AMAL CARB FIX

BY

ENGLISHMAN

I’m always hearing from people who are having troubles with their old Mk1 Amals, either they won’t idle properly or they are running way too rich and leaking all over.

Fact is, most of the problems with these old carbs can be traced to misuse over the years, owners cranking the nuts down too hard when bolting them on, and other general ham-fisted boogering.

When the retaining nuts are over-torqued, the flange connecting the carb to the inlet manifold becomes bowed, allowing air to enter and causing all kinds of grief when it comes to idling and setting the mixture. If the retaining nuts are seriously over-tightened, the carb body itself can become warped, leading to fun, such as the throttle sticking wide open!

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Another thing is the float bowl, people using too much force to screw the bowl on can seriously warp the surface, leading to leaks that leave those nice brown streaks on your freshly-polished engine cases.

Fortunately, there is a cheap fix for all but the worst cases of ‘owner involvement’, and you will have a valuable tool for working on other parts too!

You’ll need a piece of plate glass, I bought a 12”X12” piece for the princely sum of $4, a slightly larger piece would work better, but it sufficed. g

Plate glass is flat. Very flat in fact and an ideal low-budget basis for a facing tool. I picked up a couple of sheets of wet and dry 400 grit, a can of Disc Adhesive (used usually to stick sanding discs to power sanders, this stuff is STICKY) and a can of WD40 for lubing purposes.

Start by spraying the adhesive on the back of the sandpaper and center it on the glass and allow to dry. Disassemble the carb and check for warpage on the flange, either by holding a straight-edge against it, or by placing the flange on a flat surface and see if it rocks back and forth. Chances are, if the carb is 20 years old, it’s going to need a little help. ive the sandpaper a dose of WD and carefully begin rubbing the flange down. A quick once-over and inspection will reveal the high spots. Be careful not to favor one side or the other, you don’t want to end up with a nice, flat flange, but at a 30 degree angle to the carb! Likewise, it’s not necessary to take the whole face down an inch or two, a light circular/figure eight movement should rapidly remove the high spots, continue until most of the surface shows sanding marks.his should be repeated for the float bowl, invert the bowl, give it a quick circle or two on the sandpaper, and check to see how bad it is.

 

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These two things will do wonders for your sucking air/leaking gas problems. Just remember, be gentle, it’s easy to remove metal but virtually impossible to replace it.

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This method also works well for fixing warped cylinder heads. Of course if it’s bad, then you are going to have to take it to a machine shop, but next time you have your head off (so to speak), you can pull out your piece of glass and face off the gasket surface in the same manner as before. This will help if you are having problems with blowing head gaskets.

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Back to the carbs, another common problem is caused by boneheads screwing the pilot air jet adjuster all the way to the bottom and then some! Once the bottom of this air passage is messed up, it’s really tough to get the mixture right. If adjusting this screw seems to have no effect on the running, chances are there is a piece of crap in the airway. Try compressed air to dislodge it, I have had good results using the non-aerosol carb cleaner, the type you immerse the carb in rather than spray on, such as is available at NAPA stores. Be careful with this stuff, it seems to be quite toxic and if you forget to remove the adjusting screws, the rubber seals will be swelled to twice their original size when you pull it out of the chemicals.

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While the float bowl is off, place the float, pin and float needle in the bowl, hold the pin on both sides and turn the whole thing upside down. If the float hangs down past the lip of the bowl, this may be the cause of some rich running. What’s that you say? You can’t adjust the float height in a Mk1 Amal? Wrongo!

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Pull the banjo bolt out from the bottom, remove the banjo (put it on your knee if you wish) and the plastic fuel filter (good time to clean that puppy out too). The float needle has a brass seat that is a press fit in the float bowl. If the float height is too high, it is possible to lower it by using a drift and punching the seat a little higher in the bowl from the bottom. Higher seat = lower float height. Similarly, you can drift the seat down a little from the top if the float height is too low. What’s the correct float height? Good question, I have had the best results setting it so that it is just at the lip of the bowl when held upside down. Once again, be gentle, find a drift that is the right size so you won’t damage the carb. Neither I nor THE HORSE BC will be responsible for some bonehead pounding the seat out of their bowl, so you attempt this at your own risk.

If your carb has one of those nasty plastic float needles, toss it into the parts bin and get one of the viton-tipped brass versions, they seal MUCH better!

Check the throttle slide for excessive wear where the protrusion fits in the slot in the body, if it’s not very square-looking, the throttle will rattle around a lot at idle and make the bike sound rough. Also check it for side-to-side wear on the body and make sure it’s not sticking at the top of it’s travel. Chrome slides are available, as are chrome inserts for the carb body itself.

If you aren’t running the choke assembly, blank off the spare hole in the carb top, you don’t need the extra air being sucked in through there.

Once you get these carbs figured out, you’ll be happy with the simplicity and performance of them, hell, a rebuild kit is only about $4 and any swap meet contains a lifetime supply of spare parts for them!

 
© Charlie Horse, LTD.

586-992-9803