If what I wrote above is right, what I still don't understand is:

1) How did the bike run so well for over 200 miles? Great power, great mpg, no cutting out or dying (except when the bike was hot AND the fuel level in tank was low, say after ~95 miles riding).

2) How/why is there excess fuel above the inlet valves on cylinders where those valves are closed? The fuel did not drain from these places with the sump plug taken out, so it makes me think it is getting there through the carbs somehow.

But the floats were seating properly and the carbs weren't leaking, and also, the volume of fuel above the valves in cylinders 1 and 3 was far greater than the amount filling the float bowls of those cylinders' carbs when the float needle valves had closed.

Mysterious!

I still think it is a stuck float. Have you replaced the needles and seats yet?

Hi vfrman. I've not bought new needles and valve seats yet (it's £120 to do all 4 carbs, + postage), because I have doubts about the stuck float theory for these reasons:

1) I set all the float heights and checked they operate smoothly;
2) The bike ran very well and then this happened over 48 hours with the fuel tap in the OFF position, where I put it immediately after stopping the engine (and it doesn't leak);
3) The fuel tap diaphragm looks like it is warped, and there was fuel in the vacuum side of the tap and in the vacuum hose;
4) I filled the carbs on the bench and measured the amount of fuel from each float bowl and it was the same (small) amount in each one;
5) The amount of fuel in the cylinders/block/sump was enormous, way more than in the 4 float bowls put together.

It's hard for me to see how a stuck float would make a bike that ran really well on Thursday, was stopped and fuel tap turned off immediately after Thursday's ride, and just sat with a non-leaking fuel tap in the "OFF" position, be flooded on Saturday.

On the other hand, there was fuel above the valves on cylinders where the valves weren't open, which I can't explain on the basis of the leaking fuel tap vacuum diaphragm... So maybe both things are happening. I'm a bit lost...

Less than fifty quid.
http://www.davidsilverspares.co.uk/VFR4 ... rt_224142/

If the tap is sealing in the off position and all float valves are sealing then no fuel can flow unless a float is sticking or punctured.
This could fill a cylinder in minutes, but the engine would run poorly and stink of fuel in the exhaust.

If the vacuum hose allowed fuel to seep into cylinder number 3 over time and fill it and that carb up to the breather tube connected to cylinder 3 with its valves closed, then that cylinder would have fuel above the valves.

Run hose from tap fuel line and vacuum spigot into 2 trays and leave over night to check for seepage.
You may have fixed the problem if the diaphragm was not seated.

What arsey30 said...

The best way to figure out which one is leaking is to hook tubes to each float drain and open them up. Have a separate container for each drain tube. Let it sit for a while to see which one is leaking (I would unhook the vacuum line from the no 3 cylinder so you know it is the float for that cylinder. You can put the vacuum line to it's own container to verify that isn't leaking as well.

Thank you all for all the help and advice, I really appreciate the time you have all taken to help me with this. I hope in future I will be able to help someone out as decently as you all have helped me.

Thanks for that link, hunter. I bit the bullet and bought 4 of them, as I reckon it's better to eliminate the needles/seats as possible causes, since every time this happens it costs me £30 in oil after all...

Right I am going to post the detail of what I did today underneath in case it is useful to anyone, but long story short:

- All four float bodies have tiny pinholes around the seams & metal/plastic joins;
- Carb 2 float valve was intermittently sticking in "closed" position;
- All four seats have separated somehow, i.e. the threaded brass part and the filter/mesh part have come apart and the filters are now stuck in the carbs after the threaded parts lifted out.

I am going to try to find some sort of petrol-resistant sealant to coat the float bodies with, because they're £36 per float at CMS and at David Silver, and I'd really like to avoid paying £150 for 4 floats!

As for the seat assemblies separating, does anyone know why this might be happening? I know they come as separate threaded bits + filters, but there's no way to get the filters out of the places they sit in the carbs (without wrecking them), so I assume they're not supposed to separate in normal use.


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OK so the detail for those who might find it useful for any reason:

I was going to do the drip-tray tests suggested above, but since I had the carbs off the bike I thought first I would check the float operation once more. All the floats moved freely except for the one in carb 2, which was very sticky, but not always - sometimes it would get stuck when pushed up gently into the seat, sometimes not. When I unscrewed and removed the seat, only the threaded part came out - the filter part remained stuck in the body of the carbs.

I dug it out with a staple and some tweezers, which ruined it. Then I undid the seats from all the other carbs, and they all did the same thing, i.e. separate into the threaded bit that lifted freely out and the filter/mesh bit which stayed stuck in the carb.

Is this normal? From the picture on the CMS page here http://www.cmsnl.com/honda-vfr400r3-199 ... 011mr8003/ it looks like the seat/filter is actually an assembly; however there doesn't seem to be an obvious way of getting the filters in and out of the carbs if they're not attached to the threaded brass part. And when I first took the carbs apart this separation didn't happen, the whole seat/filter assembly just unscrewed and lifted out together, nice and cleanly.

Before I dig the rest of the filters out and ruin them in the process, I wonder what you knowledgable people think about this?

Also I dunked each float into very hot (just boiled) water and held it underneath. They all gave off very fine streams of very tiny bubbles from spots close to the "seams", and also from near where the metal part goes into the plastic. I held them under for quite a while but they still floated up when released, and I couldn't hear any liquid sloshing about in them when I shook them afterward.

So I reckon the floats are getting overweight and that is why the valve isn't closing and that is why the engine is flooding. This might also explain why it ran well for a while and then happened after sitting for a while - gradually the floats were gaining weight, so the float bowl levels were gradually getting too high. At the end of my last ride the float bowls were overfilled and that ended up in the manifolds of cylinders with closed intake valves (1 & 3), and in the combustion volumes of cylinders with partially open intake valves (2 & 4). From on top of pistons 2 & 4 some of the fuel trickles down into the crankcases and sump. Next attempt to start it = failure, cos it's flooded, as observed.

The little filter mesh is separate on the geuine float valves. You can get cheaper copie valves that have a built in filter.

There is no need to ruin the mesh filter if you remove it properly.

If tap was off and not leaking, punctured floats will not allow fuel flow, and a high level in the float bowls will not be enough to fill a cylinder and flow into others.
If there is no fuel in the floats, they will work correctly.

The squared top edge of the filter does sometimes screw up into the carb body to form a spiral that wedges in the hole.
I push a close fitting rod into the filter body and unwind it as pull it out.

When i had a fault on mine only happening when attempting to start the bike after 24 hours or the next day it would clatter once & not turn over when i think back i could hear the conrod bang it turned out to be hydraulicing on fuel that had leaked into a cylinder and stopping the bike turning over, a full carb strip and new fuel tap kit sent from RO fixed it- i was supprised it did no damage to the engine.