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Took her for a test drive today & a few yards down the road she started popping again. Took this video:

http://www.youtube.com/watch?v=Gh04ZBYmrPE

The black smoke implies it's running rich. I've just replaced the idle mixture screws & the air filter is a K&N style one. Will put the original air filter back on, I think & then start twiddling the screws again (as they're only screwed out 1 1/2 turns)

Judging by the condition of the idle screws you removed I suspect that the idle screw seats are also buggered. I’d suggest handing the carbs over to a good carb man for a rebuild.


re: K&N air filters may want to read the following threads


viewtopic.php?f=4&t=3144&hilit=+air+filter+stock

viewtopic.php?f=4&t=4746&hilit=air+filter+stock

Thanks, Ed. Just been reading in another post that the idle screws should be 2 turns out for the tamper proof style (like mine) - I'd only done then 1.5 turns out each.

Think I'll put the stock filter back on again and see if things improve

Right, stock filter reinstalled and screws turned in and then back out 2 revolutions...

Engine now running on 1 cylinder (#4, as it happens). I'm wondering whether I've iced up the carbs or something - just topping the battery up and will then get a hot air blower in there to see if that fixes things. Definitely getting a good spark

Should have guessed that the rich running last night might have fouled the plugs. They weren't that bad but a quick clean up and got it running again. Not running great at idle, but better than before

Are the butterflies closing completely? I had to manually apply pressure on one of my carbs to fully close the butterfly. It seemed to move a few millimeters more than the other. I then tweaked the actuator arm to compensate and fully close the carb before adjusting the idle screws.

Once they are fully closed, its time to work on the screws. Youre in a bit of a different spot than most as the chambers could be messed up. Backing them all off 1.5 turns may not actually be the same across the board. You will have to try each to find a sweet spot as Linden mentions.

Think they are. Trying to get hold of the sync tool at the moment so I can get them sorted. At least the engine is running now - was worrying this morning when I couldn't get it to start at all

Right on. These carbs are tricky - especially from someone like me who has only worked on SU carbs in the past. I can see where simple experience makes a huge difference. Learning curve is steep.

You must first sync the carbs before you can do idle mixture adjustments. A good engine usually syncs in around 6-9 kg/h @ 850-950rpm.
If the idle screw seats are damaged your mixture adjustment can be as little as 0- 1/4 turn. If you have worn out or twisted brass throttle shafts you are wasting your time.
One last tip, inspect the pump bores. The old style leather pump plungers are like a wick to moisture when sitting resulting in pump bore damage due to oxidation.
I just finished up 6 carb overhauls from 2 very different regions and all of them had to have some restoration to pump bores to repair water damage, and 2 had to have worn out brass throttle shafts replaced.

Jim, what am I looking at to determine pump bore damage? Rust?

Jim LaGuardia wrote: bore damage due to oxidation.

Aluminum does not rust, it turns to aluminum oxide(powder). This leaves a deep stepped pitted area.

I got hold of a sync tool and got the readings the same. Still wasn't running well so took the plugs out (NGK BPR6ES) and found that they were hardly sparking. Tried with the old, non-resistive plugs and a vast improvement.... for a while.

Still running rich, though, so think it's fouling those plugs too. I've turned all the mixture screws out another 1/4 turn - should I give them a bit more? Thinking about getting a colortune so I can see what's really going on in there

At least I've had it running well for a brief moment

**I've turned all the mixture screws out another 1/4 turn - should I give them a bit more?* *

The idle screws also affect the over-all mixture. If you're running rich,going IN is the direction to go if the problem is the idle circut. If you can run them all the way in and it still keeps running then it's an internal issue. Time for dissassembly and clean.

Ah, that could explain things... Will clean plugs (again!) & screw IN. Think the problem is at idle because before the plugs fouled it ran like a dream at higher revs

I would double check the coil, spark should jump at least 1/2". That would also explain the plug fouling.