Tuning

[Deleted User]

Silk 700 now with M.O.T but not much use on road as yet.

head skim by approx 45thou to get flat. Concentric mk2 38mm carb with standard jet’s, all jets and carb are new and clear, will not tick over without choke, did several miles this am with ignition retarded by 2mm from 5mm btdc. and with choke on, which ran much better and a reasonable tick over. ( pazon ignition system)
having checked everything and measured ports on later model I find that the inlet track has been opened up by approx 8mm in height and also 8mm in width along with inlet manifold to match. the transfer ports are knife edged and the transfer ports look to be much wider too. I can adjust the mixture screw with no effect. So a few questions.

1. do you advance or retard ignition with higher compression

2. is there a good way of guessing jet sizes as i suspect they need to increase considerably

3. will the 38mm carb be enough

HELP

[Deleted User]

Check for an air leak first. 🙂

[Deleted User]

Hi,

My dad had a BSA that did that and it was an air leak on the top of the carb where the threads had been mashed by some past imbecile. Check your top cap and make certain it is sealing properly.

Thanks

James

[Deleted User]

Thanks but i can be certain that there are no air leaks, i should have said i have already checked this as i had this on an rd yamaha years ago where the inlet rubber and balance pipe had a split in them.

[Anonymous]

What size pilot jet have you fitted, and what is the float height? Both have to be right to get correct fuelling at tickover throttle setting are are the first suspects if you can get it to tick over with the choke closed.

1. With higher compression you usually retard the ign (make it fire nearer TDC)
2. Main jet has no fuction at tickover. Do a web search for Amal tuning hints and you will get the info you need, but you start by running the bike flat out and doing a plug chop to get the main jet right, then work your way down through the needle jet, choke cutaway, pilot jet and the the air bleed screw.
3. Err – might be too much, the larger a carb the lower the inlet tract velocity at low revs (like tickover) so the poorer the starting and slow running performance, especially noticable when opening the throttle fast from a tickover.

Cheers,
Rich

[lewis onions]

I had similar carb problems with mine. Try 480 main jet (this is the factory fitting, honest !). I also recommend a proper throttle slide – Teflon coated and with a hole at the top to prevent the creation of a vacuum and the associated sticking. I was told to use a chrome-plated brass slide but I am not sure that they are available. Try Hitchcocks, although carb wizard Don Payne no longer works there:

https://www.hitchcocksmotorcycles.com/home

Lewis

[Author]

Posts