revtothelimit - motorbike forum

Is the thickness of 1/4mm a typo? Should it read 1/4"? Are spares easily got before you start repairing the old one?

cheb wrote: Mon Apr 03, 2023 12:17 pm Is the thickness of 1/4mm a typo? Should it read 1/4"? Are spares easily got before you start repairing the old one?

Not a typo, it's the wall thickness of the float bowl itself, there is no 'flare' to provide a larger mating surface for the cover. (it might be more like 1/2mm, but it's no more than 3/4mm ).

Have you pictures of the thing?

cheb wrote: Mon Apr 03, 2023 4:23 pm Have you pictures of the thing?

I'll try to take some tonight.

This is the carb float bowl.



This is the nick on the mating face.



This is the "cover". (Its a fancy "big reservoir" version)

I can't remember what it's called (I'll try and find out) but I've used a reddish RTV in the past that's made specifically as a gasket cement for fuel pump to tank interfaces. Maybe some of that? I'd be tempted to fill the nick with JB Weld and then use the RTV to stop petrol getting at the JB Weld...

Personally I'd back JB Weld to stand up to petrol, as long as it's fully cured.

Screwdriver wrote: Mon Apr 03, 2023 7:00 pm

Rockburner wrote: Mon Apr 03, 2023 4:08 pm

cheb wrote: Mon Apr 03, 2023 12:17 pm Is the thickness of 1/4mm a typo? Should it read 1/4"? Are spares easily got before you start repairing the old one?

Not a typo, it's the wall thickness of the float bowl itself, there is no 'flare' to provide a larger mating surface for the cover. (it might be more like 1/2mm, but it's no more than 3/4mm ).

If you're very careful, you could solder it.

<edit> looking at the picture, surely a thicker gasket would solve the issue. Cut one out of some EPDM.
<edit.2> Scratch that. Looking at Google, not EPDM for petrol...

What sir needs is a nice Viton (FKM) gasket.

I'd have thought proper gasket material (i.e. not paper) would be best. Cardboard will Shirley allow the passage of petrol, wouldn't it?

Screwdriver wrote: Mon Apr 03, 2023 8:07 pm

mangocrazy wrote: Mon Apr 03, 2023 8:02 pm I'd have thought proper gasket material (i.e. not paper) would be best. Cardboard will Shirley allow the passage of petrol, wouldn't it?

Wood fibre is a remarkable material, you can soak it in oil to make it swell and remain impervious to fluids. Better off with a more robust modern plastic but cardboard is an excellent material for many engine gasket applications.

To block the passage of oil, perhaps, but petrol is a lot more searching and will leech away any oil over time. For an application like this I'd use a modern plastic-based material, as you say.

Screwdriver wrote: Mon Apr 03, 2023 7:39 pm Back in the day, I'd cut mine out of cardboard. You can use anything petrol resistant that has a bit of flex to it.

The gasket that comes from AMAL is a waxed paper.

I think a rubber gasket would compress better and allow a greater disparity in the mating surface.

Ordered up some 1mm viton sheet last night, going to try to make a rubber gasket as a first fix attempt. Might be safer than trying to grind down the float-bowl 'face', at least as a first try and then we'll see how we go.

I"m not actually sure 'why' the bike has a Monobloc in the first place tbh.... Obviously the bike is a 'bitsa' special, but I'd have expected the carb to be something more suitable for a JAP speedway style single.... might ask around on the JAP FB pages to see what other people are using.

It was very likely just something Dad had lying around, or one that a mate had lying around and offered to him. It's bodged onto the head.

Get a nice Gardner on it.

I spent 20mins talking to the owner of that bike about how the carb worked, he even gave me photocopies of all the documentation to peruse over night (It was a race weekend at Cadwell and we were parked across the road from him)

So how does it work? Looks cool!

FWIW, the 1mm Viton didn't seal the leak on the float bowl, but did reduce it to a very minor weep. So i smeared some blue Hylomar on the seal and its worked a treat.

Eventually I'll probably get the float bowl ground down slightly, but for now it seems good.

Mr. Dazzle wrote: Thu Apr 06, 2023 9:51 pm So how does it work? Looks cool!

The bottom fuel pipe is connected to a tube with the needle on the end. Instead of the needle lifting from the main jet, it lowers... basically when you twist the throttle the needle drops out the bottom on the carb, that's why the pipe has so much free play on it.

Come to think of it Ricey, I might....might...have some bits of monobloc in the garage. Deffo inherited a bunch of concentric bits, including bowls. Can't remember if I've got a similar stash of monobloc. The 5TA has a monobloc carb on.

I'll have a look and see if I've got a bowl, if nothing else you could have a practice at fixing one?

Mr. Dazzle wrote: Fri Apr 07, 2023 2:12 pm Come to think of it Ricey, I might....might...have some bits of monobloc in the garage. Deffo inherited a bunch of concentric bits, including bowls. Can't remember if I've got a similar stash of monobloc. The 5TA has a monobloc carb on.

I'll have a look and see if I've got a bowl, if nothing else you could have a practice at fixing one?

That may well prove useful, thanks!

I think on the Monobloc, the bowl and body are all one piece, so only if you're absolutely sure you won't need it yourself!

I am still to take it for a ride, but it seems to me that it's spitting and banging a bit too much and i think it might be running rich.

It's fitted with a 107 needle jet, which, according to this info: https://vintagebikemagazine.com/technic ... cles/amal/, is a bit unusual, so i may get a .1065 needle jet and see what it's like.

Took the Japton around the block a couple of times....

First few hundred yards were fine, then it started playing up again. Wouldnt fire, but would eventually backfire then pick up again for a bit, then repeat.

Managed to nurse it home.

Doesnt help that the choke lever won't stay open!


I think its running a tad rich:




Its also pushing a bit too much oil i think.