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750 Custom wont run past 5000 rpm

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evanoman

750 Custom wont run past 5000 rpm

Hey guys I am new to the forum. My dad and I purchased my first bike, a Honda CB750C this spring from a guy who said it had run the year before but leaked gas and just needed a carb cleaning/overhaul. My dad and took on the task this summer and thoroughly cleaned the carbs and replaced the carb points. However when we went to start it it ran rough and wouldnt take gas. Thinking it was a jet problem, we went and replaced the jets and the deceleration diaphragms upon the recommendation of a local repair man. We got the bike started and it idled well but when we revved it over 6000 rpm in neutral and 5000 under load the bike would cut out, lose a cyclinder or two, and then die. We have triple checked the carbs and had several sets of eyes look at them and clean them so it definitly isnt the carbs. We also checked to see that the airbox and carbs had a tight seal and that the boots are in good shape. We made sure everything was sealed off and yet the bike ran the same. Any of you know anything it could possibly be? I have paid $550 for the bike and about $400 in cleanings and parts so its getting to the point where I cant afford to bring it in to a shop because I am trying to make it through school haha. Any input would be awesome, thanks!

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Re: 750 Custom wont run past 5000 rpm

Administrator

Welcome to the forum. Unfortunately I do not have much experience with the DOHC bikes. I am sure someone here might be able to help more then I. However I would suggest the timing advance.
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1977 CB 750-A X 4
1977 CB 750-K
1976 CB 750 F
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1973 CB 350
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Rodgil

Re: 750 Custom wont run past 5000 rpm

In reply to this post by evanoman

Make sure the vent in the petrol tank cap is not blocked. If it is, air can't get in, so after a while, fuel can't get to the carbies. Try running it with the petrol cap loose. Also, if you haven't already done it, check the fuel flow from the petrol tank, with the fuel line disconnected, to rule out a blocked fuel tap or filter.