1200 carbs

[slate]

Hi  i am restoring a 1200 ulimatly for my daughter, but the single solex is goosed the ball valve under tthe accelerator jet is jammed and refuses to come out, so i thought about fitting twin carbs. It has a six port head and i wondered if a manifold from a 1250 with twin carbs would fit.

Pete.

[Nick Jones]

The 12/50 has basically the same single solex as the 1200.  You can fit the 2 x 1/25" SUs from a Mk1 or 2 Spitfire though.  You'll also need the exhaust manifold as it's cast in one lump with the inlet.  It is possible to sae the inlet off (in several pieces) if you are feeling really energetic though.

NIck

[slate]

Hi Nick

thanks i wil start looking. is this what im  looking for.

Pete

[slate]

Or is this the bit i need

[Andy66]

You'll need both. As Nick said, the 12/50 exhaust & inlet are cast in one lump with the Solex carb sat on top. You'll also need the shorter front 12/50 exhaust pipe. Have you sourced a set from a 12/50 that's been converted to twin SU's???

[Andy66]

Pete,

Just a thought..... you're probably better off just trying to find a used solex carb to keep things simple. You'll have no more oomph with the SU's alone, just less money! I fitted a 12/50 manifold and downpipe to mine with no other mods and to be honest couldn't feel any difference. It still couldn't 'pull a hen off it's nest!!'

Andy

[herald948]

Arguably the most significant change between 1200 and 12/50 was the camshaft; without that, just the change of manifold likely wouldn't do much of anything. As for dual SUs on an otherwise stock 1200, one often loses low-end power but gets a bit more at higher revs. Without the cam and other recommended changes, it's probably not worth the effort. As Pete said, find another Solex or fix yours!

[stillnotfinnished]

my solex will be looking for a new home soon..... see 'herald needs more go'
ed ;)

[heraldcoupe]

herald948 wrote:

Arguably the most significant change between 1200 and 12/50 was the camshaft;

Initially yes, but later 1200 engines were identical in spec to the 12/50, except for the long branch manifold and downpipe (still with a single Solex). In that late spec, the 1200 was quoted at 48bhp, the 12/50 at 51bhp, so the manifold and front pipe would appear to be worth 3bhp.
Cheers,
Bill.

[Andy66]

Bill,

Just out of curiosity my engine no. is GA 212400, if it's the original lump, I presume 'late' engine.

Cheers,
Andy

[herald948]

heraldcoupe wrote:

Initially yes, but later 1200 engines were identical in spec to the 12/50, except for the long branch manifold and downpipe (still with a single Solex). In that late spec, the 1200 was quoted at 48bhp, the 12/50 at 51bhp, so the manifold and front pipe would appear to be worth 3bhp.

True, Bill, but don't forget there was also a change in compression ratio from 8.0:1 to 8.5:1 in that particular "late" spec. So while it's possible the long branch manifold was worth 3 hp on a later spec., engine, it might not have been worth more than 1-2 hp. on the early engine. Besides, I doubt a 2-3 hp increase, even having started with only 39 hp, is very noticeable except via stopwatch! >:

[heraldcoupe]

herald948 wrote:

So while it's possible the long branch manifold was worth 3 hp on a later spec., engine, it might not have been worth more than 1-2 hp. on the early engine.

But we don't know whether the OP has an early, late or some mid-period engine.

Quote:

Besides, I doubt a 2-3 hp increase, even having started with only 39 hp, is very noticeable except via stopwatch! >:

There is quite a noticeable difference in tractability when the 12/50 manifold is fitted, with no other modifications being made. This has been with late 1200 engines, and I agree that the difference is most likely to be less noticeable with an early spec engine.
When 12/50 manifolds were more easily obtained, this was a very common 'nuts & bolts' improvement in the UK. That manifold is now much harder to obtain, which is something of a shame,
Cheers,
Bill.

[herald948]

heraldcoupe wrote:

But we don't know whether the OP has an early, late or some mid-period engine.

True! Even then, we also don't know if, after the better part of 50 years, said engine might have been rebuilt to a later specification with a CR boost and/or a change in camshaft, etc.!

heraldcoupe wrote:

There is quite a noticeable difference in tractability when the 12/50 manifold is fitted, with no other modifications being made. This has been with late 1200 engines...

That does not surprise me at all. What does surprise me is that Triumph bothered to continue with the older manifold, which I have to assume was more restrictive, once that 18/58/58/18 camshaft and 8.5:1 CR were commonised across all the Herald 1147cc engines. I mean, that older manifold wasn't much different than the one dating back to that of the 803cc Standard Eight and the 948cc Standard Ten/Herald engine, except for the size of the carburetor and minor change to headpipe flanges.

[Paudman]

heraldcoupe wrote:

When 12/50 manifolds were more easily obtained, this was a very common 'nuts & bolts' improvement in the UK. That manifold is now much harder to obtain, which is something of a shame,
Cheers,
Bill.

Bill - any opinion as to how do the currently available tubular manifolds perform in relation to the old 12/50 one?
Colin

[slate]

Thanks for al the info. considering the cost involved in coverting for only a modest gain, i think a reconed solex is in order  as mine is past it.

Pete

[heraldcoupe]

490 wrote:

Bill - any opinion as to how do the currently available tubular manifolds perform in relation to the old 12/50 one?

It's not possible to make a real-world comarison as there is no discrete single-carb inlet manifold to go with the tubular exhaust. The exhaust side of the 12/50 manifold however, is physically similar to the cast item used with the factory TC setup, and matches to the same downpipe. Whether that means the performance characteristics are similar, well, I wouldn't want to assume anything!
Cheers,
Bill.