Carboresizes

[BiKenG]

In case you can't work out the subject line, I'm trying to find out the bore sizes of the different CBX carbs. I've been wading through all the carb threads and very interesting they've been, but I've not seen the actual bore sizes quoted. Could someone please assist me here. I'm sure there are those here who have this information in their head, but if I ever did, it's long been forgotten. So...

Are all the carbs the same bore size?
If so, what is that?
If not, what are the different sizes?

Hope someone can help.

[daves79x]

CBX carbs are all the same bore, which is 28mm at the slide, 32.3mm at the head side.

Dave

[Warwick Biggs]

Yes, Dave but there are some CV's around that have been bored out and there is a wide variation in bore sized of CR's set up for the CBX. As far as I'm aware they range from 29mm to 35mm and more importantly, because Keihin neglects to provide any external distinguishing features it is possible to find mis-matched banks of 6. I would recommend that where a CBX is not fuelling properly that the internal diameter of the bore on the head side be always be measured as a matter of course.

[daves79x]

In probably 300-plus sets of CBX carbs I’ve worked on, I’ve never run into any of this. It should be very obvious if a carb has been bored. But then again, what do I know?

Dave

[FalldownPhil]

In probably 300-plus sets of CBX carbs I’ve worked on, I’ve never run into any of this. It should be very obvious if a carb has been bored. But then again, what do I know?

Dave

More than most of the rest of us know combined !!
Best,
Phil

[Warwick Biggs]

I hate to be the first and I'm advised that I'm not, so mix ups can and obviously do happen.

And while discrepancies in bore sizes may seem obvious, I can also assure readers that a few mm's difference internally is easy to miss when you make the assumption that they are all the same. When you are looking for discrepancies it can seem obvious, that is true. Perhaps it might surprise a few experts who are not expecting it as well? Which is another reason to raise the possibility. Rare is not the same as never and never say never.

I possess a set of CV's (restored by Old School) that I had bored out by RPE to match my ported head (2mm o/size).OK, that is for racing but where they might eventually end up is impossible to know. They worked well with the head they were matched to but still suffered from the CV vacuum lag and ultimately had to be replaced for race use. I also unfortunately possess a mis-matched set of CR's that were sold as a set of six 31's in a bank. After much stuffing around trying to get them to fuel properly I measured them. They turned out to be 5 29's and one 31. Innocently, as far as I can determine. Between a conspiracy and a stuff up its nearly always the latter. and as I opined above Keihin don't help by neglecting to have any measurement or distinguishing feature on the body of the carby.
R.

[EMS]

I may not be an expert like any of the guys at RPE, but frankly, I cannot see how you can bore a set of CBX CV carbs. You have a throat diameter that faces the cylinder intake and behind that, you have a variable diameter opening that represents the venturi. If you decide to mess with that, you need to really verify your flow characteristic and have some in-depth knowledge on how your altered venturi affects your required jetting. Besides, I have no idea, how an increased "bore" which is most likely a cylindrical feature will retain a venturi function.

[Mike Nixon]

I have seen a number of bored carbs, still possess some, have bored a few myself. Back in the 70s folks bored SOHC carbs after epoxying the outsides. The ones I bored were first-gen Harley Evo carbs, was very common at one time cause the early fixed venturi Keihins had a lot of spare mass. I wouldn't bore carbs today. Virtually all multicylinder bikes have carbs that border on being too large already for good intake airspeed. Unless you're doing something to the engine that will make up for that, bored carbs or even just bigger carbs are not going to inspire... And cams are only going to make things worse as they slow intake also. You want a litre bike that runs like a 350 until 9,000 rpm? Me either.

Warning, opinion!: Unless you are one of the rare sort that truly does ride 10/10ths, simply getting the half-dozen common glitches sorted in your CBX will present you will all the performance you will ever want or need.

Couple challenges. One, the slide in the CBX carb defines the venturi, and increasing the venturi leaves no retention material for the slide. And none of the VB series slides are any larger. Not to mention you now have to bore the vertical direction in addition to the horizontal. Been there done that. You want to make your stock carbs perform better than you ever dreamed of? Get the engine right mechanically, then the carbs squeeky clean and properly adjusted, and that baby will fly!

[Larry Zimmer]

Many thanks, Mike. (And, all well said)

[Gearheadgregg]

I ran Webers on my Turbo Sandrails for years , they have interchangeable throats, Increase the Throttle plate on the CV Keihen would also be needed , Seems like a lot of work for a CV carb ? why bother , I found it impossible to tune my 40 mm CV that came with my Magnacharger kit , I had a stumble that was so miserable i had gave the CV to my friend for his Harley... NC , Found that the Idle circuit on the CV was to small , Installed a 45 Flatslide Mikuni and seen that it had 3 idle orifices in the throat that gave a much better signal for transition, I must say i have the stock CV's on my CBX and with said here with a proper clean and adjusting Jetting they work good for street use , what baffles me on my CBX is why would the previous owner Port the heads so large and keep stock 28mm on a Drag bike ? , Me wonders if they had ran something different and removed before sale ? I noticed when i purchased new Rubbber insulators that the bead in the inner rubber was restricting the larger port so i matched them to size, sorry to be long winded but it all has to flow and the things i see make no sense. Greg

[Warwick Biggs]

I'm certainly no expert, just an amateur club racer but as I learnt, CV carbs, no matter how you set them up are simply unsuitable for racing because of the vacuum lag. However, I did observe that Hugh Robinson at RPE spent a lot of time machining parts and testing on the dyno. He is hardly the only one to adopt this approach. Sadly, I don't think RPE are still trading and I haven't heard from Hugh for quite a while. He was/is a great machinist and an artist with a mill.

Lots of experimenting, fiddling and dyno checking can produce increases in power but doesn't necessarily produce a quicker bike and as I have also learnt, it doesn't necessarily pick up fundamental problems like mis-matched carbies. Mike's comments above are pertinent and very apt in this context. Its no good having the most powerful bike in the paddock if it doesn't run cleanly. In retrospect I'm surprised my bike ran at all. It was most cantankerous when accelerating out of the slower corners. Cylinders would drop off and then suddenly chime back in as you moved off the idle jet onto the main. That produced some memorable and sphincter tightening slides!

I recall the time when expansion chambers were being developed for 2 strokes, when back pressure and thermo dynamics was little understood, it was pretty much all trial and error by guys like Warren Willing at Suzuki. Warren was a racer first and only developed his skills as a top GP tuner over time well after Kaader's and Ernst Degner's pioneering engineering derived from rocket research pointed the way.

There is a famous poster of Willing on an OW1 at SKyline at Bathurst in the mid 70's where he had his TZ flying high in the air and almost vertical at 170mph. I think he brought the same audacity to engine development.
R.

[Warwick Biggs]

Since my last post on this thread I have learnt that all Keihin CR smoothbore carbs are 29mm internal diameter on the inlet manifold, irrespective of bore size. However, they are stepped out to match the bore at the other inner end.

However, there are a number of different custom mods to these manifolds for race use. Some have been bored out all the way through to match the bore size and some are tapered to match the bore. So another possible issue affecting fuelling could be mis-matched manifolds as opposed to carbs.

Put another way, it may be necessary to check the inner bore size of the carbs as well as both ends of the manifolds. This is in addition to jets, needles and floats, if your CR's are presenting fuelling issues.
R.