Carb alignment with hole in seat tube

[shaunstaples]

Just wondered if everyone else’s carb is out of line with hole in seat tube?
Mine is nearly half a hole out, with carb towards r/h side of bike. Existing monobloc is tight and not enough room for studs to be used in manifold. I can just get a gauze on with a jubilee clip for a crude filter.

[shaunstaples]

I’ve bought a concentric because it’s about 1/4” shallower but the fuel banjo is fouling where the seat tube angles forward under the hole.
In the last picture I’ve got an 1/8” packing in at the flange, the carb is near enough to get a rubber tube on.
If everyone else’s is the same? But I’m worried that something else is out of line causing it?

[Groily]

It was because of some of the issues you mention Shaun that I tried to persevere with the original Type 76 carb on mine, until I got so tired of leaks that I went 376 Mono half a lifetime ago.
Your lower pic shows alignment of the Monobloc pretty close to what I've got. Hard to be too certain, to a millimetre, but 'looks about right' to me. The lower lip of the air intake on my carb is about level with the lower edge of the hole in the frame member. (But, mea culpa, I don't have a hose or filter.)
I seem to remember when trying a 626 Concentric that it didn't want to play, same as your fun and games, but memory is a bit fickle so I'm not going to say more.

Ref the studs vs bolts thing, I have managed to retain studs and nuts with a 376, but it is tight and the studs can't be longer than flush with the nuts, and the nuts can't be too deep and there can't be a thick washer 'twixt carb and manifold. If there's one thing I hate, it's having to use bolts into ali for something that has to come on and off.
For a while I had to remove the battery carrier to wiggle the thing onto the studs from the drive side with encouraging cursing as a background theme tune, with the feed banjo on top of the float chamber removed, but then I got bored and ground a crescent moon off the leading edge of the battery carrier to get clearance. Still have to take the battery off, but can get the carb off in about 10 mins if I must.

[shaunstaples]

Thanks Bill, it’s reassuring to know “it’s not just my then”.
With the 1/8” packing in it lines up better but not perfect for a hose.
I’ve managed to get another manifold with same number on it as mine, and I’m going to have a go at facing both faces of the manifold with a 1/16 offset to see if I can replicate the packing.
The manifold I got has an o ring grove cut in it whereas mine looks like it has been machined off in the past, maybe to help carb removal?
I would love a pre monobloc, but my pension won’t stretch that far!

[Groily]

I would love a pre monobloc, but my pension won’t stretch that far!

They look good Shaun, but not sure if you WOULD love one in practice. The factory went Monobloc for good reasons. I know some people's pre-monos are pristine and leak-free, but even with a 'new' one from Surrey Cycles some years back (£££ even then), I couldn't keep it drip-free for very long at a stretch. And I don't like things that trickle petrol over magnetos . . !
Good luck with mods to the manifold, could be a good plan methinks. Wonder how much the congestion in this area prompted the revision of the frame geometry to 'vertical' later on.
It also annoys me, incidentally, how tight the run of the advance and retard cable has to be to clear the manifold. One time, I made a cable with the adjuster somewhere at mid-point rather then at the mag, which helped a bit. Another time I made an auto-advance to sit in a much-reworked spare camshaft gear so I could use a fixed cam mag. But nowadays I just put up with the foibles, mutter mutter and carry on. Leaves me more time to fight the regularly recurring oil weeps at the dynamo to timing case joint!

[ajscomboman]

The set up for carb mounting using a mononloc was bolts originally due to clearance issues. My 55 Mod 20 used them and whilst not as easy as studs they aren't too bad. The hole never lined up with the carb, the optional hose was an off set type. Later twins had to go back to bolts again due to clearance.

[raffles]

carb fitting becomes easy if you use the smaller headed front mudgard fitting bolts

[Duncan]

If you look at a twin engine head steady you will notice that it is not symmetrical but is off set to one side, this means that the barrels and heads are not evenly spaced around the centreline of the frame leading to the carburettor being offset slightly too.

[shaunstaples]

They look good Shaun, but not sure if you WOULD love one in practice.

It would be nice to experience the bike as it was back in the day, but loads of other things I need to spend my Christmas wad on...

carb fitting becomes easy if you use the smaller headed front mudguard fitting bolts

I was thinking of thinning the bolt heads, the concentric is much tighter where the casting bulges for the tickler and that's on both sides.

The hole never lined up with the carb, the optional hose was an off set type.

The offset hose, is it to align side to side or up and down? mine is down a bit and nearly 1/2 hole to R/H

If you look at a twin engine head steady you will notice that it is not symmetrical but is off set to one side, this means that the barrels and heads are not evenly spaced around the centreline of the frame leading to the carburettor being offset slightly too.

Could it be possible for it to be fitted the wrong way round ?

[shaunstaples]

I’ve been preparing for getting the faces machined tomorrow if all goes well.
Hoping there will be enough clearance on the studs if I open holes up to 5/16”, I’ll try and drill them while it’s bolted down on offset face.