Jensen, I too knew John. He was great friend with my fellow collecter Les, who lives a couple hours away. He and John were at my house on several occasions, doing engine work, and building wheels. He is missed by many, and as you said, his knowledge was incredible!
davomoto
30 HP out of an CB72 ?
Hi Dave,
The world is small, especially for great people. He did like going to the US, was always enthusiastic about it. His last year he spent mostly finishing a few projects, mostly bridgestone's I helped him with a few CYB parts for his racers. Yes, he knew so many things, but the worst thing is that he never wrote something about his knowledge, he wasn't an Internet guy either. Jensen assembly of Japanese motorcycles requires great peace of mind (Pirsig)
Jensen
Have you ever pondered why Honda put those long inlets on the CB160 / CB175 but put the carbs almost directly on the head on the CB72 / 77? The 160 seemed to form the basis of later Honda twins in lots of other respects but none of their other bikes (except, I suppose the 50s and 90s). They could have put long air hose rubbers on the 160 just like the CB72. I have always wondered... Regards G '60 C77 '60 C72 '62 C72 Dream '63 CL72
'61 CB72 '64 CB77 '65 CB160 '66 Matchless 350 '67 CL77 '67 S90 '77 CB400F
I know we're veering a bit off topic here, but I've wondered about that too, but not specifically about those bikes. For example, does pulling the carbs back from the head affect the power? I know that when I tried to use Mikuni carbs on my bike, they wouldn't work because I had them bolted directly to the head, but the stock petcock configuration didn't allow you to move the carbs back at all. Apparently (or so my mechanic told me). bolting those particular carbs directly to the head didn't give the gas enough room to 'atomise', yet the Keihins and my Amals work fine this way. But, if I took either the Keihins or the Amals and say, pulled them away from the head by 3", would that make any kind of performance difference? Vince
The issues that I can think of are that a longer inlet resonates at a lower frequency so would normally be used for an engine where you need the torque to peak at lower revs. The atomisation issue may be carb / distance dependent but the use of an insulating spacer will help to keep the carb body cooler. The answer to this could be found with a gas flow rig but I've always wondered about those small twins. In theory a long pipe provides bigger frictional losses on the gas flowing through it so for out and out performance the shorter the better. G '60 C77 '60 C72 '62 C72 Dream '63 CL72
'61 CB72 '64 CB77 '65 CB160 '66 Matchless 350 '67 CL77 '67 S90 '77 CB400F
G-man, Vince,
I can answer this question in only one way, but it all comes down to physics, and I choose not to use that in this forum. But I will make the intake a little longer (40 to 50 mm). I'm doing this with a rubber inlet to disconnect the engine vibration from the carbs. I will use 26 mm carbs, probably CR26 carbs or, in case I don't like the on/off effect which goes with these carbs, move to standard CB77 carbs. I make the intake a little longer to compensate for the bigger carb. When you're carbs have a bigger diameter the engine will not create enough low pressure (vacuum) to suck enough petrol from the carbs in the lower rpm regions. Creating more resistance by making the intake longer, compensating for this effect, will make the low rpm regions better. I aim for usable power and a stable tick over. Jensen assembly of Japanese motorcycles requires great peace of mind (Pirsig)
Hi all,
I have visited this site over the years but never noticed the Forum, so this is my first post. IIRC, CB72 came with a variety of pistons including a race kit piston which had quite a high dome. I see some on ebay from time to time, but they are not very common. Personally I prefer a lower crown and flatter combustion chamber for better flame propagation, so I tend to machine the top of the block and or the bottom of the head to raise compression. The CB72 has more than enough valve area but has poor intake port shape. Both ports are set low in teh head and the short turn radius is very tight, which is not good for flow. In addition, the rate of expansion in teh exhaust port is too high. A good clean up of both ports will help as will raising the compression. For a street bike I'd stay away from those RD carbs - not because they are 2 stroke, but because they are a little large for the street. In fact the RD has needle jets that are designed for a 4 stroke and they work well. Most Mikuni carbs do have a 2 stroke primary type needle valve and it does a poor job of fuel atomization. For a cam I would recommend the Megacycle street grind. It has enough lift and duration to work well. I prefer a crank mounted electronic ignition, but raced for years with stock points - well they weren't stock, but they were points ;-) Even on a street bike the 2x3 cross is a worthwhile transmission improvement and of course use a new cam tensioner roller and a new primary chain if you can find one. I've been lucky to not break a primary on my 350 racers, but that was probably luck rather than good judgement on my part. Always spend time to get everything rolling smoothly with minimal friction and balanced one side to the other. I wouldn't worry too much about what HP Honda managed at the crank on a good day in Tokyo or how they adjusted for losses. 30 is a stiff target but it's possible. Of course the bike will still be heavy as a 350, but that's another story.
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