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Hi all this is my first post after visiting this site often for lots of great info! I was lucky enough to be given my grand father’s 72 t120 back in 2008. My uncle gave it to me in pieces with a couple service manuals with a “good luck, you got lots to learn”. He was right I’m always learning new things and love it! It came to me with just under 5000 original miles and after rebuilding it I put another 5000 miles on it. Last autumn I imported it to Belgium after it sat in my parents garage for 5 years. Now I’m trying to bring it back to life. It runs good as far as I can tell and starts easily, one or two kicks. I had new clutch plates installed before I sent it over and I just picked up a set of new AMAL premiers. My question is about the pilot air screw adjustment. With the old carbs I would always turn out the screws 1 1/2 turns and that very close but with these it barely starts with stuttering and spiting and wants to stall. It runs good at half a turn out maybe a little more. Does this sound strange to anyone? I’m running screened velocity stacks with 190 main jets. The bike came with 180 mains and these worked great before when I was riding around nyc and philly before I moved to Belgium and stored the bike. The exhaust are the 1 1/2” earlier model type with stock 72 mufflers. The head was drilled and tapped to take the thread in spigots instead of the 1 3/8” push in pipes. Timing is good, valves are set and tank is sparkling clean inside. I just replaced the wiring harness but that’s another post in itself but I don’t think there’s a relationship there...? But I don’t know.
Thanks for any advice in advance!
Jamie


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Hey Jamie, I just got my bike running again after 5 of sitting as well. Are you letting your engine warm up fully before letting it idle on it's own? Are you using a choke when starting?

I have the chokes in my carb, but never use them, but it does take a little while to warm up before it'll idle on it's own. I noticed if you set the idle when the motor is cold, once it does warm up it'll idle pretty fast. And 1 1/2 turns out on the air mixture screw is the conventional wisdom for a good starting point for further adjustments. I've found Bushman's carb tuning secrets to be a good resource for dialing in a bike with twin amals. Here's a link. http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html

I'd give everything a once over, check that your cables haven't gotten out of adjustment, height of the slides in the carb bodies etc. I found after sitting so long things needed a little bit of tweaking after cleaning the carbs. Good luck.

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Have a check to see what size pilot jet is fitted to your bike, it’s most likely the #17 which is 2 identification rings near the tip at the business end. If this is the case you want to try a #19 which is the next size up, As a rule of thumb a jet size is Is equal to about 1/2 to 3/4 turn out. So it should put you about right on the mark (1 1/2 turns +/- 1/2 a turn)

I’ve just done this with mine (A65) tonight, one thing I did notice is changing the pilot jet has also influenced other areas of the mixture, so my next job will be to try lowering the needle as it’s now started running rich at 1/4 throttle. The pilot jet is effective throughout the full range so you might eventually find yourself revering to that 180 main jet, unless your elevation previously was above 3000 feet where in Belgium I would think that you wouldn’t be that high.


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68’ A65 Lightning “clubman”
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Thanks for the replies! Cory, I use the bushman’s carb tuning guide too and it’s always worked in the past. Cables are good and seem to lift off the Throttle stops at the same time. I am not using the chokes and have the cable holes blanked off. I guess I was assuming that the new premier carbs would be set up exactly like the originals. I went for a ride the other day 40 miles or so on country roads and the spark plugs were showing signs of richness. Not wet or oily but mostly black definitely not tan or brown. I will check which pilots I have. Is it wrong to assume that the pilots would be much different between the originals and the new premiers? The floats seem to be set right. The needle is set in the lowest position (top groove) and they are 106 as was the originals. Plugs are nc3 like originally set up. But I still don’t understand why the adjustment of the pilot would be so different from the old carbs 1 1/2 turns vs 1/2 turn out is very different no?
Thanks again for all the help
I hope I can contribute one day on not just the receiving end!


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I've just fitted a new premier carb to a mates T100C, and also found it ran best with the mixture screw about 1/2 turn out.

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Hi Jamie, The premier carb looks similar to original. However, there are many differences when you examine them closely. The 2 small holes on throat floor are larger. Bowl is built differently. Float level on premier is not .080" below gasket surface. It is just above & parallel with gasket surface. However the actual liquid level remains unchanged should you test with manometer should you have one.

Sellers often sell what they have, not what you had. Many sell a "ready to tune" carb. See AMAL web site for details.

