Freestyle Balance between carb signal and flow

[Pablo]

I'm always learning when it comes to carbs. I know lots of guys will just throw larger carbs on their engine just because they think bigger is better...not always true though. I've ridden plenty of big bores and some lower cc strokers that are tuned great with oem 38's. My question is: What displacement deserves what size carbs? I know the huge 1000-1200cc engines can handle the largest carbs (48-49's) because they provide the needed signal for those large throated carbs, but at what point are u either choking ur engine or drowning it with different sized carbs? This is kind of a Pandora's box type question and each builder has their preference for a particular engine build, but so many engines are stripped and rethrown together by knuckleheads. I always assumed that BB engines are fine with 38's and 44's (possibly 46's), 800-900cc SS and stroked can handle 44-46 (?48's) and 900cc+ will best be served with 46-49mm carbs.

I'm asking since I'm running 48's on a midsized stroker (862) and thought a smaller carb 44-46 may better serve this engine. These BJ 48's are crisp and work fine but wonder if I have enough signal strength to have this setup at its peak. Sort of a string of conscious thought here...I don't care what brand carb u think is best or what builder u think is great either. I already have that figured for myself.

 

[OCD Solutions]

I think it all comes down to which is more easily tuned. My first lesson on this topic was my 98 GSXL. It comes with OEM I-bodied 46's but I couldn't get them dialed in once I upgraded to the big bore kit and Coffman pipe etc. I picked up a set of 46 Novis and it actually got worse. So here is a 980cc twin cylinder with 46's but not enough signal at lower rpm to pull enough fuel thu them. Research showed that the Novi 44's would have been the better choice. They flow more than the OEM 46's ever could and had much more signal than required.

I believe that a good tuner could make just about any carb setup work but that smaller carbs with more signal are easier to tune and a better choice for those of us with less tuning skill. A smaller set of carbs, dialed in perfectly, will make better power than a larger set of carbs tuned poorly.

 

[SuperJETT]

My opinion: signal strength is a Group K marketing deal. Your best bet is to go with real world results which are that Novi, Full Spectrum, and Blackjack carbs all work very well, they just have to be tuned properly.

I went to a pwc carb seminar by Herb Kane back in '95 I think. He was the guy responsible for the SBN book and the development of the SBN plus setting up factory calibration (tuning, but he wouldn't use that word) for the oem skis. He knew those carbs inside/out and never once mentioned signal strength but did talk about atomization.

 

[OCD Solutions]

I understand signal strength to be just a term that group K uses to describe the siphoning effect that draws fuel thru the fuel circuits. It's the same principle that draws detergent thru a sipe line on a pressure washer or water from your bilge thru the stock siphon tubing.

Understanding the principle behind the effect is much more important than the term used to describe it.

 

[N3vrSat1sfied]

subscribed, I am curious as well. I debated all winter if I wanted to rebuild a set of 38's and use Jetmaniacs recommend jetting on the BB781 or run my already good oem 44s. I decided to run my 44's. I plan on getting the 38's ready as well and swap them out one day at the lake to compare.

I don't want to direct this thread towards a particular engine but here is my theory on my setup:
Chris and Blue are marketing these motors more towards the budget builder and using the 38's provides an cheaper/easy swap for someone with a running superjet. A 760/64x motor comes with 44's from the factory, so I would think that the bigger carb would be better over the 38s on the 781.

 

[High Speed Industries]

Your carbs are only as good as the guy tuning that's tuning them.

 

[Pablo]

I totally agree with y'all, but for certain there is more draw/signal strength (whatever u want to call it) with larger displacement engines. There has to be some kind of guideline for total amount of flow (cfm's) pulled in through intake track coupled against the velocity of the flow that will reach a zenith for a particular displacement. I understand the Bernoulli principal. Would anyone know if a stroker or a SS have a different "signal" when they are same displacement?

 

[OCD Solutions]

Sometime this summer I am going to replace the taper bored 38's on my wife's 760 with OEM 44's. She doesn't want any more power so it will be interesting to see how they perform in comparison.

I would really like to bolt on the Boysen intake and 46's from my SS865 and see just how much more I can get out of the 760.

I imagine that I won't make much more power but rather just move it around in the rpm band.

Sent from my SCH-I535 using Tapatalk 2

 

[Pablo]

Duplicate

 

[wanderer1]

When I was shopping for carbs for my 927 I spoke with Tim,Ed,and Art. Signal strength was lightly discussed by all 3,espescially at low rpm because the 66E has so much vibration there. What was talked about more was carb size because of the large intake on the 66E based motors. All 3 have been helpful explaining what size will do what for my application, I.E. 46 will work but 48 will pull harder and such. Don't waste their time but ask them what and why since they make them and each has their own proprietary circuitry. All three are very helpfull and all 3 make carbs that we could only dream about in the late 80's. On a side note, Ed was actually forthcoming about possible deficeicies with his carbs in my application,not just saying "hey mine are the best". Also,first time using the Mikuni DF62 fuel pump instead of individual pumps on carbs, I think thats what I'll be using from now on. That little sucker is awesome. Also just realized that I was only talking about higher $ carbs. I have found that the Mikuni SBN 46's are hard to beat from 701 up 900,those are great carbs out of the box when jetted IMHO

 

[SuperJETT]

Great post right there. One of the cleanest running skis I was ever one had 'plain' 46's, super smooth and strong.

