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Rapide carburation

ernie

VOC Assistant Secretary
VOC Member
Oh dear, JTAN is down.

So here is another carburettor question.

I am the founder of the "Tilt your stub mounted 276 carburettor in order to adjust the fuel level in the float bowl" club, which Tom Gaynor has recently joined. For years I enjoyed a sooty rear spark-plug and fuel consumption of 40 mpg. I tilted the carb on the stub lowering the float bowl about a gnat's testicle. Result - not quite so sooty a rear plug and fuel consumption of 55 mpg.

All carb settings are standard for a standard B Rapide. Front plug is nicely brown with no tilting.

So now a few questions:

1 Who else has resorted to this tweak to adjust fuel level?

2 Are there different lengths of float needle to effect the adjustment? Or are there different float bowls? All the ones I have look the same.

3 Is the cause of the rising fuel level wear on the needle?

4 In my box of assorted carb bits I found 3 different shouldered nuts which screw onto the bottom of the carb body (see my picture). The middle is the one usually found on our carbs. The longer one on the left would also effect a lowering of the fuel level in the carburettor. Has anyone fitted such a one?

Final question to check if Graham reads my post - is the spelling checker running mine or yours?

Best regards

Ernie
 

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Tom Gaynor

Well Known and Active Forum Website User
VOC Member
Answers to questions:

1) Sid Biberman, it was he who told me about it.

2)I don't think so, although I've seen needles with two notches in them.

3) Needle wear would affect it, but the dominant factor is that the tilt of the carb alters the level, so set it wrong, and it'll be wrong. Right is the point at which fuel stops leaking out of the hole in the body level with the top of the base nut.

4) Never noticed that before. Worth knowing. Thanks.
 

shadhubh

Active Forum Website User
Non-VOC Member
shouldered carb nuts

Oh dear, JTAN is dow

So here is another carburettor question.

I am the founder of the "Tilt your stub mounted 276 carburettor in order to adjust the fuel level in the float bowl" club, which Tom Gaynor has recently joined. For years I enjoyed a sooty rear spark-plug and fuel consumption of 40 mpg. I tilted the carb on the stub lowering the float bowl about a gnat's testicle. Result - not quite so sooty a rear plug and fuel consumption of 55 mpg.

All carb settings are standard for a standard B Rapide. Front plug is nicely brown with no tilting.

So now a few questions:

1 Who else has resorted to this tweak to adjust fuel level?

2 Are there different lengths of float needle to effect the adjustment? Or are there different float bowls? All the ones I have look the same.

3 Is the cause of the rising fuel level wear on the needle?

4 In my box of assorted carb bits I found 3 different shouldered nuts which screw onto the bottom of the carb body (see my picture). The middle is the one usually found on our carbs. The longer one on the left would also effect a lowering of the fuel level in the carburettor. Has anyone fitted such a one?

Final question to check if Graham reads my post - is the spelling checker running mine or yours?

Best regards

Ernie

The one on the right is the ones fitted to my Shadow carb,,gives more clearance for main jet,, should be stamped with "M" on the bottom, indicating the transfer hole in the carb bowl has been drilled out larger, cant remember to what size, not a lot but sum ! on reflection ,,I 'm talking about the Banjo bolt
 
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Vic Youel

Well Known and Active Forum Website User
Non-VOC Member
Hi Earnie,

Your pictures of the different “Mixing Chamber Union Nuts (MCUN)” got me thinking. I have a similar selection of bits in my carb spares box.. I have short and long types for both the 276 and larger diameter 289 series of bodies and an extra long type too for the 276 just like your ones.

Looking in my various catalogues:

My old Amal data lists a type 6/062 MCUN for the vast majority of the many 276 or earlier 76 variants including the ones used on B and C Rapides. There are two other types listed (6/210 and 6/033) which are used on a few of the 276 types of carbs. The Latest Burlen catalogue lists the 6/033 as a short one and the 6/062 as a long one.

My old Amal data lists the 29/078 MCUN for the Shadow/Comet 289 series. Similarly the Burlen Amal catalogue confirms this part number.

Turning to P B Bickerstaffs superb reference only one type of MCUN is featured which seems to be the long one.

I have concluded that the middle one of your picture is the correct type of MCUN for Shadows, Rapides and Comets…..ie the long one as described by Burlen and that is what I have also fitted to my Rapide and Comet. Type 6/062 for Rapides and 29/078 for Comets and Shadows.

Hope this helps……incidentally I had never noticed that the bottom bolt is occasionally stamped with an “M” and has two large ¼ inch diameter holes as opposed to four 1/16 inch holes on the standard one. Burlen list both types.

Hope this is helpful

Vic
 

ernie

VOC Assistant Secretary
VOC Member
Thanks Vic - I believe all is as you say.

I got a new float and needle from Burlen. The float is plastic and same dimensions as the current copper one. The needle does indeed have two notches. The top one, which I believe is the one to use, has the dimension to the top about 1/16" than the one presently installed, effectively lowering the fuel in the float bowl by that amount. I have installed same and we shall see what transpires. The second notch is 5mm below the top one. Not sure what use that is. My estimation, from looking here

http://www.hitchcocksmotorcycles.com/amal/pre_mono_ht_3.html

is that the fuel level needs to be high enough to just submerge the pilot jet fuel feed hole in the bottom of the jet block.
 
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