Topic: Need some carb advice on Keihin CR carbs

[Swoop]

Hi guys,

I bought a set of carbs for my Seeley that I was told were CR31's.
When I measure the outlet I'd of the carbs it measures 33 mm id.
Are these CR 33's? Any other way to check?
Is this too much carb for my motor?
Motor has ported head, oversize intake valves, lightened crank w/carillos, had springs, Harmon D grind, 836 Wiseco's and free flowing 4 into 1.
I currently have some bored out stockers that are performing quite well.

I appreciate your help.

Thanks

Steve

[gschuld]

I know a few road racers that very much prefer CR29s over the CR31s  they also tried when running strong 836 motors like you are running.  I would think 33s would be much more than you need.  31s at the most.  FWIW, I have no first hand knowledge on the subject, but have done a fair bit of research regarding those who have.

On another subject, I'd like to hear more about your bored out stockers.  It's a personal interest of mine...

George

[SOHC4 Cafe Racer Fan]

The CR29s are better for the street.  The CR31s would be better suited for top end speed racing application.  I assume this is a racer??

[Swoop]

On another subject, I'd like to hear more about your bored out stockers.  It's a personal interest of mine...

George
[/quote]

All I know is that they were bored 1 or 2 mm larger and from what I feel they are very close to the 29 smoothbores that I had.

This is for the street.
From what I have researched it looks like these are 33 mm CR's ..... Which will be up for sale soon as I have no plans of going over 1000 cc.

Thanks
For your help.

Steve

[SOHC4 Cafe Racer Fan]

33 is overkill especially for under 1000cc.

[gschuld]

Swoop,

    These are my round top bored carbs I did a while back compared to a stock carb( bored carb on the right obviously).  They measure out to be the equivalent to 29.4mm throat size.  Bill Benton managed to post just north of 100hp (with oxygenated race fuel) with them so that would easily equate to lower to mid 90s in HP on normal fuel.  He did this with a 915cc motor with a top of the line Mike Rieck stage 4 ported head, great pistons, etc, etc, and a RC327 camshaft.  Though the carbs were clearly not tuned for regular fuel while on the dyno, it proved that the bored carbs are capable of flowing enough CFM to support well north of 90hp on regular fuel.  It does NOT, however, prove that bored carbs are as easily tunable, or even CAN be tuned as well as properly dialed in CR carbs or smoothbores.  A set of properly tuned smoothbores or CR carbs may well have proven to make better power in Benton's 915 motor.  But since it was the only set of carbs he tested that day on that motor, that's what we have to go on.

    I'll venture a guess and say that a properly dialed in set of bored carbs or especially the 29 smoothbores would be hard to beat for street use on your hot 836 motor.  A pair of CR31s might make better power in the very upper rpm range, but generally at some expense of low and midrange drivability.  I know some racers running some very well tuned 836 based race motors, one a former multiple year class champion in USCRA heavyweight supervintage, and he has run both CR 29s and CR 31s, and says without a doubt the 29s are better for the tighter tracks he races.  If he was racing at Daytona, Mosport, etc(fast tracks) he might favor the 31s.

   Unless you really like the idea of running CR carbs, you may end up disappointed with the ratio of money spent to performance gained going with CRs over good bored carbs or smoothbores.  Especially on the street.  That said, a Seeley framed bike(I'm officially jealous ) with such a well sorted 836 would be complimented nicely with a set of classic CR carbs.  If that's the case, I'd consider going with CR29s over the CR31s. 

   Others here with far more hands on experience will likely step forward soon.

Just my 2 cents.  Feel free to post pics of your Seeley

George

[Swoop]

Hi George and thanks for your great information.....very informative!   I may well stay with the bored stockers.
Here is a pic of the Seeley just prior to my road testing.

Thanks

Steve

[SOHC4 Cafe Racer Fan]

That's a looker with all of that polished aluminum.  I guess you needed the Ohlins shocks to balance out all the gold/yellow bits on the front! 
Very nice, bike.

[gschuld]

Very nice.  She looks to be in good hands

George

[Swoop]

Thanks guys,

I had been sitting on the frame, tank and seat for 7 years while I collected bits and pieces. I put the push on this year to try and finish it.
Actually the painted front parts were to match the Ohlins.....got a good deal on them.
Still a few gremlins to sort out...nothing major.

Now on to the KO.

Cheers,
Steve

[BPellerine]

it has the distinction of being the only one in Canada too!!bill

[gschuld]

I was just up in Ontario watching Brent and Scott race at Mosport.  Well I'm glad that you got her all together.  If you get bored of her sometime, let me know.  I always wanted to race a Seeley.  All I'd need is the frame, tank, and swingarm.  You could build the KO with the rest.  Super easy build...   And just how you like it.  You could build it and be riding in a week, tops.

Seriously, I'd like to buy a Seeley frame and swingarm as the basis of a proper vintage racer, with or without an original tank.  As Seeley owners tent to seek out oyher Seeley 9wners, let me know if you run across one in your travels.  A Rickman would also work.

Good luck with your carb decisions.

George

[01Thomas]

Correct me if I'm wrong but the OP said he measured his CR31 carbs Inside Diameter at the outlet and gets 33mm.

Surely carbs are rated by the size of their venturi? That's the smallest part of the carbs' bore, not the outlet nor the inlet. So isn't it likely that he in fact does have 31's and not 33's. That changes the picture somewhat.

cheers
Thomas

[Swoop]

Hi Thomas,
Thanks to your post I went back and measured the area that you mentioned. It appears that they are in fact 31 mm. I think that I will give them a go. I will just have to modify the outlet adapters as per MRieck's earlier post.

Thanks again

Steve

[turboguzzi]

just in case it helps, VM26 can be easily bored to true round 29 ID without the trouble of the oblong as in george's pic.

they are not smoothbore, but actually for street riding, i.e. not WOT all the time, that's a good thing.

[Swoop]

Thanks for that info.

Steve

[bear]

I'm with Turbo on this one.

I have run CR's on my race motors for going on fifteen years, but if I had a street bike I would be useing something similar to what you are useing at the moment.
Easy reliable starting and low speed operation are not the CR's strong points.

Cheers,
Brian

[PeWe]

I have a similar engine (K6). 836, ported head with 34mm IN (F2 valves as oversize), cam now close to RC295.
I have Mikuni TMR32 that work really well from idle to WOT with a loud 4-1.  These carbs has a wonderful throttle responce. A little bit too good when the smallest twist will stretch my arms. Full throttle in less than 1/4 turn! Maybe 1/5 turn of handle. Low lift jetting is very important to get right on a street bike/tourer.
Drive with WOT different. TMR has acc pump squirting fuel...

I know since I made it run well on Dyno with a hotter cam that has more overlap, later cam with less overlap ( but higher lift), richer pilot + fuel screws for good low response, larger mains for more torque on 5:th gear from relatively low speed.

Back in the 80's it went very smooth with OEM carbs.
Mikuni VM29 smoothbores were tricky to jet to perfection.
Since yesterday 4-4 exhaust, HM300 replicas. Rain has stopped a test drive. It sounds nice without baffles. Less than my 4-1.
I'll see if I need to rejet.