December 4, 2009--
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Carburettors and manifolds.
[Click pictures for larger view]
Amal 626 carbs were pretty much a mess when I started. The white junk in the bowl is a corrosion product of the metal of the bowl. Only one of the bowls had this deposit, and it showed significant pitting after cleanup.
Removed the original tickler so I could install a tickler upgrade. Also checked float bowl mating surfaces for flatness. I measured a 0.018" gap on this one. Usually it's the mounting ears that are distorted, as if from overtightening. Linishing on some 220 wet/dry paper on a surface plate with some WD40 for lubrication got the surface flat again. Taking off too much metal in the wrong place could make the float spindle stand proud of the bowl and affect sealing.
The bodies respond pretty well to cleanup--carb cleaner first, followed by a phosphoric acid dip and a good rinse. The pot metal won't keep a shine very long, though.
I found the outlet flanges weren't flat by about 0.006", possibly due to overtightening. Because of the integral construction of the body, distortion of the flange suggests possible distortion of the slide bore, which could cause binding of the slide. If this were the case, the only way to try to fix it would be to bend the flange ears back into line, hopefully restoring the bore geometry. Some talented restorers have developed jigs to do this relatively easily. In my case, the slides were still good sliding fits in the bores, so I was able to flatten the flanges by simple linishing on a surface plate.
The idle mixture on these carbs is controled by the air screw on the side of the carb, and a pilot jet bushing that meters fuel. The brass pilot jet can be seen at the bottom of the hole where the air screw goes. The other side of the pilot jet is not readily accessible, so it's not possible to completely check the pilot for debris. In carbs that have been sitting this long, and with this much crud inside, it's worth making sure the pilot circuit is clean.
The raw carb bodies are such that they can be made into a left or right carb, depending on which side is drilled for the air and throttle stop screws. This means that the back side of the pilot jet can be exposed by drilling out the blank in the casting opposite the air screw. Obviously, this should be done very carefully so the pilot jet is not damaged. The access hole can then be tapped and blanked off again with a shortened air screw.
This is the setup for drilling the blank. The bottom guide slides into the air screw hole and ensures that the new drilled hole is on the same axis.
The thread for the air screw looks like 10-32, but it is really 2BA (most threads on this bike that look like 10-32 are 2BA), so the drill is a 4.0 mm, which is correct for the 2BA tap.
The pilot on one of my carbs barely let any light through, so was partially obstructed. The pilot hole is 0.016" in diameter. A #78 drill is 0.016, but I had a piece of music wire (guitar string) of the right size. I used it carefully as a probe to clear the jet. The picture on the right shows standard air screws along with two screws modified to close off the access hole on the back side ofteh carb. I used a little blue Locktite on the blank screws.
Here are all of the parts for the carbs. I'll replace all the gaskets, O rings, and the filters. The float needles will get upgraded from the plastic ones to Viton-tipped brass. Amal also recommends replacing the needles and needle jets if their condition is unknown. I'll also upgrade the ticklers to ones that don't weep fuel all over your hands.
The newer tickler works just like the old one--pressing the button pushes the fuel float down, bringing the needle valve off its seet, which floods the carb. The only difference is the sleeve that goes in first which blocks the fuel bleed hole. The larger aluminum piece is a shop built drift to help seat the sleeve. It helps to measure the sleeve and bore first to know when it's all the way in.
To set the proper fuel level in the float bowl, assemble the float, float needle, and float spindle into the bowl. Press on the tabs of the float to just seat the float needle, making sure the float spindle does not lift. This simulates a full bowl with the fuel stopped by the float needle. In this condition, the opposite side of the float should be 0.080" below the mounting surface of the bowl.
Both of my carbs had the float setting way too high. Since I know this had never een checked before, it had to have left the factory like this. I have it on good authority that the .080" setting wasn't issued until after this bike was made.
The only way to adjust the float bowl fuel level is to move the needle seat. The seat is a friction fit in the bowl body, and the normal way to adjust it is to heat the bowl and tap the seat with a suitable drift. This seemed like it may be hard to get a precise adjustment, so I spent a few hours to make the seat adjustment jig shown here. The brass drift fits over the socket head cap screw. The ball is inserted as a bearing so the drift doesn't try to turn.
The jig screw thread is 10-32, so each turn will raise the seat by about 0.031". The lever ratio of the float will translate this into about 0.240", while 15 degrees of turn on the screw will move the float position about 0.010".
Final float position. This also shows the best place to press on the float tab. It seats the needle while also keeping downward pressure on the float spindle.
The old needles and needle jets are on the right I re-used the main jets and jet holder, as they normally don't wear.
Main bodies assembled, with the pilot jet access holes blocked off:
Float bowls assembled. Need to make sure gasket doesn't foul the float. It is also important that the gasket seals around the fuel passages on either side of the float needle (only one is actually used on each carb, depending on if it is a right or left unit). This is the fuel supply for the pilot jet, and if it can suck air--from either outside or inside the bowl--it will be difficult or impossible to get the idle right.
Bodies and bowls assembled. Throttle valve, air slide, and needle are all inside. I re-plated the steel hardware, but pot metal parts don't plate well, so I didn't try.
Cleaned up the manifolds, working the inside bore, too, trying to match it with carbs and head for smooth flow. Left pic shows hardware before plating, right is after.
Carbs mounted on engine. I'll have to open them up again to hook up throttle and choke cables.
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