Tuning and Jetting Guide For Amal Mk.1 Concentric Carbs on Vincent Twins, and Singles.
There are (3) major components to the proper setup of Amal Mk.1 Concentric carbs when used on Vincents. These are: 1) Carb Modifications, 2) Carb Jetting, and 3) Ignition Timing. Each will be covered here in great detail.
Amal Mk.1 Concentric Carb Modifications.
New Amal Concentrics can be purchased today from several sources, and sometimes, they can be bought equipped with the jetting that you will specify. Regardless, these carbs whether new, or old/used need to be "blue-printed" for use on a Vincent. I learned the hard way that they CAN NOT be just mounted, and used. I found data on the proper setup of the float level. I discovered that a restrictor low speed orfice must be drilled out, and I discovered that the brass fuel feed nozzle in the bore needs to be modified (like a Norton 850' 932 carbs). Perform all (3) of these mods!
1) Float Setting: Normally the float in NOT adjustable. The float's needle seat that is in the float bowl can be moved slightly up or down to the precise location by first running very hot tap water over the seat for a few minutes, and then gently tapping the seat in either direction to move it. Use different sized rods, or drill bits in a manner that won't hurt the delicate needle's seating surface. The proper float height is .080" measured from the top of the float to the top edge of the float bowl. It is important that the stock bowl gasket be used because of it's specific thickness. To measure for the .080", assemble the float, float pin, and needle into the bowl, and carefully hold the float pin fully down as if it was installed to correctly position the float, hold the float level, and upright as if it was on the bike. I use a .080" drill bit, and I lay it on the top edge of the bowl near the float (opposite end of the float from the needle), and I "eyeball" the thickness of the .080" drill bit, and compare how far down the float edge is from the bowl edge. The float drop should equal the thickness of the .080" drill bit. The seat will need to be heated, and moved several times to get this adjustment just right. If the float level is too low, the carb will always act lean, and if the level is too high, the carb will always act too rich!
2) Low-Speed (Pilot) Jet Restrictor: The Low-Speed jet size for a Vincent is very important, and on the later Concentric carbs, a restrictor jet of about a #15 jet size is installed that limits the available low-speed jetting size. This size is only applicable to 500's! On the side of the carb is an angled screw for setting the slide height, and a horizontally mounted adjusting mixture screw. Remove the mixture adjusting screw, and spring, and look into the hole it came out of. You will see a restrictor jet with a very small hole (.015"). This will need to be drilled out with a .040" tiny drill bit. Great care must be used while doing this to not wreck the carb, or to break off the end of the drill bit! Now this restrictor is equivelant to a #40 low-speed jet (larger than you will ever need). Now, the normal low-speed (pilot) jets can be screwed into the carb body's pre-drilled orfice above the float bowl. A #30 Low-speed jet is correct for most Vincent applications. A general rule for selecting the correct low-speed jet is that the fastest idle speed found by adjusting the mixture needle while idleing should be found with the needle out between 1 to 1 1/2 turns out (prefferably 1 1/4 turns out).
3) Brass Fuel Feed Nozzle Modification: The brass fuel feed nozzle visible inside the throttle bore (the needle goes into it also) has a round cylindrical design where it stands about 1/4" above the bores lower edge. This is appropriate for small displacement motors, but for larger applications (like a Norton 850, and Vincents) this feed nozzle needs to be modified. When you look into the carb's bore you can see this feed nozzle. The complete back half (downstream) needs to be cut away. To do this, carefully scribe a mark along the lower rear edge of the feed nozzle along the body's bore surface. Mark a small dot with a "Sharpie" pen to orient the exact midpoint of the rear of the feed nozzle. Disassemble the carb to remove this feed nozzle. Carefully mark the sides of the feed nozzle with vertical lines for cutting away material. You want to use a Dremmel tool with a cut-off disc, and be very careful to remove exactly 1/2 of the feed nozzles exposed material (the rear half!!). Re-assemble the carb. The need for this is because at low engine speeds with only slight throttle (slide) opening, the in-rushing air will pick up fuel from the needle while air is rushing around the feed nozzle, rather than the air having to rush over the top of the feed nozzle. Untill I discovered this mod, I was suffering poor low end running, as well as alot of detonation, and heat build-up.
Jetting Guide For Amal Mk.1 Concentric Carbs For Vincents.
