Carb tuning is a trip

[bored&stroked]

When I built my superjet, it was a all new combo and I went with the most recommended carb setup via the carb database list because carbs are not my strong point lol. Dual 44's on a modded 760, everyone likes 125L and 115H and tune from there. Sure enough, the ski started right up, idled good, good throttle response out of the box. Well, my buddy hates reverse jetting. He was mean to me and said my ski was slow lolol. Between that and another issue, he convinced me to go normal jetting this season. So stock low speed jets, and one step up on the highs. First ride is telling me I need to step the highs down, back to stock.
So stock jetting, on a highly modified setup.

Carb's make no sense unless you have lived your entire life dealing with them.

 

[fastgtfairlane]

It really depends on which 44 carbs you're using. Oem 760 carbs do not have the same fuel circuitry as the Aftermarket sbn 44s. The oem carb will have the manual choke while the Aftermarket will not. The Aftermarket carbs also have a 4th bleed hole just below the butterfly. The Aftermarkets will also flow more fuel. I have a fully ported 735 with Aftermarket 44s and my jetting is 120 pilots, 125 mains 2.0n/s with 95g spring with 25psi popoff. Also am running oem 61x flame arrestors with one screen. The flame arrestor you use will determine where your popoff should be. With oem 760 carbs I was at a 125 pilot, 130 main with popoff being the same at 25psi.

 

[kevbo]

Carb's make no sense unless you have lived your entire life dealing with them.

They are not that difficult, I can probably get you to understand what your engine wants in about five minutes. After that you just make adjustments toward that goal. You will never ask anyone what jets do I need ever again because nobody can know this before you do the tuning.

 

[waxhead]

When I built my superjet, it was a all new combo and I went with the most recommended carb setup via the carb database list because carbs are not my strong point lol. Dual 44's on a modded 760, everyone likes 125L and 115H and tune from there. Sure enough, the ski started right up, idled good, good throttle response out of the box. Well, my buddy hates reverse jetting. He was mean to me and said my ski was slow lolol. Between that and another issue, he convinced me to go normal jetting this season. So stock low speed jets, and one step up on the highs. First ride is telling me I need to step the highs down, back to stock.
So stock jetting, on a highly modified setup.

Carb's make no sense unless you have lived your entire life dealing with them.

Slow is relative. if some ones badly tuned ski goes better than a much worse tuned ski then they are both slow but one feels fast.
44s are an awesome carb on what you have, but that jetting spec is off

 

[bored&stroked]

Slow is relative. if some ones badly tuned ski goes better than a much worse tuned ski then they are both slow but one feels fast.
44s are an awesome carb on what you have, but that jetting spec is off

Well, it used to be his ski for about 10yrs but with a non ported 701 with less compression, but also running 44's with the same b pipe. So he knows it pretty well lol. He was running 112.5L and 125H's. And jetting is only off if it doesn't run right. One thing the master carb list taught me, is everyone is different. So Im gonna try this and see where it ends up.

They are not that difficult, I can probably get you to understand what your engine wants in about five minutes. After that you just make adjustments toward that goal. You will never ask anyone what jets do I need ever again because nobody can know this before you do the tuning.

Indeed. That's why I started basically stock and am going from there. I only went up one on the highs because thats what you do to a 701 when you add a b-pipe.

 

[waxhead]

Well, it used to be his ski for about 10yrs but with a non ported 701 with less compression, but also running 44's with the same b pipe. So he knows it pretty well lol. He was running 112.5L and 125H's. And jetting is only off if it doesn't run right. One thing the master carb list taught me, is everyone is different. So Im gonna try this and see where it ends up.

Indeed. That's why I started basically stock and am going from there. I only went up one on the highs because thats what you do to a 701 when you add a b-pipe.

His ski will carb clean with that setting and it will feel buzy but it will be down in midrange torque. How the b-pipe is set up makes a massive difference as well

 

[kevbo]

Next lesson will be pump load tuning.

 

[TheBuzzard]

Well, it used to be his ski for about 10yrs but with a non ported 701 with less compression, but also running 44's with the same b pipe. So he knows it pretty well lol.

So in other words he knows the hull but he don't know poop about the motor or its tuning requirements lol tell him to sit down and stfu! If it's not reverse jetted, there is a large chance of running lean in the midrange on deceleration and siezing. 44s are 44s so don't buy into the BS on that. Nothing wrong with oem 760 44s. Properly jetted they rip as good as AM 44s lol. Suckers. These are the jets I recommended in a badass ported 84mm bore 61x cyl with oem 44s b pipe and I'd put it's power and throttle response against anybodys junk that thinks it runs better

 

[Sparkplug]

They are not that difficult, I can probably get you to understand what your engine wants in about five minutes. After that you just make adjustments toward that goal. You will never ask anyone what jets do I need ever again because nobody can know this before you do the tuning.

