The first image is from the December 1973 Big Bike magazine "Build you own 1000cc Honda" article that was written exclusively about RC Engineering, with photos and content coming directly from RC's shop.
The second image is from the well known March 1976 Big Bike magazine "Building a 1000cc Honda" leading with: "COBRA Two grand worth of instant 11.0 quarter mile times"
I have corresponded with Russ Jr a bit on the subject and he distinctly remembers Byron Hines experimenting with modifying(boring) factory round top carbs to see what he could get out of them on bigger hipo motors. They offered the service, even though the RC catalogs I have did not mention it, for $95. In comparison, a complete RC manifold Weber setup, in 1973, cost $299.
Webers were clearly the carbs of choice for RC Engineering(in drag racing anyway) with all their in house record breaking dragsters running on Gas (Hilborns went on for Nitro work). All the early single and double engine bikes, the Vance and Hines Double, the Triple, etc. ...ALL ran Webers. So as far as all out performance on the drag strip is concerned on race gas, Webers were THE choice.
That said, Russ Jr. mentioned that Byron was pretty impressed with how the modified factory carbs performed, even in strong big bore motors. Not quite as good as Webers, but not TOO far behind.
I'd LOVE to know the exact details of what Byron Hines did to the factory carbs. I don't think any paperwork exists documenting these things.
Kickonly849,
That is a very good question and I have wondered about the same thing. Based on my limited research, I would have assumed that a potential weak link for bored factory carbs would be a reduced signal and potential mixture quality at the lower rpm ranges because of the reduced Venturi effect that you mentioned. Strictly theory though. It's one of the things that I am very keen on finding out with a back to back chassis dyno session between these carbs(tuned well) and a factory set of carbs(tuned well) on a strong performance motor(like Benton's Cobra motor). All we can go by at this point regarding these carbs is Bill's minimal first hand experience. He reported that the carbs ran strong from off the line all the way up the rpm range, with no flat spots or hesitation anywhere. He seems to think that they might even idle a bit smoother(not sure why, who knows). Well on a basic level, his Cobra motor ran an 11.1 on tuned up factory bore carbs and it's a fair bet that the motor wanted more cfm that what stockers had to offer.
On Bill's cobra engine, my guess is that they would be slightly(if any) under stock carbs pulling from low rpm, but start pulling away convincingly from say 4500 on up. I would expect, again just guessing here, the percentage improvement over stock carbs would increase as the flow needs(engine size/headwork/cam) increase.
For example, again just guessing here:
An 836 with a mild port job, and a modest performance cam would likely see some benefit from bored carbs over stock at the top end, but barely measurable.
A 1080, on the other hand, with fully ported heads, big valves, 327 cam, etc, etc. will simply WANT more airflow than factory carbs can deliver. The drag race 1080 motor is asking for a MINIMUM of 35% more airflow than a factory dead stock 736cc motor at full song. Offering the engine some more carb cfms, as long as the mixture and signal is comparable, should allow the motor to breathe better, charge the cylinders better, and produce more power. Those bored carbs will NOT be able to flow 35% more air than factory carbs with only a 7% increase in total throat volume and a smoother blend/polish. Chances are that the bored carbs will still be under carbed for a really big powerful motor, just not sure where the cut off is...
RCs own statements suggests that a 900cc performance motor is about the limit for a set of stock carbs to really "keep up", with performance being more and more restricted as the motors get bigger, more powerful.
It's not easy to put a clear set of numbers on a break off point on factory carbs though. For example a stock bore road race motor with a Mike Rieck stage 4 head, Yoshi rods, 12.5:1 compression lightweight pistons, lightened crank, huge road race cam, etc that was set up to run 13,000+ rpm needs huge amount of carb cfms just like a big bore 9,500 rpm motor.
As I have mentioned earlier, this is an experiment. I'd like to document by chassis dyno, where stock carbs really start to fall off performance wise. And hopefully we can find out if the bored carbs that I did can perform better, and by how much, in a large bore motor.
I have a pretty healthy 1000cc motor coming together. When it's ready, and if Bill isn't better by then to dyno them on the Cobra Girl where the bored carbs currently reside, I'll make an effort to do it myself and document it.
If boring carbs turns out to be a viable option, I have no problem with others taking the information and specs gathered and setting up to do a run of bored carbs. Maxibore already offers the service.
http://www.x-pipe.com/motorcycle.html I spoke with Monty Campbell about 2 years ago while researching them a bit. He has offered the service since the late 70s I think. His claims are very bold, but I have search far and wide and have found little in the way of first hand feedback, good or bad. And I have never seen his handywork first hand, so officially, I have no useful opinion regarding his carb work. He certainly was not open, at least when I spoke with him, to give any details regarding what his bore jobs entail(fair enough, it is his business) so I had nothing concrete to go on. Frank (754) luckily did have a set of bored carbs, unknown who did them, and measured his for me. Thanks Frank! You helped get the ball rolling
George
