Yamaha F100A has got the sneezes

[zenoahphobic]

That’s a lead.
Check your carburettor butterflies play. It does pay to synchronise them.
Statically try to see what they look like at the approximate opening that this “miss” occurs.
You still can get cylinders to push a faulty cylinder, one that has far less opening, or far greater opening. The earlier causing insufficient fuel to be sucked; the later causing more air to be sucked. Both lead to loss (can be momentary) in power: lean sneeze or ratio fuel change. Think: 1 cylinder less able to influence 2 others. The engine more likely to run on two cylinders, than on only one cylinder.
Having said that, you can fiddle with the linkages when it is running to see the interaction, and maybe get a step closer to solving your problem. It is a method to determine whether all cylinders are putting in equal power. Be very careful though as engine needs to be under load!
Vacuum guages can’t actually tell you that there is fuel, they tell you what the restriction to air flow is. Sorry a bit of a ramble.

[Herman Melville]

OK thanks to all the people who took time to try and help me.

Seems to be fixed so I thought I would post here what happened.

The prime start body (metal unit under the electrothermal valve) has a tiny crack that I had inherited and it was glue bodged I think air was getting in. I bought a new OEM part for insane money.

Carbs cleaned so much you can see them from space.

There is an air check valve in the carb air boost line into carb 4 (only carb 4) and this looked scummy. I could not blow much through it the right way and seemed like it could blow a tiny bit the wrong way... so I had that off and ran WD40 / compressed air through it back and forward...seemed to open it well the right direction and really close it tight in the reverse direction.... so I blew out any remaining WD-40 and refitted that.

So when it was finally reassembled it ran kinda OK but there was now the odd misfire at idle on carbs 1 and 2 (misfire was at speed before and on carb 4 I think). With the air box off I could see it was popping back out of the carbs so that is a lean condition right?

I set the idle needle screws to the best I could figure - further out than stock 2 turns and where it idled better, then set the idle screw on 4 (the master) then I set the idle screws best I could on the other 3.

Then I got a carb balancer and managed to set the vacuum on each carb to be about the same (they were not the same at all) - I am pretty new to that and found it hard but got them all there or thereabouts in the end.

Once I had balanced the carbs it was finally fixed.

I dont know if I have 100% fixed the problem becuase I had to adjust the idle screws on the top 2 carbs quite a lot (to their limit in fact) to try and set the vacuum to be right. I don't know if that means:
1) I am useless at carb balancing
2) There is some deep problem I am just covering up
3) It's really fine I just don't think it's fine.

Anyway doing pretty good now so above it what I did and thanks for your support and help.

[ausnoelm]

Best guess you still have a partial blockage in the idle circuit, carbs being "clean" doesn't mean they look pretty, all passages including those in the carb body must be perfectly clear...not just "clean" sometimes mechanically poking something through them is the only way to be 100% sure they are clear.