Surfriding running dual 38s on larger motors?

[masterblaster]

Two 38's are equivalent to one 54 (you would add the areas, not the diameters).

You might want to look into the Jetworks taper bore 38's. Art says power wise they are equivalent to 44's .

not sure what you mean by area vs diameter. I have talked with art about those taperbore 38s they do sound nice,but he told me they significantly reduce run time cause they are making more air flow= more fuel. I have big carbs already,Im interested in trying to get the most ride time outa the 38s I can because they are the most efficient setup out there.

what mat posted is the main reason the 38s are more efficient than the single carb setups,yami didnt stop the single carb setup and go to the duals on 62t cases for no reason,the dual setup with 38s still makes more hp than the single 61x setup on just flow characteristics alone.I doubt its much different with most aftermarket 62t single manifolds,maybe with the exception of the atp.Ive riddin some nasty low end powered dual 38 limited 701s that got really long ride times,Im just not sure how much ride time i will get running them on a larger motor with alot more vacuum.

If i can get some adapters to put them on my boyesen intake plate I could always switch back to the 46s or 48s in a few min for riding other areas where fuel is not much concern.anyone ever seen anything that would work for this?

junkyard,those are the same ones im running but with a 80 pilot,prolly have to go up a good bit to feed this motor though.

kevbo,my big carbs are all tuned at the brink of lean as is for freeride use,zero topend use for me. If I lean the pilots even a 1/8 the motor will gasp at quick wot and then take off,yet with the long azz slow wake we have in and out,I have a pile of smegma coming out the exhaust all the time,running 40/1. these large carbs use almost the same amount of fuel in the slow wake and slow cruise to the ride spot in open water. as they do in all out ripping and rideing freeride use

 

[Cannibal]

not sure what you mean by area vs diameter.

What he means is this:

Lets say you have a 2" pipe and a 4" pipe, just off the cuff, you'd think the 4" would flow twice the volume as the 2", but let's take a look at the area of those:

Area = Pi(R^2)

Area of the 2" = 3.14159(1^2) = 3.14159"

Area of the 4" = 3.14159((2^2) = 12.56636", or roughly 4 times the area.

A 4" pipe will flow roughly 4 times the volume that a 2" will.

Carbs are the same way.

38mm carbs have an area of 1134mm, so 2 have an area of 2268mm, divide that by Pi and you get 722mm, take the sq/rt of that and you get 26mm (radius), and multiply that times two, you get 54mm (a single carb twice the size of twin 38s)

Now a 54mm carb will flow more than twice what a 38mm carb will. because of the bomb site. The bomb site takes up a larger percentage of a 38mm carb than it does a 54mm carb. From what I remember, Art sees a 9% increase in flow when he removes the bomb site (and works his magic), so taking that into consideration, a 38 will flow like a 34.5mm tube. Converting that to a single carb like I did above means a 49mm single carb should flow the same amount as twin 38mm carbs.....if that were the way it really worked....

In reality, it works like Matt said:

You are only ever drawing through one carb at a time, so one cylinder is only ever seeing a 38mm carb, not the both carbs or a "54mm equivalent (or 49 for that matter)". The only time you would see that is if both cylinders were pulling through both carbs equally, not one through one and one through the other. Even with a stock crossover setup, you will mainly pull through the carb for that cyl, and partially (if at all) the carb for the other.

So because your engine only pulls through one carb at a time, your carbs see this: Intake - exhaust - intake - exhaust. Air flows and the stops in a dual carb setup during the cylinder filling cycle. Yes it happens fast, but your reeds seal, and air flow "pulses".

In a single, your carb sees this: intake 1 - intake 2 - intake 1 - intake 2. Air flow through the carb never stops, and so you see that smoothness you talked about before. So basically, a single 44 can flow more than dual 38s...but where you run into issues is at a certain point the intake pulses are so close together that both cylinders are pretty much pulling on the carb constantly, giving you a restriction. And because intake pulses don't start and end 180* from each other, there is an overlap pulling on the carb...not as clean as 1-2-1-2. The process is not as efficient as with duals, and because of the shorter, more direct path into the crankcase, dual carbs on a 62t shine.

