Carb troubleshooting

Hi

Looking for a few ideas troubleshooting a new carb set up.

I put a set of brand new Amal premieres on to my 1973 / 750. Its one of the last detuned 750s (not combat) so the carbs are the Amal 112 pack ordered up from Andover - 230 mains / 106 needles / 3 valves and RJ17 pilots. First fire up, idles perfect on 1 1/4 turns out either side but with hesitant pick up and bad surging at slow cruise. I raised the needles / dropped the clip to bottom notch and again idles good, takes off fine and makes good power on the throttle. Still slight surging again at steady slow cruise 30mph mid gear speed although a lot better. Plugs are reading v lean (bone white) left and bit oily right ?? Inverted floats both sit 1mm up inside the lip. Throttles are balanced on the stops and on the wire. I'm going to try balancing with the vacuum gauges this weekend - maybe one side is stronger than the other and I need to lift one of the throttle bodies slightly ? I'll also verify timing and go back over the tappits just in case. Any other things to think about ??

Number 17 pilots I note. Many have struggled with those, they were even no good in my 650 T120. Changing them for number 19s has cured many an issue. If I recall correctly, Amal now supply them with number 19s due to all the feedback they’ve had.

Depends what you mean by ‘slow cruise’ though, carbs don’t really care how fast you’re going, the real issue is how much is the throttle open, as that dictates (by and large) which jet circuits are in play.

Pilot jet is mainly responsible for low throttle openings.

Eddie - I read about that, mostly for 850 ? The idle circuit runs well but not with the turn and a half optimum - I'll order a pair 19s and try them, could be an easy fix - thanks

The 1.5 turns is a starting point only, as long as it idles after idle tuning when warm set between 1 and 2 turns then the pilot is sized correctly. A slight mismatch on the slides opening will give a hesitation so try the Vacuum gauge to see if they are out of sync first before changing pilot jets.

Replacing 17's with 19's did sort out my backfiring 850 bike on cold starts, the 17's worked fine when the engine was warm but I needed to double/treble tickle to stop the cold start backfiring.

stu said:

Eddie - I read about that, mostly for 850 ? The idle circuit runs well but not with the turn and a half optimum - I'll order a pair 19s and try them, could be an easy fix - thanks

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That’s why I mentioned that even my T120 needed number 19s, they generally runs leaner jetting then a Commando (it only has 200 main jets)...

Wow. problems with a set of Premiers. Whoda thought?

yawn .

blaisestation said:

yawn .

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I'll say.

Fullauto said:

Wow. problems with a set of Premiers. Whoda thought?

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As if no one has ever had an issue setting up a Mikuni.
Your endless attacks on Amals is nauseating.

MichaelB said:

As if no one has ever had an issue setting up a Mikuni.
Your endless attacks on Amals is nauseating.

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Nauseating. That's hilarious. An Amal man for sure. You haven't got time to defend them. There's work to be done on your carburetion. Whether you realise it or not.

Have you checked for good seals at intake manifolds/head and manifold/carbs and no leaks at the balance hose? Vacuum leak will cause the surging you mentioned. An oily plug would tend to indicate a valve guide/seal issue rather than a carb issue.