Tuning carbs

I have just rebuilt an 850 mk11a engine with new valves, guides, springs, piston rings and camshaft a PW3.

Before this the plugs would be of the correct colour after a run. Now, because I am still running in and the revs are below 4K, I have noticed the right hand carb is running lean (white), the left is ok. As its not on the main jet yet - I think - how do I correct this leanness?
Will raising the needle position cure this as they are both set to the middle ring, or is that not wise having one needle higher or lower than the other
The engine runs fine and the carbs are balanced.
I have very little experience on carbs, so any advise would help.
Thanks
chris r

I you still have points-and-coil ignition, make sure both sets of points are set exactly the same as they are independently adjustable.

Nope , boyer ignition.

If the R crab slide/bore leaking more air than th L then likely can raise R needle to get closer color tune. Might also try to raise float level in R carb. You can get a sense of float level by amount of turns out of pilot screw for best ildle on each carb. Ideal is 1.5 turns out so less turns out imply hi fuel level and visa versa.

Your issue could be a simple matter of an incomplete run-in, you didn't say how many miles you have put on since the new top-end. If your motorcycle was running evenly before the work miles may be your friend, but there is a maxim that says new parts accentuate the faults of old parts; tuning a worn engine will make it run worse.

However

If the carbs you have brought into question are as old as the engine cases consider replacing them. There are tons written on this forum about Amal Mk 1 concentric carbs, the bottom line is that pre-Premier Amal carbs were poorly thought out from a material point-of-view as the carb bodies and the slides are of the same material which means galling which leads to accelerated ware which causes air flow around the slide which means un-balanced random mixtures for the cylinders at random RPM. Pre-Premier Amal carbs respond well to chrome slides if they are "young" enough. The problem is that the chrome slides cost about $30/each and if the carbs have begun to wear its good money after bad.

Based on the RPM you allude to be using during your run-in the needles and needle jets are the major players. It is not uncommon for the needle jets and needles to ware un-evenly.

A bad coil may also account for the difference, especially true if your wiring harness is the one that your motorcycle was manufactured with.

Before you start spending your retirement I suggest that your investigation start with a leak down test, if that gets a check mark and your engine fasteners are properly torqued and valves set correctly engage the services of a chassis dynometer with EGA; the technician can tell you exactly where you are rich or lean, if the calibrated parts that fuel the problem RPM are in good shape then your carb(s) are, in fact, too worn.

Bill.

hobot said:

If the R crab slide/bore leaking more air than th L then likely can raise R needle to get closer color tune. Might also try to raise float level in R carb. You can get a sense of float level by amount of turns out of pilot screw for best ildle on each carb. Ideal is 1.5 turns out so less turns out imply hi fuel level and visa versa.

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Respectively speaking hobot, I think your suggestions are addressing symptoms rather than effecting a cure.
That said, I don't really think you should be concerned anyway. Both plugs are white on my Norton but show no signs of over heating do to leanness. No little black pepper looking specks, no melted electrodes, no little balls of aluminum, nothing. Looking into the bore with a Mag Light likewise shows only a happy lightly deposited piston. I think further running in will slowly get the darker plug whiter.

If you are running your old carbies the first thing I would replace is your needles and jets, they wear oval from vibrations and should be replaced ever 3 years of full time ridding without failure, a lot of Norton owners negelet the needles and jets and cost less than $50 for a carbie kit, I have done this with all my old British bikes and my carbies have got long milage out of them and running great as well one carbie can be turned diffrently to the other carbie so having one needle in a diffrent position can happen to have it running right, but do replace your needles and jets reguley (every 3 years if its a everyday ride), they might not look worn but chance are they will be.

Ashley

Try a cooler plug on the seemingly leaner side.

Check for any possible air leaks around the flanges and manifolds, particularly on the right.

I don't recall reading anything that says that both carbs need to be identical in setup to function.

And it does beg the question, if one goes to such length in rebuild, carbs should most certainly be included in the process.

Is there a way to effectively measure wear on needles and needle jets?

I'd like to see the dimensional difference between new and worn.

You will nomaly see wear marks on the needles but looking with a magafine glass you nomaly can see the oval shape in the jets, but I justreplace them anyway every few years to make sure, there is a lot of vibrations with hard manifolds which makes the needles vibrate and the jets wear after sometime.

Ashley

If you didn't have one side leaner than the other before the rebuild, I'd be looking for an air leak between the carb and the valve on the lean side. (loose bolts ?) If the needles and needle jets wear, the mixture gets richer in the mid-range and the motor will be sluggish as it picks up. You should check both plugs by looking right down inside them to see that the 1mm black ring is still there on the porcelain right down where it meets the metal. If it ain't there you will probably burn something. The easy way to check if it is too lean in the midrange is the motor will often cough as you ride changing up and down through the gears. It might be something as simple as your idle setting. I usually screw the adjuster towards leaness until I get the spit-back through the carb, then back off a bit.