Keihin FCR parts suppliers in U.K.

Well, the thing is, these carbs offer super precise measurements of fuel at all possible throttle openings. But when that pump kicks in, it’s basically just lobbing in a load of very poorly metered and poorly atomised fuel… So, on the dyno, it can make it difficult to read the AF data and make decisions about settings etc.

They are big carbs, and have a very quick acting throttle, I firmly believe the pump is there to protect against them being whacked open to much / too early. This is especially the case for dirt bike riders who only have two positions on their twist grips. But I think it can also apply to old Norton lumps too.

I’m not a dirt bike rider, but when I ride mine on the track, I’m also a bit guilty of ‘whacking it open’ so I suspect that I might benefit from keeping them working. I suspect I’d be hitting flat spots regularly without them.

But definitely for the dyno, for trying to understand the AF ratio at specific throttle openings and RPMs, disconnecting the pump would seem to make sense IMHO.

Fast Eddie said:

They are big carbs, and have a very quick acting throttle, I firmly believe the pump is there to protect against them being whacked open to much / too early. This is especially the case for dirt bike riders who only have two positions on their twist grips. But I think it can also apply to old Norton lumps too.

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Well - after disabling the pump I tried all sorts of acceleration, etc (including whacking them wide open - in a variety of gears and rpms) and was not able to induce a stumble or hesitance (that I could detect) of any kind.
That doesn't mean that acceleration might not have been stronger with the pump, but I don't think so.

robs ss said:

Well - after disabling the pump I tried all sorts of acceleration, etc (including whacking them wide open - in a variety of gears and rpms) and was not able to induce a stumble or hesitance (that I could detect) of any kind.
That doesn't mean that acceleration might not have been stronger with the pump, but I don't think so.

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It’s all relative I guess. At the end of the day there is no way a 35mm carb is going to flow optimally, and therefore meter the fuel optimally, at WOT from very low rpm.

I would suggest you can ascertain this yourself, I’m quite certain you’ll accelerate faster if you ‘feed’ the throttle on vs WOT from launch. Even my 650 Triumph will ’bog’ if the 30mm Amal’s are whacked open at too low rpm. That’s what the pump is there to protect against I believe.

However, as to whether its actually needed by all riders in all applications… that’s another matter. IMHO it’s not a debate about whether the pump is good or bad, it’s a ‘different horses for different courses’ kinda thing.

Over the years on the Laverda forum, there's been regular debate about disconnecting the accelerator pumps on the old dellortos. Most of the time there doesn't seem to be any down side (that's what I found) and you get better mpg and a lighter throttle. However one forum member did the same on his big Husky dirt bike and tried to loft the front end over a log, resulting in bogged throttle and landing on his ear! The accelerator pump got reconnected on that bike!
There's also a school of thought that they allow the bike to run a bit leaner, ergo if you can't tell the difference then you're running rich everywhere? Obviously this approach has its limits, especially on old air cooled bikes.

There’s a flip side to the theory that they allow leaner running… it probably is correct for a race bike becasue they are always either accelerating or decelerating. But on the road, that weakness might expose itself when holding a steady partial throttle opening.

So, my thinking is that the carbs need to be set for the right mixture anyway, and the pump is then only protecting against a throttle being opened too fast for the carb to deal with properly.

I've never done WOT from dead stop with the throttle fully closed with the 35mm FCRs on my 750. All that would do is wear my rear tire out quicker. I'm not sure if it could take all that air at once anyway. It will take WOT from 2000RPM, which IMO is closer to the operating range the FCRs were designed for. I would think racers are getting the RPMS up and slipping their clutches at launch.

I get over 60mpg riding in mixed HWY, rural, twisties, and city combined with the acceleration pump functioning. I'm sure that would decrease on a race track or in city only riding. Not real important though, since I don't race and try not to get stuck in stop and go traffic.

Zero issues on the HWY with steady throttle, or going very slow as long as I am in 1st going slow. If I try to go 5 mph in 2nd, it's a bit jerky unless I use the clutch a little. The FCRs work well on my Norton 750 street bike with the accelerator pump. My motor is not stock, it stands straight up like an Atlas, and I reshaped Commando intake manifolds to work on a straight up motor. Combat cam, and head was ported by Fred Barlow. It's different, so not too many or any P11's to compare it to. And very few Commandos to compare it too with the Fred Barlow head. It has never run as well as it does now with the FCRs, which means something bad is about to happen.

Did you ever figure out what was causing the fuel weep?

Schwany said:

Did you ever figure out what was causing the fuel weep?

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Still suspect it’s the float needle searing O ring as mentioned in post #8. Trouble is, the supposed genuine OEM replacements don’t fit and I won’t know until I’ve replaced them and tested them.

Fast Eddie said:

Still suspect it’s the float needle searing O ring as mentioned in post #8. Trouble is, the supposed genuine OEM replacements don’t fit and I won’t know until I’ve replaced them and tested them.

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That is disappointing. The Sudco Keihin User's Manual lists part number 021-047 for the Valve Seat O-Ring. Is it not the right one?

You could fire off an email to the Sudco rep and find out if there is something unusual about the Norton 35mm FCR carb set. That listed part number is supposed to fit all, whatever that means.

Schwany said:

That is disappointing. The Sudco Keihin User's Manual lists part number 021-047 for the Valve Seat O-Ring. Is it not the right one?

You could fire off an email to the Sudco rep and find out if there is something unusual about the Norton 35mm FCR carb set. That listed part number is supposed to fit all, whatever that means.

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Dunno, have others coming so will see if they’re different.

The old ones were pretty shot though when I removed them, so that’s a good sign they could be the root cause.

As mentioned earlier, the float needle seating o rings (supposedly ‘genuine’ from Allen’s Performance) were too big for my liking, if I forced the housing in it ‘cocked‘ so would have probably casued the issue you mentioned to me at Cadwell Ralph.

So, I got correct ones, along with some other bits, from ‘Franks MX parts’ (mxdude on ebay) in Holland.

With the correct o rings, the searing fit in perfectly, no danger of becoming ‘cocked’ either.

Haven’t rebuilt or tested them yet mind, so more later …

New o rings fitted. Carbs now fuel tight !

Also put in several new o rings, accelerator pump diaphragm, gaskets, needles, etc.

Then discovered one accelerator pump jet blocked, quite fiddly that, but got it squirting nicely in the end.

Just waiting on some basic hardware (pipe, clips, filters) arriving and I should be good to fit ‘em on.