Does anyone out there have any specifics on tuning the Amal carb on my 73 Ossa Explorer? I asked this question in the Classic Trials section, but recieved no answers. Is the one air/fuel screw an air screw or a fuel screw, and how many turns out should it be to begin with? My Clymer manual doesn't cover the Amal. Can't seem to make plugs last long. I'm running Denso IW24's.

On a different note, I noticed not one person suggested an Ossa, for 'Best Twin Shock Ever' I like mine and thought they did quite well in their day. But I guess the early 80's produced some far better bikes. Obviously from the response, Fantic was the favorite.

Edited August 31, 200916 yr by Gordo

It is an air screw on the MK1 Amal. If you screw it in it shuts off the air, screw it out it lets more air through. Try 1.5 turns out for a starting point.

If your Mk 1 AMAL is making the motor run rich from 1/4 to 3/4 throttle, it most likely has worn jet needle and needle jet. Those parts wear rapidly on that carby and replacements are available from AMAL carby suppliers

It may also have a leaking float needle valve.

Your OSSA questions will probably be answered better in Twinshock because Classic is usually used referring to pre-1965 bikes

The wrong plugs will foul quickly. But it's hard to find a listing for OSSA. From what I can tell you're probably using the right plug, although if you're going through iridiums that fast you'll soon spend more in spark plugs that the bike is worth.

Perhaps a bad condenser? Or just that the carb is running too rich or perhaps the oil/gas ratio is too much.

Right now the air screw (now that I know it's an air screw) is out less than one turn and I am running aviation fuel and Golden Spectro at 52:1. I'm due for more fuel, so if you folks think a better oil or ratio is better, let me know. I'm only using Golden Spectro because it's what I always ran as a kid. The bike is basically an MAR, with an Explorer tank. It is used for trials only, so maybe my ratio is wrong. 52:1 is what my 85 TY350 calls for. I'm wide open to you folks giving opinions.

Thanks

That sounds reasonable ratio.

And the spark plug sounds ok.

Is it a Pioneer or MAR?

Spark plugs sounds ok. Gas sounds good. Other than fouling plugs how does it run?

It is a 73 Explorer. The Explorer was basically a trials bike with a larger seat, tank and counter shaft sprocket. The bike starts and runs well during an event, with a new plug. Over the winter, I think I'm going to buy a Mikuni for it.

Edited September 2, 200916 yr by Gordo

If it runs fine up until the plug loses spark it is possible that the problem is lead fouling from the AVGAS you are using combined with a too hot or too cold heat range plug. Have a very close look at one of your fouled sparkplugs (magnifying glass) and see if you can see anything unusual ("hairs" or yellow/orange patches on the nose can indicate lead fouling).

It is a waste of money using iridium plugs in a bike that fouls plugs.

Another possibility for frequent plug fouling is is your bike has been run on unleaded fuel previously, running on leaded fuel may be causing some combustion chamber deposits to come off and stick to the plug nose.

Should I not be concerned about the type of fuel? Should I just use 93 octane unleaded? I was concerned about the additives in pump gas and racing fuel is not a local option. But I can get it if it's a better option.

AVGAS is fine if the sparkplug heat range is right. I don't know what pump gas to recommend otherwise because your pump gas is likely to be quite different to our car fuel.

We can buy 98 Octane unleaded car fuel from the bowser, has no ethanol in it and it works fine in all my air-cooled trials bikes. Standard car fuel here is 92 octane with no ethanol and it does not go quite as well in the bikes as the 98 octane. We also have 95 octane with ethanol which I only use in my car.

I suspect all your unleaded pump gas varieties will contain ethanol and that the ethanol may damage your fibreglass fuel tank.

My recommendadtion for your OSSA is to use an NGK B6ES plug and AVGAS premix, and if it fouls another plug, work out what is causing the fouling by inspecting it closely.