Carb Theory & Practice

[Storbeck]

My experience was that when I switched my 781 from 44's to 46's it didn't improve anything, and when I switched my 898 from sbn46's to 48's it didn't improve anything.

I'm not claiming to be any kind of master tuner (or rider), so maybe it was there and I couldn't tell or maybe there is something I could have done better to take advantage of the bigger carbs, but that was my experience.

I would worry a lot more about tuning the carbs, ignition curve, pipe water/temp, and most importantly pump before I put carbs bigger than sbn46's on anything. My opinion.

 

[sjetrider]

I’m not saying all aftermarket carbs are made equal but there are a few brands out there that will eat a sbn46 setup alive.
But you didn’t answer my question Kevbo, where do you personally draw the line? I will attest from personal experience that a set of 50mm FS’s hit and pull harder on my 760 than a set of 48’s and a set of 46’s wouldn’t even be close. Would I put them on a stock 650/701? No. Would I put 48mm duals on a ported 701 with decent bolt ons ? Absolutely, if it was in budget.
Ain’t nothing wrong with 44’s/46’s and they make good power but I’d say night and day comparatively to a set of 48’s and up on most setups.

I have a set of FS 48's on here for sale but not even crickets are looking at them. I took them off this 701 setup (not because they didnt work) but because I have this freshy set of 44's laying here and felt I could make them work pretty close to as good as the 48's.
I may change my mind and put them back on here. To be honest, if this were my primary ski and on my GF's whip, they would have never come off LOL.
She thinks her boat hits harder than ever right now, Im pushing to get ALL I can out of this 701.
Once I get the 44's DIALED, I will put the FS 48s back on and dial them and see what happens. I have not run 44's in YEARS so just trying something kinda new. I will say the FS are WAY easier to tune.

 

[sjetrider]

My experience was that when I switched my 781 from 44's to 46's it didn't improve anything, and when I switched my 898 from sbn46's to 48's it didn't improve anything.

I'm not claiming to be any kind of master tuner (or rider), so maybe it was there and I couldn't tell or maybe there is something I could have done better to take advantage of the bigger carbs, but that was my experience.

I would worry a lot more about tuning the carbs, ignition curve, pipe water/temp, and most importantly pump before I put carbs bigger than sbn46's on anything. My opinion.

I kinda hope you are correct. Why use high $ carbs where they are not needed. I really hope the 44's end up close to the performance of the FS 48's on this modded 701 setup.

When I used to mess with 44's and 46's was back in the enhancer and B-Pipe days so its a new game with better pipes and timing curve options.

 

[sjetrider]

Someone early on in this thread explained why the aftermarket carbs are easier to tune than the Mikuni Retail and OEMs, After messing with these I am pretty sure he nailed it LOL.. But I know these things can be efficient and make power so I am chasing that dream right now.
Its been challenging getting them just right.

 

[DylanS]

44’s and 46’s are great especially on smaller setups, all I’m saying is if I had both sets or the option to buy either a set of 46’s or decent t48’s for relatively the same price I would for sure go for the 48’s.
Most of this stuff is subjective haha I’m just curious as to where everyone draws their line.

 

[waxhead]

I guess my real question for you guys is where do you draw the line on too big a carb.
What do you guys use to determine when a carb setup is too big for the engine it’s bolted to?

Well i ran 48 novis on a 550 sx and picked up a huge hit in the low speed. So i'm not the guy to answer this as I will always fit the biggest cars i can. As long as they are decent carbs with great signal

 

[Storbeck]

Well i ran 48 novis on a 550 sx and picked up a huge hit in the low speed. So i'm not the guy to answer this as I will always fit the biggest cars i can. As long as they are decent carbs with great signal

Awesome.

Did it get you anything on top?

 

[waxhead]

Awesome.

Did it get you anything on top?

No nothing up top, But the bottom end came up a lot, I ran a 550 at the worlds in 2018 and 2019 with 46mm carbs. It really helps with the hole shot

 

[sjetrider]

Guess I’ll be pulling the 48’s off the market soon lol

 

[kevbo]

My sizing standards are easy. It needs to hit the pipe no later than 50% open. If the pipe hits below 40% the carb is too big. If the jetting becomes sensitive, the carb is too big. If I have to lower popoff and jack with the pump pressure, the carb is way too big!!!! I might as well make a fuel injection system at that point.

 

[waxhead]

My sizing standards are easy. It needs to hit the pipe no later than 50% open. If the pipe hits below 40% the carb is too big. If the jetting becomes sensitive, the carb is too big. If I have to lower popoff and jack with the pump pressure, the carb is way too big!!!! I might as well make a fuel injection system at that point.

48 novis on the 550 it was at 20psi pop off

 

[kevbo]

The outboard guys are using EFI and a sliding plate over the reed block that completely uncovers the entire area. It has already been developed and is not difficult to tune. It is actually much easier to tune with outboards since they have no tuned pipe to compensate for. I think someone in this community can make something work. Im happy to take a small power penalty and use a slightly smaller carb that is very easy to live with.

 

[sjetrider]

Lower popoff helped. The Bun Pipe doesnt seem to give any gains over the PFP.
No more gurgling bog. But she is week coming out of the setup wake, couldnt tell before because of the bog. Back to the prop.
then I will try the full specs. and see how much gains there really are there. Mid to top this thing rips.
The prop i have has been pitched down from 6/13 to 5/12, going to go to 5/10 and hope it needs no welding.

 

[sjetrider]

if anyone has a skat 155 5/10 or 5/11, hit me up , i have a Stock pile of props but nothing that low, would do some trading if someone wanted as well.

