30 HP out of an CB72 ?

[teazer]

A couple of other thoughts.

Intake length is a function of rpms and the speed of sound and valve opening and closing times. If you use the time worn formula for intake length, you get a very long intake from the valve to the bellmouth.

G man explained it pretty well. The lower the rpm, the longer the intake system.

Revs- we tend to think of these bikes as being rev hungry beats but most CBs will not rev to 12,000 or even 11,000. Valve float is a problem and so are the thick piston rings which flutter above about 10,000. Yoshimura discovered that valve weight was crucial and it his tuning notes he mentioned the different weights that were needed for different revs.

Piston rings flutter at certain piston acceleration rates. Thinner rings flutter at higher revs than thick ones. The rings on a CB72/77 are relatively thick and will flutter at about 10k or less.

Good valve springs are essential. R&D used to sell them with Titanium top caps. They probably still do.

[jensen]

Hi,

Saturday I did a run on the dynoyet, after breaking in the engine.
It performed well, as it throwed out a dazzling 33 HP @ 11.500 rpm.

Later, during this month, I will get this thread complete to give overview of the steps I took to accomplish this.

Jensen

[jensen]

Hi,

I decided to use the carbs of a Kawasaki KR1s, the reasons for this choise is simple, I had them lying around, and I had access to a lot of needles and jet's. These carburettors are are flat-sides, so a very smooth bore at high revs. At the the Kawasaki the carburettor is attached to a membrane inlet, so I had to make two new manifolds, which I did. The only draw-back is that the flat sides are
a bit "bumpy" in the lower regions. If this is going to irritate too much in the future, I'll try 26 mm CB77 carbs.

I wantedto use the "normal" air filters, but instead of the paper element, I used (wet) foam. In an old thread I described how to do that (that was a thread in the old style mailings). The (wet) foam gives much more through put, but filters the air better than the paper elements, especially the fine dust. Next to that I want to maintain the bike properly, the paper air filters are expensive. The foam filter only require a wash down every once in while.

I decided to go further with the desired 45 mm extra length between carburettors and the head. I shortened the hose behind the carburettor that 45 mm, so it looks if it a standard set-up.

The carburettor is hanging between two pliable pieces of rubber, so the float works like it should work and doesn't resonance. The carburettor doesn't interfere with the gas tank, but the free space between petrol tap and the carburettors is small.

To my surprise the intake noise is (much) louder than the stock set up, and that's a very good thing.

It took me 4 to 5 hours of jetting these carbs properly, mainly chancing needle's. I took the best working Kawasaki needle set as a example, and slightly chanced them myself on my newly (but second hand) acquired Emco compact 5 turning machine. I'm still not convinced that this is the best set-up, of the carburettor, but it is much better than the standard setup. Especially picking up from the stationary jet is somewhat abrupt.

But it never runs too lean, nor to rich, and the mixture is ideal from 8000 rpm to 11.500 rpm. From 3000 to 8000 rpm it is running a little rich and in the lower regions it is also good.

I also had the chance to compare RON 95 and RON 98 petrol (low respectively high octane petrol), and like I mentioned in the fuell type thread, I see no differences in the temperature of the head nor the mufflers and pipes. The power curves are exactly the same too, so my conclusion is, as-long the engine isn't pinging, high octane petrols is a waist of money.

Jensen

[G-Man]

Jensen

Really good stuff! I am looking forward to the summary of what you have done. Also interested in your air filter design.

I have bought a nice CL72 engine as a spare but now I think that I might build a lightweight special with it. I shall go as minimalist as possible and build a fun bike that I can ride around the lanes.

Must finish some of my other projects first, though.

G

[Phil-UK]

Hi Jensen

Good stuff ... well done !
Look forward to more information and hopefully some photos.

Could do with a rolling road with gas analyser around here to get my CB72 sorted at the
top end .. mine feels like the Power Jet system isn't working properly.

Keep up the good work,

Phil