First question for the group; I've seen inline 4 carbs swapped out for a 2 carb system, with aluminum intake manifolds, etc. what's the pro's and cons of this, from a performance and efficiency standpoint? better or worse than the inline 4 carbs?
I believe that they come form Murray's Carbs and supposedly work very well on the street. Murray knows more about carbs than most and I suspect that the kit is better than most will achieve with pods and jets in OEM carbs. I would guess that CR carbs would make more power, but they were never designed to be used on a street bike. That doesn't mean they won't work, but like most things in life will be a compromise.
That said, Mr Honda and his friends at Keihin managed to make modified OEM carbs work on the RCB endurance racers. The OEM carbs are not easy to get right, especially with an aftermarket exhaust without a lot of dyno time.
Problem with building these old bikes is that you spend a lot of time and money, and some decent 17yo rider on a used craigslist 600 blows your doors off.
At least they look way cool.
12-08-2018, 04:57 PM
Originally Posted by TrialsRider
& Did they keep the air box or go with felt pod filters? lol
Neither. Open velocity stacks IIRC.
Way back in the day the late Leon Moss of LEDAR who was a bit of a motor and carb expert, tried to get a CB900 to run with an aftermarket exhaust and he ended up with external tubes to equalize pressure in the carbs. I don't remember all the details but I think it went from rich to lean and back again as the throttle was opened.
I have no idea if that was all one big pressure differential issue or an exhaust resonance pressure wave timing thing, I have no idea.
That does raise a question as to how owners of cars with SU carbs (more or less the same design) get around those same flow/pressure wave issues.
I would brace the frame (doesn't have to complicated, some well placed proven ones), then figure out how to fit modern rubber in decent sizes to the front and back. 110, 120 front, 150-170 rear. That likely means a fork swap but not necessarily USD. That way you get bigger rubber up front and better brakes, too. Set the geomtery, brace the swingarm and space for the larger rear wheel. Quality bushing for the swingarm, bearings for the steering head.
Update the fork internals if possible, upgrade to nice rear twin suspenders.
Ride it for a while until you get bored of the stock power, spend more money on engine mods. Those are pretty well sorted...
12-15-2018, 07:51 AM
Trials, I'd have to say no, my current carbs do not give me nice even throttle. idle to mid is a bit of a sputter. WOT gets me to about 90 mph tops and seems solid. though I had one time when suddenly had loss of power going freeway speeds. pulled over, shut it down, waited a minute, restarted it and it's run fine ever since. I've not touched the carbs since I bought it, other than keeping clean, high proof petrol in it and stabilizer in storage.
i'm definitely moving to the flat side CR carbs for the rebuild. want to move to pods and the CVs are shit for that, from my experience. tried it on the 900 project and couldn't get it tuned. I've not worked with the CRs but my understanding is they're better designed for PODs.
but appreciate the thoughts. will stick to the 4 carb set up. I've seen the aluminum billet intake manifolds for the CB750. are they worth it or should I stick to rubber?