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[stinkwheel]

So they basically solder the rings inside their lands? Interesting.

[Bullet Whisperer]

So they basically solder the rings inside their lands? Interesting.

Yes, that's basically it.

[Creaky45]

Many interesting posts here thanks. To answer your questions, yes the head is tight. I had it off not long ago to helicoil the decomp thread and re-tightened it after a few runs. Fuel flow is fine as far as I can tell. A while back I bought a brand new Mikuni and had such problems trying to tune it even with the help of this forum that I went back to the Mikarb, and apart from this latest problem it has been perfect. And, Vince, I'm not sure what you mean about the Zip noise. There was no noise or backfire when it slowed and stopped but I did have my earplugs in and the exhaust is quite loud. So far I think the 'stuck valve' theory sounds good but I might whip the head off in case. I checked the tappets cold just now and the exhaust spun freely (just right) but the inlet was a bit tight so I fixed it. Strange, I thought it would be the other way round. In lockdown and can't go for a ride for a few days. Lucky I have a spare bike that never breaks down. A 2019 650 Interceptor. Luxury!

[windmill john]

. In lockdown and can't go for a ride for a few days.

I still get my bikes out, need to keep using them. But I don’t flout the law, I use them to pick up groceries etc.

[vince]

Hi, if you have never experienced a seized piston, it has been described as a zip sound as the piston is rapidly slowed and the back wheel locks followed by a tonk tonk noise as the piston breaks free. Once heard never forgotten. The sticking valve goes phit bang . If you have have the head off to inspect, check the valve stem it should have a shiny contact area . Vince

[Creaky45]

Hey Vince, No zip sound and no phit bang either, just a voice in my head saying, "not again", and, windmill john, I could do that but if /when I broke down the ice-cream would melt.

[Creaky45]

Tuning update. After doing plug chops at different speeds I was still getting a very lean looking plug. I borrowed a P2 off a mate and it made no difference. So I put my old original muffler (loooong one) back on. Still the plug was just as lean. Then I remembered. When I put my new 2.5 slide in I also changed the needle to a brand new one I bought from our host ages ago. I compared it with my old one and while they are the same length the new one is quite a bit thicker, hence it would not let as much fuel round it. I put my old needle in and voila!, nice coloured plug and it goes like a rocket. I will put the loud muffler back on and do some fine tuning now. So it seems changing the muffler made little or no difference to the mixture and nor did the P2 as opposed to the P0. Strange. How many types of needle are there? They don't seem to be marked either. Also found out from my mate that my P0 was actually a P8 which I think is the leanest (standard) needle jet.

[stinkwheel]

The exhaust backpressure will however make a big difference to the stability of the idle.

[ChrisD]

For those using the Mk2 Concentric, here's my story so far.
Jets that work in the 32mm Amal Concentric Mk2 for my 1996 535cc Classic are given below. Bike has MrH’s larger exhaust valve and now large 1 ¾” scrambles inlet valve, opened and matched tracts, high capacity oil pumps, compression at ~8.5:1, performance cams, exhaust on dot but inlet retarded 1 tooth, 3-way pinion set to advanced, alloy barrel, metal-to-metal head joint to help remove the heat, twin-plugged, shorty silencer (PN91007) cut open with long-thread fibreglass forced into annulus, Boyer Bransden electronic ignition (NB: this won’t work with Iridium plugs). Tried a shorty EMGO rev-cone megaphone with which it goes better BUT is sooooo loud. Rear sprocket now 19T. Tachy too! Much larger breather diameter at 10mm (not big enough yet).

AMAL CONCENTRIC MkII 32mm throat.
Codes = 2932 R300, Made in Spain.
Slide = 2.5 (tried 3.0, too weak).
Needle jet = tried 105, 107), use 106 after adding K&N filter and Scrambles inlet valve.
Main jet = tried 220, 240, 250, 260), use 230 after adding K&N filter and Scrambles inlet (still not sure about final jet size).
Pilot jet = 25 (tried 30, 35, 40, 45) – maybe rethink this.
Choke = 45 (tried 25, 30, 35, 40) – maybe need to rethink this.
Needle = 2A1.
Needle clip = #2 (tried all positions, fixed on #2 after fitting K&N filter and Scrambles inlet.
Air jet = 3.5mm (tried 2.5mm).
The air inlet OD is 57.5mm. OEM air box inlet diameter is 47mm but I now use an AceCafe cylindrical airbox with a 4”x2” K&N E3120 filter.
NB: jets are cheap compared to overheated piston damage, so keep testing.

I have never attained the max speed. With GPS measurement, it reached over 130kph on flat, with no wind but I'm not confident of the roadholding so chickened out whilst still accelerating (I'm OK with my BMW at 100+mph). At 4000rpm, cruises comfortably at 110kph, almost 70mph which it has held OK for an hour. I've given up worrying if it will last at that speed- I expect it to outlast me.
Hope this helps someone.
ChrisD (in Cape Town)

[Creaky45]

Good post Chris, I have got an Amal 32mm so I may just give it a go on my bike using your settings etc. I only have a standard head. High comp Mr H. piston and iron barrel bored to 535. Loud muffler and POD filter. Goes like a rocket now with its Mikarb so I should probably leave it alone. But, my motto is " If it ain't broke, fix it till it is"