35 FCR flatslides maybe need tinkering

Hello all,
I have a set of 35mm Keihin FCR flatslides on my 850 MK3 and have been more than happy with them for maybe 8 - 10 years since I installed them.
I have PW3 cam, bigger inlet and exhaust valves, Maney barrels etc.
I have never needed to touch them and done around 35,000 miles with them on.
However lately the bike has been running poorly, esp on a steady throttle. Its jerky and flat in certain speeds, seems ok on acceleration, which indicates to me its not the pumpers causing issues. I have changed the coils, checked for air leaks in manifolds and even removed aircleaners.
The plugs are very black and as I have changed nothing in the carbs, am looking for advice as to where to look.
I have worked with amals no problem, but have no idea where to start or even what is what on these.
Any advice as to what wears or what to check gratefully received.
Cheers

2 things to look at IMO:

1. The float needle housing sealing o ring perishes over time. When this happens, fuel by passes the float needle even when it is fully seated. Thus causing flooding. Your black plugs would support this possibility. These are special o rings. Standard o rings are too fat and will cause issues trying to force the housing into place.

2. With your mileage the needles are suspect, they wear surprisingly quickly IMO.

Both of the above = more fuel = black plugs and poor running.

What Fast Eddie (Nigel) said although I haven't had mine long enough for them to start going off.

If you are not riding your Norton much and have older fuel in the tank, the fuel can be problematic.

My plugs are dark if I start my bike in the garage and run it without going for a ride. If I ride and keep the RPMs high enough they clean up and basically look perfect. They are on the dark side if I short shift and try to keep the RPMs down.

You might have some junk in the low speed air adjuster, which looks like a jet at the rear of the carburetor. If there is any blockage your neutral throttle and low throttle positions are probably going to be on the rich side. The slow air adjuster/jet is on the left side at the back mostly covered visually by the air cleaner adapters and is slotted so it can be adjusted in and out. In is less air in the mixture, out is more air in the mixture. There is (or should be) a rubber gasket under the air cleaner adapters. The gaskets will not fit back in place easily or at all, because fuel stretches and expands the rubber. You can use something to absorb as much fuel from them as possible and blow some hot air over them to possibly help get them to shrink a little. Can be a futile exercise though if in a hurry. I buy new ones if I know I'm going to be taking the air cleaner adapters off. I've never tried not using the rubber gasket with the air cleaner adapters. Others may say the adapters don't need the rubber gaskets, but I sort of doubt it, or the gaskets would not be there in the first place. I know people don't use the rubber gaskets if running the plastic velocity stacks. It wouldn't serve any purpose with the velocity stacks.

Last thought is check your overflow/atmosphere tube and make sure it isn't pinched shut or have bugs living in it.

Edit: The rubber float bowl gaskets are usually not a problem. I've never rebuilt my FCRs, but I have had the float bowls, accel pump, and air cleaner adapters off a few times making mild modifications on one thing or another. Working on them is a long way from rocket science, or I wouldn't be able to do it. Obviously much easier to work on off the bike.

Fitted my FCR's in 2008 and only recently gave them a birthday - rebuild kit included new float needles/seats/o-rings. The pump diaphragm also came in the kit and the old one did look tired.
That solved fuel leak/8 stroking and difficult cold start issues and now getting 60MPG again on the last run. I was too mean to include the needles but may add those to my list based of FE's comments.
If you can work on Amals then these aren't much worse, exploded views are easy to find online.

FWIW, I just went through a similar experience. I'll spare you the details but first thing I did was replace the plugs. No change. Then tested the plug wires on a multimeter - ohm'd out perfectly. Then went perfectly nuts checking everyhing I could. No joy.


Finally I replaced those tested-good plug wires. Problem solved.

YMMV, but again, FWIW.

Tx - B

I would have thought faulty plug wires would cause missing before rich readings on spark plugs.

I had the same problem a while back , did an overhaul , the main problem was the slides have a piece that lifts out , wear plate ? They were stuffed .

dero said:

I had the same problem a while back , did an overhaul , the main problem was the slides have a piece that lifts out , wear plate ? They were stuffed .

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Good call. That is probably very likely to be the issue on a set of FCRs that have been used on a paint shaker Norton twin for 35,000 miles. I can see that happening for me in the future should I rack up than many miles on my Norton since I installed them a few years ago. At the rate I'm riding it'll take 15 years, and be past my use or freeze date.

acotrel said:

I would have thought faulty plug wires would cause missing before rich readings on spark plugs.

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In my case, both. The missing caused me to pull the plugs, and one of them was really sooty.

Thank you all for your input and experience, somewhere to start!

Fast Eddie said:

2 things to look at IMO:

1. The float needle housing sealing o ring perishes over time. When this happens, fuel by passes the float needle even when it is fully seated. Thus causing flooding. Your black plugs would support this possibility. These are special o rings. Standard o rings are too fat and will cause issues trying to force the housing into place.

2. With your mileage the needles are suspect, they wear surprisingly quickly IMO.

Both of the above = more fuel = black plugs and poor running.

Click to expand...

My mate adjusted the screw at front of carbs and it now (well when he did it) idles well. I have taken the float bowls off and then retaining screws for the float and the float needle seat to check the o ring behind the seat. My question is: how do you get the seat out? It is a hex flush in a round hole?

johnny Lagdon said:

My mate adjusted the screw at front of carbs and it now (well when he did it) idles well. I have taken the float bowls off and then retaining screws for the float and the float needle seat to check the o ring behind the seat. My question is: how do you get the seat out? It is a hex flush in a round hole?

Click to expand...

IIRC you remove a retaining screw that holds the housing in place and then it pulls out. Obviously, it could be pretty stuck with age…

I’ve never done it in situ though BTW… lack of space and visibility would make it a PITA I reckon…

While you're at it it would be with looking at the vacuum release plates ( and seals)

Cheers