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Louie
Louie, Technician
Category: Motorcycle
Satisfied Customers: 12675
Experience:  25+ yrs. experience
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I have a 1981 honda cb900custom engine idles fine but bogs

Customer Question

I have a 1981 honda cb900custom engine idles fine but bogs [email protected] rpms what could the possible cause be
Submitted: 2 months ago.
Category: Motorcycle
Expert:  Louie replied 2 months ago.

Hi and welcome to JustAnswer, I’m Louie. Sorry you had to wait awhile but I’ll do my best to help.
Initially and to avoid asking you to do something you've already tried, can you please tell me what / any and all troubleshooting / specific steps you've already attempted other than the above; "...Rebuilt carbs changed fuel lines..."?

Is the described symptom of "...idles fine but bogs down@4500 rpms ..." present even before the carbs were rebuilt?

Customer: replied 2 months ago.
No it definatly idled at a higher rpm but i never had the chance to actually rev the engine. The bike sat for 30 years when i bought it so i removed carbs and has them rebuilt
Expert:  Louie replied 2 months ago.

Have you checked the spark plug burn color?

...test battery voltage when idling? ...and when attempting to rev?

Customer: replied 2 months ago.
Plugs were changed i did remove them and checked for any abnormalites they appeared they appeared to be the same color. I did check to make sure the wires were getting a spark it did seem like the 2 middle exhaust pipes were not as hot as the outer ones
Expert:  Louie replied 2 months ago.

Appreciate the additional information.

Still and on:

  • Plugs - ...dark/blackish? ...(light) brown/almost whitish?
  • wires were getting a spark - spark test;
  • seem like the 2 middle exhaust pipes were not as hot as the outer ones - drain carb bowls to compare fuel qunatity and quality?

And is the air box + filter installed? ...fitted correctly/tightly? ...with no possibility of an air leak?

How about the battery voltage? ...when idling? ...and when attempting to rev?

Customer: replied 2 months ago.
Ill have to check the voltages, the airbox and filters are not installed yet plugs looked like they were normal color according to the book
Expert:  Louie replied 2 months ago.

Very significant information; "...airbox and filters are not installed..."

Strongly consider installing. Their absence would result to a massive air leak causing a very lean mix. The engine is starving for fuel but getting too much air. This would explain if not contribute largely to the hesitancy and the seeming flatspot @ 4500 RPMs.

Customer: replied 2 months ago.
I will give ot a try and see of that corrects the problem thanks
Expert:  Louie replied 2 months ago.

Of course, it is still possible to have uneven quantity of fuel inside the carb bowls and/or the needles are not rising adeuqately. But yes, let's start with the air box and filter.

Customer: replied 2 months ago.
Quick responding to my questions
Expert:  Louie replied 2 months ago.

...and thank you for your kind understanding of automated-systems / machine-generated followups, ads & offers.