Poor fuel economy after rebuild

Need advice on what may be causing dramatically reduced fuel economy
on an '85 Honda Nighthawk CB700sc.

I owned this bike for 18 years and kept it perfect working order.
Originally the bike delivered 42mpg. 10 years ago I had a DynoJet kit
installed which knocked the fuel economy down to around 30mpg. Four
years ago I quit riding the bike, and let it sit.

Recently, a nephew approached me about purchasing the bike, so I took
it to a local Honda dealership to get it back into running condition
(ouch did that hurt). They did a superb job, which included complete
teardown and degunking of the carburetors.

The bike runs great, but is now delivering 20mpg. No leaks apparent
anywhere in the fuel system. Any ideas as to what might be causing
the problem? Could the mechanic have assembled something wrong?
Think visiting a certified DynoJet shop might solve the problem?

Jim

 

1). Depends on who's doing the MPG calculations, then and now.
2). You say no fuel leakage, have you checked(carefully) for any
crankcase(vacuum) leakage?
3). Have you spoken to the maintenance shop where this rebuild
was done?
4). Does the bike roll free(no brake dragging, bindings, corroded
bearings, driveline constraint)?
5). If chain drive, could you have forgotten about a sprocket change?
6). Tank venting OK?

Beside the carburetor work, what else was done?

Gary

 

See answers below:

It's me and now my nephew. Feel confident in the numbers. Top it
off; drive until hit reserve; gas it; divide gallons into miles using
calculator. e.g. 3.7 gallons into 80 miles = 21.62mpg

I'm depending on my nephew for this; not sure how carefully he
visually checked for fuel leaks. Don't know how to check for vacumm
leak; will refer to dealership.

Yes, they think it might be worn needle/seat in carbs. Want the bike
back to diagnose.

Good point; easy to overlook. I will mention this to my nephew.

It's a shafty.

Dont' know; will mention to dealership.

Standard service interval stuff plus change all fluids. Hydraulic
clutch and brake systems completely rebuilt. It was about $1,200
worth of work.

Thanks for the tips, I'll pass these on to the dealership. My other
idea is to take the bike to a local independent shop that is certified
in doing DynoJet kit installs and have them check out the current kit
and retune and/or replace worn parts. Thanks again for your
assessment.

Jim

 

That's really about all I can suggest. I can't imagine a 750 getting
a consistent 20mpg. I have a Valkyrie, and while I can certainly
drive it down to 20mpg, and below, it is normally reported at
somewhere near 35-40mpg.(probably idling)<g>

The Valk did also develop an air leak, which caused a severe
increase in fuel usage, but it also caused very poor performance.

I think your planned course of action is appropriate, let us know
the details as they unfold. It sounds like the shop is reputable enough
to stand behind their work.


Thanks - Gary