QuestionQUESTION: The bike has been sitting in heated garage since 1990 so I had the carbs have cleaned and tuned. The fuel pump (? the one the end carb) replaced with new one ordered from Honda. New air filter.The problem is it runs great until you get up around 55. Try to give it gas and it falls on it's face. If you remove the air filter it runs perfect.
ANSWER: Your bike doesn't have a fuel pump, so I'm not sure what part on "the end carb" you are referring to.
If the motor runs better without an air filter, I would check the air filter and airbox for obstructions. Check the area where air enters the airbox, make sure there is nothing in the opening and surrounding area that can interfere with airflow.
---------- FOLLOW-UP ----------
QUESTION: First, let me Thank you for responding. Now let me see if I can clarify. All work was done by a mechanic. The part I was referring to resembles an accelerator pump on a car carburetor. The diaphragm was dried out and cracked. It was on the far left carb I think and was ordered from Honda. Anyway the carbs were removed, cleaned, and adjusted. Fuel line replaced. New air filter, plugs, wires, and oil. We tried gutting the material out of the old air filter and covering it with a foam for increased air flow with no improvement. I found a site on the restoration of an 81 CB650C that mentioned this same problem and they solved it by removing the cover and using washers to the edges of the air filter in place. I haven’t tried this yet. I am hoping to find the problem causing this so I can keep the bike in 100% original working condition. It only has 15,000 miles and is in beautiful condition. I don’t want to coble it. The mechanic has suggested new rebuild kits even though he felt all original o-rings, jets, etc were fine. He suggested in his words, “for a lack of having anything else to try” and that maybe someone had changed (to larger?) jets in the past. I’m pretty sure the bike never had any previous work; I bought it from a friend the original owner.
AnswerAccelerator pump! I should have known that's what you were talking about. Anyway, it sounds like you've pretty much done everything possible to cure the problem and I'm out of ideas.
The only other possibility lies in the air filter itself... it is not uncommon for the paper elements to "close up" over time and restrict the airflow. It's kind of a dry rot thing that is very difficult to detect by sight - I had a similar problem with an old CB650 that had been sitting unused for a couple years, the filter looked good but when held up to a bright light I could not see through it. Changed the filter for a new aftermarket one and the problem was solved. Might be worth a try if you bought an OEM filter from Honda.
