Topic: Anyone with Keihin CR Roundslide carbs on their CB750?

[jerimiah797]

I'm thinking of picking up a set of these. They make a quartet that's set up for the CB750. They have them in the regular (29mm) or oversize (31mm) sizes. I'm not sure what size to get. Anyone used these? Likes/dislikes? They claim a 10% performance increase off the bat, which would be great, but mostly I'm thinking that they will be a nice way to modernize the engine/throttle performance of the bike.. not sure I want to muck about too much with re-jetting my stock butterfly carbs if these are really that awesome.

Thanks for any info.

-Jerimiah

[jerimiah797]

Here's a link if you haven't had a chance to check them out:
http://dynoman.net/bikepages/sohc/carbs.html

[Sam Green Racing]

Go for the 31s, They work well on a stock motor with a good ignition system and free flowing exhaust.
The reason I gave for going for the larger bore is, if you ever decide to go larger displacement, they will be ideal.
One thing to take in mind though, these are racing instruments and are quite difficult to get spot on for slow riding.
Allthough the CR in my avatar can be ridden slow, it will soot up the plugs and show a rich condition.

Sam.

[jerimiah797]

Good to know. I've got pods on the intake right now and a fairly open Kerker 4-1... Oh and a Dyna S ignition with the 3-ohm coils. I feel like my carbs are about the only thing holding me back right now.

-Jerimiah

[Sam Green Racing]

That and your cylinder head, see this thread and have a word with Mike.

http://forums.sohc4.net/index.php?topic=40311.0

Sam.

[RRRToolSolutions]

I still don't know what I should....

Like why Buzz was so darn determined that going to 31's on my big-bore 836 with cam and ported head would be a huge mistake - huge he said.

The 29's and 31's are exactly the same price - he had both in-stock and I could have had either, but he strongly endorsed the 29's as being the faster in every real-life and road race track situation. The man has been around racing for 40 years - I listened and bought his 29's. I love them and I have never wanted for more carburetion that I know of.

For anyone looking at the stock 28mm carbs in comparison to the CR 29's - you see something immediately obvious - you can almost stick your hands into the 29's venturi - it looks twice the diameter of the stockers. Why they call the Honda carbs 28mm I  don't know that either. It narrows down to about 22 in the center and the slide is tiny - clearly the CR's are kick-a$$ and will make power.

I'm extremely happy with mine. Do read some of the old post here though, the installation can be simple of something of a real PAIN.
1) put your manifolds on the bike first (new ones are even better) and line them up to exact parallel - flat perfectly straight
2) get your throttle working on the bench - don't learn that your bracket has to be threaded or drilled once on the bike
3) figure out what you're going to do about the idle setting screw before the install. There is no access once the tank is fitted - remember these are road-race carbs and normally run on stripped-down bikes without stock seats and tanks.

Regards,
Gordon

[Gamma]

What type are the air filters Gordon?

[RRRToolSolutions]

K&N's - They are sold to fit the CR's and do so very nicely. I could find the numbers if it will help.  - Wait - Buzz has them listed on his website -

www.dynoman.net

Regards,
Gordon

[MRieck]

What type are the air filters Gordon?

You will need them Gamma.

[Terry in Australia]

Yeah that 29mm vs 31mm thing has me slightly confused too. Mike got me some 29mm CR's because he couldn't get 31's at the time, and they work fine on my 836cc engine with Mike's stage 3 head, Megacycle 125/75 cam etc, but according the the documentation that came with the carbs, they recommend 31mm CR's for my engine capacity, and larger.

Now apart from that, my confusion stems from the fact that the recommendation for carb improvement on my other infatuation, Suzuki's magnificent GS1000, (and Kawasaki's Z1 900 or Z1000) DOHC "Musclebikes" of the era, was always Mikuni's venerable 29mm "Smoothbore" carbs. Does this mean that the Mikuni's flow better than the Keihin CR's perhaps?

