Talk carbs to me

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?

Again what is the typical rule of thumb regarding higher lift cams, regarding jetting?

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!

Well, it has been a very long time since I've been involved with an over bore, and it was always on big block car engines. But the principles should be the same.

If anything, the jet sizes should remain constant, assuming the bike was running fine before. You are simply sucking more mixture through, at a constant air/fuel ratio. If your engine is running odd, and you think it is a mixture issue (plugs are the key for reading this), you will probably want to go down a little on the jet size (Holleys were notorious for this). The increased lift cam will help move the air in/out, but watch your clearances and lash. And, in the old days on big blocks, it seemed a slightly increased adjustment to the timing (dwell) was necessary, firing an additional degree (or so) after TDC. Electronic ignition makes this last point moot. Also, on a small motorcycle engine, it is probably not a concern.

If you want it perfect, pay the money and put it on a dyno with an air/fuel sensor - an experienced operator/mechanic can get the jets perfect. Not sure about dyno costs in the UK. Also, use old tires: dynos eat tires for lunch.

BTW, here's an MCM linkie with a bit of info.

Thanks, thats exactly what I've been looking for I don't believe I've been jetting the wrong way round!

I found that I needed smaller jetting after I installed my big bore but then I needed bigger jetting after I installed my 30mm carb.

Turns out my issue was valve clearances, it's running now only at around 3/4 to full wot it bogs down. it's a pz30 carb with a 105 main. Do you think it needs bigger or smaller Main jet

Well we have different engines and setups so we probably need different jetting but I have a 115 in mine so that would probably be a good starting point, also you have a performance exhaust whereas I do not so you may need something even bigger.

are you running a 30mm inlet carb with a 26mm rear?

Will try a bigger main tomorrow, just need to sort out my choke as its a 1/8th away from being fully open and I'm unsure if this is the issue contributing the bogging down.

30mm inlet with 46mm rear I believe.

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Originally Posted by spencer2004

30mm inlet with 46mm rear I believe.

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46mm across as is flange or in diameter of carb?

Do engines make a distinct noise when under fueled or overruled as I'm trying to work out what way things are going asy colortune is broke.

The outer diamater of the bit that the filter fits on to is 46mm.

No idea about the noise, I just made an adjustment then rode it to see if there was any difference in power.

Will give a 115 a go, hard to tell what's going on but the issue is 100% main jet now I got a few sizes laying around, shame my mikuni 30mm packed up on the gasket side as I would of preferred that to this sheng wrey pz30.

what's your needle position?

Trying to save some time on narrowing the setup.

Mine's in the middle position. I think I did try raising it a notch higher once but it was too rich.

Brilliant I will try your setup, are you using keihin or mikuni jets?

I think my main is a Mikuni. I had to get a bigger pilot jet and that was a nightmare as I could not find one anywhere on the entire internet so I found one that looked simular and luckily it fitted, I did have to cut it in half to make it fit however.