Amal 420

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fleetingcontact
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Amal 420

Post by fleetingcontact »

I'm putting a 102 bitsa together, this one will be a bit more ED than AC, and I am intending to use a 420 carb which came from a 170 which I scrapped. I think the jet I need to re-jet it to is a no.90 (which with a bit of luck is the one in a 416 with bits already missing). Can anybody confirm this?

Also, the 420 comes with two screw adjusters where the 416 just has blanks.
Amal 420.JPG
I assume one is a mixture screw. Can anybody tell me which one it is, and also what the other one is for!

Thanks
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Charles uk
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Re: Amal 420

Post by Charles uk »

Slow idle, (tick over) you will see the end projecting out under the throttle slide. The other is the pilot jet adjuster screw.

If this was a 170 carb it will have a 140 jet, you won't be breathing enough air to need a 90 jet, start with a 95 & work up.

The amal carb for your 102 is a 416 (16mm) with a 95 jet, your using a 420 (20mm) which is over 50% bigger, slower air speed, less suck on the jet, perhaps you should start with the 140 as you've got one.

I think the only jets commonly available are 140, 115, 100, 95, 90.

That is a pumped 420 which has a smaller gallery above the needle valve than the gravity fed carb, it might not feed enough fuel without a pump, check carefully.
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blokewithaboat
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Re: Amal 420

Post by blokewithaboat »

Are you sure that carb will work on your 102?
I reckon it might be a little on the large side myself. Will it actually fit onto the inlet stub of the 102's cylinder?

What bits are you missing from your 416?


As for adjustment screws, 1 will probably be a pilot screw (fuel mix) and the other an idle speed adjustment screw. Fuel mix screw will probably have a sharper tip at the business end and idle screw will have a blunt or rounded tip where it butts up against the bottom of the throttle slide.

Correct me if i'm wrong but weren't 420's normally fed their fuel from a pump as was the case with the 170 you scrapped? Will your 102 have a fuel pump? I'm not that clued up with 420's so not sure if this carb will work with a gravity fed fuel system as it would be for older seagulls.

Another thing, normally the 420 carbs (not sure about 418's) were paired with an ignition cut off to stop the engine. With a "conventional" carb you just close the thumb throttle right down to kill the engine. Will your ignition system have a similar ignition kill button? Its one thing getting the engine to run, stopping it creates another headache.

416 is much easier for this application and be a lot simpler to set up for the 102 much like the 46n 2jet that many 102's had originally.

ooops, just tried posting while Charles was replying, but yeah. What Charles says
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fleetingcontact
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Re: Amal 420

Post by fleetingcontact »

Ok...

Thanks Charles and Bloke.

So just because they look the same on the outside...I was under the impression that the main difference was the jet size...I was wrong.

Anyhow, the synopsis I'm taking from Charles is start by trying it as is and see what happens, but I have a very decent 46N that I will use to make sure all else is good to begin with.

What's missing from the 416 is the needle jet and the compensator. I could get replacements but the carb itself looks manky. Still, I expect it could clean up. Or find another 416...anybody fancy a swap?
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Charles uk
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Re: Amal 420

Post by Charles uk »

I wonder what would happen if you undid the idle screw Jon? & swapped the insulation bush over.
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fleetingcontact
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Re: Amal 420

Post by fleetingcontact »

So, Blokewithaboat, it didn't fit onto the 102 block. But it did after the insulation bush was swapped. Charles. Good thing I had a spare. What do you you think the result would be if I undid the idle screw?
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Charles uk
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Re: Amal 420

Post by Charles uk »

This screw controls the tickover speed, if you undo the the screw a little, it will stop when you close the throttle.
Pull the cap from the throttle body, remove the throttle slide you can then see the screw, undo it till it lets the slide close all the way.
Most screws are pointed & will adjust tickover speed & a few have parallel ends but they have a slightly different throttle slide but I think they were only used on the inboards

These are quite low tech 2-stroke motors, no rocket science!
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fleetingcontact
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Re: Amal 420

