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Three times CB72 carbs, all different in details

jensen

Veteran Member
Joined
Jun 1, 2020
Total Posts
1,213
Total likes
377
Location
Blaricum, The Netherlands
Hi,

In the process of cleaning carburettors with the ultrasonic cleaner (see my thread on the CB400f : ) I stumbled into a box of CB72 and CB77 carburettors. In total I 8 sets CB72 and 2 sets CB77 (and a few C77 dream carburettors as well). I started with the CB72's and the first three sets where all different.

Overview :


early style.jpg
Early style CB72

later style.jpg
later style CB72

latest style.jpg
latest style CB72
 
Interesting changes. On CA95's the early ones were round bowls. I think they, eventually, may have done away with the power jets also. Were there any jet changes over time?
 
Yes, the CB72's, have also two type of bowls, and I believe the round type is even earlier. I must have a few set's somewhere. All the above carburetors have power jets, you can see that in the pictures below. Yes, the jetting is different, as are the needles and other small parts. Some differences are minor and some differences are huge.

backside type 2.jpg

backside type 3.jpg

backside type 4.jpg
 
My early '64 CA77 carb does not have a power jet. I need to check my friends '65 as there were several changes during that time. Nonetheless, I would be curious as to what jetting was used in the power jets on PW22's for the 305's.
I see there are brass jets on the sides of yours and I assume this is for fuel. On the spare CA95 I'm looking at there is no removable jet on the side (only a cap screw) but there is one in the air nozzle above and parallel to the venturi. It is quite tiny on the outside and is unmarked but appears to be in the 120 or less metering size. It draws air from a flexible tube to the air filter.
 
To me, the dreams never had power jets, the same for all CB77's officely sold in the US. The power jets are only found on CB72's and maybe, very early CB77's for other country's then the US.

The powerjet system sucks fuel from the from the bowl directly (the copper pipe into the float bowl, see pictures above). The jet that you see from the side regulates the amount of fuel. on the back side there another jet that regulates the air in the pipe. This air is sucked out of the air filter directly via a hose. Air and fuel mixture are sucked by the vacuum and released before the slide. The higher the vacuum, the more fuel is bypassing the regular metering via the main jet and needle. The system is designed as an accelerator enrichment to distribute more fuel at fast acceleration. However, when closing the slides, vacuum is high, and extra fuel will be sucked in, leading to a very rich mixture at engine breaking. The system is not delivered to the US due to regulations.

I experimented with fuel metering jets and air jets to get the system in balance. I choose for a little smaller jet for the fuel, and a larger jet for air, this way the system operates best on the dyno. Effect can be seen clearly on the dyno curves. One thing I learned is not to break on the engine but with the brakes. The enrichment of mixture is way to high and causes non- burned fuel in the exhaust system.
 
Thanks Jensen, that is some really useful information, especially concerning rider technique for bikes so equipped. I was just looking at the carb from a 62 project CA95 bike but will have to explore my rider, a 63 CA95. Jetting info for those is scarce and your suggestions will be handy.
 
btw, if you look careful at the pictures in post #1, you'll notice that the last one, which I labeled as latest, is actually the round bowl type, the other two are square bowl types. The carburetors are taken from bikes off different years, maybe the carbs where replaced at some time, I have to look into that more closely. I'm also making a list of all small parts per carburetor, including jet sizes.

The set in picture one, post #1 cames from a '61/'62 bike, the set in picture two from a '63 bike, and the set in picture three in post #1 comes from a late '66 bike.

The '61/'62 bike is a nice example of Honda efficiency, it has parts of both years.

Frame is '62, (very early number), gastank is '61, speedo is '62, shocks has aluminum shrouds ('61), aluminum front fender ('61), seat is '62, mufflers are the short stainless steel version ('61), engine covers are '61 winker switch (optional) is typical '60 (was actually a part intended for the CB92).

It's not a Frankenstein bike (two bikes build together), I traced back the daughter of the first owner, and I saw pictures of this bike just after being bought. It's truly a factory-off Frankenstein bike, a '62, with a lot of '61 left over parts if these fitted.

30 years ago I started with collecting CB72's, the goal was to have one from each production year. The goal is accomplished, and I added this bike as an in between, on top of the '61 and '62 I own. I also have a '60 CB72 engine (almost complete) and some parts, this bike was the first one of it's kind, being brought to the Netherlands as a on-off for a yearly fair. It seems that the rest of this bike is somewhere in Belgium, at least that's how far I could trace back the history of that bike.

The story is that this bike was "lost" after the fair and went to a dealership. This dealership used the bike for racing (because this '60 bike had magnesium break hubs and a few other special parts). Soon the bike was crashed a few times during racing, and the story goes that the engine was swapped for a later one. I have this engine with a very interesting engine number, I also do have the headlight housing and the rev-speedo counter.

I took the engine apart, and found many differences when comparing with later engines. This engine has the rotating gear shift mechanism, you can change from 4th gear directly into 1st gear (and looking at the damage, this is exactly what happened at speed). The crank is build-up different, gears are the same, crankcases (sand-cast) are different (much thicker that later ones), Cylinder head side covers and top cover differ, the list goes on and on.

I gave up on completing the engine for now, because parts are very rare, if they exist.
 
That is very interesting indeed on the early 72's. The magnesium hubs sound very rare, and I've not heard of a rotary shift in twins before. Practical for stop and go but risky on a race bike for sure.
Bold and innovative engineering was not always matched by the realities of early production. I imagine parts changes and availability determined each units actual make up. In a sense, back then, all of them were more or less one-offs. Sparse documentation adds to the mystery.
Bill Silver has written much of the US 305's and it is interesting to hear about the early European models.
 
Bill Silver has written much of the US 305's and it is interesting to hear about the early European models.

Yes, I know, I have both books he wrote (paper version), I really like it, and much info can be found.

And after the carbs, the petcocks where cleaned tonight. After a good hot Ultra Sonic bath the petcocks came out decent, and while inspecting all parts I found that I have two types of petcocks, an early version '61-'62 (left in the picture below), and a later version '62-''67. All parts are different, but the bowl from the '61 fits on the later petcock and vice versa. The tab won't fit, it's smaller ('61), the rubber is different, spring is different etc. Body's are completely different, and while the early petcock ('61-'61) has a steel small nut that connects to the gas tank, the nut's from the later ones are aluminium, at least a non-ferro metal.

5ad2e25f-7dec-4854-b07a-e85a2dffa3de.jpg
 
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