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Need help with Amal carbs #773049 05/08/19 6:58 pm
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johnpl Offline OP
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I am the owner of a 1966 T 120. I just recently got the old boy running again but I am having a problem with my lack of knowledge when it comes to tuning. I can only go about 50-100 miles and my new spark plugs are a dark sooty black. I remove and clean them only to have the same problem reoccur. I am currently using NGK B7ES. Seems to be running rich. Is this adjustment controlled by moving the needle on the slide? Any suggestions would be great,

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Re: Need help with Amal carbs [Re: johnpl] #773051 05/08/19 7:14 pm
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kevin roberts Offline
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B7ESs are a bit hot, in my experience. i use B8ES. but that shouldn't be the problem. put a timing light on the spark plug wires, doesn't matter about timing, just watch the flash to see if it's sparking more or less reliably. if it is, then look at the carbs to see whether it's too rich.

you can adjust the mixture in several place on the carb-- idle screw setting, needle position, needle jet, needle, and main jet.

the 63-70 workshop manual says this:

[Linked Image]

is this what you're running?

how old are the needle jets? if they're not new, throw them out. i'd get new needles too.



every day you do not take a chance is a day of your life that you will never get back.
Re: Need help with Amal carbs [Re: johnpl] #773094 05/09/19 1:04 am
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Tridentman Offline
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Do you have chokes fitted?
If so--after warming up---is the choke lever in a cable taut condition?

Re: Need help with Amal carbs [Re: johnpl] #773106 05/09/19 3:34 am
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Mark Z Offline
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Under normal riding, let's say, 1/4-3/4 throttle, your mixture is primarily controlled by the needle and needle jet, as you surmised. You should see what you have in there and which notch the needle clip is in, and compare with the factory settings.

If it's not something else, like ignition or choke setting, you could try dropping the needle a notch. Note also that needles and needle jets wear after many miles of use, causing an over-rich mixture, and may need to be replaced.

One way to isolate the idle mixture is to: 1. Start and warm up the engine, so that it can be restarted without choking or tickling. 2. Change out spark plugs for brand new ones. 3. Restart the engine and let it idle for a few minutes without touching the throttle. 4. Remove and look at the spark plugs. Idle mixture is controlled by the pilot jet size (if yours is removable) and air mixture screw.

Once having the idle mixture sorted, then look at plugs after a ride. If you have a place to do this, as you're cruising along, pull in the clutch, kill the ignition, and coast to a stop. If not, then just pull in as usual and don't let it idle long before killing the engine.




Mark Z

'65(lower)/'66(upper, wheels, front end, controls)/'67(seat, exhaust, fuel tank, headlamp)/'70(frame) A65 Bitsa.
Re: Need help with Amal carbs [Re: Mark Z] #773200 05/10/19 12:25 am
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johnpl Offline OP
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Sounds like a good plan. I do not have a choke on this bike. Any idea where the factory setting for the needle would be located (which notch), # 3 as listed above on this post?
As a follow up, I have not changed or replaced any parts except gaskets on these old amals. If I pull them off what parts would you recommend I replace?

Re: Need help with Amal carbs [Re: johnpl] #773204 05/10/19 3:00 am
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Mark Z Offline
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Originally Posted by johnpl
Sounds like a good plan. I do not have a choke on this bike. Any idea where the factory setting for the needle would be located (which notch), # 3 as listed above on this post?
As a follow up, I have not changed or replaced any parts except gaskets on these old amals. If I pull them off what parts would you recommend I replace?


The factory settings are as listed above by Kevin. Do you have Monoblocs or Concentrics? If Concentrics, the needle position is "2".

In general, carburetor parts other than gaskets and O-rings are not replaced unless there's a problem. Needle and needle jet wear depends on mileage. You can replace them out of hand if you like, but they might not need it.


