Other keihen vs. mikuni carbs??

Pablo

Pablo

sqeez bth levrs & lean bk
Site Supporter
Location
georgia
This has always been a background thought in my head. It may be heresy for me to ask since Mikuni's are so prevalent for the basis of modified carbs, but here goes.

Growing up in the late 80's I rode mx bikes quite a bit and pretty much read the words off of any motocross mag I could get my hands on. I know mx and pwc applications are different, but I distinctly remember reading over and over how superior Keihen carbs were to Mikuni carbs (in mx circles). Why are Keihen's pretty much ignored for aftermarket/high performance mods on pwc's?
 
kevbo

kevbo

Location
dfw
Keihin did not have as much vendor support as Mikuni. Plus their "big" carb was called a 42 where Mukuni had a 44. They both have the same choke area but the uninformed will always buy bigger. Keihin exited the market once the boom was over.
 
Waternut

Waternut

Customizing addict
Location
Macon, GA
The Keihin CDK II's are great carbs and they sip gas compared to the mikuni's from my experience. The accelerator pumps on the 42mm versions are fantastic too for our style of riding with all the low speed to full throttle pulls we do. Cars and boats all seem to have some kind of accelerator pumps but I haven't really figured out why jetski's don't. Either way, I don't know if the 42's would have enough flow to accomodate the stroker and big standard stroke engines. I seem to remember GroupK having a good article on Keihin's at one point but I don't remember for sure.

You know me and I'm not a big advocate of following the pack but when it comes to carbs, I do like to have a general starting point for tuning and you just aren't going to get that with a Keihin on a Yamaha motor.
 
cjbchopper

cjbchopper

Location
West Michigan
Octanes came from the factory with 40mm cdk II Keihin carbs. Those carbs really seemed to limit the motor. I went to a pair of the blue sudco 42s and got a nice improvement in power. From there, went to a pair and dual 46 SBN's and the power gain was HUGE. From there, I never bothered playing with keihins. Keihins seem reliable and smooth, but the SBN seems to make a lot more power IMO. Most of my local tuning buddies strongly agree.
 
Cannibal

Cannibal

Tasty Human
Location
Summit Lake, WA
Jump On It said:
I recall John at JSS saying that he really likes the 40mm I series for our application...
Click to expand...

I had talked to Art about this too, he really liked them for SJ application as well, but nobody makes a manifold. He also can make them even better by doing a thin tube atomizer on them as he no longer recommends Blackjacking these because he's found they can sometimes load up under certain circumstances. He was also getting better flow from the thin tube atomizer than he was from the Blackjack mod because of being able to open them up even more.

I currently have Blackjacked 40 I bodies on my SXR. I will probably see how they work after my engine rebuild and maybe have him thin tube my left over stock carbs someday.

There was someone selling a 61x case that had 750sx intake welded to it. I though about that as a cheap way to give some 40 I's a test on a SJ. I need to see if that's still available.
 
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kevbo

kevbo

Location
dfw
Mikuni I-body carbs have larger venturis than SBNs relative to the butterfly diameter. The I-38, I-40, and I-42 have the same bolt spacing and are easily adaptable with a protec 90 degree manifold and linkage, I never measured I-44 and 46. They wont pull fuel as well as an SBN with the same choke area so be prepared to use larger main jets. An SBN44 will usually require smaller main jets than an I-40. With this in mind a giant I-46 may not work on smaller engines without a restrictive flame arrestor.
 
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