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new float needles, now bike stumbles at speed

brd

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Fort Worth, TX
So, in an attempt to find a niggling fuel leak, I pulled the carbs and replaced the float needles in the VFR yesterday.

Put her all back together last night, changed the oil this morning and headed off to work.

As I pulled out of my neighborhood, I noticed a hesitation. On the highway, that turned to stumbling, but never cutting out or shutdown.

Hrm, this is what I get for not doing a shakedown ride. I had to get to work, and it didn't seem too bad, so I pressed on.

Up to about 4-5k rpm, it was fine, and from there up to 8k was just wishy-washy. In high gears, full throttle would do nothing acceleration-wise. I can be WOT in 6th and the bike does not accelerate at all.

I did find that in 3rd, though, it would accelerate nicely from 8k to redline. I cruised there and back mostly in 3rd and sometimes 4th gear.

My first thought was that the tank wasn't venting right, but I opened it and filled up with gas and there was no woosh or sucking noise.

Aside from the float needles and oil & filter, the only thing I (intentionally) changed is adding a longer hose to the carb overflow vents. It had a short 3" hose that just dumped excess fuel onto the engine. I ran a 2' piece out down the bottom of the bike where I can see it with the bodywork on.

Fuel hoses and tank vent hoses are on correctly. Petcock is to full "On".

Could there somehow be air in the fuel line, pump, or filter?

:help:
 
Brian,

Call Patrick at Motorcycles Unlimited tomorrow (Wed). his number is 281-444-0545. Tell him you are the guy with the VFR from TWT with carb probs. He said he'd walk through some stuff to try with you. Before lunch would probably be the best time as he gets busier in the afternoons.

Good luck!

Adios,
 
Brian,

I wish I had some magic advice. I have performed simular surgery on my ZX6e and ZX9r, but luckily I did not have the problems. I might suggest it could be vaccum leak. You might want to check to see if there is a leak around the boots between the carbs and the intakes.

Keep us posted

regards,
Tom
 
Sounds like a vacuum leak to me. Like the diaphrams on the needles are not getting opened up due to them either being pinched or not properly sealing off.
 
I didn't mess with the carb diaphragms or slides at all, just pulled the bowls and replaced the float needles. Also, the floats are non-adjustable and don't seem to have fuel in them.

It does sound like some sort of vacuum deal. I thought I had found the problem in a pinched fuel vent line, but that didn't totally clear it up.

Then I disconnected both the vent and overflow lines from the bottom of the gas tank so that I know there are no obstructions in them and went for another ride. It was better, but under load in high gears it's still lugging and down on power, though it will at least accelerate now.

As soon as the bike cools down, I'm going to pull the tank off again and check out the carbs and pull the rear sparkplugs and see what they say.

I also noticed that there's a pop or two while engine braking, which never happened before.
 
I'll make you a deal on a 2001 VFR 800 :-P

Adios,
 
Well, I think I fixed it. Apparently, adding a longer hose to the carb overflow vent tube is not a good thing.

I went out and pulled the tank. Airbox looked good, boots and everything seemed okay, all the plumbing fine. I verified fuel flow was okay (and soaked my sidi boots :(). Then I pulled off the hose from the vent tube and put it back together.

I got out on the highway and flogged it. Despite rush hour traffic, I managed to get in some top gear testing. No more stutter.

Weird. I guess the longer hose caused a restriction in the rate the fuel can flow into the carbs by reducing airflow in/out to the float bowls?

Does this make sense to anyone? If not, then I guess I just accidentally fixed whatever else it might have been by taking things apart and putting them back together again.


Oh, and while I had the airbox open, I desnorkelified the VFR. Why didn't I do this years ago? :twisted:
 
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