Carburettor Amal 375

The carb in this bike is probably fine because I've started the motor before and it ran nicely. However that was throttled simply using the stump of broken throttle cable so I will have to fit a new one (Hard to find but I got lucky) and for this the carb needs to come off. I might as well strip and clean it out at the same time. The Amal 375 carb is a fairly substantial lump.

Unusually in the bike, the carb is held on by 2 Bsc bolts rather than studs. Sadly the heads of these have been mullered at some point in the past. Luckily it did grip in a ring spanner. I'll file some of the burring off to improve fit as Bsc bolts are going to be hard to find.

Damaged bolt head.

Other bolt not so badly damaged.

Carb was easily removed so I took some pics to show the various fitting positions before removing the air filter- mine is simply the air filter case and contains no filter.

The fuel inlet was secured with a bolt that looked wrong- its a metric thread whereas the original should be BA1. It should also have a fibre washer fitting into the recess in the hose union. I've ordered a 1BA bolt and a gasket set for the carb which should provide the missing washer. I removed the bajo and cleaned the filter beneath. It did have some sediment but wasn't by any means blocked.

I removed the front plate which also seals the float chamber

The float just lifts off its pivot pin and the needle valve then drops out- in my case its an all plastic needle which is less than satisfactory in my book. Some diagrams show a pivot sleeve bush but there was none in my carb. I will have search in case I've dropped it or it is retained in the float pivot tube mounting.

Float removed.

The fuel inlet filter was unscrewed and  the needle valve retrieved from the float chamber. I didn't detach the filter from the inlet since it was a very tight fit and would probably be damaged. However I can tell its perfectly clear so further disassembly isn't necessary.

Inlet fuel filter and needle valve.

I could then remove the  jet holder complex from the bottom of the carb

Float chamber small amount of sediment. Unscrewing the jet holder from the base

This is a 2- jet complex- the needle jet unscrews from the top of the holder. It was clear but I blew it through to check.

needle jet unscrewing from jet holder

The main jet cover unscrews from the base of the jet holder and reveals the main jet  beneath.

The main jet also unscrews- note fibre washer on jet holder.

The top of the carb unscrews and lifts out- I need to replace the cable but want to keep dismantling to a minimum.

There is a rod plunger attached to a slide which serves as  a choke. The slide does lift off but I had no need to remove it. Note the throttle slide spring.

The throttle slide spring was removed and the slide itself lifts out still attached to the end of the cable.

Sadly the cable cant be removed without first removing the needle which is held by a sliding split clip.

Cable attachment to the throttle slide. note the needle blocks the cable attachment. Sliding clip visible.

Checking the clip shows that its fitted in slot number 4 from the top.

Removing the clip frees the needle and allows the cable to be slipped out.

Cable removed

The cable nipple slips out through the adjuster on the carb top and the new cable slips back in its place.

Making sure that the needle is clipped into groove number 4.

I could then examine the carb body, firstly removing the  the small silver screw peg that locates the jet block. I'm not intending to remove the jet block but wanted to make sure everything around it  is clean.

jet block locating peg removed.

I also removed the air control screw. Firstly this needs to be fully screwed in whilst counting the turns- in my case this was 1.5 turns. You can then remove it and use the fully inserted position as  a guide to restore its setting on reassembly.

The pilot jet cover and washer are then removed

In theory the pilot jet should now unscrew- however the  screwdriver slot was damaged and since I think this is clear anyway I didn't try harder to remove it but simply blew it through with carb cleaner..

Small amount of debris in the jet cover which was easily removed with a spray of carb cleaner.

I put all the components in the sonicating bath and cleaned them for 30 mins before reassembly. This was literally the reverse of the process above except that the float chamber gasket was damaged and I had to cut and tap out a new one from gasket paper. I think this will work but in fact I have a gasket set on order which will include a new one anyway.

Reassembly used new gaskets and fibre washers. I renewed the float needle using a viton tipped item

I also found that the new 1BA bolt couldnt secure the fuel inlet tightly enough and fuel leaked from this union. I concluded that the metric bolt had distorted the thread such that the BA thread could no longer grip. I tapped the valve assembly to M6 and used an M6 bolt and this fixed it.