Talk Carbs to me

[golf-sib]

I've rebuilt the engine, valve clearances set and I jetted up assuming it was the correct thing to do, now I have a non starter. Can someone give me hints on carbs in general as there's barely any useful information on mr google.

Ie I found only two pages amongst many that says if you increase the cc of the engine which I have the jets will now supply more fuel due to more vacuum, so rather than jetting up you may have to jet down, is this correct as I've been jetting up and having little success?

Also what is the rule of thumb for bigger carbs, I previously had a bigger carb running before the big bore kit and managed to tune that reasonably well using a colortune, when you increase the carb size do you jet up or down?

I know these are stupid questions but asking google gives me the usual tosh of generically tuning a carb nothing to do with how more cc effect it nor the effects a bigger carb has on jet sizes. Also any more additional information from peoples experiences with carbs will be useful, as I've discovered mikuni label their jets by cc/min but it doesn't help in anyway at all as applying injector calculations are completely useless, its a shame no such calculator exists for carbs.

Thanks in advance!

[danny_p]

it's not just the jets you need to consider,  but the air correctors and emulsion tubes, 

its often air speed thats critical

[topher]

i've no idea what kind of engine you're using this "mikuni" carb on but are you saying you've tried all sorts of jet sizes and the thing wont start? you sure its fuel related?

[danny_p]

try easy start, or propane.


one it's fired on that you can hear how it dies     then that gives you ideas on how to piss about with the carb

[gazareth]

a lot of bike carbs have a tapered needle going down in to the jet with a small circlip and 5 different grooves to put it in. from factory they are usually in middle groove. if you put circlip down a groove that will richen fuel mixture etc. I think you need to double check everything you have had apart because the engine should start on standard carb even if it has been bored out.

[notenoughtime]

What have you done to the engine and what carbs, do you have a spark? Try putting bit of fuel down the carb and see if it fires.

We need more info