Amal Carbonator Slide Numbers.

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'72TR6RV

Well-Known Member
Hello everyone

Hello everyone
Let's start over.
Amal carburetor slide numbers. On the '76 Bonneville I've recently finished. Should the left and right slides be the same number. I have a 3 and a 3-1/2.
Not sure what the mains are yet. Im not sure if the carburetors are a matched set. Left says L301 with the 930 under it. Right just says R930. The engine would run on the left cylinder at idle but the right runs decent once out of the idle circuit. I swapped the slides now the right side starts but left side will not fire untill out of the idle circuit and backfires. Left side seems to flood easily. I have cleaned the air / fuel passages thoroughly. Fitted new stay up floats and aluminum Viton tipped float valves.
Also what is the purpose of the vacuum balance hose between the two intake manifolds?
 
Should the left and right slides be the same number.
Yes. Different slides in different carbs on the same engine is wrong.

not sure if the carburetors are a matched set.
They are not.

If you look in your bike's Triumph parts book at the "Amal Carburettor T120R" pages, it says "L930/67" and "R930/66":-

. "L" and "R" denote which side of the body the air screw (and throttle screw if fitted) is/are fitted;

. "9" denotes a 900-series body casting;

. "30" is the inside diameter of the main venturi through the body from atmosphere to engine;

. "66" and "67" denote the bike and model year Amal supplied the carbs for. Carb. in hand but not knowing the vehicle, you could contact Burlen (the current Amal licencee) with the 'slash number', they would tell you the carb's original spec in terms of jet sizes, slide cutaway, needle, etc. and the vehicle it was supplied for.

. The exceptions are L301 and corresponding R300. Amal supplied these direct to dealers with 'average settings'. The idea was the dealer could bolt it to any suitable engine and the engine could at least be started, then the dealer could fine tune the carb to the specific engine by changing anything of jets, slide, needle as necessary.

If you are not hung up on having the 'correct' numbers on the outside of the bodies, it might be possible to turn what the bike has already into a "matched set".

Fitted new stay up floats and aluminum Viton tipped float valves.
Before incurring more expense on Amal parts, it is likely wise to have particularly the bodies measured for wear, because the price of genuine Amal parts bought individually can rapidly exceed the cost of a complete new carb and, if the slide bore is worn badly, a new carb or reboring and sleeving the old one is necessary. :(

Measuring bore wear needs a bore gauge (and someone who knows how to work it) to do it properly, because most wear happens on the engine slide of the slide bore just above the main venturi - the engine sucks the slide against that part of the body. Knowing the wear there, the slide should be 0.4 mm smaller diameter; greater clearance and more air is sucked between body and slide, screwing up the mixture.

The engine would run on the left cylinder at idle but the right runs decent once out of the idle circuit. I swapped the slides now the right side starts but left side will not fire untill out of the idle circuit and backfires.
The problem might be fixed just by fitting both carbs with slides with the same cutaway. Or it might be one or both carbs' idle circuits is/are not completely clear. The pilot jet is the usual suspect but the two tiny holes in the bottom of the main venturi, one each slide of the slide land on the engine side of the carb must also be clear.

If a carb is heavily corroded, it can be wise to drill out the 'block' at the 'atmosphere' end of the fuel passage so you can be sure it is also completely clear. Drilling out the block must be done with thought, as you must be able to block it again after cleaning. I use M5 grub screws and drill out the original block with a 4 mm drill (because 4.2 mm is the tapping size for M5); In the US, 10-32 grub screws might be easier to find?

Left side seems to flood easily.
If this means the right side does not, the float is depressed by a roll pin protruding from the tickler button inside the carb body. You can grab the roll pin gently with pliers and extend it a little further.
 
The problem might be fixed just by fitting both carbs with slides with the same cutaway. Or
Thanks Rudie
Correct me if I am wrong, does the 3 and 3-1/2 represent the cutout angle of the slide at the air inlet side of the carburetor?
 
Maximum height of the cutaway in 1/16", so "3" is 3/16", "3-1/2" is 7/32".
I took some time today and removed the float bowls to do some investigating. Apparently the previous owner picked up some a left and right carburetor at a swap meet. The left carburetor had a 3-1/2 slide with a 190 main jet. The right carburetor has a #3 slide with a 200 main. Needle jet is .106 which is correct. My manual calls for a 210 main.
 
Both carbs should have the same throat sizes, same slide cutaways, same main jets, same needle jets, same needles, clips in the same groove, and same float needles, same float heights.

The only difference could POSSIBLY be the exact number of turns on the low speed mixture screw.

Never mind the secondary number on the carb body, it does NOT relate to the above internals.

If identically built carbs produce significantly different burn characteristics at the plugs, you need to investigate your individual points settings (if running points), and your valve lash settings. Also ensure you don't have one air filter significantly dirtier than the other. One other possibility to easily investigate is one clogged muffler.

If all of the above are correct, and you STILL have significantly unequal burn, you may need to pull the cylinder head and see if you have significantly different valve seats from one side to the other.
 
Both carbs should have the same throat sizes, same slide cutaways, same main jets, same needle jets, same needles, clips in the same groove, and same float needles, same float heights.

The only difference could POSSIBLY be the exact number of turns on the low speed mixture screw.

Never mind the secondary number on the carb body, it does NOT relate to the above internals.

If identically built carbs produce significantly different burn characteristics at the plugs, you need to investigate your individual points settings (if running points), and your valve lash settings. Also ensure you don't have one air filter significantly dirtier than the other. One other possibility to easily investigate is one clogged muffler.

If all of the above are correct, and you STILL have significantly unequal burn, you may need to pull the cylinder head and see if you have significantly different valve seats from one side to the other.
Thanks GrandPaul
Both carbonators are Amal 930s
But with different slides and main jets.
I think the previous owner picked up a left and a right carb at a swap meet in Carlisle, PA and put them in the box of parts with the bike.
The engine number and frame numbers match. He also had 3 other Triumphs, and a Norton 850.
I got a pair of slides and the correct main jets on the way.
The bike would start but run like garbage! No wonder.
Greatly appreciate the advice and sharing of knowledge from you and Rudie. After the holidays I'll get the carbs set up and try it again.
 

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