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Re: Question: 1980 CB750F has carbs from a 900 - yes or no?

Posted by gonebiking-2 on Sep 03, 2008; 3:33am
URL: http://vintage-and-classic-hondas.81.s1.nabble.com/Question-1980-CB750F-has-carbs-from-a-900-yes-or-no-tp458280p458281.html

By hell you pose a tricky question. Its like how long is a piece of string. Remember that the CB900 is 150cc bigger in capacity and the gear ratios are different from the 750. So when  modifying an engine that wasnt designed ( technically speaking ) to take bigger carbs, other factors have to be taken into consideration, such as valve timing, compression ratios exhaust valve timing and many other items.The 900 has bigger inlet ports and inlet/exhaust valves. For what its worth, I would personally rebuild each of the four carbs off the 750 taking your time and make sure everything is 100% spot on. Then  put them back and I see no reason why it wont run properly. One thing that you may have overloked is the number of teeth on the front and rear sprockets. There seems to be an anomally with the CB750s. In the workshop manual for my 1981 CB750K it says the rear sprocket has 38 teeth and the front has 15. Problem is when I bought my bike it had 44 teeth on the rear sprocket and 15 on the front. I replaced the chain and sprockets with a 15 front and 40 rear. Made a hell of a difference in that the RPM dropped by a 1000 at 70 MPH from 4000 to just under 3000, fuel consumtion improved , top end performance was better, but bottom end required swopping cogs frequently to keep up the power up in the 3000+ area. So if I were to replace the chain and sprocket with a 15/38, I`ll have no bottom end torque at all and starting on hills will become a problem. Suggest you get someone who knows about Honda carbs and get the 750 carbs stripped and overhauled. Also check that the air filter element isnt clogged.

If you get the 900 carbs re jetted for the 750 motor, there shouldnt be any problems with burning out valves, rings, pistons etc. Its only when they are set too lean that everything runs too hot and eventually engine failure happens. Hope that is of some help and somewher to start.