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#702703 - 07/25/17 2:10 am M20 Carb Advice Needed  
Joined: Aug 2016
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M20 Offline
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M20  Offline
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Tennessee
I am preparing a 39 M20 for a long trip in August, most of the riding will be at 20-45 MPH on dirt, gravel and back roads. My carb is shot and I am thinking of using a flat slide type carb on the bike. I have seen some post about over heating of carbs on M20s and don't want to buy something that won't work. I'm look to find the right carb quickly so I can get some miles on the bike and get it vetted prior to the event. Has anyone had any experience with this or have a recommendation for a carb that will work well.
Cheers

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#702733 - 07/25/17 1:06 pm Re: M20 Carb Advice Needed [Re: M20]  
Joined: May 2004
Posts: 3,527
BSA_WM20 Online content
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BSA_WM20  Online Content
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Sydney Australia
If you are only going to be travelling at M20 speeds ( gentleman's pace ) then you will not go wrong with a 626 Concentric.
I ran one on my M20 for many years and it ran best and very reliably with it on.
You will need a very long spacer to move the carb back far enough to clear the magneto drive.
When running at M20 speeds there were no overheating problems unless I was trying to keep pace with the A65's.
Mine was off a B40 GA and ran that jetting fairly well although from ( unreliable ) memory I went up 3 sizes on the main jet.
The manifold spacer was also off a B40 GA and I fitted a 1/2" tuffnol spacer to make up the extra distance


Bike Beesa
Trevor
#702742 - 07/25/17 4:01 pm Re: M20 Carb Advice Needed [Re: BSA_WM20]  
Joined: Aug 2016
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M20 Offline
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M20  Offline
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Tennessee
Thank you for the response, I believe the terrain and my limited riding abilities will dictate the speed! This is more about reliability and keeping things from rattling off the bike. I felt like the M20 has been there and done that and hopefully it will do its part if I can do mine. I have been trying a 626 but so far I cant get the stumble out of it under load. Runs fine at low speed and when you give it more fuel it stumbles and backfires. It will only run with some air cut off at filter. I'm running a 2.5 slide, 200 main, needle in the middle. Do you remember your set up on slide and needle by any chance?

Cheers

#702746 - 07/25/17 4:31 pm Re: M20 Carb Advice Needed [Re: M20]  
Joined: Aug 2005
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bodine031 Online content
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bodine031  Online Content
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melbourne florida
Might consider a Mikuni Round Slide, make an alloy adaptor to mount a rubber flange on. Tucker Rocky & Parts Unlimited both sell carbs and spigots and pod type air filters. My 69 B-44 VS runs great with a 30mm Kuni/RS Have fun!!

#702759 - 07/25/17 7:06 pm Re: M20 Carb Advice Needed [Re: M20]  
Joined: Aug 2001
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triton thrasher Online content
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triton thrasher  Online Content
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scotland
Originally Posted by M20
I have been trying a 626 but so far I cant get the stumble out of it under load. Runs fine at low speed and when you give it more fuel it stumbles and backfires. It will only run with some air cut off at filter. I'm running a 2.5 slide, 200 main, needle in the middle. Do you remember your set up on slide and needle



What's stopping you raising the needle to a richer position?


Amateur Loctite enthusiast.
#702788 - 07/25/17 11:43 pm Re: M20 Carb Advice Needed [Re: triton thrasher]  
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Tennessee
Raised, lowered, back to the middle with no real change in performance. It seems to be a regular old carb. Mystery, I chased my tail for two days and the only thing that seems to work ok is a PWK type flat slide but it is too large 30mm. Today I ordered a 26 mm JCR/PWK that should solve the problem for now. I will rebuild the old 276 and have it on hand if the JCR doesn't make the cut. Thanks for all the responses, I will let you all know how it shakes out.

#702793 - 07/26/17 12:03 am Re: M20 Carb Advice Needed [Re: M20]  
Joined: Dec 2013
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kevin roberts Online content
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kevin roberts  Online Content

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ohio, usa
if you ever decide to go the mikuni route, here are inexpensive rubber adapters to go from AMAL's 2-inch flange to a 33-35mm mikuni spigot. these are much cheaper than the hot-shot aluminum CNC ones for big money:

http://www.usedracingcar.com/detail...er-for-AMAL-Mikuni-OKO_111984320509.html

i seem to recall that mikuni 32 and 34mm carbs use a 40mm spigot, so these would work well for a smaller mikuni.



