h4 questions

[mk1]

What do you want to know Andreas?

[su_mark]

See below spec AUD 108 which is pretty much the same as AUD 165, but for the 1070 instead. This one is a bit clearer!
Note the difference in needles, 1275 - BG, 1070 - MME. Also note (bottom right in image) jet AUC 8188 0.90" thermo jet proving the 'no chokes' theory.
Interestingly I can find no mention of this being specifically for LHD cars....

AUD 108a.jpg

[mk1]

That is a brilliantly clear copy. Thank you for sharing this part of the official SU archives on the forum.

[ricardo]

Pictures of the actual carbs (AUD165):

[almondgreen]

How does the Thermojet work ?

[rich@minispares.com]

How does the Thermojet work ?

it just removes the choke operation, its like a little brass blanking plug

[almondgreen]

Is it a fix system, or does it change when it is warm(hot)

[mk1]

It is a totally fixed system. The jet position is fixed when the carb is set up & that is that. Having this set up on both carbs was perfectly OK on a race engine, particularly back in the 60's when people expected them to be difficult to start.

As has already been said, if you use this set up on one carb & a normal choked set up on the other they start fine & you don't need to be Superman to pull the choke knob on.

[mk1]

PS. I remember selling Pete a set of H4's years ago that had the above 1 choked & the other not & he moaned a bucketfull that "loads of bits were missing" shows we all live & learn doesn't it

[su_mark]

It is a totally fixed system. The jet position is fixed when the carb is set up & that is that.

When we say 'totally fixed', that is not to say you can not adjust the jet up and down for mixture control, because you can, by means of the little screw under the jet which becomes accessible once you remove the brass cover.

[su_mark]

For clarification, see below exploded image. Items 14a-14m.

IMG_20160226_122720.JPG

[su_mark]

The term 'thermo jet' comes from the fact that the carbs that employed this type of jet used a small solenoid operated thermo starter device instead of the conventional choke system.

[steve1071]

I'm just rebuilding a set of ST H4's for my 1071. Does it matter which carb loses the choke? (Right hand drive) Also, which jet's / needles should I use on a 1071 with Downton head?

[mk1]

As you are looking at them from the front of the car the fast idle goes on the left hand side of the right hand side carb between the two carb bodies. The levers both point inwards.

Re the needle, what cam are using?

[steve1071]

As you are looking at them from the front of the car the fast idle goes on the left hand side of the right hand side carb between the two carb bodies. The levers both point inwards.

Re the needle, what cam are using?

Thanks Mark, not decided yet but probably a 544.

[mk1]

In that case you will want MME's

[abri]

On the topic of H4 carbs...

I have a freshly rebuilt 1275 S with stock AEG510 cam that's running an unmodified 940 head (apart from seats nicely cut), Minispares 1.5 ratio rockers and an Maniflow standard bore LCB. I'm currently using the standard twin HS2 carbs with M needles.

I have recently got hold of a set of H4 carbs. With the setup as described above, would there be any benefit in swapping the HS2s for the H4 carbs? The car will mostly be used on the road (99% of the time) and might do a track day in future. Obviously the H4 carbs are nice to look at but I won't fit them just for that reason. If I can expect a meaningful performance improvement I would. What do you think? Overkill??

While on the topic of needles....

Which needles would work best for this engine that I described earlier?

[Austin Costin]

I have just fitted a pair of HS4s to a stage 3 1293, they are the type with full floating needles (HS6 ?) would anyone have an idea of what needle to use ?
Cheers Paul.

[Andrew1967]

What sort of price do H4's go for (needing a rebuild) ?

Thanks

Andrew