Hi Tim and Jonathan,

You have slightly different AMACs, both of which were "current" in 1920. Tim's is the older style (new for 1915) and Jonathan's is the Type H (new for the 1920 year models). The biggest difference is the arrangement of the jet and the float bowl attachment, but they are similar in that both have a submerged jet and the AMAC sprayer. Details here: https://www.douglasmotorcycles.net/index.php?topic=6244.msg23427#msg23427

Don't worry Tim, you're not banned from going above 1/4 throttle! Once warm, the engine should take the air lever being opened gradually, together with the throttle. If you can't get the air lever open at all, your main jet might be too small (overly lean mixture). (First check for leaks in the inlet manifold.) However I usually do all this stuff while riding the bike rather than on the stand, where old bikes don't always behave at their best. My test for main jet size is a rather elementary one: I have a bit of quiet flattish road, run the bike up to a modest touring speed (say 30-35 mph) by adjusting the throttle and air levers. Ideally the air lever should be something like 3/4 to fully open when the engine is happy at this speed. If the bike wants full air and more, the main jet is too big; if it won't take 3/4 air the jet is too small.

By the way, the ignition timing has to be right too. At your 30-35 mph on the flat the engine should be comfortable with full advance (tight wire). Once started, there are very few occasions where the timing needs to be retarded - maybe just a little when slugging up a killer hill.

Cheers

Leon

This has been posted elsewhere but now has hole sizes. I hope it is of use.  The carb on the left has been modified but I don't know enough to know what the end result would be. The ring on the modified spray block served to lower it.
Jonathan

Hi Tim and Leon,

I had a quick look at the heat jacket Amac carby I have (it has 2 3/4hp stamped on the outside in very small letters). The jet in it is marked size 25. I can only guess that the jet is original. 

As the hole in the jet is 0.026" (I measured the hole diameter with a guitar string as a gauge), I presume it is an Amac jet size. The 2 3/4hp parts list shows you could buy jets from Douglas in the Amac size range from 24 to 32 - so at least the jet I have in the heat jacketed Amac is in the general range Douglas would have expected to use. I don't know how this runs on a 2 3/4hp as this carby is not on a running motor (yet..... but getting there very slowly...!).

On page 129 of "Radco's" very useful book The Vintage Motorcyclist's Workshop there is a Jet equivalents list and gives a description of two methods of adapting Amal jets to fit Amac jet holders. I have not tried Radco's methods, but it seems simple enough to do and may give you a source of jets to experiment with?

Also some info on equivalent jet sizes here;

https://www.douglasmotorcycles.net/index.php?topic=2901.0


I played with jets in car carburettors a long time ago and remember drilling jets out with various size number drills (due to lack of different size jets available for the particular carburettor in question) - it was quite a tedious process but did result in a small increase in performance so was probably worth the effort!.

cheers and have fun!

Edit:- Tim, let me know if you need the diameter of the sprayer holes as a check for yours - will be fiddly without pulling the sprayer out, but I think I can manage it.

Hutch

Leon this is FANTASTIC information thank you!

I have been going at this like tuning my 1938/42 Norton 16h and thanks to you I now know this is not the same!

I did expect to have the mixture full open after starting, and I did expect to be able to rev the engine.


The AMAC guide was not good preparation either.

The AMAC guide does recommend "tuning" with jets, do you know the size or a source?

ANSWERS TO YOUR QUESTIONS:

Hi Tim,

Are we talking the shiny AMAC you've posted elsewhere?

YES.

Your first photo in this thread looks like a pagan sunrise - it's meant to be a row of 7? small holes, which AMAC refer to as "the sprayer". The jet is submerged and just meters the petrol into a pool underneath the sprayer.

Yes this seems like alien technology to me.

Starting with the assumption that the bike has run in the past and should run again, let's just focus on starting technique.; Sadly you can't rev with the throttle and fine tune with the air.

WOW, good to know. What is it like while RIDING? Do you always stay at or about 1/4 throttle?

Engine starts? Excellent. If you open wide either lever at this point the engine will stall. Perfectly normal.

This is pretty close to what is happening right now, though perhaps just less than 1/4 throttle.

Yes, I am using a fan for static cooling.

Hi Tim,

Are we talking the shiny AMAC you've posted elsewhere? https://www.douglasmotorcycles.net/index.php?topic=9260 Is this the one with the concentric slides? The outer slide (very roughly "throttle") should be attached to the long/lower lever on the double handlebar lever.

Your first photo in this thread looks like a pagan sunrise - it's meant to be a row of 7? small holes, which AMAC refer to as "the sprayer". The jet is submerged and just meters the petrol into a pool underneath the sprayer.

Starting with the assumption that the bike has run in the past and should run again, let's just focus on starting technique.

The AMAC of the period is by no means "automatic", and although we could refer to the lower lever as the throttle and the upper lever as "air" or "mixture" the functions of the two levers are not independent. Sadly you can't rev with the throttle and fine tune with the air.

Things are even more messed up when the engine is cold, particularly with a Douglas because of the long inlet pipes.

Set the magneto lever about half way - tight wire should be advance (faster running).

Make sure there is some oil in the crankcase - say 2 or 3 full pumps from empty.

Start with throttle open 1/4 - 1/3 - 1/2, and the air lever fully closed.

Engine starts? Excellent. If you open wide either lever at this point the engine will stall. Perfectly normal.

1920 engines most often don't go "vroom vroom" in response to the throttle, particularly when cold, so don't try. Instead massage the levers slightly to see if you can keep the engine running for a minute or two. Don't thrash it - aim for a very fast idle. Small changes to the levers is the best bet, almost certainly with the air "more closed" than the throttle. Change the engine speed by moving both levers together. (The air lever is certainly not a choke as found on more modern carbs - it is not designed to be closed for starting and opened wide later.)

See how that goes. Keep in mind that air-cooled engines are not designed to run for long periods while stationary. Use a fan to blow some air on the engine if you're going to run the bike on the stand for a prolonged period.

Cheers

Leon