Into the distance a ribbon of black
Stretched to the point of no turning back
A flight of fancy on a windswept field
Standing alone my senses reeled
A fatal attraction is holding me fast how
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#702805 - 07/26/17 1:44 am Re: M20 Carb Advice Needed [Re: kevin roberts]  
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M20 Offline
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M20  Offline
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Tennessee
Thanks for the information, that could also help with the heat issue.

#702845 - 07/26/17 1:16 pm Re: M20 Carb Advice Needed [Re: M20]  
Joined: May 2004
Posts: 3,527
BSA_WM20 Online content
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BSA_WM20  Online Content
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Posts: 3,527
Sydney Australia
Originally Posted by M20
Thank you for the response, I believe the terrain and my limited riding abilities will dictate the speed! This is more about reliability and keeping things from rattling off the bike. I felt like the M20 has been there and done that and hopefully it will do its part if I can do mine. I have been trying a 626 but so far I cant get the stumble out of it under load. Runs fine at low speed and when you give it more fuel it stumbles and backfires. It will only run with some air cut off at filter. I'm running a 2.5 slide, 200 main, needle in the middle. Do you remember your set up on slide and needle by any chance?

Cheers

Really windy today so after chopping up a fallen tree it was good to hide in the shed for a while.
The 626 was still in the desperation parts bin.
Slide 3.5
Needle 106
Main 190

These are slow motors and develope most of their torque down the bottom end so the main is not real important.
When it was running the 626 it was also over geared in order to keep pace with faster bikes.
I run a looooog inlet .
The B40 spacers is a good 2" then there is another 1/2 of Tufnol which has the carb really close to the oil tank.
A firebird pill box just fits in but I have to assemble it on the carb.
The end plate of the filter is glued onto the sleeve then I assemble the filter, perforated ring and back plate to the carb in situ as there is not enough space to remove the filter in one piece.
If you are interested can shoot you a pickie of the set up with the 928 currently filled .
Big problem is the cable .
If you can get your hands on one of the curved ends they are worth while fitting.
Originally I used one because the 932 was too close to the tank and would occasionally jamb.
The rear of the tank sits on 1/2" rubbers which gives a little more space for the cable but a lot less reserve fuel aas the tank ends up with a forward slope.
Been running concentrics on this bike 15 - 20 years and cant fault them.
Without the extra spacer the fuel does boil in the carb when pushed hard , which a thoughtful caring BSA owner would never do to an M20.


Bike Beesa
Trevor
#703289 - 07/30/17 12:29 am Re: M20 Carb Advice Needed [Re: BSA_WM20]  
Joined: Aug 2016
Posts: 40
M20 Offline
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M20  Offline
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Posts: 40
Tennessee
Thank you all for the replies, I've had 3 or 4 carbs on this bike and thought i would hit the jack pot with the 26mm JCR but so far I am still chasing my tail with getting it set up for the M20. If anyone has any experience with these flat slides please let me know. I am way rich at idle and have yet to get that sorted enough to run it down the road for some real chops. These seem to be set up for a T100 as far as jetting, I'm thinking if I cant get it out with the air screw then I'm going to need a small needle jet. Again if anybody has done this before please let me know your setup. Bout ready to go rebuild the 276 again.
Cheers

#703317 - 07/30/17 9:59 am Re: M20 Carb Advice Needed [Re: M20]  
Joined: May 2013
Posts: 4,533
Allan Gill Offline
Allan Gill  Offline



Joined: May 2013
Posts: 4,533
Rotherham - S. Yorkshire
Have you not considered that the slide could be too rich, idle jet is just that its effective at idle, from there slide cutaway and needle Position. I've had a bike not run because the needle was too rich


beerchug
#703322 - 07/30/17 10:56 am Re: M20 Carb Advice Needed [Re: Allan Gill]  
Joined: Aug 2016
Posts: 40
M20 Offline
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M20  Offline
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Posts: 40
Tennessee
Allan,
Thanks for your reply. You are correct, there are several changes that could be made and I just need to work through them one at a time to see what gives the best result. I decided to send an email to the company that markets the carb. And see if they can give me some basic set up information so I can make sure I'm starting with the correct jets, needle, etc. for the M20. There seems to be a vast number of options. I hope to hear from them this coming week and will post any information they provide for others that may want to try this. It seems that most of the information on these is directed toward twins not singles and I could be fighting an up hill battle if I have the wrong jetting altogether. I will keep you all posted and thanks again for your help.


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