A10 plunger carb

[olev]

Thanks Musky,
You're right, those settings are a little off what I expected.
straight through pipes?? i'm scandalised. don't you have coppers in NSW.
how do you sleep at night?
cheers,       

[TT John]

I read this post with interest, as I have a 1957 plunger and I have been playing about with the carb and head, firstly the intake on a swinging arm head is tilted and does not allow the fitting of some carbs and because I could not get a head for my bike I used an Ariel Huntmaster head which is almost the same as the early A10 heads, with a short intake for the carb and horizontal, I am running a Amal concentric R928 using a 106 needle jet and a 230 main jet, it seems to go quite well but it seems quite juicy with the petrol, do you think I should try a smaller main jet.
BTW, I did actually manage to saw the intake manifold from a S/A head vertical so that the carb would just fit in but the screws had to be in at an angle which made it a little difficult but it ran okay.

Kind regards TTJohn

[muskrat]

G'day John.
First thing I'd do is check the fuel height. You will need to make level tube. Get a float bowl drain plug, drill and tap 1/8" BSP, screw in a hose tail, attach clear tube and bring up to the top of the carb. Turn the fuel on and see the level in the tube. A 928 should have a fuel height between 0.17" (rich) to 0.24" (lean) below the bowl/body joint.
Cheers

[TT John]

Muskrat.
Thanks for the advise but my carb is level and there does not appear to be any way to alter the level in it, plastic float.
I have been playing about with the jetting today and have reduced the size of the main jet from 230 to 140, I know it seems quite drastic but I thought I'd give it a go and after a bit of fine tuning with the mixture control and air intake, it still seems to go okay and hopefully it will be a bit more economical.

TTJohn

[duTch]

 Hey John, slight difference, but I'm running a Monobloc on an alloy head, with a 25 pilot/105needle/ 240 main and when I changed to a 105 needle jet, it made a lot of difference...!

[muskrat]

G'day John.
First I'd throw away the plastic float and invest in a stay up float (adjustable) and viton tipped needle. A level carb is a cinch to get right but on a downdraft like pictured (alloy heads) can be a fight.
As duTch suggests the needle jet and needle are subject to wear. A new pair could make all the difference.
Remember most of your riding is on the slide and needle jet (1/4 to 1/2 throttle). Spirited riding is on the needle clip and main jet. A colour tune and plug chops is the best way to get it right. I always err on the rich side.
My A7 plunger gives 60 mpg, my A10 cafe (read racer) gives 40 mpg, my 1100 Honda is lucky to give 25 mpg but fun doing it .
Cheers

[Dynamo Regulators Mike]

Hello John
For what it's worth the 928 on my Flash has a 20 pilot, no. 3 slide, 106 needle and 240 main, needle in middle position. Reckon it gives a decent mixture through the normal range (with a question over main jet as I rarely if ever use top end throttle, where it matters most). And 60 mpg or more. What sort of mpg do you reckon you are getting? If it is very low I would agree that the float fuel level is most likely culprit. It can be lowered by tapping the brass insert where needle sits from the upper side to lean up the mixture, and this makes a big difference in running. Also agree that a viton needle is pretty much essential. The 'stay up' MoD type float is easier to adjust, but the hollow plastic ones are perfectly ethanol proof, but could of course leak if damaged.
cheers,
Mike