2 3/4 AMAC main jet

[Black Sheep]

My 1913 2 3/4 AMAC carb has a 35 main jet. According to RADCO's book this is too big - it should be a 26 or thereabouts. 
What should it be?

[cardan]

RADCO is close to god: he's almost always right. AMAC suggests 27 for an early 2 3/4 hp twin, which equates to 0.029". 

A 35 jet is 0.037" - enormous.

If you don't have jet drills, you can measure the jet diameter (ignore all numbers these days) with a darning needle. Slide it into the jet, mark where is stops, and measure the needle diameter at the mark with a micrometer.

Cheers

Leon

[Black Sheep]

Many thanks Leon!
Our 2 3/4 had been running for years on the 35 main jet until its recent engine rebuild. However, there are concerns that a massively over-sized man jet could be causing oil dilution in the bores. Not a major problem with cast iron pistons but a concern with these new-fangled alloy ones.
A new main jet has been ordered.

Gordon.

[cardan]

Hi Gordon,

Hopefully it will be transformed with a leaner mix, although you might need to be a bit more attentive to the air lever. Keep in mind that because the jet is submerged on an early AMAC it only meters the fuel rather than spraying it - it doesn't have to be perfectly formed. You could just reduce the size of your current jet, either by running a  strand of wire running through it (if it really is a number 35, you'd need a strand around 0.020" diameter!), or filling the hole with soft solder and drilling it out with a jet drill. This is quite easy if you hold the drill in a pin vice and turn it by hand.

Have fun.

Leon

[Hutch]

Hi Gordon and Leon,

The Handbook of the Douglas Motorcycle for the 2 3/4 H.P. says that the jet size should nominally be 27 with 2 screens on the carburettor. This is in general agreement with Radco's information (which appears to be derived from AMAC's "Hints and Tips" and catalogue information?). Given that modern fuel is a different specification to that used back in the 1910's and 1920's then some jet fiddling may be required to get the best performance, but 27-28 seems to be a good starting point as Leon suggests.

I wonder if the carby was run without a screen at some stage which led to the increase in jet size (?) - but it does seem to be a bit on the large size to me.

Cheers

Hutch

[EW-Ron]

Its not impossible to put a blob of solder on the old jet, and redrill it with the correct size of smaller drill.

While this is a bit rough and ready, it was often done to experiment with different sizes of jets.
If only temporarily until the sizing was settled, and then a 'proper' jet obtained.
Getting a really cleanly drilled hole is a bit of a lost art.

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

[Black Sheep]

The solder and drill option was going to be the first option until I found that new jets are available. I've ordered up a 25 and a 30 but of course a 26 or 27 would appear to be the best. So some experimentation and possible/probable fine tuning the jet is the next stage.
All this coincides with the onset of winter. Gritters are not far away which might just curtail things.
I'll let you know how I get on.
Many thanks for the contributions. This old 2 3/4 is a flyer when it's going!