Post by fleetingcontact »

Thanks Charles.
Snazz.JPG
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blokewithaboat
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Re: Amal 420

Post by blokewithaboat »

Yes, but the 420 carb is too big for a stock 102 cylinder even with or without the nylon adapters in the carb flange. Have a look through the carb while its fitted onto the cylinder, remove the throttle slide and compensater and see how much of a step there is between the carb and inlet stub.
Airflow and airspeed through the carb and inlet port will have been dramatically altered if you intend to use a 420. You could try porting and polishing ports etc. which might help a tiny bit but probably not really worth spending the time or effort on, whereas the 416 is a far better match all round. Air speed/flow will be somewhere like the original 46n carb, hence a far easier option with less hassle. Much easier to set up too.


Mind you, this is a "bitsa" in the making so anything goes i suppose.

What do you have in mind for "swaps"
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fleetingcontact
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Re: Amal 420

Post by fleetingcontact »

While I'm on such a roll of useful information, perhaps you would answer another question or two...

I compared a Silver Century crankcase to a 102 crankcase and could see no difference except the model stamp. Is there a difference?

I have several crankshafts which I know came from a Century (not a Silver Century, just to clarify). Again. I have compared them to earlier and later crankshafts and apart from the fact that some are welded whereas other are not, is there any material difference between them or to put it another way is there any reason why I can't use the Century crankshafts in either a 102 or a Silver Century (if I wanted to)?
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fleetingcontact
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Re: Amal 420

Post by fleetingcontact »

Hi Bloke,


yes, I agree it would be better to have a 416 - so I was thinking of a swap for the 420 - but to be honest I'm starting to want to try it and see - even though it sounds like it'll all end in tears...

Other bits I have for potential swaps - hmmm, a bakelite plug cap? A pulley that says 'Villiers Magneto'? A swept two-blade 102 prop? An alternator?

Wanted: a late model 102 clutched gearbox and er... a 416 carb (dead or alive)...
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Charles uk
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Re: Amal 420

Post by Charles uk »

Why not fix the 416 you have, exactly what is missing?

Quite a few racers run 420's on centurys & 102s & do pretty good. I have a couple, the 416 insulation bush has a bush with a smaller hole in the middle, then you just fair in the carb throat up to the bush with plastic metal, you can also limit the amount of travel of the throttle slide with a bit of plastic tube but I doubt you'll notice the difference as you only want to run it, not water ski!
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Horsley-Anarak
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Re: Amal 420

Post by Horsley-Anarak »

Charles uk wrote:Quite a few racers run 420's on centurys & 102s
Yes but fit a pump, otherwise the carb cant keep up.
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fleetingcontact
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Re: Amal 420

Post by fleetingcontact »

Yep thats right, not intending to win races and if the result is a good runner that starts easily I'l be very happy with it.

416 missing the needle jet & compensator so no big deal deal really, I could take the compensator from the 420 and Sheridan will sell me the jet. But as stated it looks horrible and would need some time spent on it cleaning (insides too) if I was going to use it. Because the 420 spent its life inside a cowl it seems to be in 'as new' condition. And now I know that this is a proven mix of parts I'm even more into it :shock:

So all I have to do is clean up a few more bits (the bracket is looking a bit shabby) and the prop needs fettling a bit - the bush seems worn and I have a replacement - I have a new piston for it and I am toying with the idea of a new con-rod too because all I have are Century ones...the barrel is excellent but needs a re-paint - think I'l do that a bit later for fun - and as ever with 102s the fuel tank is a bit dinged so I need to attend to that somehow - and I have a brass exhaust tube which will (or course) need to be polished.
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fleetingcontact
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Re: Amal 420

Post by fleetingcontact »

HA, I have the pump from the 170 and wondered if I could fit it to the 102 cases, other end to the standard tank - stupid idea or not? I'm not at this point keen on doing the remote tank thing. I doubt I'll ever need it.
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