I would like to try a single 44 setup back to back with a dual 38mm setup for some real world testing for power and fuel consumption.

 

[Cannibal]

yami didnt stop the single carb setup and go to the duals on 62t cases for no reason,the dual setup with 38s still makes more hp than the single 61x setup on just flow characteristics alone.

One other thing to consider as to "why" they changed. In 95, Kawasaki came out with the 750 SXi, a dual carb engine (that competed with Yamaha's single carb 701). Coincidence as to why Yamaha came out with dual carbs in 96...you be the judge....either way, we are all glad they did.

 

[masterblaster]

flapjack, that was one detailed explanation bro,good job.it would be a interesting experiment what kinda efficiency you can get out of dual 38s vs single 44. It might be close,but the bottom end snap outa dual 38s isnt gonna be beat,it may outflow it in rpm but most us are not after that. also for most with 62t cases it doesnt make sense to switch intakes and carbs for kinda close performance.In my case I already have dual intakes with removable top plates that make it very easy to switch carbs fast,I just need some type adapter to make them fit,I dont think the larger intake tract will slow air flow with so much vacuum already being pulled through smaller 38s

 

[chriselmore_1]

hmmmm. now im i cant decide between some dual 38s or a single taperbored 46...... damn u guys!!!!

 

[masterblaster]

its not that hard of a choice,if you want maximum low end snap and super clean responce with longer ride times,go 38s

if you want more upper mid and topend with more rpms and a smoother power curve,and dont mind a little less ride time,go single 46. sometimes with light hulls smooth curves can make a big diff in your bodys ability for ride time as well

 

[2XLR8]

If you modify the 38's for maximum flow they will still out flow the 46 and the midrange and higher rpm will be better, by higher rpms I don't mean wide open speeds either. You dont need a flow bench or a carb swap to figure that out. Like Flap said calculate total usable flow area minus obstructions. Obstructions become more of an issue the faster the air flows. We have to keep this in mind if we are trying to feed a bigger cc engine with stock 38's.

 

[Cannibal]

No prob. There is even more to it as far as parasitic drag and such, but for all intensive purposes, you get the point.

I had talked to Art about his taperbore 38s and that's why I ended up with some (clean used ones). Just got them on and hitting the water this weekend with them. I'm only running a 701 though.

He did say that you could easily run them on a 760, and I hope to be up there someday with a 61x big bore. I don't believe his taperbore 38s will flow as well as stock 44s, but I bet they are getting close. Also, the reason he doesn't Blackjack the 38s, it ended up loading up because the signal was too strong.

For another comparison, let's look at the SXR 800 (really 782ccs). It has 40mm I bodies stock. We all know the SXR is more high rpm race oriented. Air flow requirements are higher at higher RPMs (duh). If 40mm carbs work for a high rpm 782cc engine (1mm over-bore = 801 and 2mm over-bore = 820). I'd say you'd be fine with stock 38s or Jetworked 38s (probably flow like stock 40-42s) on your engine at low to mid RPM. You might see some loss in power at the high RPM, but building a fuel efficient race boat is not really what we are after.

I am a firm believer that people over-carb for the kind of riding they do because "bigger is always better".

Steve

 

[masterblaster]

I agree,with freeride use,the diff I get between 46 to 48 carbs isnt really worth it.I just hope I can get these 38s to make use of at least most the power this motor is capable of. I just need it for low end responce and hit,so my odds are looking good on that.

 

[djkorn1]

I like IT! I'm going to use this as an 8th Grade Geometry question!


Lets say you have a 2" pipe and a 4" pipe, just off the cuff, you'd think the 4" would flow twice the volume as the 2", but let's take a look at the area of those:
Area = Pi(R^2)
Area of the 2" = 3.14159(1^2) = 3.14159"
Area of the 4" = 3.14159((2^2) = 12.56636", or roughly 4 times the area.
A 4" pipe will flow roughly 4 times the volume that a 2" will.

 

[Dustin Mustangs]

You might want to stick something in there along the lines of "neglecting friction and assuming fully developed, uniform flow" in case you have some dip know it all student. Comparing fluid flow through pipes based solely on cross sectional area is an oversimplification that isn't necessarily true.

