Topic: Road racing with K0 carbs

[gschuld]

Are any of the vintage racing organizations out there (WERA, USCRA, VRRA, AHRMA) currently banning single pull throttle cables, even if they were factory stock?

I tried to leave myself the option for going either way, but I ended up with a clearance issue with the K1 on up rack type, dual cable push/pull setup(I'm running dual early petcocks) The single pull K0 type is my preference anyway(I'm wierd)

I don't have all the class rules in front of me at the moment.

I have plans to race in some AHRMA, VRRA, and USCRA races this season. No current plans for WERA at the moment.

I'm thinking maybe it was AHRMA that discouraged them.

Thanks,

George

[turboguzzi]

regardless of the rulebook, do dual cable just for your own safety....

[slikwilli420]

Those aren't the bored out carbs you were showing a while back are they George? If so, those aren't kosher for AHRMA anyway. Either way, I think I would look into a clean rack of newer carbs. The push/pull cable does offer additional peace of mind as TG mentioned.

[gschuld]

Those aren't the bored out carbs you were showing a while back are they George? If so, those aren't kosher for AHRMA anyway. Either way, I think I would look into a clean rack of newer carbs. The push/pull cable does offer additional peace of mind as TG mentioned.

Yes, the bored carbs are among the sets I have on hand for use.  I believe AHRMA only restricts factory cb750 carbs to be unmodified in the Sportsman class(which I would not qualify for).  Classes like Formula 750 and heavyweight Superbike have no such restriction as far as I understand.  Guys commonly run CR33 carbs, much bigger than my bored factory carbs.

I'd be running a totally 1969-70 factory correct throttle setup.  Factory K0 throttle control on the handlebar, NOS factory K0 4 into 1 throttle cable, factory K0 carbs and slides.

Are we concerned that what Honda supplied on hundreds of thousands of Honda cb models, including MANY models other than the early cb750 up to around 1971, is considered unsafe on a road race track?

Examples below:


George

[slikwilli420]

These carbs kind of suck though. You would be light years ahead going with something like CR carbs. I'll be petitioning to get the rule lifted in sportsman this year. It's absurd to restrict only these bikes but no others.

[gschuld]

AHRMA 3.3.9

"All motorcycles must have a self closing throttle."


Single pull throttles are self closing by design.  No mention is made that I saw in VRRA, AHRMA, or USCRA that requires, or even mentions, pull/pull two cable throttle systems.  Granted, I'm pretty skilled at not seeing things that are there....

So unless I missed it, single pull carbs are perfectly legal in VRRA, AHRMA, and USCRA.  Just unwise as a safety issue, specifically a stuck throttle situation not having a positive mechanical ability to pull it closed.  Correct?

George

[gschuld]

These carbs kind of suck though. You would be light years ahead going with something like CR carbs. I'll be petitioning to get the rule lifted in sportsman this year. It's absurd to restrict only these bikes but no others.

Well, I have put a bunch of work into improving factory carbs for more power potential.  Modified, they are capable of flowing a respectable amount.  It's fine tuning the fuel delivery system to match the new airflow that is needed now.  This requires some testing on the dyno, I'm working on that. 

Sure, throwing on a set of CR31 or CR33s would be the logical thing to do.  Easier tuning that factory carbs can't match. Those that know me well know that I tend to enjoy shoving(or trying to shove) square pegs into round holes.  Luckily world domination on the road course is NOT my primary goal.  I enjoy the challenge...

George

[754]

 Yeah but what a sleeper if you could run carbs, discontinued in 70, running against bikes with CR carbs....

[turboguzzi]

"Are we concerned that what Honda supplied on hundreds of thousands of Honda cb models, including MANY models other than the early cb750 up to around 1971, is considered unsafe on a road race track?"

main problem IMHO is in downshifting at high RPM, the high vacuum might hold the slides a bit open, not a problem that you have in street riding, i.e. the environment these early carbs were designed for.

so "unsafe" is a heavy word, maybe lets just say that dual cable is safer?

[Terry in Australia]

The K0 carbs are great for road bikes, with a much lighter "pull" than the K1 - onwards carbs. As far as racing goes, they can't compare with CR's, which are a crappy road carb, but brilliant at WOT. I took off my 29mm CR's and reinstalled my stockers with bigger jets and a K&N filter in the stock airbox and they're much easier to live with, but the power loss at WOT is really disappointing. Of course, Mikuni RS "Flat Slide" carbs are the best carb currently available for road/race bikes, but I don't know what your rules allow. Cheers, Terry.

[gschuld]

Helpful thoughs.  I've managed to get creative and gain enough clearance to make the k1-k6 rack carbs play nice with the dual petcocks. 

Regarding factory carbs vs other options, I'd like to give my bored carbs a good effort in getting them dialed in(proper dyno effort)  If I'm not satisfied with the performance, I'll get a set of CR carbs and be able to compare them back to back on the dyno.

George

[Kickstart]

Hey George,

Sorry I'm late to this thread... as far as I know AHRMA does not have a rule against single pull carbs.  Many of the bikes in the earlier classes only have single pull carbs, for instance the cb160 I race has single pull carbs and I'm pretty sure all the other cb160/175 bikes in the GP200 class are the same way.

But, if you've got the bank to fit then that's probably better.