VF700F Dying Under Throttle

Hi All,
I hope yoh can see the video.
I finally got around to working on the 700 Interceptor
Its carbs have been cleaned, new jets, etc.
It starts fine, idles and accelerates a wee bit, then bogs down and out with anymore throttle.
Any ideas?
Thanks,
Shawn

 

My first guess that could produce that symptom, along with the fact that the carbs have been worked on, is the diaphragm(s) are damaged or installed incorrectly.

 

Thank you, I was wondering about that too. I also read (but can't find anymore) that the fours have two plunger needle sizes, two for the front, two for the back. Maybe I mixed them up? At least the diaphragm covers are easy to access, as a first step...


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So, I've removed all of the assemblies and what I found:
All the needles appear to be the same. I do not think these were included in the rebuild kit, so they would have come with the non-running bike at purchase.
The springs are different lengths. Thr two front ones are longer than the two back. Are they backwards?
All diaphragms look good, no pinholes, squishy... Couldn't tell if they were seated correctly when J took the covers off, but the little outcrop half circle was lined up correctly-ish with the corresponding hole in the carb.
When I blew air through the connectors (black plastic ones that connect to the airbox): The two rear carbs spat out fuel from thd bowls. The right front one did, but the front left did not.
I cleared that one reversed (blowing air through the needle hole). Maybe it was clogged, or maybe there was no fuel in that bowl?


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Longer springs in front.

When you install the slides, invert the rubber into a mushroom cap. Put your finger down in the carb and hold the slide up when you install the spring and cap. This greatly improves the chances of the diaphragms staying in the grooves and the cap installing properly. Sometimes you have to let the slide down just a touch to keep the diaphragm lip in the groove. With just 2 opposed screws in place, let the slide go. It should fall smoothly and somewhat slowly. Give it a couple lifts and releases and make sure the action is correct.

 

Ok thank you!
It's possible they weren't seated right. I know I did not install them ax you described.

 

after many years the diaphragms may shrink or swell, and that makes getting them back in the grooves correctly a problem and a common screwup. always test before remounting the carbs.

probably ok to use very small dots of something sticky to get them to stay in the groove if nothing else works.

 

Sigh...All back together after checking everything and making sure the diaphragms are seated.
Same issue.
It starts great, the slow circuit is ok up to 3000, 3200 rpm. I can gradually throttle up yo that point then it won't go any further. If I hold it there it will then go up a bit, down a bit, up a bit and down a bit as if there is an air leak.
If I throttle any further it wants to die.
This is the first time I have had success with the slow circuit and trouble with the fast circuit. Bummer.

 

does it run better when using a little choke ?

check that float settings are correct, about nine mm.

 

Hi, the best run is with full choke. If I let it run for a while and warm up it gets to about the same point.
I will pull and inspect the plugs next and check the floats.
Would a weak spark contribute? Probably not, if I get this far...
The fuel pump is new and so is the fuel pump relay, btw.
All four main jet needles are identical. They're supposed to be different from front to back. I did actually receive 4 new ones in the kit and found 3 of 4 of the old ones (where's the 4th?). All three of the old ones are the same.
I'm getting the hang of taking them off and getting them back on, so maybe I'll start from scratch?
Should I double check valve clearances?
Thanks guys

 

Are you trying to run with the air box off?

Also, new carb insulators?

 

The air box is all on as it should be.
The carb boots are all new. You thinking air leak? I know I have zero clearance on cyl. 3 and the boot is hard to get on. I should try the ether spray test to see what happens.

 

Yeah. When full "choke" makes it better, you are likely getting excess air somewhere. Do a spray test for sure.

 

Also, the front of the air box where it meets the plenum doesn't have any screws in the middle of that front edge. I've had one very warped there with a huge gap. With a good lower air box, I will still run a bead of Hondabond to keep shit out.

 

Ok thanks Cap'n.
After doing this , and if it doesn't reveal anything, I'll take the carbs off and check the pilot jets. They are new, but something may have got sucked up when it started.

 

be sure to check the small filters above the float needle seats.

 

If it's starting, idling and coming off idle.... It's not the pilots. That's what they do.

 

....true

 

Today was a bad VF Day. I need a hug...I ran the engine some more and it seems it is making progress, kind of. Whether choke or not it doesn't stall, but doesn't fully rev. If I back off a bit it will rev, then back off by itself
I took a break from that and worked on my front brakes. I had to stop and wheel the bike in. I had the tank resting on top of the bike and when I moved it, it fell. Nice asphaltic dent now, damnit.
Then I remembered I wanted to look at the float levels. The number 2 bowl is empty. I noticed the same on thd bench test. But by adding air into the fuel feed all four floats moved. Problem solved, whatever it was, I thought. Why would this carb not receive fuel like the others? There are no leaks, the connector tube must be unobstructed, or I couldn't have air-moved the float....Very weird.


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I took the carbs off again and found three interesting things: Carb 2 definitely had no fuel. Its float needle had fallen off the tang of the float and was probably stuck and stopping fuel. Carb 4 had a float like Jagger - shattered (shoo bee, shoo bee shattered...). They are all brittle looking. New ones on the way. Third, the fuel feeder hose ,which I never took off before, is hard as steel at the end where it attaches to the carbs. Will see if these corrections make a change.