[Super45]

Ok, thinking heads on.

Firstly there is nothing wrong with Amals, yes there are better carbs but the vast majority of 60s and 70s bikes used the Amal concentric without issue, so let's park that one.

If you bike is bogging when you open the throttle (and you have said it runs fine with the Mikarb)  it is one of two things (maybe three, maybe four) firstly too much fuel at one, secondly too little fuel and it is being delivered too slowly and thirdly a blocked air filter but we have covered that already. Lastly an air lock in your filler cap, but unlikely if the Mikarb works. How about a blocked fuel tap, remember the AMAL will be trying to draw more fuel than the Mikarb was and maybe it can't?

I think it is too lean.

So, when you open the throttle (soft snap) the needle raises and lets the fuel mixture through all at once and quickly if it isn't there really isn't much it can be other than  a blocked/damaged needle valve seat or a sticky or leaky float, is the float pin bent, and does the float move freely up and down (I guess yes) If the float has a leak it will be too heavy to respond quick enough and will starve the engine, have you tried submerging it in a cup of petrol to check it isn't leaking?

Please keep us posted if it gets sorted or chucked out of the window!

https://www.jrcengineering.com/technical-support/amal-concentrics-tuning-and-jetting/

Will keep the post updated as I go along.

firstly its a brand new carb, bought about 3 maybe 4 months ago and with test runs up and down the lane I doubt its done a mile in total use as the lane is about 100 yards total length.

Float height has been confirmed and its a stay up float fitted from new, fuel shuts off and ticker works as it should so assumption that its not holed at present as no leaks.

Brand new air filter that came with the cab kit, if you put your hands over it at tick over you can strangle the bike till it cuts out, no air leaks on carb side tested with WD40.

petrol cap has been checked and you can blow though the breather hole both ways

Petrol Tap is one from Mr H and fuel flow seems good, always fuel in the pipe even when revving the bike up.

Head gasket replaced last weekend and re sealed with automotive rtv however inside of combustion chamber is very black per the photo, no sign of any oil use so assumption is currently that its burning rich. valves are dry just angle of light on the photo 

[allanfox]

Well weak that ain't!

Now I think about it I did have to mess about with my float height to get it running correctly 

https://amalcarb.co.uk/new-amal-concentric-stayup-float

I think you have us stumped!

[Super45]

Well weak that ain't!

Now I think about it I did have to mess about with my float height to get it running correctly 

https://amalcarb.co.uk/new-amal-concentric-stayup-float

I think you have us stumped!

I know!

Stay up float already fitted and set correctly.

looking at the jrc guide on occasion it has spat back though the carb which would suggest being lean, but as the battery was getting low at that stage as it eventually died and the head picture above disprove that its a carb issue ( new battery on shopping list)

[stinkwheel]

Be interesting to see what it's like once it has a good spark. The flame front on those engines has a long way to travel, if it's getting half-arsed ignition, it might make things look richer than they are.

[Super45]

Be interesting to see what it's like once it has a good spark. The flame front on those engines has a long way to travel, if it's getting half-arsed ignition, it might make things look richer than they are.

On a fully charged battery it gives a decent blue/ white spark across the plug when against the head but the points have give me a belt before when adjusting the timing with the ignition on. 

[Super45]

Well what a weekend.

Battery charged and a bit of fiddling done, advance / retard unit checked and all ok, timing checked again and only approx.  1mm out so that was reset again.

Carb still being it's helpful self with the large flat spot off idle , discovered the base gasket was leaking so that meant yet another top end rebuild on Sunday to reseal it ( 4 hours dead start to finish  ) , just waiting on the sealant to dry now and  as my carb bits look like they wont be here till middle / end of the week due to the seller using parcel farce I'm stuck for a few days will they arrive.

[SteveThackery]

Carb still being it's helpful self with the large flat spot off idle

As I mentioned above, that is a characteristic of the Amal Concentric MK1 and cannot be cured.  However, if you fit a richer slide and set the idle mixture as rich as you can without the engine bogging down, you can often reduce or hide the flat spot altogether.

[Super45]

As I mentioned above, that is a characteristic of the Amal Concentric MK1 and cannot be cured.  However, if you fit a richer slide and set the idle mixture as rich as you can without the engine bogging down, you can often reduce or hide the flat spot altogether.

thanks I did read your earlier comment was just passing comment on the bikes current running condition, remind me as I've set the idle up per the guide that came from Amal, does screwing in the mixture screw enrich or weaken the mixture or is it vice versa?  as it just says turn either way until you reach the ideal RPM after setting the slide position?

Thanks

[Paul W]

Phil,

The idle screw is an air bleed, rather than a fuel screw, so opening it up (anti-clockwise) weakens the mixture.

[allanfox]

Screwing it in adds fuel so enrichens it and it will start to bog down, screwing it out adds air and weakens it and it will start to rev higher.  So if the pilot jet is the right size and clear the point in-between is what you are aiming for. Always start at 1.5 turns out for reference.

[Super45]

Perfect thanks Gents.

In other news the slides I ordered have finally shown up, no sign of an explanation from Parcel Farce of why they were 4 days late but hey ho some tinkering to be had at the weekend . so I now have a 2.5, 3, 3.5 ( wassell version as Amal's are ridiculously priced if I'm honest) and a 4.

Am I right that 2 is rich and 4 is lean? 


Thanks 

[allanfox]

Lower number richer and higher number weaker, I guess the larger cutaway allows more air into the mixture, as already pointed out they are are pretty basic things but so are our bikes! 

[SteveThackery]

Screwing it in adds fuel so enrichens it and it will start to bog down...

Technically it reduces air rather than adds fuel. That's why you have to keep altering the idle speed screw whenever you alter the mixture screw: reducing or increasing the idle air quantity works like another little throttle. Most idle mixture controls leave the air flow the same and just alter the fuel quantity. Yet another example of the cheap and nasty design. I think you're a hero for persisting with it!

[SteveThackery]

Am I right that 2 is rich and 4 is lean? 

Yes.

There is something I want to point out: you can fine tune the cutaway yourself by filing it.  Filing the slanted surface effectively increases the number (eg from 2.5 to 2.75, say). Just be sure to mark the slide in some way so you don't think it's still a 2.5.

You can make it richer by filing away the flat bottom surface of the slide (thus shortening the cutaway). If you go too far you might need to raise the needle a notch because removing material from the bottom of the slide is a bit like opening the throttle a bit further.

Personally I wouldn't mess with filing anything until you are pretty sure you need to.

The cutaway has the biggest effect just off idle, tapering away to around half throttle, where it has a pretty small effect.

[Super45]

Technically it reduces air rather than adds fuel. That's why you have to keep altering the idle speed screw whenever you alter the mixture screw: reducing or increasing the idle air quantity works like another little throttle. Most idle mixture controls leave the air flow the same and just alter the fuel quantity. Yet another example of the cheap and nasty design. I think you're a hero for persisting with it!

I'm a sucker for punishment, I've also persevered with the Old style Magura controls on this bike too including locating spare bits where ever possible rather than junk the lot and go to minda or the classic styled set Mr H sells!

Technically, it's a superstar model but has been converted to standard bullet casquette at some point in its life ( first Mot on record was 2022  and no paperwork prior to that) but I draw the line at finding all those front bits to put it back to standard.