As was stated the old pilot jet was .017. However the new Premier seems to do much better with .019. As was stated .017 has 2 rings, .019 has 3 rings machined into snout of jet. At the same time you have jets out. Remove drain plug in bowl & shoot some carb cleaner spray into the bore where jet goes. Let the cleaner flush back any dirt, casting flash back into bowl & out drain. Grease the jet o-ring, Helps assembly. They can cut or tear. Deburr bore edge if needed. Flush jet also.

I've worked with several Premiers now. I find the factory does a good job setting the jets. Needle clip you decide & set when you install cable. The main jet, slide is a crap shoot as it depended on how seller ordered carb from AMAL.

If the idle screw is too deep like 1/2 turn it skews air flow through idle bore & often causes odd things to happen when bike is warmed up as well. Especially at lower speeds & stop signs, stop lights. The mixture cannot be made ideal in all situations.

Removing choke won't make difference one way or the other since you blocked the cable holes. It will not effect actual starting much, but choke helps you to use more throttle without hesitating during warm up.

Velocity stacks with screens can dramatically alter air flow. So expect to have to tune your new carbs. If bike ran good on old carbs. You may want to examine old jets & slides (except pilot jet), clip position. Start there.
s
Bushmans guide is good. John Healy's is good also. Maybe better. IN EVERY CASE mark your grip in 1/8s. Critical to know where the slide is.


https://static1.squarespace.com/sta...2/1481956877861/Amal+MK2+Carb+Manual.pdf

Print & take in pocket. Try to find a decided road test to see how bike runs in all conditions. Use same road test during tuning so you can feel changes more readily.

Also verify battery voltage & charging system are good. Low voltage will make bike run oddly.
Don


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Wow thanks Don I really appreciate your response! I will definitely print out John Healy’s guide and follow through with these steps and report back how I’m doing. What a great resource this forum is! And thanks Tbird649 for sharing your experience as well! The adventure continues!!!
Jamie


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Quote
Cables are good and seem to lift off the Throttle stops at the same time. I am not using the chokes and have the cable holes blanke

but where are the throttle stops set ?
the cables have to be slack enough so that the slides rest on the stops .

and the stops are set at ... 3/4 turn of lift ... off fully bottomed slides
this is the stops initial position ... to fully engage the idle circuit .

it can take some hunting and pecking to establish 3/4 turn
.. because its from throttle slide movement ( lift-off ) .

your slide travel stops may be set to high ?
or
the idle jets are clogged ?

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To synch and set idle mix on a twin with a balance pipe between the inlet tracts is difficult, the balance pipe confuses accurate settings. I have tried to do this with the balance pipe connected and got very confusing results, unacceptable.

Treat like two singles, first disconnect the inlet balance pipe on one side, plug the open end of a pipe with a suitable screw , make a temporary pipe with another blanked end and fit to the stub where you pulled off the original BP, now the motor should run without one side affecting the other.
Put a plug spanner, small clamp and screwdriver in yer pocket, go for a ten mile ride to get the motor properly warmed through, stop at a convenient quiet spot .

Remove a spark plug , either side doesnt matter, clamp the plug and lead to the head so it can still earth/ spark.

Start the motor, it will now run on one cylinder, find the optimum idle by raising the throttle stop on the running side a 1/2 turn or so then adjusting the pilot mix . Screw out until speed rises then falters, screw in till speed falters, note the range and set pilot mix middway, it should be right when the screw is set a tad richer than max idle speed setting. Use the tacho to set idle at 950 rpm with the throttle stop screw.
Repeat for the other side.
When both sides are set, with both plugs fitted it will now idle too high, drop the throttle stop screws an equal amount each side to around 1,000 rpm.

Remove the balance pipe blanks and refit so the inlets are linked.

Thats the idle set perfect.

Now synch the cables so both slides lift at exactly the same moment, if you have a manometer/ balance gauge , use it, if you dont put a finger in each inlet bell and touch the slides , have someone open the throttle gently, you will feel one slide lift before the other, take up cable slack on the lazy side, repeat until both are dead on.

Thats it done. make sure there is a little slack in the throttle cables, maybe a mm or two .

The method above gives me best mpgs and clean pickup.

Velocity stacks with screens will mess up the main jet setting,the screens are remarkably restrictive, I strongly recommend the original air box and filter set up.

Last edited by gavin eisler; 07/22/20 10:22 am.

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