I have a set of Mikuni 29mm smoothbore's sitting in my garage and I'm tempted to try them out, but I do like the Keihins (it's really nice having new carbs that don't leak 100 octane fuel all over your fresh engine paintwork, and reason enough to buy a new set, IMHO) so I'll probably just stick the Mikuni's on EBay. If anyone knows why Mikuni 29mm smoothbores flow as well, or better than Keihin 31mm CR's though, I'd be interested to know? Cheers, Terry.

[Gamma]

Thanks Gordon and Mike,
 So 29 or 31?

[jhasewhite]

I've spent the last month tuning my 29cr's on a stock 750f motor with a Dyna 2000 and open kerker and the bike still has that "over - carbed" feel to it. Not very progressive or drivable off idle and very touchy on throttle position during acceleration.  These are race carbs, The choke system doesn't really work when you get it from Sudco, jets are pricey and there are no vaccuum ports to do a carb synch (you install your own) but they do make a whole lot of power. I would say the 31's would be overkill, even for an 836 motor.

-j

[RRRToolSolutions]

29mm - only because Buzz told me so. Again, his words were - "every track comparison we've done and seen those 29's came out on top". To see these 29's next to the stock 28's it's like drinking through a big-mouth jug and a straw - they are 29mm al of the way through whereas the only the lip is close to 28mm on the stockers. I personally don't think I'm using all of the carburetion available on the 29's.

I've got 836, big cam, Kerker, and a ported head and I have to ease into the throttle with these 29's. The bike will bog/stumble if opened abruptly anywhere less that 5,000 rpm. My seat dyno says they are only "slightly" better than stock and only at the 9,000 up range. I say this based on the stock 28's (now with Seaweb/Derek) the front would start lifting the front on accel at 8,000 in 2nd and hold it there until about 9,500 rpm. With the CR29's - the lift is still in the 8,000 area, but doesn't set down until it's in the 10,200 range.

The off-idle and mid-range was better with the stockers. It started better, drove better period. I hope Derek found the same - those were jetted and set-up nice. The CR's big venturis are over-sized and don't mix as well in my opinion until the rpms get in the 5 digit range. That's my butt dyno again.

Gordon

[lordmoonpie]

Ah but which Keihins are you referring to Jeremiah? Do you mean the lovely new ones with the choke levers that are excellent bits of kit or the ultra rare 31mm race kit Keihins with the ticklers?

Aaaah what does it matter - they're both ACE! 

[Sam Green Racing]

This is a set of modern 31s in action.

Sam.

[MRieck]

Thanks Gordon and Mike,
 So 29 or 31?

I'd use 29's on your engine Gamma.

[Terry in Australia]

This is a set of modern 31s in action.

Sam.

Geez it doesn't really seem to rev that hard Sammy, I'm surprised? 

[Gamma]

I will probably go with the 29's plus K and N's, just got to watch the exchange rate to get a good deal 

[Terry in Australia]

I will probably go with the 29's plus K and N's, just got to watch the exchange rate to get a good deal 

You're lucky you're not in Oz mate, our dollar dropped a whopping 20 cents against the greenback in the last week or so, what a kick in the nuts!

[sparty]

I still don't know what I should....

Like why Buzz was so darn determined that going to 31's on my big-bore 836 with cam and ported head would be a huge mistake - huge he said.

The 29's and 31's are exactly the same price - he had both in-stock and I could have had either, but he strongly endorsed the 29's as being the faster in every real-life and road race track situation. The man has been around racing for 40 years - I listened and bought his 29's. I love them and I have never wanted for more carburetion that I know of.

For anyone looking at the stock 28mm carbs in comparison to the CR 29's - you see something immediately obvious - you can almost stick your hands into the 29's venturi - it looks twice the diameter of the stockers. Why they call the Honda carbs 28mm I  don't know that either. It narrows down to about 22 in the center and the slide is tiny - clearly the CR's are kick-a$$ and will make power.

