Taper bored 38's on a 845cc Big Bore?

[OCD Solutions]

I was doing some research and see that Group K actually reccomends a set of true "taper bore" modded OEM 38's for use on their 845cc Superjet package and wanted to see who was actually running them in a bore that big and with what results. I guess I have 2 questions about this.

1) What is the biggest bore you have run with dual 38's and what mods were done to them for it and,

2) If you swapped up to a larger set of carbs what were the differences in acceleration, fuel consumption, overall performance that you noticed?

http://www.groupk.com/y845.htm

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.

 

[Shifty]

I have the 845cc package, it was my friends. GroupK recommended he run the 44's from the OEM 760 engine. He was not concerned with fuel consumption. The engine has never been run, I will get it in my ski over the winter but I will be running 46mm BlackJacks when I do.

 

[Talsivar]

i ran taper bored 38's on my big bore 85.5 for awhile. ran good and strong reliable. but i went to 44's and wish i'd done it sooner huge gain on low end with the 44's.

 

[Aircraftsalz]

Boyer aren't you build a Xscream SS motor? If so duel 46's, stock 46's done by Xscream. Running that set up on my 6mil/860 & Xscream SS motor.

 

[OCD Solutions]

Boyer aren't you build a Xscream SS motor? If so duel 46's, stock 46's done by Xscream. Running that set up on my 6mil/860 & Xscream SS motor.

Yeah, I'm just doing some research. I was shocked to read of 38's on that big of a bore and was wondering how much they are holding me back on my 760. I put my TS on hold and ordered a Matrix hull instead so i may have to run it with my 760 for a few months.

 

[orangefinger]

dual oem 44 's on our ported 760 suck...very happy with dual 46's TS on 771cc and these can remain with growing engine...

 

[Aircraftsalz]

dual oem 44 's on our ported 760 suck...very happy with dual 46's TS on 771cc and these can remain with growing engine...

Boyer this is what I would do. Buy the Stock 46's done by Xscream with the Boyeson intake. Then you got the Carbs set up for the 865SS or 1000cc motor! You can't go wrong with Xscream, Idok told me about the hull, congrats! Can't wait to see it at the spot.

I heard oem 44's suck, More of a Sit down (wide open carb).

 

[waxhead]

What jetting do you have on your dual 44s

 

[OCD Solutions]

Boyer this is what I would do. Buy the Stock 46's done by Xscream with the Boyeson intake. Then you got the Carbs set up for the 865SS or 1000cc motor! You can't go wrong with Xscream, Idok told me about the hull, congrats! Can't wait to see it at the spot.

That was my plan.

 

[Talsivar]

i'm not going by what i "heard" just personal experience. i'm sure the 46 blackjacks are much better. but the difference between 44's and taper bored 38's is night and day. i have powerbomb 48's too but was struggling with tunning them. As hard as it hits with the 44's i haven't felt any motivation to mess with the powerbombs. I only ride surf and can only compare the difference in low end hit. wax i'd have to look it up i have it written down somewhere.

 

[waxhead]

If i had to rate them in order it would go like this

38,s on stock ski only
44, mikunis
46 mikunis
46 novis
48 novis
I have tried full spectrums found them to not have the same midrange as the new novis ( being polite about spectrums here)
I havent tried black jacks so I cant comment on them
Some people love them , all good for them
lenzi was running novis this year didnt seem to hold him back

 

[LBE]

I have a built 760 and running tapper bored 38's. Art at Jetworks recommended that set up.
I also have a built 760 in my B1 but with stock 44's.

The motors are identical in mods, carbs are the only diff, and I can't tell the difference power wise.

I realize you are running a larger bore motor, but just thought I would chime in as I'm running both types of carbs.

 

[masterblaster]

lbe,what is the diff in the fuel run time? We tried a 701 lim setup once with 38s and 44s and did not notice that much diff as well,all the improvments in low end power seem to start with 46s and up

 

[2XLR8]

If LBE says that there is no noticeable power difference between taper bored 38's and a set of dual 44's yet people who switched to 44's from 38's report a big increase in performance, then maybe his 38's are more modified than your usual taperbore modification that is commonly done like at group K .
I am sure the more modified and or larger the engine is the more it will benefit from bigger carbs.

 

[tipafew]

I noticed a big difference going from stock 38's to 44's on my baby scream set up. I found a cheap set of Pro tec carbs and rebuilt them. They have run night and day better for me.

 

[dbrutherford]

I am not trying to bash Group K but you have to realize they are a machine shop business. So they probably make more off of boring your carbs rather than selling you other new carbs. Just my $0.02. I know YZ ran a set of LPW bored 38's on that white ski you picked up. It ripped but then again it was only a 701.

 

[steveyz250fpilot]

I am not trying to bash Group K but you have to realize they are a machine shop business. So they probably make more off of boring your carbs rather than selling you other new carbs. Just my $0.02. I know YZ ran a set of LPW bored 38's on that white ski you picked up. It ripped but then again it was only a 701.

Those were stock, rejetted 38's.

 

[ADDict]

After a phone call to Art recently, he convinced me to do the black jack mod on a set of 38's. I called him with intentions of mailing him my a/m 46 for modding, and he basically wouldn't let me. He steering me towards the 38's, and explained it all completely as to why they would be best for me. most of what he said was way over my head, but it sounded good to me.

 

[HLS30]

I had a set of 40 mm Mikunis done for my Kawasaki at Jet Works. They were done with the thin tube atomizer that completely eliminates the booster venturi. They now look like a 42 mm Keihin. The response was much better with the biggest improvement being the midrange. The time it takes to windup to max rpm is much shorter.
When I talked to Art he basically said if you are thinking of going to 44's or 46's then just do it, but if you are not then this mod is the way to go to get the most out of the stock carbs. What I do remember is him saying he did the same mod to the Superjet 38's and he said that modified they worked better than any single big carb. He thought it was foolish to spend so much money on a new modified big carb and still have to spend big bucks on a special manifold. All I can say my 40's pull much harder than they were stock and feel more like 44's.

 

[masterblaster]

didnt know he did 38s with the bj mod,thought it was a diff setup for those carbs