Amal mk2 tuning

Bernhard said:

Old Bloke said:

My 850 mk1 is being a witch to start after its rebuild, maybe it was before, I don't remember!


The symptoms are that even when cold, it does not seem to like the cold start lever on, and will start (after a dose of kicking) without it on, then idle ok. An observation is that the air screw is screwed nearly all the way in(less than 1/2 turn out) to get stable tickover, does that indicate that it needs a bigger or smaller Pilot jet, it has a "15" fitted at the moment?
I'll not mention the smoking engine when it gets hot yet

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Re; less than ½ turn out.
Most of these types of carbs work best when the pilot air screw is around 1/12 turns out. (+ or - 1/4 turn)
½ turn out suggest it too rich if the mixture is correct, could the pilot jet in the carb have dropped out :?:

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No, pilot jet is in place in the float bowl(had to remove it to check size which was 15), and blanking plug is in place in the carb body in the alternative position. Concensus earlier seemed to say try a bigger pilot jet, guess this provides more fuel, thus means pilot air screw will be unscrewed further to provide more air to balance mixture. I've got the replacement size 25 jet delivered now so hopefully be able to report progress tomoorow. Thanks.

15 on a single carb twin is far too low a pilot jet size.
This size doesn’t need almost any air for the engine vacuum to suck pilot jet mixture into the engine.
Hence my confusion on assuming there was no jet.
You should try the 25 and see if it will run at 1 1/2 turns out.

There also appears to be a smoking issue :?:

Bit of an update. Replaced the 15 pilot jet with a 25. Now idles reliably with the pilot air screw about 1.5 turns out.

Wouldn't say it has made starting any easier though, despite trying the "keep the throttle closed" technique, guess I will be looking at the float level next

The smoking thing, hmmm. No smoke at start up, after a couple of minutes in the garage, for example doing the timing, both exhausts start to smoke pretty constantly. I've had a good look through the forum, and that could be a few things, most likely Intake valve seals? I had a look in the intake valve chamber, and you can see a puddle of oil remains between the valves. Drain hole is clear, and I tried the draining, by tipping a few CC's of oil into the chamber(bike not running) and making sure it drained away. It does, but takes a couple of minutes. Who knows if that is normal? Rocker spindles are in the right way, and intake seals are, or at least were fitted, the last time the bike was run some years ago.
I see this subject has been done to death previously on the forum, so I'll consider what to do next, hopefully not to have to remove the head(yes seen the Ludwig and rope trick methods).

Was difficult starting the reason that you rebuilt the bike ? I cannot understand how you suddenly came to have the wrong pilot jet fitted. Are you using new spark plugs or the ones which have been sitting in the motor for years covered with carbon ? I use two 34mm Amal Mk2 carbs with methanol fuel, and I've never found a four stroke that needed to be started with the throttle closed - only ever two strokes. If your bike is jetted correctly, you should have to use the choke to start it when the motor is cold. The smoking problem probably has to do with the amount of oil in the crankcase or the rings are stuck in the grooves in the pistons. If you ride the bike and it smokes more after the throttle has been closed on the over-run, and then suddenly opened, I'd suspect the valve seals and the guide clearance. How old is your bike ?

bike rebuild wasn't done strictly for engine problems, it was running before it's layup, more for frame paint, new iso's etc, but it did have a leaking head gasket at the front of the engine, so head was removed and new gasket fitted. As far as I am aware, the pilot jet has been in the carb since I have had it, but I don't remember having such difficulty starting it before it's layup. Bike is a 73, but I don't know when it had rings, or any barrel work.
Is it possible that the valve seals would fail after being lying for several years, gone hard maybe?

Old Bloke said:

bike rebuild wasn't done strictly for engine problems, it was running before it's layup, more for frame paint, new iso's etc, but it did have a leaking head gasket at the front of the engine, so head was removed and new gasket fitted. As far as I am aware, the pilot jet has been in the carb since I have had it, but I don't remember having such difficulty starting it before it's layup. Bike is a 73, but I don't know when it had rings, or any barrel work.
Is it possible that the valve seals would fail after being lying for several years, gone hard maybe?

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It seems someone has misdiagnosed the rich /sooty spark plug problem by fitting a 15 pilot jet.
1, Do you have an air filter on the carb :?:
2, Is the carb slide /body worn, i.e. can you rattle the slide to and fro with your finger when closed, and half open :?:

Bernhard said:

Old Bloke said:

bike rebuild wasn't done strictly for engine problems, it was running before it's layup, more for frame paint, new iso's etc, but it did have a leaking head gasket at the front of the engine, so head was removed and new gasket fitted. As far as I am aware, the pilot jet has been in the carb since I have had it, but I don't remember having such difficulty starting it before it's layup. Bike is a 73, but I don't know when it had rings, or any barrel work.
Is it possible that the valve seals would fail after being lying for several years, gone hard maybe?