I'm extremely happy with mine. Do read some of the old post here though, the installation can be simple of something of a real PAIN.
1) put your manifolds on the bike first (new ones are even better) and line them up to exact parallel - flat perfectly straight
2) get your throttle working on the bench - don't learn that your bracket has to be threaded or drilled once on the bike
3) figure out what you're going to do about the idle setting screw before the install. There is no access once the tank is fitted - remember these are road-race carbs and normally run on stripped-down bikes without stock seats and tanks.

Regards,
Gordon

Gordon and Gamma,

I too have the CR 29s on my steed.  Buzz put me in contact with a guy who races with the CRs.  He did a real life race and road test with the 29s vs. the 31s.  The result was that the 29s were by far FASTER than the 31s.  Why?  He told me that the 31s tended to overfill the chamber and lead to less than ideal combustion which, in effect, reduced the power.  However, on a bike with more than two valves per cylinder the 31s worked better.  So it is a no brainer - 29s are the key to better power on a two valve bike.  In this case bigger is not always better. Even with oversized valves like I have the 31s would provide too much fuel and air to burn completely even at the top of the RPM range.  I would bet that it would sputter like heck down low with the 31s.  It does so with the 29s like Gordon said.  If you went with the 31s you would have to jet them and make needle changes to perform like the 29s, so why bother.  Finally, with the 29s I am running rich down low and near perfect up top, but you wouldn't run up top most of the time.  The 31s would be super rich down low and remain somewhat rich up top unless you jet them.  The 29s WILL give you a 5 to 10% increase out of the box on a stock engine, but much more with a worked head like Gordon and I have.  I am really glad that I did my homework before I bought the 31s.

Sparty

[sparty]

Go for the 31s, They work well on a stock motor with a good ignition system and free flowing exhaust.
The reason I gave for going for the larger bore is, if you ever decide to go larger displacement, they will be ideal.
One thing to take in mind though, these are racing instruments and are quite difficult to get spot on for slow riding.
Allthough the CR in my avatar can be ridden slow, it will soot up the plugs and show a rich condition.

Sam.

Sam if you and Chris were running the 29s you would see mid 11s.  I bet you are losing more than 5HP with those 31s and his big arse.   See if you can borrow some 29s for a trip to the track.  You will be impressed.

Art

[Jerry Rxman Griffin aka MuthaF'er]

Hey guys,

Let's get Mike's input on his 31's on his 900.

[754]

What about in between??

I run 30 mm carbs on stock and and 836.. with great results..

 Not keihin, and not slide type.. I tried 32's but it was harder to run off the line and throttle response suffered..

Plus I get better mileage with the 30,s than a fairly stock 78 750 K.... and that was with a tired 836..

[MRieck]

Hey guys,

Let's get Mike's input on his 31's on his 900.

They work very well Jerry. They carburete well throughout the RPM range. How well the larger carb works depends on the strength of the signal to the carb....obviously the 915 provides a strong signal that allows for good carburetion with the larger carb bore. I've used 29's on the engine (though with 33.5mm intake valves as opposed to the 34mm units I run now) and they was little difference except for a couple of extra HP on top. If you really want to learn about fuel systems I'd HIGHLY recommend "Motorcycle Fuel Systems Techbook" by the late, great John Robinson.

[Sam Green Racing]

This is a set of modern 31s in action.

Sam.

Geez it doesn't really seem to rev that hard Sammy, I'm surprised? 

It revs to 8,500 Terry, the same revs that Honda designed it to rev at as a road motor.
We could have made it rev higher, we could have built a race motor with more power.
What we wanted was a motor that could retake some of his strip records that had been lost to Z1s and the triple two strokes.
We didn't want a motor that we would be stripping down every 5 minutes, we wanted a trouble free few years of racing and that's what we have had and It's retaken his lost records.
As you know Terry, the more power you build into a motor, the more money you have to spend on building reliability in to it, Carrillo's etc.
We got what we wanted and the bike has done as it was asked with no fuss, that's all that Chris wanted.
If you want a screamer producing 110 plus HP no problem, if you want a 400+ blown nitro drag motor no problem (speak to you later Jay ) It's all about how fast you want to go and how long you want it to last. Now is there anything else you want to know while I am rambling? 

Sam.