{.bLanK}GoD wrote:I have my doubts about your worn needle/atomizer theory.
Have you checked your float levels?

I agree. For what it's worth I am inclined to think it's actually fuel starvation. Possibly tank venting, fuel tap / line flow rate or float heights to low.

If the float valve seats are in need of a clean they can stick closed and cause starvation.
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it wouldn't be fun if it was easy!

If your carbs are of check the rubbers around the boost bottles for splits
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Why do things simply when you can complicate them

Thanks for the suggestions, all appreciated

The feeling when riding is not a leaning off but a kind of 'fatness' the bike is struggling to deal with. .....hard to describe but it feels like the bike is drowning rather than being strangled (lean), if that makes sense. For me a lean condition feels like a relatively smooth dry vacuum off the throttle...no one home, but rich is all vibration and hesitation eventually clearing. The plugs also read rich and the rear of the bike has a splattering of mixture after a reasonable run.

If it is the float height then it would seem to be too much fuel in the bowl, which may be causing the problem but it only happens on an extended closed throttle from high revs. This is why I thought it must be the vacuum created by the high revs drawing fuel into the bike somehow thru a closed throttle.

Will hopefully have the parts and time next weekend to work it out.

This is what arrived from cruzinimage

No needle 'neck' (sits on top of the emulsion and extends into the throat of the carb) which is a part I was after to check my theory. It looks as though he has tried to supply powerjets for different models as he has included 2 x 72's and 2 x 75's...although one kit only came with 3. In terms of quality the jets look 100%. The needle looks made from alloy so not sure about that. All seals look indistinguishable from the originals apart from maybe the top plate





Needles are stamped correct for the MC21, not the MC18, but have put them in




Emulsion tubes look good but only have 8 holes rather than the 16 of the originals (assuming they are standard) so I put the originals back in.




This is the slow jets compared. Both stamped 38 but the new ones are clearly longer so I put the old ones back in.



While not 100% the value and service seem very good. $25 for 2 kits! I thought $25 bought one kit so ordered 2 and now have 4!

I had real trouble with my 18 running rich withe the drilled airbox lid , in the end i've fitted smaller slow jets .
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it wouldn't be fun if it was easy!

Put the carbs I rebuilt in yesterday, and it appears the problem is fixed. Being lazy I changed too many things to say for certain what it was. I suspect one of 2 things. After opening up the carb body I noticed a blocked solenoid feed as indicated in the pic and this, if blocked would stop air being sent to the emulsion tube affecting a rich condition?

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Otherwise its as Wobb suggested ie knackered float valves letting in fuel with vibration. This would mean that BOTH sets of carbs have the same problem so I will pull apart the other set and check the solenoid feeds and replace the valves.

Without knowing exactly how the air solenoid circuit operates, I reckon that blocked feed pipe would almost defiantly have been the problem.

Well in the interests of a complete explanation for anyone interested the other set of carbs had nothing blocked and caused the same problem so that seems to point to the float valves not sealing. I also polished the valve insert with a cotton bud in a drill with brasso (metal polish) as its non replaceable.