Hitchcocks Forum

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Amal / Meteor Minor flat spot





Hi all, in the review for the Meteor Minor De Luxe (The Motor Cycle 26th June 1958), It states that htere is a slight 'flat spot' on the throttle opening just off idle. Well, Kevin, being a faithfully restored bike also seems to have this annoyance. It is worse when stopped. I go to pull away and if I remember to blip the throttle just a little, then all is ok, but if I just slowly open it a little to get the revs up, then it quite often just stops dead and I am left stalled at the lights, Very embarrassing.



Is there anything I can do to the Amal monobloc to get rid of the flat spot?






"Kevin looking 'flash' in France"


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REOC 15084

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This is the age-old bane of the pox-ridden Amal, although this fault is far more common on the Mk1 Concentric. The mixture goes weak as you crack the throttle.



You can't cure it directly, because it's a design fault. However, you can get round it by making the mixture either side of the flat spot richer. It's a bodge, but that's Amals for you.



Richening the idle mixture will reduce the problem (that's the mixture "below" the flat spot). Then fitting a richer slide will richen it up "above" the flat spot. Between them you may be able to make the flat spot almost disappear. Obviously altering the slide might make it richer than you'd like at small throttle openings, so try just tweaking the idle mixture first.



You often end up with a compromise: a slightly richer idle than ideal, and a much less severe flat spot. That's how most Amals end up. Better still would be to consign all Amals to the recyclers, and fit a Mikuni.

Hi Scaly, you could try a richer throttle slide [less cutaway] and / or a richer pilot jet, although the problem is probably occurring as you come off the pilot circuit and onto the slide, so the slide would have my money on it, although a pilot jet would be a lot cheaper and easier to try first. On a different note, make sure there is no slop in the auto advance unit Regards, Paul.

One thing people miss is the float height. Look here

If the float is too high it can give a flat spot because it's too rich. If the float is to low you get a flat spot because it's too lean.

Perhaps the carburetor needs a Miss Shilling's Orifice fitting to it:-

Scalyback, it could also be wear on the slide/throttle body, especially if it's an old carb. Despite what Thack says it's perfectly possible to get a smooth transition from tickover to part open throttle on a monobloc or concentric if in good condition. Maybe it's a quirk of the inlet tract design that causes this on your bike if the tester reported the same? I wouldn't know as I don't know know much about RE twins. What I do know is that when it had the original Keihin EFI on it my C5 always did exactly what your RE twin is now doing; it now has a lovely crisp pickup with the Amal conversion fitted.

As Thack says, this is the curse of Amals, Monobloc and Concentric alike, not surprising really as they share the same design of pilot circuit. However, recent experiments have given me an idea. I've been trying to eliminate a rich spot just off idle and onto the slide/needle phase. (This is on my new Series 2 Interceptor with new Amal Premier Concentrics.) It was suggested by Tim Busby that the 107 needle jet fitted as standard would be the culprit and that a 106 might do the trick. This has certainly sharpened up the response at this crucial transition point on my bike. So, for your situation, why not try replacing the 106 needle jet almost certainly fitted for a 107? My theory (I'm open to better experience,) is that the well under the needle jet acts as a reservoir to richen the mixture at the point of transition and maybe the larger jet would allow a bit more fuel through? Worth a go for the small sum a 107 jet would cost?

REgards, Mark

I side with Rattlebattle on this one, when Amal carbs wear whilst the slide rests on the stop you set up the pilot mixture, as you start to lift the slide it then tilts allowing air to pass round the slide in the worn area this is what causes the glitch.

font color="#336600">Thanks so far guys, so I am either looking for a 107 pilot jet, or to replace the carb with a new amal?

I admit they seem more difficult than later ones, but as the rest of the bike (excluding trafficators) is concourse, then I should try to get the right one. I will try for a 107 pilot first.