34mm Amals on Standard 850

Anyone on the forum running 34mm twin Amal Mk2's on a standard 1973 850 and if so what would your needle clip, jetting and slide sizes be?
regards Mike

I have only used methanol with my 34mm Amals on my 850. If I was using petrol, I would start with the same slide cutaway, needle jet and needles with the same taper as those you normally use with Amal concentrics. I.e #3 cutaway, 0.106 or 0.107 needle jets - the needles are probably a different length for the bigger carbs. But when you buy the carbs, they should come with needles. Any petrol needles will work. The needle taper is not important unless your ports are too big. If you start with the clips in the centre groove - ride the bike and if it coughs, raise the needles one groove at a time until you can open the throttle without getting a cough.
You need to be aware of which jets affect the three stages of throttle opening. Good luck, but you should not need it. Be systematic and keep thinking.
What jetting works well on somebody else's motor will not necessarily work well on your's. Comp.ratio, fuel octane rating and ignition timing and your exhaust, affect the jetting.

Does that mean your motor is a standard 850 or a hotter than standard race motor?

My motor is not far removed from standard. The cam is standard but with a little more lift and I run it 12 degrees ahead of normal. to get more torque. Even with the methanol fuel, my 2 into one exhaust system runs vey hot and loud. WIth methanol, I also run the ignition advance 4 degrees ahead of the petrol setting. My jetting is as lean as possible right down the needles, but without getting the cough. My main jets are a bit too rich, but that does not matter on smaller circuits.
What I find with the 6 D Mikuni needles, is I cannot whack the throttle open - I need to feed it on. But that way I gat the quickest acceleration rate.
With petrol as fuel, the story might be a bit different. Petrol has a lower latent heat of vaporisation than methanol. A big carb might have more effect with methanol, than with petrol. - the temperature effect ?
What has really amazed me is that with such a low level of engine mods, my 850 is competitive against much hotter bikes.

I don't know how many different taper needles Amal make for their carbs. But I think the Japanese have always been ahead of them. With the needle jets, the slightest bit oversize makes a very big difference. And that is with methanol - petrol is twice as critical. If you are that bit too rich, the bike still goes OK, but it might be much slower than it could be. If you look at the manuals for Japanese bikes of the 70s, every model has different taper needles. They must have put a lot of work into getting them tuned right

I have a single mk2 Amal on my 850. 3 1/2 slide, 20 pilot (between 1 1/4 and 1 3/4 turns out on the air screw), .106 needle jet 2A1 needle on middle groove, 280 main jet. But only it's a single carb, so may run different to twin? Also a K&N filter. Tickover is a bit lumpy at 1200rpm, but it rides smooth and plenty of pull.

The taper on the needles is there to compensate for the loss of vacuum as you open the throttle.. The needles in speedway bikes which run on methanol, usually have rapid tapers. But those bikes are ridden differently to road race bikes. A bigger port size means the vacuum in the carb is likely to be less as you open the throttle, so the taper on the needles needs to be quicker. However the slightest bit too rich at any throttle opening, means significant loss of power. I use the slowest taper needles and feed the throttle on in a controlled fashion.

The only thing I had to change from the as bought settings on my twin 34 mk2 Amals was the pilot jets from #25 to#20. And later the main jets after I fitted a S3 cam.

Yes, I tried a 15, 25, then 22, to see if it evened out my idle. Ended up back at the 20, which gives me a sweet spot in the recommended range of adjustment of the air screw. I live with the lumpy idle.

Mart UK said:

Yes, I tried a 15, 25, then 22, to see if it evened out my idle. Ended up back at the 20, which gives me a sweet spot in the recommended range of adjustment of the air screw. I live with the lumpy idle.

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What ignition do you have?

Boyer, one of the newer versions (2013).

Boyer MkIV did make my motor idle on the lumpy side. The TriSpark classic twin with the same carburetion idles smoother on the same motor. I do not suggest changing ignition for a smoother idle. I could tell you why, but then I'd have to explain myself, and that is just asking for trouble here without a dyno readout charts or graphs.

If you have had the experience of jetting a two-stroke for methanol, jetting a Commando motor is easy. Methanol fuel makes jetting a four-stroke easier because the jets flow twice as much fuel as they do with petrol, so the errors in jet sizes have less effect. As a percentage the errors are much smaller, and methanol can be run slightly rich without losing as much power. With a Commando engine on petrol, the two sizes of needle jet are 0.106 and 0.107 inch. If you use methanol at10 to one comp and 4 degrees further ignition advance., the two sizes of needle jet become 0.116 and 0.117. With a 0.117 needle jet and the slow taper needles, I could not get my motor to cough by lowering the needles. So I made another two needle jets out of brass hex using number and metric drills to get a half size. My needle jets are now 0.1165 inch in internal diameter. If I lower the needles one notch I get the cough.
I think this would be almost impossible to do if I was using petrol as fuel. I have never played around with the air jets. But if you think about it - on the longest straight on most race circuits, if you have a 4 speed close ratio box with high overall gearing, there are usually two gear changes in the straight. My motor runs best between 5,500 and 7000 RPM. In effect you mainly use the area of throttle opening in which the jetting is controlled by the last one-third of the needles. You probably don't use the main jets as much as you might think.
With methanol fuel, that half of one thou of an inch in needle jet size is the difference between fast and slow. With petrol, the situation is twice as bad. You would need needle jet sizes in quarters of one thou.

In the 1950s, a few Australian riders road-raced in Europe. They found the Brits in Europe could get their Manx Nortons going as fast using pool petrol, as our riders could in Austraiia using methanol fuel.
I was around when the 250cc TD1C Yamahas began beating the 500cc Manx Nortons. The Yamahas used petrol. Methanol in a two-stroke is something else.

Brooking 850 said:

Anyone on the forum running 34mm twin Amal Mk2's on a standard 1973 850 and if so what would your needle clip, jetting and slide sizes be?
regards Mike

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Looks like that is a no then.

storm42, Looks like it and to stop any off thread topics, I should of stated that is a road bike on petrol!!!
I have plenty of carbs to choose from , just that I have a brand new set of 34's I would like to try.
I have a set of 32 Concentric Mk 1 Premiers with a single pull gantry that came off my race bike (now has 34. mm Smoothbores) and ran that really well with a RH10 head , so have them to go to if need be.

When fitting any different carb, you always need to start at the beginning and work to the end to suit the motor mods and exhaust system. Somebody else's numbers might put you in the ball park but getting best performance is up to you.

Brooking, I'm interested in your outcome as I have some Mk2's sitting around.
Let us know how it turns out..

MichaelB yes will do

Brooking 850 said:

storm42, Looks like it and to stop any off thread topics, I should of stated that is a road bike on petrol!!!
I have plenty of carbs to choose from , just that I have a brand new set of 34's I would like to try.
I have a set of 32 Concentric Mk 1 Premiers with a single pull gantry that came off my race bike (now has 34. mm Smoothbores) and ran that really well with a RH10 head , so have them to go to if need be.

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I had a Sudco Norton kit with 34mm Mikuni carbonators on a 750. Very irrelevant.

I agree with Al. Too many variables to bother telling somebody what works on a different motor. The topic has probably been discussed many times. Search might return a few hits that would give you what you are looking for.

Excuse my Boyer ignition comment above. I was confused. I though Mark started this thread, and he said something about a lumpy idle and a Boyer.