Topic: benefits of Keihin CR vs Keihin factory round tops on a cb750

[gschuld]

TurboD,

Don't be discouraged.  I for one am very interested in your input, and testing.  I would love to get you a set or CR carbs to test along with a bored carb or two.  I had the same reaction to the hot bike article.  Their assessment of the round top carbs somewhat contradicted itself in regards to results in different places in the write up.

Brent,

I have never heard of anyone recommending bored keihin round top 28 over CR29s for a road race application on a 592 motor.  But went to CR 31s once building a 675 motor.  Interesting.  I wouldn't have guessed.  I'll have to look into Tom Marquardt.  It's interesting to me that a road race 675 would need CR31s to feed it.  That's more carb than I though you'd need at that size.  My small engine is a mild 836, and very likely less power output than that road race 675, so perhaps I am underestimating the airflow needs of the larger engines.  Sounds like bored round tops at 29.4 or even 30.5(about the limit) would be restrictive on anything more powerful than a mild 836 ideally on a road race track.

A 10,000 plus rpm 12.5:1 compression, ported, big cammed 890 motor would ideally want what CR33s?  ...CR35s?

Alan,

Your comment on CR carbs being easier to tune is certainly worth consideration.  That would be a big plus especially if regular tuning beyond being dialed in initially is a regular deal.


Pops911,

Yes, I was thinking about the VERY different application between road racing and drag racing.  Funny I don't recall CR carbs being very popular with drag racers, but are generally THE first choice for road race carbs on powerful sohc4s.  Must be a reason for that...  and I wouldn't mind learning WHAT specifically makes them so desirable for their intended purpose.  I like to learn...

George

[TurboD]

POPS In my original comment I covered and took into account the throttling elements that you are referring to.

POPS I've lost those kinds of races far too many times to count. Lol

[Bill/BentON Racing]

Yeh D ,we want to know. Appreciate your input as always,  Bill

[cbr954]

I am running CR31 on my 970 F2 headed street motor.  Noticeable improvement in acceleration over the stock carbs I was running.  Only thing I dont like is the cold enrichment setup, it easily floods the motor on startup, wish they just had choke plates but then again they are developed for racing.  On the upside the needle position can be changed in about 1 minute per carb...very easy to adjust.  Going to be running CR33 on my drag bike with 1030 motor, will see how that goes if it ever stops raining and snowing every weekend!

[bwaller]

George, Yoshimura used vintage 31mm Keihins on his CB500 based (maybe up to 592cc) race bikes early on. This does seem large but he certainly made them work so 31's on Tom's 675 would not be. Tom claimed he liked the power delivery with the bored 28's and had tried the CR29's too. More than that I just don't know but Tom was a very accomplished rider & tuner.

[napoleonb]

I for one am very interested in this thread, if I can be of any help please let me know.
Some time ago i took my CR31's apart (except for the bearings) and started measuring these up.
https://grabcad.com/library/honda-cr750-cb750r-keihin-cr-race-carburettor-1

It's a real pita but am nearly half way, just to help me grasp the secrets i and bought a CR/FCR book from Japan.

[gschuld]

Brent,
 
   Naturally, I'm hesitant to generalize carb size need based on CCs only.  I'm sure a full race vintage Yoshimura sub 600cc race motor winding out to what 12-13,000 rpm or more with a very high rpm power band needed a pretty big airflow numbers to keep up. 

   I'm curious as to what a set of bored carbs can handle, as in at what point is the power level too much ...mild cammed 836, more....  I did a set out to 29.6 and I can do a set a bit further to 30.5.   If I went with the high compression, ported, big cammed 890 I imagine that the motor would at high rpm want well more than even a set of bored 30.5s could handle, probably CR33s or CR35s would be needed for it.

"Tom claimed he liked the power delivery with the bored 28's and had tried the CR29's too.  More than that I just don't know but Tom was a very accomplished rider & tuner."  That's a very encouraging statement... at least in reference to sub 600cc sohc road race motors.

George

[scottly]

Here is a back-to-back dyno test of 30mm carbs vs stock 28mm, on a 736, also from the stone age.

[TurboD]

Going from what is seen and written in the picture, my guess is they are comparing stock carbs with stock air box with stock jets vs new carbs with bigger jets with nice velocity stacks. And Only just barely 5hp? I wonder what the hp gain would have been with stock carbs that were properly jetted along with nice velocity or even pods, my guess more than 5hp...

[gschuld]

I was wondering more or less the same thing.  The specific details of the text parameters for both the stock carbs and the 30mm throat webers will make a sizable difference.  I believe even RC Engineering didn't encourage customers to buy their weber kit unless they had at least an 836 with a cam upgrade.


Regardless, I enjoy collecting old articles such as these for my archives.  Scottly, would you mind giving me the magazine info for that article.

Which magazine, year and month? 

Thanks,

George

[scottly]

This is where I got it from, George. Lots of cool old stuff.
http://forums.sohc4.net/index.php/topic,126071.0.html

[teebee67]

One of the advantages of Keihin CR carbs is the fact that they are "smoothbore" carbs thus reducing turbulance through the carb. Some tuning methods rely on smoothbore carbs to be really effective.

[gschuld]

This is where I got it from, George. Lots of cool old stuff.
http://forums.sohc4.net/index.php/topic,126071.0.html

V
Thanks.  Found one and it's coming my way.  I love the old stuff.

George

[POPS 911]

gschuld : Are you a Road Racer, Drag Racer or a ECTA-LSR as your comments are a MIT grads work with lots of facts and figures. Great job on those blended carbs and that took much super fine good work to get them in that order. Round slides and flat slides carbs will be a thing of the past with FI making all two wheels turn up the power [ did not NASCAR show the folks ] a wrench and a jet in hand is no match for a RACE PAK AND A LAP TOP. Would really like to see your bike race at event, as I see only the best in technical things place on your bike.

