1987 vfr700 carb tuning question

So I pulled and cleaned the carbs last week and the bike is running much better. But, I still have a little stumble at low RPM (between 2500 and 4500) with very little throttle opening, basically at speed maintenance throttle openings. Normally I wouldn't even worry about it and would just avoid that RPM range, but this is my wife's first bike and while she is getting comfortable with riding she spends a fair amount of time there. So, the question is, what can be adjusted to smooth things out?

The carbs have not been synced, I don't have a manometer or the angled tool to access the adjusting screws. I also did not mess with the idle mixture screws while the carbs were off and being cleaned. Are those the items that need to be looked at, or is there something else?

 

yes, you could try opening the idle mixture screws a bit (up to about 3 1/2 turns out) as well as a proppa synch, important at small throttle openings. Stock or aftermarket exhaust ??

Idle mixture screw passages need blowing out with compressed air while the idle jets are removed. :wink:

 

Idle screws also need to all be set exactly the same. Are your spark plugs in good condition? Air filter clean? Valves adjusted correctly? The thing I would do is to pull my spark plugs and see what they tell me. You can tell a great deal about how your motor is running by the condition of your plugs.

 

Thanks guys. Exhaust is aftermarket, Yoshimura 4 into 1, and was on the bike when we bought it. The spark plugs look good, very good infact. Air filter is nice and clean. I haven't checked the valves.

The stumble is pretty minor, and I suspected that it is due to the idle screws, or the passages, but just wanted to make sure I wasn't missing something else.

Looks like the carbs will be coming off one more time to remove the idle screws, blow out the passages, and reset the idle screws to factory specs. (I'll check what they are currently set to before removing them.) I also think I will swap the float bowl covers so the idle screws can be more easily accessed. Hopefully that will square everything away.

 

Those are not idle screws on these carbs, those are air/fuel mixture screws that do affect mixture mostly in the idle range to about 1/8 throttle.
A thorough cleaning of all orifices with pressurized air, carb cleaner and wire is in order on those carbs one more time. After you set the butterfly valves according to the manual you will want to synch them when you put them on with a manometer to dial them in.

 

IT's usual for a 4 into one setup on a VFR to have a small flat spot (usually about 4500 rpm) unless careful carb tuning has been done.

in your case, experimenting with washers under the slide needle would be the place to begin.

 

Dukiedook, that sounds like the throttle range, and I stand corrected. The screws are the air/fuel screws. I have bench synced the butterflies, and am hopefull that they are close enough since I don't have access to a manometer. If I still have issues after getting everything cleaned out, I'll have to look into getting a hold of a manometer.

squirrelman, thanks for the tip on adjusting the jets for the exhaust. If, after everything is cleaned and the air/fuel screws have been adjusted, we have a flat spot, that will be the next thing on the list.