 

[Matt_E]

x2. I've also had great results with the standard SBN 46.

 

[waxhead]

When I was shopping for carbs for my 927 I spoke with Tim,Ed,and Art. Signal strength was lightly discussed by all 3,espescially at low rpm because the 66E has so much vibration there. What was talked about more was carb size because of the large intake on the 66E based motors. All 3 have been helpful explaining what size will do what for my application, I.E. 46 will work but 48 will pull harder and such. Don't waste their time but ask them what and why since they make them and each has their own proprietary circuitry. All three are very helpfull and all 3 make carbs that we could only dream about in the late 80's. On a side note, Ed was actually forthcoming about possible deficeicies with his carbs in my application,not just saying "hey mine are the best". Also,first time using the Mikuni DF62 fuel pump instead of individual pumps on carbs, I think thats what I'll be using from now on. That little sucker is awesome. Also just realized that I was only talking about higher $ carbs. I have found that the Mikuni SBN 46's are hard to beat from 701 up 900,those are great carbs out of the box when jetted IMHO

Yes hard to go past stock 46 mikunis. They will run on most skis from 701 up.
A good easy to carb to tune and get good power out of.
I have used 48 novis on a 701 boat with great throttle reponse is just takes more tuning to get it thats all

 

[Freakrider]

Some of us Tampa riders run mikunis 46's with case and reed stuffers along with the intake manifold ramped with jb weld on the long turn radius to maximise air velocity. It works very well on several skis and helps regain air speed of heavy ported cases.

 

[CARBONX2]

A general guideline is to stay below an intake velocity of 320 CFM(CFM is crucial to better atomization), 280CFM is recommended for peak power. CC's and rpm of motor can get you in the ballpark of the CFM, but how well the other the parts of the engine work together(EXH, Porting, Timing, Reeds) affect the CFM too.
2-stoke literature available about carb sizes is usually for bikes, go-karts, etc. all of which have a very small obstruction in the carb due to usually being a slide type carb. If you use something like TSR software to give you a recommended size, you need to factor in the obstructions/constrictions in a PWC carb. If you took an 44mm annular discharge carb(44mm at base/butterfly), the shaft/butterfly obstruction gives a flow area equal to a 40mm. The venturi diameter must also be checked too to see if it is 40mm or larger, if it is smaller then that is the actual size of the carb. A 44mm AD carb(40mm venturi) should be able too handle a 450cc cylinder(900cc - twin carb).
Having said all that, a big factor is how well the carb is tuned.

 

[kevbo]

My experience is mainly with smaller engines (650-771) and sbn 38, 44, and 46. All my combos were leaned until response was lost and richened until restored with the top screw tuned just rich of peak rpm. This is what I found, OEM 38s with stock flame arrestor can be tuned very clean at part throttle without sacrificing response. They also make good peak rpm and top speeds. The down side is that they require full throttle inputs for any hit and the what you get is less than that of a larger carb. Moving to OEM and retail SBN44s with an OEM flame arrestor on the same 701 engine. The 44s required a popoff and pilot combo that sputtered a little around 1/4 throttle for best response. The response and hit was noticeable better, peak rpm was up slightly and top speed the same. I consider these best for surf/freeride, they were the easiest to tune and had great overall performance/economy. The retail SBN46s were on a 760 with an OEM flame arrestor. They were also fine but needed to be very rich at part throttle, any attempt to clean up the low end resulted in loss of response with the main jet/top screw leaned near peak rpm. They were simply a little big for this application but hit the hardest with the right tune. The only down side was fuel consumption on long rides, 38s will have a gallon left when your 46s run out. I expect that the 46s would behave very well on a larger engine. You can make an oversized carb work with low popoff and a restrictive flame arrestor. They usually hit the hardest but seldom net as high peak rpm because they go rich sooner than a correctly sized carb. Big I-body carbs can work too but have a lean spot around 3/4 throttle, this was the main problem GroupK was working around on the last twostroke sitdowns. SBN dont have this problem until someone modifies them poorly.

 

[Pablo]

Thanks kevbo and others that are way more knowledgeable on this topic. I've neen thinking on this some more too. Most of my thoughts on stuff like this are off the cuff so cut me some slack before anyone flames me.

There's got to be a point of diminishing returns on putting a larger intake and carbs on smaller bore motors. Is there not a rule of them for displacement and recommended intake volume/carb diameters to ensure proper Venturi effect? Same thing vice versa with large bore motors that are chocked with small bore carbs. No one is throwing stock 38's and oem mani's on a 1L setup and similarly no one is (I hope not) putting FS 49's/hogged out mani's on a basic/near stock bore engine. I wish I had a better foundation or solid understanding of how much is too much. The mid displacement setups in the the limited to big bores seem setup for 38-44's, 800-900cc range seem as though they can handle 44-48's, and big boys handle 46-49's. Is one any better or worse than another, or is completely a matter of proper tuning/jetting with lots of acceptable overlap in carb sizes that work? I've always assumed there's a specific carb size that best suits each displacement range.