Bike stumbling bad after warmer up

So quite recently my bike (95 vfr) has been stumbling bad when trying to accelerate from a stop. When it's cold it works perfect, once warmed up it idles just fine when stopped but giving it a bit of throttle and it almost dies. I have to increase the throttle very slowly, the idle drops like it's going to stall and then it finally kicks up and away I go. I can't stab the throttle from idle either as it just makes it stumble. Once it get going it's completely fine, cruising to WOT or low rpm cruising.

What would be causing this?

I also noticed that when I start it up cold with the choke and the idle is high, I'll turn off the choke and it drops down to normal idle but if I try to put the choke back on right after, instead of kicking up again, it starts to stall. I can have the choke on for a minute and it high idles just fine but if I turn off the choke and put it back on immediately it kills it. Is that normal?

 

if the carbs are clean and temperature is above about 65F. try starting without any choke ( or try about 20%) and use idle speed adjustment screw to stabilize warm-up. probably best not to use choke to boost idle speed, screw only. about 15 seconds is the longest you might need any choke if things are right.

or maybe carbs just need a routine cleaning, eh ?

 

Well the choke isn't the real concern, it starts just fine and choke works when I start it. Just that I noticed if the choke is on and it's idling high (because of the choke) if I turn the choke off and then back on, instead of going back to idling high, it just doesn't do anything, which I found odd.

The main concern is thenear stalling from a stop once it's warmed up. It's runs great until it's warmed up, then every stop is a chore. Trying incredibly slowly advance the throttle so it doesn't stall and then it revs up all of sudden.

Are you sure just a cleaning would fix the issue? I thought there might be some adjustment to be made somewhere. Whatever controls the transition from the idle circuit to the (cruise?) circuit

 

yeah, you could play round with the idle mixture screws if you reviewed the fsm first AND have the special tool. problems could suggest dirty idle jets.

 

Dirty idle jets eh? Would it still idle fine until I increased the throttle?

Where is the idle mixture screw? I already had to increase the regular mixture screws this year year as I was having a bad surge, the surge is near gone now after increasing the mixture screw but could use a bit more tweaking but this problem with the near stalling off idle without feathering the throttle just started lyesterday out of the blue with nothing changed since the last time I rode it. It was about 3-4 weeks since I rode it last and it was perfect

 

maybe bad fuel or clogged filter if it began suddenly.

 

whats the history of the bike mechanically? How long have you owned it? the bike is pushing 30 years old, if the carbs where never removed and properly gone through and over hauled with possibly OEM rebuild kits, your just kind of chasing your tail. rubber parts break down over the years and need to be re-freshed or your never going to be able to trouble shoot your problem. There are people on this site that can over haul your carbs if you remove them mail them in.

 

I am not sure you have the same problem as I had but I had my carbs totally gone through, balanced the carbs etc
It would start just fine and run good until it was truly warmed up and than it would want to stall at a stop sign or if you blurped the throttle it would hang at 2500 rpm for a second or two
I pulled the carbs and went through them and found nothing
I than went back to the shop and by chance grabbed the intake rubbers to make sure they were tight
Two of the intake rubbers I could rotate even tho the carb clamps were tightened up to the stops, so I am sure I was getting air sucked in past the carbs when she was warmed up
Just got the new intake rubbers , so we will see what happens after a rebalance of the carbs

 

Carb rubbers sounds durty, they are "insulator bands" I believe, sounds more scientific. You must replace these things after 3 decades of use, unless you want to flip the bike fast and then you can boil them in hot water.

 

If your clamps and screws still have the little collars installed, ditch them. Over time the clamps no longer have the ability to tighten sufficiently with the collars, even with new boots (oops... Insulators). Just do a couple practice "tightenings" to find where it is nice and snug but not overtightened. On a few bikes, I could still easily spin the clamps when bottomed against the collars. These bikes are very finicky about intake leaks.

 

So... this is the picture you chose when talking about carbs? ;-)

 

LOL I know, I let you down. I was looking for a set of those master pieces on my work bench and I can tell you I was damned if I could find them this AM. So, I ended up sticking a picture of the rear brake yab that I did of yore. I know I have a "REP" to protect/keep up. Peace
"

 

Hi NateV8

I have exactly the same issue on my 1995 VFR750, done 75,000 miles & the issue started after being hardly used for a couple of years.

I've done everything on this forum & in other places, cleaned the carbs, blown out the jets, checked & tightened the boots, etc, etc, but I've still got the same problem as you.

Did you manage to find a cure for the fault?

 

^^ i suggest that you carry a spare sparkplug so you can quickly check for sparks when your engine quits just by grabbing a wire. pulse generator coils are just one component that can run ok until engine gets hot and then fails.