As you know, not any (2) motors, and their operating enviroments are going to be identical. These jetting guidelines should either be correct for your bike, or at least be within one jetting change from correct. Careful observations, and adjustments must be made after an initial impression has been realized. I have discovered some things that may influence other bikes. One is the size , and flow rate of your pet-cocks. These are bigger carbs, and under hard riding, they may need more fuel than your pet-cocks were flowing before. I have had great success from the late Norton style pet-cocks with the "paddle-type" lever. Don't get the cheaper Taiwan units. Spend the extra money, and get the much better British made units! Pet-cocks can definitely affect your main jet size!.
1) Slide: I like the 3.0 slide for quicker throttle response than the standard Amal 3.5 slide. Use "hard-chromed" slides if you can find them. The last ones I found were at (Clubman Racing). The "Pot-Metal" standard style slides just won't last as long, but they work great.
2) Needle: I use the standard Amal needle with (3) needle clips. I've found the middle notch to be best for higher elevation, and in some cases, the lower (richer) notch might be best. This is a very subjective setting, where you will need to drive test each setting to find which feels the best. This setting affects the feel of normal "inner-city" type acceleration (not high, or low speed running).
3) Needle Jet: The standard size needle jet is a #106. I have had better success with a richer #107 needle jet. This adds a tiny bit of richness to the mixture at idle, just after idle, and everywhere along the needles taper. This increase was very beneficial to my Vincent as well as my Norton 850. I have found that it is easier for me to remove my bowls, and upsize a needle jet instead of removing the carbs to change a needle clip position when going from my home at 7,000" elevation to sea level. Often people are changing low-speed, and sometimes main jets to cure a mid-range jetting problem, where a change in the needle jet was the only cure. I've found that bikes that ran fine decades ago on older style fuel usually need an upsize of at least one size for the needle jet to react to today's newer fuels. I've found this to also be true for my Mikunis! What people don't tend to realize is that the relationship of the needle jet orfice to the needle's shank diameter is actually another jet size! This relationship affects the jetting from idle to just under full throttle.
4) Main Jet: I have found for a basic Rapide, or Shadow motor that a #220 jet for higher elevation, and a #230 jet for sea level works best. This size may need to be adjusted if you are running straight through muffler, velocity stacks, hotter cams, etc.. I use #250 jets in my 1,164 cc "Big Bore" motor with Mk.2 cams, 10:1 compression, and a straight through muffler.
5) Low-Speed (Pilot) Jet: I found the #30 Low-Speed jet to be correct for a basic Rapide or Shadow motor. I am using a #35 jet in my "Big Bore" 1,164cc motor.
6) Float Level: The float level at the top edge of the float measured at the dge away from the needle and seat should be .080" below the top edge of the float bowl. This has been covered above!
Ignition Timing Specs:
1) Spark Plugs: For all Vincents applications (as you know), magnetos are sometimes weak, and often need some help by the spark plugs to get things going. I've found that the plug gap should never exceed .020" gap. Magnetos have a hard time firing past a gap greater than .020". I've found that "Gold-Paladium" (or even "Iridium") plugs fire much easier on a weak magneto, and offer better all around performance. I like NGK B8EGV "Gold-Paladiums", but these might be a little too cold for some Vincents. The best standard type plug for a Vincent that I've found is the NGK B7ES set to .020" gap. The "Iridiums" only come in the #8, and #9 heat ranges, and the better #7 heat range is not available.
2) Magneto Point Gap: The Magneto Point Gap should be an average of .012" measured between the opened point for the front cylinder, and the opening point of the rear cylinder. An example: If the opening measures .011" for the front cylinder, while it measures .013" for the rear cylinder, the average would be at .012". I do not like the idea of slightly changing this adjustment to make small changes in the timing. A weaker mag will benefit from slightly tighter point clearances rather than looser. The tighter point clearances allow the mag coils to recieve a slightly larger charge from the magnets (in simple terms).
3) Ignition Timing: Todays newer fuels seem to require setting the full ignition timing back a few degrees from the stock setting. I've found this to be true on all motors today. I've had good luck with no more than 34 degrees full advance. As you know, it is near torture to get a Vincent timed correctly. I found a handy way to mark your left flywheel with a TDC, as well as a 34 degree timing mark for the front cylinder. The next section will explain this.
