Amal Mk2 Concentric

Hi Chaps - please can somebody help !!!
I have a 920 Mk2A Commando that was fitted with a single Amal Mk2 Concentric 36mm - starting was always a pig thus I fitted a 34mm version 25 Pilot Jet, 50 cold start jet. The bike generally starts OK but then spits back and misfires at just off idle. I upped the pilot jet to a 35 with no real improvement. I fitted a new Amal 'stay-up float' and found that the fuel level was about 7mm from the top of the float bowl which seemed low thus I adjusted it to achieve about 5mm depth however then the float fouled the underside of the main carb body and thus flooded. I refitted the original nylon float and this gave a fuel depth of 5mm but on refitting the carb it flooded (fuel peeing out of the two black vent tubes) - I fitted another spare nylon float and even a spare float bowl and needle valve (viton-tipped), blew out all jets with compressed air and still the same result (flooding). I checked over and over again that there is nothing on the underside of the main carb body fouling the float i.e. preventing the float closing the valve and can't find anything. I've spoken to the techy guys at Burlen and they are also completeley 'stumped'. The best I seem to be able to achieve is to refit the stay-up float - it doesn't flood but the bike will hardly run at all - i.e. it's unrideable !!!!



Any ideas ?



Cheers.

Mark, I am not actually helping but comiserating with you.

I fitted two MK2s to my Norton in April. Started OK, but spat back a lot just off idle. Also in the many times they have been in bits and had different parts/adjustments made, the left hand carb has flooded possibly in excess of 12 times when put back on, whereas the right hand carb, which has had the same adjustments made has never flooded. I asked Amal about this and they said, you'll have a bit of dirt trapped.....but I have not!

Put the original carbs back on, and it runs fine.

Am going to Spain in ten minutes on Norton, I will deal with this issue when I get back.

Just thought I'd let you know that I have had very similar probs, and it's a real P****r. :twisted:

I've spent a lot of time playing with MK2's and am just about sorted with them after a very long time.

A couple of pearls of wisdom-

Take a look at the float valve seat and compare it to a MK1. What you'll see is that the the MK2 has a bigger valve area than the Mk1. The problem is that as standard they use the same float. The result is that when you bounce the Mk2 around, and Commando's do a lot more shaking than most bikes, there's a much greater tendency for the Mk2 float valve to unseat and flood the carb. Answer is to fit the larger floats 622/196.

The next one is slides. There are 3 types - plain aluminium, ptfe/anodised aluminium and chrome plated brass. Both aluminium types are no good in my experience. The fit of them is poor and doesn't give an adequate shut off of the air path in the fully closed position which means the pilot jet doesn't control well as its swamped by the influence of the slide.
Fit the chrome brass type 2928/170.

I know from personal experience that without both the above you can spend a lot of fruitless hours trying to dial in Mk2's

RichJ is right about this. I would also add that the air bushing has to go and a rubber band dampener must take some of the carb bodies wieght.

Hi Chaps - thanks for the advice - I have dual springs helping to support the weight of the carb (in intake tube used to 'give-way' too regularly without them). One question to Norbsa - when you say "the air bushing has to go " what are you refering to ?

Cheers,

On this site: http://www.hitchcocksmotorcycles.com/ Than go to the top, click on Amal than on mark 2 than on the big size than see item 41. It’s a bushing that lives behind the bell mouth that is screwed down to the carburetor body. It restricts the air flow in the idle circuit. It may have been taken out all ready it’s just a loose slip fit. Stopping the rush of fuel at shut down seems to be a bigger problem than enriching the idle circuit. Hence the two cycle larger floats less gas to flood it out.

I've met quite few people who have tried MK2's and given up in the end. Les Emery at Norvil apparently stopped selling them because he couldn't get them to work well.

I've persevered mainly because I like a challenge or alternatively I'm too stupid to know when to quit.

I'm not 100% convinced about removing the air jet. I tried that when I was using aluminium slides and thought it gave some improvement but after sussing the aluminium/chrome slide problem I put the air jet back in to get things back to a more standard setup, too many variables being played with at the same time just adds to the confusion.
The air jet is still fitted and everything is pretty close to where I want to be, dropped the pilot to 20 recently and just need to go round the loop of slides and needle one more time.

I'm back from Spain (see my earlier post) well actually about 3 months ago, but have just got around to investigating the poor running of the Amal MK2s that were fitted. Despite swapping a No. 3 slide for a 2 1/2, and altering the needle position several times, the running was so poor that I reinstated the Amal MK1s for the Spain trip.

mwoo wrote;

The bike generally starts OK but then spits back and misfires at just off idle.

Click to expand...

Basically, I too could not stop the engine spitting back when going from a nice reliable tickover to getting the Norton moving, and it would continue to spit back ocassionally whilst running along at about 50 mph in the cruise (about 1/4 throttle) and the plugs suggested that it was running very weak, which was of concern, despite putting the needle in its highest (richest) position.

A couple of months ago whilst ordering a new sprag clutch, I asked Mick Hemmings about my carb probs, and he suggested swapping the location of the pilot jet from the float bowl to the carb body, the usually preferred position for 2 strokes. I have done this and the spitting back problem which was exceptionally annoying to say the least, has gone. It was still running a bit weak (white plugs) when cruising at about 1/4 throttle, and so I have gone from a 106 needle jet with the needle at richest setting to 107 needle jet with the needle in the middle position. So I am assuming that for my Commando, moving the pilot jet position has been "the fix" for the spitting back just off idle problem.

Now it is running well at this (approx 1/4) throttle opening, and throughout the rev range, and the plugs although on the "light" side are more than acceptable now.

There is one anomally though which I assume is caused by the spark plug heat rating, and that is that I have run the engine with two types of plugs, Champion N7YC and NGK BPR7ETX, and what I have noticed is that the Champion plugs always show a shade darker than the NGKs when I do plug chops!