Who knew ?

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Harry44
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Who knew ?

Post by Harry44 »

I've struggled to get a steady tick over, and get rid of a flat spot just as I open the throttle from closed. I've reset the float level swapped jets raised the needle changed the throttle valve pilot jet needle jet. So what's left ?

The carb spacer that's what. it was about 3/4 of an inch thick and 28 mm diameter.

I bought a 30mm one about 1/4 inch thick and its a transformation.

A nice steady tick over no flat spot and it now responds to the advance retard lever as it ought to.
It used to struggle below 30 mph in top now it'll do 15 mph on full retard (insert your own joke at this point)

I'm a happy tinkerer :D
si is does non opportunus vos postulo a maior pango
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clanger9
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Re: Who knew ?

Post by clanger9 »

Well done, that can be a tricky one to find! Was there any "step" between the spacer and the carb body (or manifold)? Anything less than perfectly smooth here will usually upset idle/off-idle running...

There's a densely-written (but very informative) article on the archives about this very issue:
http://archives.jampot.dk/technical/Car ... mplin).pdf
1989 Moto Morini Dart 350
1993 Ducati 750SS
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Harry44
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Re: Who knew ?

Post by Harry44 »

Yes there was a step a whole 1mm. Now its perfectly smooth

And the answer to who knew is Clanger :D
si is does non opportunus vos postulo a maior pango
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cfaber
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Re: Who knew ?

Post by cfaber »

Harry44 wrote:Yes there was a step a whole 1mm. Now its perfectly smooth

And the answer to who knew is Clanger :D
Is it possible that the old spacer insulator never sealed?
Fix it until it's broken!

Otherwise how will
anything get done?
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Janet
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Re: Who knew ?

Post by Janet »

If you mean "who knew" that a spacer with the wrong thickness and/or bore would make a difference, I did.
If you mean "who knew" what yours should be, I wouldn't have had any idea.
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Harry44
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Location: SOMERSET UK

Re: Who knew ?

Post by Harry44 »

cfaber wrote:
Harry44 wrote:Yes there was a step a whole 1mm. Now its perfectly smooth

And the answer to who knew is Clanger :D
Is it possible that the old spacer insulator never sealed?
It's always possible. But I tried several times to seal it, with and without gasket sealant with and without gaskets.
Can this material go porous ?

The symptoms all fit with the tech article posted by Clanger so it seems the likely cause.
si is does non opportunus vos postulo a maior pango
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cfaber
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Re: Who knew ?

Post by cfaber »

Harry44 wrote:
cfaber wrote:
Harry44 wrote:Yes there was a step a whole 1mm. Now its perfectly smooth

And the answer to who knew is Clanger :D
Is it possible that the old spacer insulator never sealed?
It's always possible. But I tried several times to seal it, with and without gasket sealant with and without gaskets.
Can this material go porous ?

The symptoms all fit with the tech article posted by Clanger so it seems the likely cause.
To know for sure you'd have to run it and try the propane or starter fluid trick
Fix it until it's broken!

Otherwise how will
anything get done?
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