Amal Concentric Mk2 on a Commando

Hi Mark,

I had a similar problem getting mine to run although mine is a 750 and the carb is a 32mm Mk2 bored out to 33mm. I think your slide is too low (ie no air getting past it) so try opening it up on the throttle stop and once running, adjust it back down once it is warmed up. I recall my pilot jet is in the float bowl and the position under the body is blanked off with a plug. Starting was good but with velocity stack type bell mouth (no air filter) I had some sticking thottle problems (probably icing)

Yours Dave Evans

Also Mark check out this web page

http://www.hitchcocksmotorcycles.com/amal/mk2_conc_ht.html

For some detailed info on pilot jets

Dave E

Hello Mark,

This is a frustrating situation for you. Also, there may be other factors contributing to it. For example: What cam is fitted andhave you confirmed that it is timed & lifting correctly? Also, seeing a good spark with the plug against the head doesn't necessarily mean it's sparking well under compression (despite what eBay sellers say, when flogging an old magneto!)

I'm tempted to guess that the fuel level in your float chamber is low. The last time my domi had the same symptoms, it turned out to be the float needle sometimes sticking shut.People thought I was firing a cannon inthe Mallory paddock! There is some useful information on setting up a Mk2 concentric, including setting the float height (albeit 'statically' - I think modifying a float drain plug for a U-tube would be better!) at:

http://www.knucklebusterinc.com/tech/AmalMK11Tuning1.pdf

That's my two penn'orth!

Paul Wilson

Mark,

The suggestions I made in my last message were regarding the problems with the 34mm carb.

What is the jetting on the 36mm unit? When 'over-carbed', a motor tends only to be happy flat-out. I'm trying to imagine how your 920 would cope with a (nearly 1 3/8") choke. I would imagine the carb would have to be 'over-jetted', in order to match the airflow at higher revs, giving you the richness at lower revs. Again, the float level needs to be right to begin with...

That makes 2.5 penn'orth!

Paul

OK Mark,

In the mean time, I thought of something else today, which will be worth looking out for.

I would recommend confirming that your fuel delivery, from the tank to the carb is not compromised. What brought this to my mind is what happened when I went to check out a (single carb) 650 a few years ago. I had been primed by the owner that it was in good shape, ran well and would behard to fault (with the old chestnut that he'd been offered close to his - high - asking price already). There's no point in going into what a 'toilet' that bike was; but the type of flannel the guy used is very common these days.

I thought I may as well hear it run and the owner set about his demonstration. After a fair bit of popping & banging, the guy stood for a good 30 seconds with his thumb on the tickler. Eventually the bowl was flooded & it started (rattle, rattle, clatter...blimey!).It wasn't long before the popping returned, along with flashes from the carb. Finally, one good bang and the carb was on fire! Lots of rags got it out, and that was that.The fuel tap had only been allowing a trickle of petrol through. I never bothered to find out why.

2 1/2 hrs isalong drive for a short fireworks display!

Paul

Mark

It sometimes pays to look beyond the obvious when trying to sort out this kind of problem.

The fact that your engine starts absolutely fine when cold would suggest to me that there is not a problem with either the fuel level in the carb or the flow to it. Fitting a big carb or high lift camshaft cancause a starting problem but this generallyaffects both acold and hot engine.

A hot engine that refuses to fire up again easily is probably having a fuel mixture issue or a sparking problem. I would be looking at the possibility of air getting into the system somewhere between the carb and the cylinder head. Once the cylinder head has got really hot, the heat transfers quite quickly in the direction of the carb withtwo effects.

First the heat vapourises off a lot of the fuel in any fresh incoming charge, leaving the plugs trying to fire up an air/fuel mix that is too weak to ignite. Check that you have good insulating spacers between the manifold and head. They need to be at least 2.5mm thick.

Second possibility is that of leaks in the manifold. Very common in Commando engines due to the shaking they give the carbs. Also the engine heat will make the tiniest gaps expand from pinhole intotunnel size.Check all the joints carefully for air gaps and don't rely on O-rings for a good seal. Use a little Silicon RTV on all the face joints.

My bike had a similar problem and would not fire up when hot.I wrappeda rag soaked in cold water around the mainfold for 5 minutes and and the engine started easily. I changed theheat insulators for Norton items, raised the main jet by 20 andapplied some Siliconto all the joints. Problemsolved.

Finally. Remember that some electrical components don't like getting hot either. Condensors especially. Check these items out if you have any in your circuitry.

Hello I will up my pennyworth in two Phill talks some good sense. I have seen these Commandos fitted with SU or Zenith Carbs as they are variable jet 34mm. Once fitted these bike can do up to 70 mpg -that's good long distance riding.

Hello Mark,

I hope you have made some progress with getting your 920 running better,

over the weekend.

May I ask a favour?

I can only speak for myself; but it would help me a lot when keeping track of this thread,

that you make it clear which carb is being discussed.

I didnât do that myself, in my first message.

I managed that all by myself!

Now, I think there is a danger of that spreading between posts.

I wonât need a title at the beginning of every sentence but just enough to avoid confusion

(mine, in particular!).

In your first post, it was completely relevant to give the story of the two carbs.

I think you are going to continue this scenario by trying to set everything up with the 34mm

carb?

The one which has only produced the pops & bangs.

Good luck!

Paul

Previously wrote:

.......................I must say that I'm not entirely convinced by the claim that the Mk2 cold-start system is completely separate from the pilot jet etc. as the pilot circuit is operative at all times (i.e. not isolated when the cold-start lever is engaged). This seems apparent by the fact that adjustment of the pilot airscrew does affect cold starting. Amal documents also refer to different locations for the pilot jet - they recommend it is fitted in the float bowl for 4 strokes and any application with down-draft mounting and recommend it is fitted into the underside of the mixing chamber for 2 strokes. They offer no explanation - has anybody any ideas ?

Good on you Mark.

Well sorted!

I canât answer anything specific to MK2 concentrics, never seen one.

Try calling Surrey Cycles.

The last time I heard of someone using those Colour-Tune things, a bloke bought one and set about tinkering with the S.U. carbs on his MGC (must have been around 1978).

He reckoned heâd got them right and set off for a weekend at the coast.

Came back on a breakdown truck with burned out exhaust valves.

We all âHO-HO-HOâd about Colour-Tune for a few weeks.

Then he came âround and said heâd just had a medical for a new job and they told him he is colour-blind.........!

Paul