 

[Flash-FX]

When is the carburetor too big?? Basically the tell tale indicator is a low pop off, because of a weak signal. Making things hard to tune for crisp response. Must resort to borderline tricks to fake out mismatched parts/pumps and over ported cylinders...

Kevbo quote: "Im happy to take a small power penalty and use a slightly smaller carb that is very easy to live with". Yep, all day long!

 

[DylanS]

So what I’m hearing is there is no definitive factor for a on the market carb being too big outside of “the signal is too weak” which can either be tuned out or simply mitigated with more restrictive arrestors.
So, the claim, “a carb too big” should be changed to “a carb to big for me to want to tune it to run right” lol.
Where an engine gets on the pipe can be tweaked by a handful of different things, how can you pin it directly to the carb itself? Is the carb really too big if you can alter those other things, retain the big carb, and make more power at the desired rpm range?
I’m sure it’s possible to go so comically big that you will see a negative I suppose I’m more curious as to where you stop seeing a net gain at x’ccs with x sized carb and if there would even be a way to crunch the numbers in a way to semi accurately predict it.

 

[Jhucke]

So what I’m hearing is there is no definitive factor for a on the market carb being too big outside of “the signal is too weak” which can either be tuned out or simply mitigated with more restrictive arrestors.
So, the claim, “a carb too big” should be changed to “a carb to big for me to want to tune it to run right” lol.
Where an engine gets on the pipe can be tweaked by a handful of different things, how can you pin it directly to the carb itself? Is the carb really too big if you can alter those other things, retain the big carb, and make more power at the desired rpm range?

This is the correct assessment, everything is based off feel in this industry. Tuning an EFI system and picking a throttle body size, which admittedly I know very little about, you're looking at engine load, volumetric efficiency of the engine, theoretical mass flow rate of the throttle body, intake air velocity, etc. All data that most likely next to no one actually has empirical data for. At least from people that actually participate on this forum.

 

[DylanS]

No idea what would be involved In it or how accurate they are but if you could find a way to mount one of those on board gforce/braking/rpm trackers you see in the onboard footage of road bike/cars you should be able to track acceleration forces to get a rough idea of how a “freestyle” ski is pulling. Could’ve been done before or maybe they aren’t sensitive enough just food for thought.

 

[E350]

Just a couple of thoughts from the peanut gallery here.

This discussion is being approached from two separate perspectives.

First, those with experience running large carbs on engines.

Second, those interested in the theory of large carbs on engines.

I started to put a SBN 44 on a 550sx using a single carb adapter manifold.

@waxhead stated in my thread on the topic that two SBN 38's would provide more bottom end power than a single SBN 44. So, I have changed plans to run dual 38s using his dual adapter manifold.

From a pure speculation perspective, it seems to me that an engine is primarily an air pump. It pumps air in the intake and out the exhaust. Given the diameter of the pistons and the length of their stroke, there is only so much air that the two pistons can suck into the engine's cylinders on each stroke.

Since the carb's function is to introduce gasoline into the air being sucked into the engine, it seems to me that a larger carb may not:

1. Atomize the fuel sufficiently as a smaller carb might?

2. A larger carb might be slower to introduce fuel into the engine as fast - causing a lean condition at higher rpms - because the Venturi effect will be less strong in a larger diameter throttle body than in a smaller diameter throttle body?

3. How would dual SBN 38s provide more bottom end on a 550sx than a single SBN 44?

What do you guys and gals think?

I respect the experience here. I bought a B pipe from @Flash-FX for my FX-1 which I haven't installed yet. He was kind enough not only to send me the pipe but the jets along with it and instructions on how to tune the carb for the pipe.

@waxhead knows allot as does @Blue etc. Wouldn't we like to see them participate more in this discussion?

 

[DylanS]

If you were to say compose a chart to try and help people find the sweet spot on carb size for their setup it would have to be such a insanely large chart with an insane amount of R&D and mostly done by the same trusted group of people.
You would need to test every size engine with various styles of intake manifolds, reed cages/reeds, exhaust systems, etc etc etc to make an actual accurate chart on what size specifically works the “best” with each setup.
And then you’d need to plot out curves to show horsepower and torque for each setup so that people could look at it and decide what they consider the “best” setup is for them.
You can only crunch numbers and run programs on this stuff so much and the real world is almost always in some way, minute or not, different than what can be predicted.
^ this is why it hasn’t been done me thinks. And people who have dedicated themselves to this amount of R&D spent a lot of time and money on the information and you won’t catch most of them giving it out for free.
So what you’re left with is subjective speculation from a group of guys on the forum that each have their own unique experiences with totally different conditions and setups that are all trying to guide someone in the right direction. Sure you can look at CFM other things and make an educated guess but that’s only a starting point.
A 2 stroke engine is an air pump and bore and stroke aren’t the only thing that play into how much air it can take in at any given rpm. You have to account for the supercharging nature of a properly setup 2 stroke engine with its given parts as well.
Very few people can look at an exact setups specs that they’ve never setup themselves and definitively say exactly what needs to be tweaked to get what the rider is after to the T.
I am a believer that you’d have to be stoutly over sized on carbs before you’d see the negative on most setups but that’s based on my personal experience and preference. I like that “light switch” feeling and less of a smooth pull which may be easier to obtain with a smaller oem setup.
What to me may seem like a rocket ship may to kevbo seem like grandmas Prius or vice versa.
Because we have no way of knowing what feels like it pulls harder without riding each others skis first hand it’s all speculation and even then it’s based on feeling unless you’re talking top speed under uniform conditions.
Thank you for listening to my Ted talk.