Mark Z

'65(lower)/'66(upper, wheels, front end, controls)/'67(seat, exhaust, fuel tank, headlamp)/'70(frame) A65 Bitsa.
Re: Need help with Amal carbs [Re: johnpl] #773219 05/10/19 8:14 am
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R Moulding Offline
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Somethings to bare in mind. For starters your bike should be equipped with Monoblocs. If it has Concentrics fitted the factory setting will be different. Also if your bike is actually a 66 T120 it will be fitted with 389-203 carbs, these are inch and an eighth. If however it is a T120R supplied to JOMO it will be fitted with 389-95 carbs, these are inch and three sixteenths.

The only jet that tends to show signs of wear is the needle jet, this is recommended to be replaced every 12000 miles. Start with the needle jet.

Rod


So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth;
And pray that there's intelligent life somewhere out in space,
'Cause there's bugger all down here on Earth!
Re: Need help with Amal carbs [Re: johnpl] #773221 05/10/19 8:46 am
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gavin eisler Offline
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Sooty plugs and old AMAL Monoblocs.

Check in the float bowls.
Three good possibilities.
Punctured floats, remove and check .
Missing float spindle spacers, floats are held in position by a short brass collar which fits over the shafts.
Easy to leave out.
Poorly seated float needle valves, remove needle valves and replace with Al viton tipped type.

look at the fuel feed banjo unions, these should NOT have a fiber washer between union and carb body, ( modern repops do have this , but if fitted to old bodies causes over rich high fuel level), an easy mistake to make.

If all this checks out ok start looking at pilot air screw settings and needle clip heights.


71 Devimead A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
The poster formerly known as Pod
Re: Need help with Amal carbs [Re: johnpl] #773235 05/10/19 11:20 am
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Beach Offline
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Needle jet and needle as stated before. The jet can wear to a larger size causing a rich condition.


Bill
1974 Norton Commando
1966 Lightning
1965 Lightning Rocket
1966 Norton Atlas
1967 Norton Atlas
1948 Panhead
Re: Need help with Amal carbs [Re: kevin roberts] #775016 05/29/19 1:02 am
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johnpl Offline OP
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So I finally got around to pulling out the jets in the AMAL Monoblocs 0n my 66 and the main jet size is 200, not the 206 as listed above. Would this cause a rich condition? any suggestions on who to obtain AMAL parts from?

Re: Need help with Amal carbs [Re: johnpl] #775038 05/29/19 6:38 am
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Allan Gill Offline
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206 looks like a miss print as mainjets usually go up in tens, also 260 on a monoblock usually converts better to around 220 for a Concentric.

If your using a 200 then that would make it very lean instead. All good suggestions as above, check needle and needle jet and replace if history unknown (these are a service item!!) also check the float is “floating” and not holding fuel also that the float height is correct.


beerchug
Re: Need help with Amal carbs [Re: johnpl] #775113 05/30/19 12:27 am
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GeauxTwins Offline
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Did you check your air filters?
Happened to me!


Trophy 67, A65T 70, T100C 70, TR6R 71, Daytona 72
Re: Need help with Amal carbs [Re: GeauxTwins] #775119 05/30/19 3:13 am
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johnpl Offline OP
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I removed the air filters for this test. Plugs still sooty so I order replacement needles and needle jets today. Maybe I'll get lucky and solve this one!

Re: Need help with Amal carbs [Re: johnpl] #775132 05/30/19 9:30 am
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R Moulding Offline
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As Allan sugested a 206 main jet is a misprint. However you need to identify your carbs before you start buying jets. As I mentioned earlier two different Monoblocs were fitted in 66 depending on the market the bike was built for. If you have 389/203 carbs you should start with a 260 main. If you have 389/95 carbs you should start with a 330 main jet. Check the mounting flange of the carbs you will find the number stamped there.

The main jet is not currently causing your rich condition! The main jet is doing next to nothing until you reach 3/4 throttle and above.

Before you do anything else, identify your carbs so you can buy the right parts to return them to their factory settings. This will give you a good base line to start from. Then go to the AMAL website and download the tuning guide. Once you understand the process you will better understand the advise you are given.