Click to expand...

It seems someone has misdiagnosed the rich /sooty spark plug problem by fitting a 15 pilot jet.
1, Do you have an air filter on the carb :?:
2, Is the carb slide /body worn, i.e. can you rattle the slide to and fro with your finger when closed, and half open :?:

Click to expand...

I do have an airfilter on the carb, it's an aftermarket round K&N type, the one supplied with the RGM kit. The carb slide is in very good condition, it's a chrome plated brass item. The carb body is also in good condition, and is a good sliding fit with the slide.

These are some good threads on the Mk2 Concentric. I don't know if you've mentioned what size carb you're using. If it's a 36mm carb then it could be that balking goes with the territory using a bigger carb. If it's a 34mm and the manifold and other hardware could accept a Mikuni then you might consider a swap. Just the price of a chromed slide, jets, and gasket set would cover the price of a new Mikuni.

amal-mk2-concentric-t6854.html

http://www.nortonownersclub.org/noc-cha ... /195560758

http://www.nortonownersclub.org/noc-cha ... /103233081

http://www.britbike.com/forums/ubbthrea ... ber=322384

rpatton said:

These are some good threads on the Mk2 Concentric. I don't know if you've mentioned what size carb you're using. If it's a 36mm carb then it could be that balking goes with the territory using a bigger carb. If it's a 34mm and the manifold and other hardware could accept a Mikuni then you might consider a swap. Just the price of a chromed slide, jets, and gasket set would cover the price of a new Mikuni.

amal-mk2-concentric-t6854.html

http://www.nortonownersclub.org/noc-cha ... /195560758

http://www.nortonownersclub.org/noc-cha ... /103233081

http://www.britbike.com/forums/ubbthrea ... ber=322384

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Thanks for the links. A couple I had not read before. my carb is a 34mm, and already has the chromed slide, so I'd prefer not to have to buy a new carb just yet

Old Bloke said:

Bernhard said:

Old Bloke said:

bike rebuild wasn't done strictly for engine problems, it was running before it's layup, more for frame paint, new iso's etc, but it did have a leaking head gasket at the front of the engine, so head was removed and new gasket fitted. As far as I am aware, the pilot jet has been in the carb since I have had it, but I don't remember having such difficulty starting it before it's layup. Bike is a 73, but I don't know when it had rings, or any barrel work.
Is it possible that the valve seals would fail after being lying for several years, gone hard maybe?

Click to expand...

It seems someone has misdiagnosed the rich /sooty spark plug problem by fitting a 15 pilot jet.
1, Do you have an air filter on the carb :?:
2, Is the carb slide /body worn, i.e. can you rattle the slide to and fro with your finger when closed, and half open :?:

Click to expand...

I do have an airfilter on the carb, it's an aftermarket round K&N type, the one supplied with the RGM kit. The carb slide is in very good condition, it's a chrome plated brass item. The carb body is also in good condition, and is a good sliding fit with the slide.

Click to expand...

This is a shot in the dark, try running without the air filter and see if the sooty plugs still perish and it gets easier to start when hot.

Bernhard said:

This is a shot in the dark, try running without the air filter and see if the sooty plugs still perish and it gets easier to start when hot.

Click to expand...

I can do that, willing to give anything a try. Incidentally, I can't remember saying I had "sooty plugs" although I probably have!

Old Bloke said:

Thanks for the links. A couple I had not read before. my carb is a 34mm, and already has the chromed slide, so I'd prefer not to have to buy a new carb just yet

Click to expand...

It has a chrome slide, but it sounds like it might be the wrong one. New slide and new stay up float kit = price of a new carb, (jetted). :shock:

Ah, I see what you mean now, doh. If a mikuni cost the same as an Amal chromed slide and a float, then I might consider that.

Old Bloke said:

If a mikuni cost the same as an Amal chromed slide and a float, then I might consider that.

Click to expand...

It hurts to think of running a Jap part on our British Iron, but it's hard to argue with success. Mikunis are easy to tune in that there's all kinds of jets/nozzles/needles/slides for them, and they don't tend to wear like the Amals. At 9700 miles, my old girl is to the point where I carry a small screwdriver to correct for LSS (Loose Slide Syndrome), so a Mikuni conversion is something for me to ponder over the winter months. It's embarassing to have to turn the idle screws whilst sitting in traffic. Sigh...