Dommie 500 with concentric

Rohan

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After some eons, have dragged out the concentric for an early dommie,
and to my surprise its got a 250 main jet.
Thats the jet size listed for a P11.
Anyone know what main jet size a dommie 500 likely should have ?

Dommie 500's were never sold new with concentrics, they'd gone by the time the concentric came along.
So there are no specs to consult on this.

Went along to see a Norton a while back that was 'running good' with a new one,
but it was coughing and spluttering and chugging black smoke, scratch that idea.

P.S. Mine is a L626 B 9, anyone know what that would have been in a past life ?
A 626 is 26mm, obviously - early dommies had 1 inch carbs.
 
texasSlick said:
(agree?)
Click to expand...

No.
Monobloc jet numbers are/were way bigger - 450 is quoted for something.
At some point they were standardized with concentrics though ??
Which just triples the potential wrongness....

At some point I had a monobloc on an old dommie Model 7.
Spares back then were unobtanium, but it oozed petrol everywhere and chugged black smoke (much) on the move,
and it went back on the shelf.
 
Rohan said:
texasSlick said:
(agree?)
Click to expand...

No.
Monobloc jet numbers are/were way bigger - 450 is quoted for something.
At some point they were standardized with concentrics though ??
Which just triples the potential wrongness....

At some point I had a monobloc on an old dommie Model 7.
Spares back then were unobtanium, but it oozed petrol everywhere and chugged black smoke (much) on the move,
and it went back on the shelf.
Click to expand...

They used a 1" monobloc, do you have a 928 for the Domi, and wouldnt that affect the main jet size too?

Amal 376
Bore 1"
Pilot 30
Cutaway 3.5
Main 240
Needle .11
PositioN 2
 
gortnipper said:
do you have a 928 for the Domi,
Click to expand...

No, its a 626, which is as near to 1" as they made in concentrics.
Since it feeds into a 1" manifold, not much is to be gained by going larger in the carb ?
And its the all iron low compression motor, so its no fireball...

I see that Triumph 626 concentrics use a main jet of 170 or 180, a dommie would be similar ?
 
Rohan said:
texasSlick said:
The data will be for Monoblocs, but maybe they crossover. I always thought the number referred to the cc flow ... thus a 240 main jet in a monobloc flows the same as a 240 in a concentric (agree?)
Click to expand...

No.
Monobloc jet numbers are/were way bigger - 450 is quoted for something.
At some point they were standardized with concentrics though ??
Which just triples the potential wrongness....

At some point I had a monobloc on an old dommie Model 7.
Spares back then were unobtanium, but it oozed petrol everywhere and chugged black smoke (much) on the move,
and it went back on the shelf.
Click to expand...

From: http://amalcarb.co.uk/main-jet-size-180.html

"All genuine AMAL jets are calibrated to a flow rate. The number e.g. 100 indicates that it will flow 100cc`s of fuel in one minute at a specified head. Beware of imitations as they are just drilled to a size. All our jets are branded and packaged with the official AMAL logo."

Well, that is what they are saying now, dunno if those numbers applied to fuel flow rate back when monoblocs were fitted to the old dinosaurs, but I suspect it was so.

Slick
 
So a Norton Atlas (with monoblocs) should have 350 jets ?? ?? ??

So no, I'd never jet a concentric from the monobloc settings......
 
So both the 624 and 626 use by default:

3 Cutaway Slide
140 Main Jet
106 Needle Jet

Maybe drop Amal a line?
 
If you go to the first link I gave you, the Data table lists the Atlas with 420 main jets.

I read Amal's statement "All genuine Amal jets are calibrated to a flow rate ..... " to mean .... all jets regardless of carb model in which installed.

Therefore, I think, (I think, means I'm not 100% sure, but it stands to reason) that if you find the carb setup as fitted to your Dommie in the table which I gave you a link for, the main jet size used would be a starting point for your concentric conversion.

Slick
 
texasSlick said:
If you go to the first link I gave you, the Data table lists the Atlas with 420 main jets.
Click to expand...

I did. The line I read, for 1965, says 350 main jets......

The trouble is, the concentric for a Norton P11 says a 250 main jet.
And the same for the Atlas...
 
gortnipper said:
So both the 624 and 626 use by default:

3 Cutaway Slide
140 Main Jet
106 Needle Jet
Click to expand...

They are smaller (main) jets than Triumph used in 500's.

gortnipper said:
Maybe drop Amal a line?
Click to expand...

I believe that the man I went to see with the new Amal - that was chugging black smoke - had a new carb from Amal.
Thats why I'd like to find someone who has a sweet running one, and go from there.

However, seeing what they suggest is a good suggestion.
If I am not going to buy anything, will they talk to me ?
 
Rohan said:
texasSlick said:
If you go to the first link I gave you, the Data table lists the Atlas with 420 main jets.
Click to expand...

I did. The line I read, for 1965, says 350 main jets......

The trouble is, the concentric for a Norton P11 says a 250 main jet.
And the same for the Atlas...
Click to expand...

The 420 Main was for an Atlas Sports w/ 389 - the plain jane 65 Atlas lists 350.

I saw 220 for a 67 Atlas w/ 930, using a no. 2 slide

http://www.inoanorton.com/docs/TheNortonCarburetor.pdf
 
Hi Rohan,

For twin 30 mm Concentrics on the Dommie 500 race bike I used between 180 and 210 main jets depending on weather and altitude.

Timing 28.5 deg full advance, NGK BP9ES, 10.25 CR, 98 octane RON.

We won a hell of a lot of races with those settings over 5 seasons and 100 plus events.

John
 
Thanks folks, considerable diversity of jet numbers here.

With all iron head and cylinder, factory 6.5:1 compression and hp in the 20s, this is no highly tuned beastie.
Might shoot for Triumph type settings, and see from there.
With the number of times this is likely to be flat out on the autobahn, could probably leave the main jet out - for safety's sake.
 
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