I agree with this. Too many variables. Learning engine theory is the best bet. Otherwise you will get yourself involved with a bunch of speculation. The guys who build good engines, get paid to do it
And they don't give their secrets out.

Mikuni says the highest popoff with the smallest needle possible with proper running is the way to attain the best tuning. The 1.5 needle can flow more fuel than the engine can ever burn. Yet you see people running 2.0 2.3. 10 psi popoff. All that.

Hardest hitting 701 I ever rode was from a guy who's never even been on this site or a Facebook page. Grains of salt

 

[kevbo]

It N/S size wont matter much, if any as long as the carb is supplied with a solid stream of fuel. I do like to use 2.0 or larger N/S on single carbs just because they experience double the intake cycles and the total jet area is more than 1.5MM. You can run oversized pilots and low popoff very successfully if you are fast with the throttle. The engine will fourcycle horribly if you are slow and smooth with the throttle. Thats why its so difficult for different riders to understand why something that works so well for them may not for another. Another thing to consider with really good running engines is the fact their exhaust ports are going to be very wide and flat. These aren't goint to last 300 hours.

 

[Sparkplug]

It N/S size wont matter much, if any as long as the carb is supplied with a solid stream of fuel. I do like to use 2.0 or larger N/S on single carbs just because they experience double the intake cycles and the total jet area is more than 1.5MM. You can run oversized pilots and low popoff very successfully if you are fast with the throttle. The engine will fourcycle horribly if you are slow and smooth with the throttle. Thats why its so difficult for different riders to understand why something that works so well for them may not for another. Another thing to consider with really good running engines is the fact their exhaust ports are going to be very wide and flat. These aren't goint to last 300 hours.

According to mikuni. The manufacturer of the carburetors. It DOES matter. But people do what people do. They tinker and find things that work for particular setups. They convince themselves of results and atrribute that to their own genius. Negating the fact that engine theory is over a century old. And not up for conjecture. It's black and white.

 

[kevbo]

I have run N/S from 1.2-2.3mm, they all worked fine. Or maybe I was delusional??

 

[kx454]

I have run N/S from 1.2-2.3mm, they all worked fine. Or maybe I was delusional??

Not delusional at all. I tried everything back in the day on my SX-R. It ran best with 13 PSI pop-off. Still ran great into the mid-30s PSI pop-off as I remember as well. Tried the reverse jetting deal on it with a set of 44s. It ran better everywhere with traditional jetting. Never blew up, never seized, and always started like it was fuel injected.

 

[bored&stroked]

I will say this, with the previous reverse jetting the throttle was extreme on or off. Up to 1/2 throttle, it was very controllable. Once you hit 1/2 or above, it was just full throttle. Lightswitch response, and I hated it. Day 1 trying it the Mikuni way, the lightswitch was gone, and I could modulate the throttle in a controlled way.

 

[Sparkplug]

Like I said. A bunch of conjecture. From a bunch of amateurs. None of you make your living building engines. Mikuni makes their living building carbs. I include myself in this statement.

 

[kevbo]

You need to go to the Mikuni forum then. Everyone here is too dumb to help you.

 

[Sparkplug]

You need to go to the Mikuni forum then. Everyone here is too dumb to help you.

I didn't ask for help. I posted the mikuni manual that SPECIFICALLY states that tuners will recommend larger seats. And that is wrong. But I guess you didn't read that. O well. What's that they say about a horse and water? They ride jet skis on it?

 

[kevbo]

This tuner has used them all and couldn't find any difference. Theory and reality are having a standoff.

 

[Sparkplug]

This tuner has used them all and couldn't find any difference. Theory and reality are having a standoff.

How many engines do you sell a year? What's your business called? How many racers or riders have you put on a podium? There is like 4 or 5 guys on this site that have the real knowledge. The stuff that takes decades of dedication. And they don't give it out willingly. The rest of us are just playing mechanic at home with the world's simplest form.of propulsion besides a bicycle.

 

[kevbo]

Just because you are young with limited knowledge doesn't make everyone else inept. I have some very fast race cylinders that I would never use on a pump gas rec boat. The exhaust port is too wide for long piston/ring life and its too tall for good response on cheap gas. Too much of a good thing happens quick with a twostroke.