POPS BK911 NHRA

[gschuld]

Very kind words POPS.

Thanks.  Frankly I like a bit of everything.  I like vintage cb750 hipo stuff in general, period, in all forms really.  I clearly have a "thing" for RC Engineering history, motors, etc.  This lead me down the path with drag racing once a certain dragster frame decided it wanted to follow me home

Am I a drag racer ...no.  I doubt that I will ever really get "into it".  But there is an enjoyment involved with learning the history, assembling the necessary parts, fabricating others, and putting together something rare, vintage, and fast like an old single engine dragster and putting in a big bore early motor full of vintage speed parts assembled, or reassembled and improved by the best around(That's you MRieck )

Vintage road racing is something that appeals to me more on a personal level.  Learning about the history/mechanics of it all is fun all in itself.  Then putting a vintage road racer together, making it handle/brake well, and learning to become a more proficient rider would be very satisfying to me regardless if it becomes a long term passion or not.  Being around others running similar style bikes on road courses dedicated to vintage bikes sounds like fun(My wife and I went to an AHMRA road race event, and had a blast).  With drag racing, you generally have a handful of vintage bikes surrounded by modern stuff, cool, but not nearly my thing.  I really enjoy being around the history and vintage aspect of it all if that makes sense. 

Land speed racing.  Well, that would just be for giggles on the dragster.  Maybe some day, something to look forward to at some point down the road.  In the end, I enjoy learning about vintage racing, in all forms.  Being able to participate in them myself, even if it ...could ....be short term, would be a worthwhile and enjoyable experience.

Perhaps I should consider it a bucket list thing for a guy who is young enough to start on it early and has the time to cross off lots of things... Perhaps something will really stick...  Road racing is the most likely motorcycle wise.

It's funny you mentioning that round and flat slides becoming a thing of the past in favor of laptop controlled fuel injection, along with programmable electronic ignition.  I agree 100% with what you said.  But from my personal perspective the more something is considered outdated and a "thing of the past" the more I appreciate and am drawn to it .  Once again with respect to my mechanical interests, I am a troglodyte.

George

[Jerry Rxman Griffin aka MuthaF'er]

Here is a back-to-back dyno test of 30mm carbs vs stock 28mm, on a 736, also from the stone age.

Uhhh, I live near Denver at elevation..... Where are those instructions they refer to? Guess you might have to come visit for a tune session one day! 

[Old Scrambler]

My RIDER"S GUIDE is oil-soaked..............been on my bench since it was new   Not all Gospel...........but it was proven advice in its day.

[turboguzzi]

gee guys, i leave you for a day or two and this is what happens? I been slashed here in the past for expressing an opinion without  seemingly having enough experience at least by that person's standards and it wasnt funny. But this is the internet and for good or bad, everyone can have an opinion, it's up to us to filter and decide what information is relevant and to whom to listen. And BTW, the same logic applies to articles form magazines, not every printed word is holly... Even Kevin Cameron makes some dubious statements sometimes and he's my hero...

Back to the the subject, as TeeBee says, CR are smoothbores, that means that at WOT air has no turbulence inducing gaps or edges in it's tract. In a same size non-smoothbore (as most street carbs are) the slide has a floor that at WOT is slightly recessed out of the flow. This   recess will cause some turbulence and hence a drop in max flow. So the key CR flow advantage happens mostly at WOT while in partial throttle situations, the lack of that "floor" or rather "roof" in the slide makes it carburate a bit less clean with partial throttle. The nice thing is that in road racing, both conditions are important, so it's almost a matter of rider preference and type of track. Will it help my cause if i told you that this very weekend, the outright winner of all vintage classes in my championship was an overbored 650/740 wearing stock 650 carbs (bored out to ?) and yes, these are not smoothbores for all i know. Marco, the rider and his tuner father have been beating with their 740 all the open class bikes for the last year and a half, and ruling of course the 750 class against some fancy Ducatis...   
Had the pleasure of following him (sort of) around the esses in last weekend's race and he's an amazing rider. 
and BTW, no bracing on the frame whatsoever, go figure...

[BPellerine]

Here is a back-to-back dyno test of 30mm carbs vs stock 28mm, on a 736, also from the stone age.

strange when you look at all the diff setups for webers and the diff jetting,135 mains on a stock 750?but this article confirmed that they did make the kits at some point for the 736.they seldom list pump size etc.bill

[bwaller]

Hey TG, I don't need to tell you...some guys ARE just plain fast no matter what they ride, good for Marco. The rest of us fill out the grids. 

[calj737]

Hey TG, I don't need to tell you...some guys ARE just plain fast no matter what they ride, good for Marco. The rest of us fill out the grids. 

If it weren't for winners, there'd be no losers...

[scottly]

Here is a back-to-back dyno test of 30mm carbs vs stock 28mm, on a 736, also from the stone age.

Uhhh, I live near Denver at elevation..... Where are those instructions they refer to? Guess you might have to come visit for a tune session one day! 

You'll have to bring the bike to AZ, Jerry. It's only 4500' here, but it would be easier to transport the bike than the dyno. Contrary to popular belief, power will decrease with an increase in elevation, and no amount of jetting will restore it. I've ridden my bike over Monarch Pass (11,300' for you flatlanders ), and while gutless, there were no problems with fouled plugs.

[Retro Rocket]

TG, I like that tank and tailpiece, do you happen to know what they are off..? Tank kind of looks like a HB1 and the tailpiece looks early Honda RS / NS / NSR....? 

[turboguzzi]

DIY by marco's father... that's a dad!

[bwaller]

Not much offset. Wonder what the front end belonged to?