What carb set up?

Tim, bit short on info here but I have the same bike with same exhaust system, using the RGM single (932) Amal Mk 1 conversion. Also the earlier paper air filter before the black box.(As on the 750's)

Main jet is 260 (Thanks John Dunn, it made all the difference) I use the 4 ring needle on the middle notch but I can't remember if the slide is 3 or 3 1/2?

Not that I use ethanol but I have an ethanol proof stay up float fitted because you need to increase the fuel level on one carb or grind to a halt in traffic. You also need the correct spray tube.

About all I can remember except that mine has been running great and starts first kick using 20/50 oil these past few years. Ave MPG on E0 is 68 mpg and plugs a nice light brown.

Previously tim_harris wrote:

Hi, I have recently bought a 75 Mk3 Commando which has the reverse cone silencers fitted, without the balancing pipe between the exhausts. I realise that originally it would have had the "black cap" type fitted. Also got the black box type air filter fitted.

My question is, it seems to be running a bit rich as the plugs are quite sooty, so I thought I'd check the needle positions etc. The trouble is, looking at the Haynes manual, it states the main jet should be 260 and the needle clip should be at the top. The Norton Riders Manual says main jet 230 needle at lowest notch, and the parts manual says main jet 220.

Can anyone tell me what the relevant jets, needle positions etc., should be bearing in mind the exhaust/silencer type fitted. I don't really want to change them as I prefer the look and noise

Thanks in advance,

Tim

Hi Tim. I run a 77 Mk111 it was fitted with the twin amals and the black box air filter assembly and I to suffered with sooty plugs from time to time. I wont delve into the jetting and needle settings as others are far more experienced than I although I tried several variations, my carbs were premier and ethanol proof although I never put the stuff in. Not undermining you technical experience here but the breather from the oil tank feeds into the air box and can in time saturate your air filter. Also if your bike does wet sump, even a little you will soot up your plugs, mine did and still does unless I drain the sump if it has been standing for weeks.in the end I opted for a single Mikuni and all is fine and lovely although I loved the look and feel of the twin carbs when you could get them balanced that is. Good luck.

Mark.

I've just had a look at the Amal Reference Catalogue and it gives the following settings for a standard Mk3. Main jet 200, throttle slide 3.5, needle position 1, Needle jet 106.

All other 850 Commandos have a 3 cutaway on the throttle slide and position 3 on the needle. Early 850s have a main jet of 260, the 2As (with the black air box I think) are 230.

The main jet will start to come into play at larger throttle openings. the No. 3 slide will run richer than the 3.5

Pilot jet is fixed but still has an effect up to about 1/2 throttle.

Ensure your plugs are sooty due to richness and not burning oil. Oil will show as blue smoke and richness as black.

Quite often plugs will soot up due to rich slow running as you are slowing down to shut down. Try a plug chop. Drive at about 60-70, pull the clutch in, press the kill button, check the plugs for soot.

The premier has a better slide material and a screw allowing access to the, normally hidden, pilot jet. It has stay up floats also to deal with ethanol in fuel. How much more expensive are they?

Just a part of the cost for having ethanol in your fuel, multiplied by the number of bikes you have. Then add on the cost of additives to help deal with the ill effects. Stabalisers etc.

If anyone says that ethanol fuel is cheaper, I'll tell you it is not, It is a subsidy paid from your taxes!

Hi Tim

You have done the right thing buying the better Premiers. Amals from the last 30 years have suffered from a weak pilot jet (fixed) fuel supply which means reduced pilot air and a tickover dependent on the slide stops.

Rich symptoms all appear at low throttle openings: for example flooding from the float jets and worn needle jets past 10,000 miles. With open exhausts the Amals need to be slightly richer with a 3 1/2 cutaway. If the needles are right then it will run nicely at 60 mph.

The main jet really does not matter as long as it is not too small and you are not trying to hold it at 90mph. If it is too large then the needle jets limit the fuel until they are wide open.

It is vital to balance the carbs exactly to get a clean pull away response. Buy a pair of vacuum gauges (eBay) and connect each to where the rubber balance pipe fits (You have to fiddle with the vacuum pipe damping to get steady readings). They are well worth the investment.

Norm.

3 1/2 cutaway is weaker than a 3 Norman