 

[grahammy]

I think you're coming from the wrong tack on this. The engine sucks the air and thus the fuel. All the carbs do is atomise the fuel with air on demand. If you take too much fuel out, you will lean the mixture and cause running issues or detonation at worst. Small carbs will just reduce the air flow by causing a restriction, but this will only be at higher revs.

Why not run the EPIC ignition, and keep the same carbs. You can then run 1 ignition curve as the main powerful one, using full fuel usage, and use a secondary curve well backed off so that the engine doesn't produce the power and hence won't suck the fuel out of the carbs

 

[masterblaster]

i do have a epic ign and have tried a few diff curves,not for fuel consumption,but it doesnt matter,this motor has really high vac and uses alot of fuel through big carbs no matter what,even in the slow wake. big carbs dont meter as seamlessly as 38s,so it will be enteresting to see what the setup will do with them,waiting on the 5 mill crank to get back from crankworks for rebuild now.

 

[kevbo]

A standard unmodified 44sbn has the best dimensions for general freeride use. Its 43mm plate area and 39mm choke area makes for a strong enough signal to work with small engines yet provides enough flow for good power. There is a lot of Freudian psychology in the jetski world, bigger is always better. This thread marks the first step beyond that. Fortunately the twostrokes' exhaust note gives enormous feedback as to the fuel mixture and carbs are easy to measure. Once you learn what to look/listen for you can tune/design your own custom carbs instead of throwing $$$ at a brands and buzzwords.

 

[Matt_E]

A standard unmodified 44sbn has the best dimensions for general freeride use.

According to whom?

 

[Flash-FX]

Here's a quote from the Group K's website referencing the Yamaha 845 kit. Harry has done some testing in this area, and is not afraid to shed some light on this subject.

Carburetion

(Since the 845 has such huge bores, it also has more “inlet signal” strength than any other SuperJet engine package ever built. In layman’s terms, inlet signal strength is the actual vacuum that the crankcase uses to draw fuel through the jetting circuits of the carbs. This vacuum is generated by the upward movement of the pistons. Having a lot of “signal” is a big advantage, because stronger signal results in less temperamental carburetion, and less fine tuning for changes in altitude and weather conditions. The strong signal of the 845 kit allows for very precise jetting of nearly any carb you install on it, and it will require very little fine tuning down the road when weather conditions change significantly.

The stock SuperJet comes with dual 38mm SBN Mikuni carbs. Group K routinely modifies the throats and jetting of these carbs to increase air and fuel delivery for modified engines. These modified throat 38s are a very popular alternative for 701/760 SJ owners wanting to get the best of both performance and fuel range, and they also work very well on the 845 kit.

During our testing, we also used the larger 44mm carbs (and manifold) that comes as original equipment on the 760 Yamaha engines. While we consider these carbs to be a bit too large for a 760cc application, they worked perfectly on our 845 kit. Since there is such a big focus on getting enough fuel and air to the 845, these oem 44 carbs are a very practical and effective choice for owners that want to get the most from their 845 kit. Unfortunately, the 44s do have considerably higher fuel consumption than the modified throat 38s. Given that, owners who have a high priority for fuel range should consider fitting their 845 with the modified 38s.)

I run 46's....

 

[Matt_E]

While Harry seems to be a super-nice guy (and I have done business with him on multiple occasions), his advice isn't the first I'd run to when it comes to high HP motors.

 

[Big Kahuna]

Why not just take a nicely ported 701 with OEM 38's and be done with it. I dont get it. You build up a big motor, you run fancy ignition and you want to put on smaller carbs to try and save gas. Stick some 38's on there, and jet the carbs for your application.

 

[Scorn800]

Why not just take a nicely ported 701 with OEM 38's and be done with it. I dont get it. You build up a big motor, you run fancy ignition and you want to put on smaller carbs to try and save gas. Stick some 38's on there, and jet the carbs for your application.

I'm with BK on this.
This whole thread is backward thinking. Why build a fancy motor and rob power with small carbs. Just stay with a small motor and smaller carbs to use less gas.

 

[Boris]

Funny thing is, my small motor with large carbs uses less fuel than a stock SJ.

Do big carbs always equal to higher fuel burn numbers ??