How to Straighten Your Amal Carburettor Float Bowl.
Written by BritBike forum member: tridentt150vHow to Straighten Your Amal Carburettor Float Bowl.
After some time and use, the bowls on AMAL Concentric
Carburettors can warp, usually due to ill-fitting gaskets and/or overtightening the two retaining screws. Many owners resort to sanding the bowl back to flat with emery on a flat surface. This is wrong DON’T TO IT
; all you are doing is unnecessarily removing metal from the bowl gasket surface, metal that cannot be replaced and causing irreparable damage.
The correct way of fixing any warp is to make or borrow a tool like this:
1. Top tightening nut.
2. Thick washer
3. Carbie bung drilled out.
4. Threaded rod
5. Bottom lock nut.
6. Flat base with angle iron for holding the tool in the vice.
When assembled without a float bowl the tool looks like this.Notes on the tool.
The main flat area has to be thick steel plate and should be at least 60mm (2 ⅜") square. The one pictured is 8mm (5/16") plate and 90mm (3 9/16") square for extra working room.
The centre hole is drilled and tapped for the threaded rod.
The angle iron welded to the base is used to clamp the tool in a vice freeing both hands. Note that the welds are only on leading edges of the plate to minimise any weld pull/heat distortion.
Threaded rod and locked in place with a nut is preferred because it can be replaced if damaged or adjusted for height if needed. A welded bolt could be used but you will lose modular flexibility. And if you don’t get it exactly perpendicular then your tool is useless.
An old carbie bung is required, if you use the tool without one you risk distorting the thread in the float bowl. This affects bung removal, replacement and sealing. The drilled out bung stops this.
Use a large thick flat washer, it will not distort and helps spread the force applied.
The top nut is used to apply the pressure needed for adjustment.
Packing washers may be needed under the bung if you cannot screw it all the way in..Using the Tool.
The float bowls are a soft metal, they can bend and re/shape very easily. Go slowly and methodically, small incremental adjustments will help avoid over straightening and having to correct over adjustments or mistakes.
To use the tool, place the float bowl upside down over the threaded rod. Screw the bung in finger tight and all the way in – you need to protect all of the threaded section plus you want the force of adjustment transmitted through the bung to the shoulder of the bowl, not applied via the thread - then using the washer and top tightening nut assemble the tool and then finger tighten the top nut. Do not force the bung in any further than it would normally sit when on the motorcycle. You may need to use the packing washers if the bung will not tighten to the shoulder.
Using a set of feeler gauges, measure the gap between the float bowls gasket surface and the flat plate, the greatest gap will usually be midway between the two bowl retaining screws.
Note this gap.
Select two feeler gauge blades that are 1 to 2 thou bigger than what you measured….by going slightly bigger you are catering for any spring back. Eg if you measured 0.008” air gap between the bowl gasket surface and the flat plate, then select a 0.009” to 0.010”.
Now loosen the top tightening nut say one turn and slide two feeler gauge blades in either end of the bowl and under where the bowl retaining screws are. This step is fiddly and you will need both hands, the benefit of the angle iron used to mount the tool in a vice works here.
Once feeler gauges are in place, tighten the top tightening nut down firmly using a spanner. Remember its soft metal, don’t overdo it. The pressure from this should straighten the bowls gasket surface.
Once you have done this, loosen the top tightening nut remove the feeler gauges, and tighten the top tightening nut finger tight. Then re measure the air gap between the gasket surface and the flat plate.
If there is still an air gap, redo this same process, changing the feeler gauge thickness at either end of the float bowl as needed.
Gentle heat could help. Methods can be a heat gun, hair dryer or using boiling water. There are other harsher heat sources such as a propane torch, but using them requires experience. Remember the float bowl metal is a soft alloy and easily damaged if abused.
If you make the tool without the angle iron, don’t use the bolt head as a substitute in the vice jaws. The bolt/threaded rod has to be 90 degrees to the flat plate for the tool to work, care needs to be taken not to affect this alignment or distort the flat plate
For general information, the above bowl had a measured gap of 0.008, I first tried using 0.009 feeler gauges but this didn’t fully work. I then used 0.010’ and it worked perfectly.Feel free to comment and why not write your own article and shar your knowledge!