Rod


So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth;
And pray that there's intelligent life somewhere out in space,
'Cause there's bugger all down here on Earth!
Re: Need help with Amal carbs [Re: R Moulding] #775157 05/30/19 3:41 pm
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johnpl Offline OP
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Thanks Rod. I checked this morning and the two carbs are stamped389/95. Now I am in the process of tracking down what the factory parts should be. My pilot jet and needle jet match the sizes shown above so I assume they were the correct items? (Pilot jet 25 and needle jet 106). The main jet I took out was marked 200. That seem quite a bit different than the 330 you mentioned.

Re: Need help with Amal carbs [Re: johnpl] #775160 05/30/19 4:14 pm
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Allan Gill Offline
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Rod, was the T120R originally with bell mouths? There’s usually a jump in jet side depending on filters or bell mouths.


beerchug
Re: Need help with Amal carbs [Re: johnpl] #775184 05/30/19 6:55 pm
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R Moulding Offline
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Hi Allan. It was actually the other way round in this case. The T120 came with bell mouths whilst the T120R had filters. The reason for the different jet sizes being that the T120R switched early in the season to using the same carbs as the TT bikes, these are the 389/95 carbs the OP has fitted and are inch and 3/16 instead of inch and 1/8 used on the T120.

When I took the same carbs off my 66 they were fitted with 220 mains. My guess is that as the needle jet wears and the bike starts to foul plugs the uninitiated instantly start reducing the size of the main jet. Probably because of advise from the expert in the pub.

When I took my motor apart it was 60 thou over and full off enough molten aluminium to cast a set of NOS pistons!

Pilot jet 25
Needle jet 106
Needle D
Throttle cut away 3.5 (reduced from 4 in factory bulletin)
Main jet 330

Rod


So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth;
And pray that there's intelligent life somewhere out in space,
'Cause there's bugger all down here on Earth!
Re: Need help with Amal carbs [Re: Allan Gill] #775185 05/30/19 7:00 pm
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johnpl Offline OP
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Rod, just finish spending two hours on line searching AMAL carbs for the factory settings. You were right on track according to the AMAL web site. I was amazed on much misinformation has been posted regarding this item.
I do have one additional question regarding the carb slides. I seem to have quite a bit of clearance when I move them side to side in the barrel. They are a lot looser than the slides on my 81 Yamaha's Mikuni's. What type of condition results from worn slides and/or mixing chamber? I really don't have any thing to compare this observation to. Any way to measure or check for excessive wear?

Re: Need help with Amal carbs [Re: R Moulding] #775187 05/30/19 7:23 pm
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johnpl Offline OP
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Rod, I can appreciate your post on reducing the size of the main jet to try and compensate for fouled plugs. I have to laugh at myself cuz that is the direction I was headed thanks to advise I received. (also at the pub)

Re: Need help with Amal carbs [Re: johnpl] #775190 05/30/19 8:05 pm
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Allan Gill Offline
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Loose slides will create a leaner condition, as more air can flow past the sides of the slide, there is less vacuum drawn on the jet, thus less fuel is drawn through the jet creating a leaner mixture.


beerchug
Re: Need help with Amal carbs [Re: R Moulding] #775193 05/30/19 8:35 pm
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johnpl Offline OP
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Hey Rod, did the 1966 T 120 TT come fitted with bell mouths? I have never seen a picture of one except maybe after market.

Re: Need help with Amal carbs [Re: johnpl] #775243 05/31/19 7:46 am
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R Moulding Offline
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66 TT bikes came with filters, according to the sometimes loose factory specs west coast bikes had the one piece black filter and east coast had the seperate pancake filters. This was the same for the T120R.

Wear in the mixing chamber and slide is very common, something to do with the two metals being too similar. The other common problem is the mounting flange being distorted from over tightening onto a softened insulator block, this in turn causes the mixing chamber to warp and the slide to stick. However it's a nearly 50 year old machine and you have to decide where to stop. It's very easy to go from simply replacing a couple of worn / incorrect jets to spending a $1000 on two carbs ( Trust me! ).

Rod


So remember, when you're feeling very small and insecure,
How amazingly unlikely is your birth;
And pray that there's intelligent life somewhere out in space,
'Cause there's bugger all down here on Earth!

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