set of 44 Mikunis on a 62T with those exact settings, but if it worked for you, that’s awesome! I would suspect you had a very small restrictor in the return that was pushing up fuel pressure or an overly restrictive flame arrestor

 

[sjetrider]

Pump tuning with a 155 and a 701 is crucial to get the engine to perform well. Also what elevation are you? I'm at around 650-700ish. I also wonder if the center bleed chamber I have maybe needs more fuel than say a regular superjet chamber or a powerfactor pipe that has a straight through design..a riding buddy with a similar setup had to go up a jet or two when he put a blaster mod chamber in his v3 tigercraft from a superjet mod chamber but don't recall how much. The center bleed design definitely puts more pressure back to the engine than the straight through style pipe.

I’m at 550 feet elevation
Pipes do make a difference obviously. The Bun vrs PFP on my 900 wanted slightly leaner pilot. On the 701 it really didn’t seem to want anything different.

 

[sjetrider]

I agree with Wax on having a minimum overall jetting in mind.
That’s why this 701 / 44carb setup confuses me. 227.5 over all. 3/4 turn out bottom and 1 turn top.
It is case ported and the intake side filled /ramped. I guess it just draws really hard.

With the 48’I was 252.5 over all 1/2 turn bottom and 1 turn top and she was slightly dirty on bottom.
So they felt like they needed lover bottom jets to me. 22 pop off and I reduced the return jet to 60 from 75 which helped some. Telling me maybe a lower pop off would help.

I don’t know why this little 701 seems to draw so hard. Same reeds and manifold as my 900

 

[Storbeck]

Yes. When I was racing sxr800’s in superstock (04) I did a set of sxr cases to fit those reeds. I have pictures I will have to dig up.

@MTRHEAD still would love to see those pics!