Amal Concentric 26mm, good choice for NC 250?

[narrow_monza]

I have an Amal 26mm Monobloc carburator that I plan on installing on my Narrow Case 250.
I was wondering if it's a good choice for my bike. Also, I have 2 types of intake flange spacers for it, a rubber one and a phenolic one.
Do I need to use both? And if so, which one goes on the carburetor side?

photo(19).JPG

Thanks

-Adrian

[Nick]

No need to use both spacers, either one will be fine. The very important thing to know about Amals is that they will NOT tolerate over-tightening. The bodies will warp and the slides will stick. Thus, when mounting the carb, only tighten the lock nuts (and you must use locking nuts) tight enough to flatten the O-ring.
Jetting for a Triumph 500 Daytona would be a sensible starting point as that model uses two 26mm Amals.

The same goes for the float bowl screws and the screws holding down the top. Just snug them up, the lock washers will keep them from going anywhere.

And of course, check the mounting flange for flatness and surface it using wet/dry paper on glass or surface plate if needed.

[DewCatTea-Bob]

[quote= narrow_monza ...
" I have an Amal 26mm Monobloc "

____ I believe that's the model-name used for Amal's older carb.model prior to their 'Concentric' model which you've pictured.



" I was wondering if it's a good choice for my bike. "

____ What bike-model is it from,, and which cyl.head-model does your 250-Duke have, a stock Scr.head ??



" I have 2 types of intake flange spacers for it, a rubber one and a phenolic one.
which one goes on the carburetor side? "

____ I'd figure that the rubber-one would seal best but, it doesn't resist the heat as well.
Do both have the same ID.size (as the carb, or port) ?


Hopeful-Cheers,
-Bob

[narrow_monza]

____ I believe that's the model-name used for Amal's older carb.model prior to their 'Concentric' model which you've pictured.

++++ I stand corrected, I meant to say "Concentric" Mk1.

____ What bike-model is it from,, and which cyl.head-model does your 250-Duke have, a stock Scr.head ??

++++ Yes It's a stock SCR head.

Do both have the same ID.size (as the carb, or port) ?

+++ Yes they both match the ID size of the carb/port. However I was concerned about the proximity of the carb body to the cylinder head.
Perhaps I should be using a longer metal spacer?

[narrow_monza]

No need to use both spacers, either one will be fine. The very important thing to know about Amals is that they will NOT tolerate over-tightening. The bodies will warp and the slides will stick. Thus, when mounting the carb, only tighten the lock nuts (and you must use locking nuts) tight enough to flatten the O-ring.
Jetting for a Triumph 500 Daytona

--- Yes I learned that the hard way in the past

would be a sensible starting point as that model uses two 26mm Amals.

The same goes for the float bowl screws and the screws holding down the top. Just snug them up, the lock washers will keep them from going anywhere.

And of course, check the mounting flange for flatness and surface it using wet/dry paper on glass or surface plate if needed.

-- Are you referring to the surface on the carb or on the cylinder head?

[DewCatTea-Bob]

[quote= narrow_monza ...
" ++++ Yes It's a stock SCR head. "

____ Then it should have a 28.5mm intake-port !



" +++ Yes they both match the ID size of the carb/port. "

____ I guess we're once-again back to WHICH they both match - the 26mm-carb, OR the cyl.head port !?
But it really doesn't matter since both are the same. _ So-then I'd suggest just using the rubber unit, (only because I don't have any experience with using the other type).


" I was concerned about the proximity of the carb body to the cylinder head. "

____ That's not much of a concern, as others have mounted such Amal-carbs directly to the head-flange/port-manifold without any spacer (since those carbs include built-in O-ring seals).


" Perhaps I should be using a longer metal spacer? "

____ Such is really not needed, but can have an effect on engine-performance.
__ Fellow-member 'Moto Mike' chose to use such an extra manifold to mount an Amal to his 450.
Here's a link to a thread concerning the closely related topic... viewtopic.php?f=3&t=330&#p2148


Dukaddy-DUKEs,
-Bob

[ducwiz]

Hi,

- Adrian, you should avoid the rubber spacer. It is much too soft, letting the carb flange to be warped when it is tightened. Even the steel flange on the manifold used with the Dell'Orto VHB 26 often showed considerabla warping when used with the rubber spacer. And - warping the flange could also warp the carb's body (concentics seem a bit weak here) and in turn cause jamming the slide's smooth action.
The AMAL concentrics are sealed with an O-ring sitting in a grove in the carb's flange, which does the job well if the flange is (still) even. If you want to use the rubber spacer anyhow, insert stud bolts into the head and use self-securing nuts to fix the carb on the head; gently tighten them until flange and spacer are in tight contact. Use some sealing compound (silicone type) between the backside of the spacer and the cyl. head.
And please, post the entire ID no. of your carb, which would ease a precise identification. Hence, if this carb was made for a 2-stroke engine, it will give you possibly lots of trouble during the tuning process.

- Bob, are you shure that a std. w/c 250SCR head has the 28.5 mm inlet diameter? I remember having worked on such a bike, and still have a genuine DUCATI gasket set stocked in my cellar. This contains a rubber spacer with a 26mm bore. The head I worked on was equipped with smaller valves than the MK3 models, 37/33 mm as far as I remember, and camshaft marked "white".

cheers Hans

[DewCatTea-Bob]

[quote= ducwiz ...
" Bob, are you shure that a std. w/c 250SCR head has the 28.5 mm inlet diameter? "

____ YES, the n-c & w-c 250-Motocross/Scrambler cyl.heads have the same intake-port diameter as the 1962-64 'Diana Mark-III' cyl.heads ! _ (Whereas the 1965-70 'Mark-3' cyl.heads have a 29.5mm in.port.)



" still have a genuine DUCATI gasket set
This contains a rubber spacer with a 26mm bore. "

____ I believe that's the size intended for a Monza (with 26.5mm in.port).



" The head I worked on was equipped with smaller valves than the MK3 models, 37/33 mm as far as I remember, and camshaft marked "white". "

____ That would be consistent with a Scrambler-head, (although their intake-valves were usually 36mm).


Dukaddy-DUKEs,
-Bob

[ducwiz]

Hmmm ... but why does the Ducati factory add a n/c Monza spacer into a w/c 250 set ?

..

[narrow_monza]

I'm going to take some inner diameter measurements and post my findings.
Thanks!