Topic: Anyone out there have bored factory carbs?

[gschuld]

I'd like to hear anyone's experience with bored factory round top carbs.  What motors were they on, tuning, who bored them (Maxi-bore, other?).  Always looking to learn.  Anyone know who, besides Monty Cambell(Maxi-bore), bores SOHC factory carbs anymore, or did?

I am fully aware of the other HIPO carb options (Webers, Mikunis, CRs, etc) that are far more popular of a swap, but I would like to know more about the bored factory carb option. 

Thanks,

George

[Big Jay]

We did hundreds of them before the 29 Mikuni smoothbores came out.

[754]

Rectangular opening ? The ones i have came with that style of cut.. Got them from Joe, have not tried them yet..dont know who cut them..

[gschuld]

We did hundreds of them before the 29 Mikuni smoothbores came out.

Jay, 

    Good to know!  In your experience, how well did the bored carbs perform, assuming they were tuned correctly and on a motor powerful enough to justify the extra airflow to begin with?  My specific application would be for a ported head 1000cc RC built motor with, for arguments sake, an RC 315 cam.  I presume that the biggest advantage of the Mikuni 29s over the bored factory carbs would lie in the low speed realm as the Mikunis have accelerator pumps.  Presumably, the more duration the cam, the more accelerator pumps help out the bottom end.  So, for example, if I used a reasonably short duration RC 295 cam in the 1000 motor instead the benefits of an accelerator pump would be minimized greatly.  Conversely, a big, long duration RC 327 cam would likely really be helped on the low end by the pumps.  I am on the right track yes?  What I am getting at is whether bored carbs have a weakness in the low speed department that would potentially effect my ideal choice of cam(likely RC 295 or RC 315).  My aim is for the bike to have reasonably civilized around town manners.

    Back to bored carbs.  Would you mind giving me a basic rundown of the process of boring factory early carbs?  Where exactly is the material removed to enlarge them?  Walls and roof of the throat?  Floor?  Are there any modifications to the fuel metering setup?  Does the boring tend to cause problems with the tuning process in relation to the otherwise identical non bored carbs?  Did APE mark the carbs in a way that made them easy to identify externally (for example:  Monty Campbell stamped "maxi-bore" into the side of the far right carb body after boring a set).  Do you recall when APE first started boring cb750 carbs?

     Does anyone, by chance, have any flow bench numbers for factory round top carbs, 29 Mikunis, bored carbs? 
 
Frank,

     Do you have any pics of your "rectangular opening" bored carbs?  I assume they a flat sided in all four sides but with smaller diameter rounded "corners"  Correct?   I'm sending you a PM in a minute...

   
FWIW:   For those who don't already know, I am trying hard to keep as many parts as possible in the 1971 or earlier vintage for my 1969 RC engined Diecast bike.  This is specifically why I am exploring bored factory carbs.  It was fairly popular back then in HIPO applications before larger carbs became readily available.  In the end, I will favor period correctness over convenience, peak performance, and/or expense.   Weber conversions are a possibility, but I am exploring bored carbs for now.

Thanks,

George





 

[Jay Ace]

mine don't seem too bored. always have something to do

[gschuld]

mine don't seem too bored. always have something to do

Well looking at the thread title again...   I guess I asked for that one .

Keep your fuel/air mixers happy everyone, nobody wants to deal with bored carbs!

George

[Big Jay]

First one I saw do this was Eraldo Ferracci. Then we did it. We used an end mill slightly smaller than the slide. Took material off the top and then the floor.   Width stayed the same.

When the 29 smoothbore came out it just made more power everywhere.

Then we quit doing that and started taking Mikuni dirt bike carbs and boring them out for the Kawasaki KZ guys. we took 36s out to 37.5.

Did a mountian of thoise until the RS Mikuni 36 and 38s came out.

[gschuld]

Jay,

    Good info.  Any idea of the time period when Eraldo Ferracci and subsequestly you started boring these carbs?  I'm trying to get my history straight!  We would be talking early 70's correct?  1971?  72?

    Curious, did you guys do any measurable testing(dyno?) back then to for tuning or to show an improvement?  So no other modifications other than the physical opening up of the top and bottom radius(1 to 2mm on each end?), presumably with a careful blending in to smooth out the contour before and after the cut?  No tricky idle circuit mods or anything? 

Do you think that a set of bored 69 K0 carbs would work well on a 69 RC 1000cc with an RC 315 cam?  A forum member says that he has managed high 10s in the quarter mile running bored early K carbs!  My motor won't be making monster power or anything compared to a modern big bore build.  Certainly not enough to run in the 10s anyway!

I presume that your customers were happy with the modified carbs?  Any serious complaints about tuning difficulties with them?


Sorry for all the questions, but you seem to be the guy to ask....

George
     

[bwaller]

By the way, the MIC 29mm smoothbores don't have pumps.

[754]

Here they are..someone do the math,these are pretty big..
 25.3mm wide by30.8 mm tall. the corner radius is .312

[kmb69]

Here they are..someone do the math,these are pretty big..
 25.3mm wide by30.8 mm tall. the corner radius is .312

A 29mm Mikuni calculates to 660.606 square millimeters.

Frank's numbers calculate to 725.327 square millimeters or 9.8% more area than the 29mm Mikuni.

[gschuld]

Frank,

     I'm intrigued by your bored carb set.  The boring seems to be much more aggressive compared to what I have come to understand was the norm.  Thanks a bunch for sharing the photos and dimensions.  Would you mind terribly taking a photo dead on through the throat with the throttle fully closed and one fully open?  That would be great.

Kmb69,

Thanks for the math...  Sounds like these carbs are maxed out.  I'd like to know how they perform...

bwaller,

Thanks for correcting me.  My knowledge of Mikuni carbs is about zero...   I always reserve the right to become smarter.  Being on this forum is certainly helping

George

[754]

We should calculate .591 x pi .squared..and see if that is over 30 mm..

[joe p]

Frank What main jets are in it?

[754]

Will check today, cant recall..

[kmb69]

We should calculate .591 x pi .squared..and see if that is over 30 mm..

I am not sure what you are asking.

A 30mm throat calculates to 706.858 square millimeters.
A 31mm throat calculates to 754.767 square millimeters.
A 32mm throat calculates to 804.247 square millimeters.

So the bored carbs have a larger area than a 30mm carb but smaller than a 31mm.

[754]

Thanks.

[754]

 Checked the mains, they were marked 125, I put a drill in that was .047, and it had slight clearance. I assume 125 means 1.25 mm which off the top of my head would be around .0485..
 The pilot was huge, not sure what size they usually are but a .076 drill had clearance so It looks like around 2.0 mm.

[joe p]

Thanks!

joe p

[gschuld]

We did hundreds of them before the 29 Mikuni smoothbores came out.

Jay, are you still set up to bore a set of Early 750 factory carbs?  Interested?  Price? 

George

[brandEn]

This thread is interesting. Subscribed.

[POPS 911]

I have a set CB750  SOHC =  RC 28.9MM bored out old style carbs with thick alum. velocity stacks that came from Texas = $250 PLUS SHIPPING....... Like new.

POPS BK911

[gschuld]

Pops,

    Could you send me a few pictures of them and whatever you know of their intended use and track record?

Thanks,

George

[POPS 911]

PM me your e-mail.

POPS BK911

[gschuld]

Does anyone here have any first hand experience with Monty Campbell's carb boring service?   http://www.x-pipe.com/motorcycle.html  He's been around since the mid 70s doing it, but I haven't heard any reports of his work. 

George