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Amal Mk1 Concentric - Running Problems


rusty ken
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I'm hoping that someone can give me some advice or pointers on how to solve a carburation problem on my and#39;53 350 matchless which is fitted with a 26mm MK1 concentric carb.

 

The bike had been running fine but I started having problems on her last outing when she started 8-stroking and "hesitating" when I opened the throttle.  My initial optimistic thought was she was running rich due to a combination of higher altitude and exceptionally warm weather.  The problem was still there when I got her home, and on further examination I found the nuts holding the carb to the manifold (head) had loosened  a little, and that she was drawing air (weak rather than rich???).  I've since had the carb off and refitted to solve this.

 

It also seemed like a good time to fit an air filter.... 

 

Needles to say if that had solved the problems I wouldn't be writing this.

 

I'm now in the position that the bike will start and will tick over as long as the throttle is completely closed.  As soon as I open the throttle a whisker, the engine dies. If I let the engine warm up I can coax her past the initial hesitation (with difficulty) and get a few more revs and it runs fine, but when I she goes back to idle there is still a big hesitation just as I start to lift the slide. This makes her impossible to ride in a section, as every time you open the throttle from idle she either hesitates (dies) or stalls.

 

I've checked the timing, points (magneto with manual advance/retard), plug, and tappets, all of which appear fine. 

I've screwed the air filter off, but it doesn't solve the problem. 

I've replaced the needle and needle jet with new items, and have tried adjusting out the mixture (raising/lowering the needle, larger/smaller needle jet, larger cutaway slide).  This has had an effect on running, but hasn't solved the hesitation problem. I've gone back to the needle, needle jet, and slide that were originally fitted.

 

I have a few ideas on where to look next and am hoping to get a concerted block of garage time in the next couple of days, but I'm starting to grasp at straws and any help or opinions on possible causes or solutions would be most welcome.

 

Thanks.

 

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Hi ken,

 

Firstly, is it possible to prove it is the carburettor - have you an old one you could fit just to prove it?  It reminds me of a problem I had with a minute hole in a carb which affected the float level, which became obvious when I had my Ajay ticking over, but hesitating on the pick up, just like yours, then one time I was trying it and couldn't reach a tool so leaned the bike a little to the side and the problem went away.........that was because the fuel level was different.

 

Just a thought.

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firstly motorcycles are simple things .... so dont expect complex solutions.. your bike was probably running poor becasue of the loose nuts on the carb.. then you fitted an air filter.. go back to how it was.. tighten the nuts only .. then look at fuel air mixture which is where the issue is.. strip carb down to its last nut and bolt.. clean and blow out every orifice/jet/ filter with canned air from a plumbers merchant.. fit a new float a black one and away you go.

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Good thinking totalshell,

 

"fit a new float a black one and away you go."

 

I had one where there was a small hole in the float and fuel got in and changed the level that it 'floated' at, on an Ariel that I restored the flat brass that the cut off jet sat on had actually worn a small depression that changed the mixture..........

 
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Might help, I bought a small ultrasonic cleaner from aldi for £19.99. It's not big enough to fit the whole carb in but I do one half then turn it over and do the other half. Amazing how clean it comes out. I found that dirt in the carb reaks havoc. Some just wouldn't come out with blowing air through the jets.

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Hi, I haven't worked on an amal carb, but this reminds me of a friends sons 50cc yamaha. Cleaned the carb checked the float, looked for air leaks, all the usual stuff. Eventually found a blockage in the fuel feed to the secondary, or intermedeate jet port. This feeds fuel into the carb just under or after the trailing edge of the slide (its usually a tiny little hole) to stop leaning the mixture just as the slide opens and manifold pressure drops. If the carb has this jet and it is blocked then just as you accelerate the motor will have a dead spot or stop completely.

Hopefully one of us has been helpful, good luck.

P.S. breakclean,or carb cleaner in a can is quite useful, and. nylon fishing line to poke through without scratching those small galleries.

 

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the key thing is get you float changed for a black "stay up" one. the way modern fuels are now with the high ethanol content they become porous and fill up with fuel. then they just sink and this makes your bike run rough.

also i've also had two float bowls dis-taught recently. you cannot even see this unless you put the bowl on a flat surface. but because of this even with a new gasket it has a minute gap that lets air in. which isn't good.

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Hi minislim could have a point, you initially said that the retaining nuts had worked loose - that could have distorted the mounting flange, you might be able to see it with an engineers metal rule against a strong light and it only needs a microscopic distortion because you are getting maximum suction on the flange with the throttle shut, then if you can get the revs up it could run better, 'cos with the slide up there's less suction on the flange and inlet.

 

Trouble is the alloy used for carbs is not the best quality.

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Thanks for all the suggestions. Having worked through everything I finaly swapped out the carb body, which seems to have solved the problem.

I think this implies a blockage in some of the internal air/fuel ways, just need ti find where, but it may be the push I need to buy a new premier!

Thanks again for all the comments.

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