4) Setting Up a Degree Wheel, and Marking the Flywheels: Since a Vincent has no marks for timing anywhere on it, timing is a very difficult job, and thus, many people just guess at it. This is very irresponsible. Timing is crucial for maximum power, milage, and the prevention of detonation. I've seen many ways of doing this, and my method works very well for me. First make sure your magneto point gap is set correctly. Then prepare a degree wheel. Get a degree wheel (a large one), and make a centrally mounted spindle out of copper tubing that mounts firmly to the center of the degree wheel that will be used to slip inside the right crankshaft end's hole after removing the "Quill". I don't want to get into the details of making up this assembly. The copper tube spindle should be able to rotate within the crankshaft end, but also to stay put without moving. Make up a pointer out of wire, and attach it to a magneto cover screw hole. To install the degree wheel, first the exhaust system needs to be removed, and the oil lines removed. This is a good time to do an oil change! The purpose of all of this is to get a reliable stamping onto the left flywheel that is visible through the left case oil drain hole. Once the flywheels are stamped, the degree wheel will never be needed again for ignition timing. The next step is difficult, and sometimes requires the aid of a helper. Bring the front cylinder to very near TDC "firing". This can be found by rotating the motor while feeling the air entering or leaving the cylinder through the spark plug hole. It is best to put the bike up on it's rear stand, and put the bike in top gear. Now use the rear wheel to rotate the motor forwards, and backwards. As the motor is rotated, and air is leaving the spark plug hole, you are approaching TDC! When you can rock the motor back and forth slightly, and feel the air push out, and pull in slightly. You are now very near TDC "firing". Now rotate the degree wheel to show TDC (0 degrees). Now you will need a dial indicator, and maybe a little extension on it to feed down the front spark plug hole in order to while contacting the piston top, and never contacting the sides of the threaded spark plug hole. Now, you have to try to "center" the degree wheel by taking readings in crankshaft degrees while approaching TDC from both directions of rotation. These readings are taken in both rotational directions, so as to take up any rod bearing slop. Since the degree wheel has been roughly set near TDC, find a comfortable position for the dial indicator to rest against that will guarantee that it will not again move during your degree wheel readings, or new readings will need to be taken. I hold my dial indicator up against the heads fins in a very stable manor with the probe centered down the spark plug hole. Now the crankshaft can be moved in small amounts forward or backwards a few degrees while watching how far down the dial indicator probe drops. I like to get the motor very near TDC by feeling the air at the spark plug hole, zeroing the dial indicator, and then rolling the motor no more than .050" drop on the dial indicator (this is about 12 degrees rotation). Then carefully roll the motor towards TDC, and at .030" from the zeroed TDC, take a degree wheel reading (this is usually around 8 degrees BTDC). While carefully holding the dial indicator, roll the motor past TDC to about .050" down on the dial indicator. Now roll the motor backwards to the .030" point before the zeroed TDC setting on the dial indicator. Now take a second degree wheel reading. Again, it will be near 8 degrees ATDC. You now have a range of total degrees between .030" before, and after TDC. Add both numbers for the total, and devide it by 2 (bisect the total). Finish rolling the motor backwards carefully this halved amount of degrees, and you are at true TDC. Now carefully move the degree wheel a small amount to show exactly zero degrees-TDC. Be very careful to never accidently bump the degree wheel untill you are completely finished. Now the left side flywheel can be seen if you remove the left case oil drain plug. Carefully place a stamped mark (centered in the hole) for front cylinder TDC. I like to use a small flat chisel to make a vertical hash mark. You can use the degree wheel to verify that you are, in deed, at TDC. Now roll the motor backwards to 34 degrees before TDC (BTDC), and put another stamp mark. I like to use a spring loaded punch for a dot for the full advance 34 degree timing mark. Now that this is done, the degree wheel will only be needed for cam timing in the future. The next time you want to set your timing, you take out the front spark plug, and find near TDC by feeling the air end it's out-rush, and back-up the motor to the stamped 34 degree timing mark. Now you go through the gyrations of loosening the ATD bolt (Auto Advance Device), and trying to find a point where a cigarette paper will just pull through the points with a little drag when the motor hits the 34 degree timing mark (assuming that you have the ATD locked to full advance, and taking out all of the cam gear slack). As you see here, this a job best left to the professionals! Rookies become professionals by mastering the task, so be very diligent, and you can do it yourself.
James R. Mosher