Flat spot off idle

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IMO this still goes back to exactly what is meant by a "flat spot." If you wick the throttle to WOT, none of the cicuitry except the main jet comes into play because the needle is lifted completely clear almost instantly and therefore serves no metering purpose. If the flat spot occurs as you are opening the throttle more slowly, then the needle is part of the transition.

So I would ask again, when does this flat spot occur? Does it occur as you are roling the throttle on in what might be called a normal manner or does it occur when wicking to WOT? If it's occuring as part of normal acceleration, then it can be corrected with proper needles/jets/float level, etc. But if it is occurring only when you rack the throttle from idle to WOT, it's a characteristic of non CV or non accelerator pump carbs.
 
MexicoMike said:
So I would ask again, when does this flat spot occur? Does it occur as you are roling the throttle on in what might be called a normal manner or does it occur when wicking to WOT? If it's occuring as part of normal acceleration, then it can be corrected with proper needles/jets/float level, etc. But if it is occurring only when you rack the throttle from idle to WOT, it's a characteristic of non CV or non accelerator pump carbs.
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Sorry, I missed this question the first time around. The flat spot occurs as I slowly increase the throttle opening, and also as I reach a cruising speed in any gear and slightly throttle back. If I hold a steady throttle of less than 1/2, the machine burbles, mostly on the right cylinder, and I can improve it by depressing the tickler on the right carb.
 
I wonder if your fuel lines may be partly obstructed. I assume you run with one petcock open. Perhaps try running with BOTH open and see if things improve? If so, I'd really suspect some obstruction in your fuel lines - clogged filter, fuel line deteriorating internally, etc.?
 
ludwig said:
Pushing down on the ticker raises the float level , so wouldn't it be logical to look there first ?
All the different settings have a certain amount of overlap , but at small throttle openings the main jet size is of NO importance .
His description indicates a lean condition , so changing from a 3 to a 3.5 slide is unlikely to help .
A Mk2 850 has std a 3.5 slide , but with less restrictive air filter and exhaust , I find it is better a 3 slide and start from there .
Raising the needle ?
at up to 1/4 throttle opening , the conical part of the needle is not even in the needle jet , so that too has little effect .
The position of the needle only matters at middle throttle openings .
I did many test runs with an onboard AFR meter ( lambda sensor ) and found that the setting : 260 main , 3.5 slide , needle middle notch ..would run well , but become way too rich at higher speeds : 75 mph and up ..
Other things like valve timing matter too .
The transition between carb and manifold must be smooth : even a small ridge or step can upset the mixture at small openings .
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Good explanation and diagnosis. I think you should try as Ludwig suggests. I assume the needle jet is tight in the carb body, and that the machine responds well to adjustments of the pilot air screw. It is very simple and quick to check both of these, then you can eliminate them in your trouble-shooting.
 
Stock jetting for the 1975 Mk III Commando is: 220 mains, 106 needle jets, with the needles in mid position.
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Yes, with the restrictive beancan silencers and plastic air box. Standard settings with MKI and MKII 850's (older carbs without cutaway spray tubes) were 260 mains and needles all the way down. My 74 MKII wants the needle in mid position, else there's a "flat spot" at low throttle, just like geezer is experiencing.
 
BrianK said:
I wonder if your fuel lines may be partly obstructed. I assume you run with one petcock open. Perhaps try running with BOTH open and see if things improve? If so, I'd really suspect some obstruction in your fuel lines - clogged filter, fuel line deteriorating internally, etc.?
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I've tried it with both taps open, no improvement. New BAP taps from Old Britts, new fuel line.
 
ludwig said:
slimslowslider said:
Flat spot cured by operating the tickler?
Had the same issue (on an Amal Mk2 though, operated the choke to get away).
Just changing the slide from 3.0 to 3.5 cured it.
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That's odd .. 3.5 is leaner than 3 ..
Size of main jet shouldn't matter at small throttle openings .
Is the transition from carburettor to manifold smooth ( no step ) ?
You could try to raise the float level a bit ?
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You are dead right there, Ludwig :oops:
Been scratching my head how I got this wrong. Looks like I mixed up 2 carb issues I had (age is playing up here)
Sorry if I have mislead anyone here.

First one was when some 6 years ago I changed to proper 3.5 slide for Mk3 when I changed to single Mk2.

Beginning of this year I struck this flat spot after a top-end rebuild and carb clean-up.
Gone now after adjusting idle as per normal procedure, but then raising the slide stop (revs increase) followed by turning in the air screw a bit (revs decrease again). I guess the richer mix at idle helps in the transition to where the cutaway comes in.
Did not check the float height, have put it on the list of things to do.

I would imagine that the effect of the type of air filter would be hardly noticeable at idle and above.
Pressure drop is proportional to the second power of the air velocity across the filter surface, ie the air flow.
At full throttle it is another matter of course.
Some say K&N's are really beneficial here. Do not know myself.
I would be interested to know if this ever has been objectively measured in comparison to the standard Norton (clean) filter.
 
Jets came in from Old Britts today, I just placed the order on Tuesday. Company in from Arizona means that I won't get a chance to put everything back to stock to begin testing until later. If I get some time I'll be right on it.
 
Found some time to work on it. Final setup, 260 mains, 3 slides, needle in top notch. Air filter definitely makes a difference, it's too lean without the K&N, ok with it. Now I have idle, acceleration and top end. Still a little burbly (I think lean) at steady throttle of less than 1/2, but I can live with it for now. I may hunt around for a stock airbox to see if that helps any. Thanks to everyone for the suggestions and help with this seemingly endless problem.
 
Norton got it very right with their bulky air box and paper filter replaced before needed. Too bad its a painful long task to do. K/N makes air box filter beside the clamp ons. In searching this basically read that good paper both flows more air and filters better than K/N. I will do this experiment on factory Combat before end of summer but maybe you can beat me to it so we can learn the pecking order of various combos on mixture with your known current fueling baseline.
 
I've never had a bike so picky about its carburetor. I called another friend, long time Norton owner currently without a Norton, told him I had it mainly sorted, he said yes but for how long. He claims a Norton can wear out a pair of Amals faster than any other Brit bike.
 
cyclegeezer said:
I've never had a bike so picky about its carburetor. I called another friend, long time Norton owner currently without a Norton, told him I had it mainly sorted, he said yes but for how long. He claims a Norton can wear out a pair of Amals faster than any other Brit bike.
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It's the shaking.
 
swooshdave said:
cyclegeezer said:
I've never had a bike so picky about its carburetor. I called another friend, long time Norton owner currently without a Norton, told him I had it mainly sorted, he said yes but for how long. He claims a Norton can wear out a pair of Amals faster than any other Brit bike.
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It's the shaking.
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That's what he thought too. Long term, I'll probably buy a pair of new ones to put on, or a pair of flat slides.
 
Thought I'd add a final note to this thread. I bought a new pair of Amals and finally all is well. 260 mains, #3 slides and needle clip in top groove. Back to stock air cleaner too. Smooth acceleration from idle, flat spot gone. Now to work on the other problems, none of which were as pressing as getting it running right.
 
I'd suspect that the standard filter has played the biggest part. Those who dislike flat spots on Commandos are best advised to avoid the products of messrs K&N
 
Ha final note on a Commando thread. Banish the thought.
Consider playing with notching out back/lowering of the Amal spray tubes for some accelerator pump off idle response.
 
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