Intermittent bogging.

1986 VFR7000- All stock with a 4-2-1 exhaust and a period-correct Yosh slip-on

I've been having the most annoying issue the last couple times I've taken her out. Seems like after riding for about 15 minutes, the bike will begin to idle rough and then upon accleration (moderate or otherwise) it won't really "catch" and begins to bog down. I can throttle through it and get the bike to pick back up after about 6k but as soon as I roll the throttle off (no matter how slowly) the bike will die. Haven't done a whole lot of checking yet due to a busy weekend and being sick yesterday but I'd like some suggestions before I really get into it. I recently put new contacts on the fuel pump and replaced all fuel lines and filter maybe 1500 miles ago.

Thanks in advance!

 

Could be float needle/s not seating properly and flooding out one or more cylinders. Try finding which cylinder/s stop firing, and inspect those plugs. Darkish black=too much fuel or weak spark. Try draining carb bowls into a paper towel to check for rust from the fuel tank.

 

i would suspect dirty carburetors, e.g. clogged pilot jets affecting the transition ports, or the cross-drilled holes in the emulsion tubes.

http://vfrworld.com/forums/showthread.php/22449-Cleaning-carbs

 

Good suggestions all around. The tank was urethane sealed just before this season and the carbs were cleaned and reassembled professionally as well. I can see just from looking down in the tank that rust is most likely not my issue as there is absolutely none present. I'll post my findings as soon as I get to it.

 

If the bike runs fine when you first start, then craps out after about 15 minutes when it is fully warmed up........you could have an ignition coil or pulse generator coil beginning to fail. Also you might try bypassing fuel pump relay to eliminate that often-troublesome component, although pump itself could be the problem. Be sure to verify that fuel filler cap vent is open.

If the bike starts easily, idles smoothly and low and has good throttle response, probably not a dirty carb issue.

 

My thoughts exactly in regards to the carbs being the culprit, I'm going to bypass the fuel pump relay this evening once again (should've just left it that way the first time) and see how that goes. I think coils are still readily available though, if that's what it takes to squash this. I may wait 'til tomorrow for a test ride. It's actually kinda cold here today.

 

There is a vent tube in the tank up behind the filler tube--it may be that they covered over it with the sealer and now it can't flow any fuel after 15 minutes due to no air coming in to replace the fuel (creates a vacuum in the tank). Do you notice a big whoosh sound when opening the fuel cap after one of these 15 minute events? go for a ride and check to see. Might be able to run a steel cable (old speedo cable) up thru the tube opening and clear it out.

 

Nah, I've put 2300 trouble-free miles on it since getting the tank done. It could be clogged, but definitely not sealed over.

 

Do you mean inside the actual flip cap? I think the tank vent is built inside the cap. Usually when it gets clogged you'll be cruising along and the bike will start sputtering just like when you need to switch to reserve. Vacuum builds in the tank and the carbs start to starve for fuel.

 

No that is not what i meant. There is a one-way vent valve in the fuel cap below the key, but i was referring to the metal vent tube built in to the tank and known as the fuel tank breather tube in the manual.

 

The overflow tube under the cap just to the side that drains down to the ground? I don't think that would have any effect on the bike running, it's just to drain any excess gas if you overfill it. (edit: yes, more likely for water since it's on the left side)

 

No it's not the drain line, that is another metal tube designed to drain rain water from the area around the cap. There is a separate tube designed to vent the tank. If you ever cut open a tank you can see the internals, or get a small mirror and flashlight to see what's in there and how it is routed, the screens on the fuel tubes, etc...

 

Yeah, I'm familiar with the tank internals on my 86 and I've seen pictures of these tanks cut open. Your talking about the 86/87 bikes, right?

Are you saying there are 2 internal vents in the tank? One built into the cap plus one more?

And your not talking about the main and reserve tubes inside the tank attached to the petcock, right? I'm really stumped. You mentioned running an old speedo cable through it, where does the vent tube exit the tank? I only know of the 2 tubes under the tank, the main out line and the drain line and then you have the key hole.

 

One more thing, is the cap vent only one way? The tank needs to let air out to reduce pressure as the fuel expands as well as let air in to prevent a vacuum build up. I assumed it was a 2 way vent.

 

I'm looking at the manual and what they are calling the "fuel tank breather tube" is just the water drain tube. They gave it a misleading name.

 

They gave the voltage regulator a misleading name too since it often doesn't.

Where could i find photo of tank internals ?? I'm still trying to figure out occasional air or fuel locks whenever tank goes below 1/3rd...........after 24 years !!! Only way to clear it is bbblowin air backwards into the tank. (Clean tank with no visible debris.)

 

I'll post you some pics tomorrow once I get back home to my PC.

I posted this 2 days ago on another thread where you mentioned your problem.

I'm sure you've put fresh fuel line from the pump to the tank, so it should have good vacuum there. I'm also guessing you checked the cap vent and made sure it's flowing air.

Have you inspected the petcock? It may be leaking air internally not allowing a strong enough vacuum.

Another possibility is that the main or reserve tubes in the tank are not solid. if there is a rust hole near the top of one of those tubes, you won't get a strong vacuum. Or if someone has poked around inside the tank they may have bent a tube and it's leaking between the pickup screen and where the tube is welded to the petcock area.

You could try disconnecting the main line from the pump and attach a hand operated vacuum pump (mityvac) and see if you can pull fuel though with the tank at differing fuel levels. Maybe listen for air sucking inside the tank if it's not sealing.

Have you tried measuring the vacuum that the pump is creating? Maybe it's seals are bad and not creating a strong vacuum?

 

Here are the pics. It's all I have.

 

Does your air box have a few holes on the top for more air to come in?

 

No, my airbox is un-altered. But I've moved past this problem and on to a whole new predicament! Luckily however, "Winter is Coming" so I have plenty of time to do my suspension and wheel swap and figure out why the hell cyl #2 is flooding.