E: Engine 1951 Comet Carburation

Monkeypants

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I run two old triumphs (Val Page 1936 Mk5, 1950 Speed Twin on worn out pre monoblock carbs plus a 1957 Thunderbird with a Sinners Union and they run and start well and tick over like swiss watches and it all about starting and usng the bikes daily

They will run quite well on worn slides as long as one replaces the needle jets now and then.
I had very worn Concentrics on the Rapide. It started to burn a tremendous amount of fuel, so I installed new Premieres on it, problem solved.
Afterward it occured that clapped out slides should lean things out, not make the bike run rich. The poor mileage wasn't due to worn slides, it was due to worn out needle jets.
A few years later I had the same scenario with a Norton twin. That time I just replaced the needle jets and fuel economy went from 35 mpg back to 50-55 where it normally is. And the bike runs fine with quite loose slides.

Glen
 

Bill Thomas

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I have had bad Tepid starting for years, Not with the Dellorto, But that needs choke, With the fast idle.
I have found they need more fuel !!, Concentrics, Mk1 and 2 so Flooding or choke with the Mk2 Just for a few seconds then put the choke off.
Being a Mechanic all these years, I could not let myself flood a warm engine, But that is what I have found,
Maybe the petrol ??.
 

Chris Launders

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Erm Have you put fresh fuel in the tank as this modern stuff goes off? and when you check you have not flushed any muck in the carb and clean the jets and check if the float is working as should as most problems are down to the float not working right.
I has been the same since I built it almost 20 years ago, with both Amal MK2s and the Mikuni's, it has a twin points set up and Dyna coils , I even had twin plugs in at one time, I think the main problem is that as it's a chopped engine the engine only turns over once on the kickstart and isn't drawing enough fuel through the cold start system of those carbs, whereas I can flood the MK1s with the tickler.
 

Roslyn

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Yes Nulli, when they developed the Premier carbs Burlen have eliminated the main weaknesses of the Concentrics, that is relatively high wear rate of the throttle valve and body and pilot orifice almost impossible to clean out when clogged up.
As they are on the market for at least ten years now I thought everybody in our little world was aware of their existence!
Thanks anyway Nulli.
My point is that every time someone enquire about carburetor problems the ultimate solution for some seems to be: "fit Mikuni".
I just want to counteract this!
Fit a new Amal instrument since they are available (lucky us) and virtually indistinguishable from a period one. The carb problems will be cured (unless they are actually ignition ones!) and the machine will still look and perform like an old British Bike!
But this is just my opinion, in the end it's up to the owner to do what he like with his machine, of course.
I ride an old 1950 Speed Twin sprung hub and does well for a old bike not bad at 80-90 mph have done over 100 mph on it but not often as the front single sided 7" brakes are not that good at stopping at that speed but that was pushing it bit not bad preformance for old british bike they can tweaked
 

greg brillus

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If you are using a bike with an AMC type gearbox you generally need to position the engine in a different spot for kick starting, that is just easing it over TDC and then kicking will not start very well. I start with the k/s lever at 90 degrees to the ground and then ease it over TDC and keep turning it further to the bottom of the kicking stroke.........then kick it with full effort and it will start much better. The kick start stroke versus mainshaft turn ratio must be different.........Pretty sure on a stock twin the mainshaft turns about 5/6 th's of a full turn.
 
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erik

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IMG_5027.JPG
 

erik

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This is where the new pilot jet lives.On the other side of the carb it is as it was ,the airscrew! Erik
 
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