1960 A10 carb talk.

[Beeza jon]

Hi all.

I've recently taken on a 1960 A10. It's in a pleasantly non standard trim, and runs well, although it spent a few recent years quietly hiding under a sheet in a garage. As a result, I'm gradually going through it. Naturally,  a part of the process has been cleaning the fuel system (including the tank). On inspection of the carb it appears to have a 420 main jet, and a No3 slide.
The bike picks up very well, and I have a feeling it may have a slightly "hot" cam. So, here's the thing, will the 420 serve me well, or should I return to the standard 250?
I've assumed that the larger main will of course affect mpg, but will the return to a 250 require anything other than refitting?
My thanks in advance for your replies.

[limeyrob]

Leave the jet alone and go by the plug colour. And only make changes of one increment at a time, then check again.  If it was rich it would not run well and the most likely outcome of going from a 420 to 250 is a holed piston or scored bore.

[DAVE BRADY]

Hi,

It may help to know what carb. is fitted.  If it has a the sporty 357 cam it may also have larger carb.

Dave.

[idie]

Has it got an alloy head and a 389 carb? If it has and it it is up to Super Rocket spec then a 420 main jet is correct.

[Beeza jon]

Hi all. Thanks for your contributions.
It has got an alloy head and a 389 carb and I had begun to think that it has been modified towards S.R spec. I will leave the 420 jet in place, and give an update in the near future.
Oh, and the plugs are  a healthy tan colour.

I have fitted an air filter,  as the thought of fine particulate entering the low mileage rebuilt engine felt a bit uncomfortable.  The P.O said that the engine had only done about 700 miles from rebuild.
I'll treat it as a new mill, and do my best to run it in with some sense and understanding.

[idie]

If you are fitting an air filter, bear in  mind that it will run rich. You might find that you will need to drop the main jet to a 350.

[Beeza jon]

Thanks for that. I had wondered if I might need to,  as I was aware that the filter would have an effect. I'll keep a keen eye on the plugs.

[Beeza jon]

I thought this may be of use.
I'd wondered if my bike had a sporty cam. This info (service sheets, 701) confirmed it. My engine number is higher.
As its "from source " it provides a useful point of reference (though i realise that subsequent work by others over the years  may have brought changes).
Good to know though.