Weird: '97 VFR suddenly won't take throttle above 4K otherwise runs perfectly

Hi Guys,
I took my '97 (carbureted) out yesterday and it ran perfectly for about 20 minutes. I did notice that it was showing poor charge on its aftermarket ammeter but didn't worry about it since it sometimes has done that in the past with no problems resulting. I was planning on going for about an hour ride, so I stopped and got a couple of gallons of gas. Rode for about another 5 minutes and cracked the throttle on a bit and the bike would not pull at all above 4k RPM. The ammeter continued to show red (it is a red, yellow, green type) so I figured my R/R was going out on me and rode home with it running perfectly as long as I kept it below 4K and didn't crack the throttle more than about 1/8 open.
Last night I installed a Ricks Motorsports R/R I have had on hand for just such an eventuality and took it out for a spin. It seemed somewhat improved but only slightly. I then added 4oz. of Seafoam to the 3 gallons or so of gas in the tank, took it for a 5 minute ride to get the treated gas in the carbs and let it sit overnight.
When I took it out a while ago it still had the same problem; it still bogs severly above 4.5K and won't rev freely above that. Even though it will get to 8K it sounds then like it might only be running on 3 cylinders. The thing that has me totally stumped though, is that it runs absolutely perfectly up to 4K. It has perfect throttle response, idles smoothly and is just as wonderfuly torquey as it ever has been. It is almost as if the carbs high speed circuit is gummed up, but with the low speed jets working fine it seems like the bigger orifices in the needle-main should be clear.
I have never run into a problem where a bike would be that perfect at lower RPM and that bad up higher. I honestly don't even have a good sense of if the problem is fuel or ignition given the odd symptoms, sudden onset and perfect low speed circuit performance.
Hoping that someone has had a similar situation and can cast some light on the situation...
Thanks!

 

You see any black smoke from exhaust ?? If so, possibly a leaky float needle drowning out one cylinder. Otherwise you need to check charging voltage across battery terminals with engine running about 5000 rpm and need to see a minimum of 13.5 volts on the voltmeter. Yes, low voltage does cause running problems.

If the bike runs OK when cold but begins misbehaving after 15 or 20 minutes, i'd suspect a failing pulse generator or bad ignition coil. Could also be failing fuel pump relay, which you could bypass for testing purposes.

 

Check all wires. Look for signs of heat at the connectors. Did you ever do the VFRness?

 

Update:
Since my original post at the top of this thread I've checked for spark, which is good and hot on all cylinders and checked for output from the fuel pump which is also fine. The voltage at the battery at 4K RPM is 14.2 and there is no black smoke from the exhaust when the problem is happening. (While I had the plastic off I replaced the plugs, even though the old ones all looked fine, and changed the oil and coolant so it wasn't a waste of time.)
My best guess now is that the problem is happening when the bike is trying to transition from the pilot circuit to the needle jet and that the vacuum operated slides are not lifting properly. It is very possible be that they are gummy since the bike had been sitting, started only briefly once in the shop for a couple of months. I might have been wrong about it running fine for the 20 minutes before I gassed up and noticed the problem because if I never cracked it on above 4K prior to my gas stop I wouldn't have observed the problem.
So, I'm thinking it is time to pull and clean the carbs, but that is a bit of a PITA and want to check with forum members for anything else that I should look at before diving into that chore.
Any thoughts will be greatly appreciated.

Jay

 

Had a similar issue with an 83 VF750F showing the same symptoms. One or more of the carb diaphragms has at least a pin hole in it. All carbs are not actuating evenly, hence the lack of pull above 4K - about when the carbs swap over to main jets. I had to replace 2 of my 4 diaphragms and then all was right with the world. I had my performer back! Just my $.02 Good luck.

 

Just due to age, gen 4 diaprams remain alot more dependable than gen 1s...........but, yes. Gen 4s carbs are EXTREMELY well-constructed compared to anything earlier, and gen4's semi-flatslides seem to stick during storage far less often than the round slides.

You can remove the air filter (using a bottle feed setup for fuel) and watch the slide action moving while the engine is running, and slides should rise evenly, but engine NEEDS airbox and filter to run right with standard jetting.

 

Thanks for the tip Squirrelman, I was just going to try lifting the slides by hand and see if they felt sticky and check to make sure they closed from open with a nice click, but direct observation while running seems like the best way to go.

 

Update, and to me surprising answer. Well, when I checked the slides as suggested by Squirrelman they all were lifting nicely, so I bit the bullet, pulled and went completely through the carbs. What I found was that the pilot jets were all perfect, but the small crosswise holes in the emuslion tubes were almost all plugged. I've cleaned bunches of carbs but never seen a case where the pilots were perfectly clear and something else was plugged up. Live and learn... The bike is running strong at all throttle openings now and has that great torquey feel from 3K up to where the top end rush kicks in around 7K. I just thought someone with a similar situation might benefit from my solved puzzle.

 

Emulsion tubes of the mains or pilots?

 

The Mains!

 

i've cleaned a few carbs and never seen that either. Were the main needles stock OEM with a gradual taper, or aftermarket with a step?

Or once in it's past life the pilots were cleaned by a previous owner, but he neglected to look at the cross-drilled holes of the main emulsion tube?

Good catch to find it and thanks for sharing.

 

I have the aftermarket Factory jet kit made to match the K&N air filter installed by the PO, so I'm not sure if the needles are stock or came with the kit, but they are gradually tapered with no step. The carb interiors were pretty clean with no significant gummyness, and every orifice in each of the pilots perfectly clean, but the crosswise holes in the main emulsion tubes plugged with a grayish to white powdery substance. Since I don't ride sometimes for weeks at a time, I treat my gas routinely with the recommended amounts of both Stabil and Sea Foam. I recently had to clean the carbs on my FJ1200 which receives the same gas treatment but is ridden even less often, and the pilots were blocked but the crosswise holes in the main emulsion tubes clear. This is what I think of as the normal scenario. Oh well...