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Discussion Starter #1
Greetings all:

I have a 1998 Honda CBR600F3. I have taken it to 22,000 at the present time. All great miles with never any issues. Over the years, I have always kept up with the required maintenance per the Honda service manual as well as, (for what is worth), the "Clymer" maintenance service manual. This is not my primary bike so it sits for a few weeks (even a month), before I ride it again.

Need some "expert advice" as I am at a loss concerning a slight carb/stumblel and or hesitation at 1,800 RPM and 4500RPM. If I rev the bike (keep it in a lower gear for example) the hesitation is not there. In cold weather ridding, the hesitation is only noticeable at 4500RPM---go figure?


Here is what I have done at present:

-Replaced spark plugs---looking at the old ones revealed each cylinder is running fine based on the visual inspection of the plugs.
-Replaced spark plug wires--they looked good, but did it anyway
-Replaced fuel filter
-Synched carbs---they were in spec however
-Replaced one carb needle as there was a slight chaffing
-Cleaned all carb intakes---they looked good before I began however
-Cleaned and re-oiled air filter (K&N)
-Checked engine compression---all in spec
-Brought carbs to Honda dealership to bench test tiny orifices to see if there were any obstructions---there were not

The only Mods done that could possible effect performance are:

A Vance and Hines slip on system
K&N filter charger
A Factory 3 degree ignition advance

However, the above I installed six years ago at 3,000 miles and never had an issue until now.

I picked a Honda techs brain (and after telling him what I have done), he stated: "Possibly need to add shims to the needle valves." He reports: "This is a trial and test (add one) ride the bike, (add another), ride the bike etc."

Of course this will become costly as it is like "exploratory surgery"---not certain...but lets give it a try.


Swapped the K&N for a stock Honda OEM filter---still the same issue, as some have said CBR's don't like aftermarket filters.
Perhaps I will try the swap of the Advance timing next for the stock setup---again someone mentioned this.

Carb hesitation/stumble is present under all riding conditions it seems. At 1800RPM and 4500RPM with a normal throttle roll on.
As I mentioned, in cold weather ridding (Maine, New Hampshire) the 1800RPM hesitation/stumble is not their.
As I also mentioned, revving the engine (keeping the bike in a lower gear) the hesitation/stumble seems to be minimal if not present at all.

The carb hesitation/stumble seems to be exacerbated in hot humid ridding conditions (July, August). So it seems to be a air fuel mixture thing...but I am to certain of course.

I feel there must be some options to look at as i am out of ideas. Honda's are so damn reliable. Anyone have any thoughts on my dilemma? I have posted on a few other sites to no avail. This site seems to be more tech savvy with more advanced experts and this is what I am after.

Wish I could connect my laptop to diagnose the issue, but we are dealing with an old school carb bike.

Thanks in advance all!
 

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i had a very similar experience with my '95 vmax. i finally took carbs off and cleaned them myself. replaced all four pilot screws (air/fuel mix adjusters-one for each carb) and that totally fixed the problem. i finally enjoy riding this bike again.
im am no carb expert but i do know that there is another valve in there attached to the float bowls, i think its called a needle valve, that can wear out too. not sure what symtoms you would get from a worn out needle valve though.
 

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the needle valve shuts off the flow of fuel to the carb. when the bowl is emptying the valve opens and lets fuel in. as it fills up it shuts it off. (so to speak)are you getting any abnormal fuel smell when your bike acts up?
 

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Discussion Starter #4
the needle valve shuts off the flow of fuel to the carb. when the bowl is emptying the valve opens and lets fuel in. as it fills up it shuts it off. (so to speak)are you getting any abnormal fuel smell when your bike acts up?
Thanks for your thoughts. No abnormal fuel smell. I completely rebuilt the carbs or shall I say, had Honda go through them to no avail as they reported...no issues. Because it is the dead of winter I just brought them the carbs off the bike and nothing else. Once we had a dry day I re-installed them and test rode the bike...still the same hesitation. :idunno:

I am turning my attention to the air solenoid. A few on this forum mentioned it can stick. Does this sound like the culprit? Has anyone else heard of this.
 

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ALL YOUR MISSING IS A REJETTING FOR THE SLIP ON AND AIR FILTER . I DID A SLIP ON AND HADE THE SAME THING YOU GOT, AND A JET KIT FIXED IT. ANY TIME YOU MESS WITH THE INTAKE AND/OR EXHAUST A JET KIT MUST BE DONE SORRY FOR THE CAPS
 

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Discussion Starter #6
ALL YOUR MISSING IS A REJETTING FOR THE SLIP ON AND AIR FILTER . I DID A SLIP ON AND HADE THE SAME THING YOU GOT, AND A JET KIT FIXED IT. ANY TIME YOU MESS WITH THE INTAKE AND/OR EXHAUST A JET KIT MUST BE DONE SORRY FOR THE CAPS
Thanks for your thoughts. However, I had the slip on pipe and air filter installed for 7 years and never had any issues until last year with this mysterious carb stumble issue.
 
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