Dual Carb questions RE: Choke and Float chamber

[Andymoor94]

Hi guys,

Needed a single H1 Carb, only thing available at semi decent price was a pair of them for a frogeye - Some may have seen them on eBay!

I want to have a crack at using them together, for no practical reason other that a bit of fun before becoming sane again.

Below are a couple of pictures that I'm questioning. Any help appreciated!

[philthehill]

See the link below for an exploded view of the H1 carb set up.
https://www.moss-europe.co.uk/shop-by-m ... 58-66.html

You will need item No: 25 to join the chokes together. You can see the full set up top left of the link picture in the circle.

The pipe is the breather/overflow from the float chamber. They are usually a lot longer than shown so as to get below the exhaust manifold.

Whilst nice to have the carbs will give little if any improvement in performance.

[Andymoor94]

See the link below for an exploded view of the H1 carb set up.
https://www.moss-europe.co.uk/shop-by-m ... 58-66.html

You will need item No: 25 to join the chokes together. You can see the full set up top left of the link picture in the circle.

The pipe is the breather/overflow from the float chamber. They are usually a lot longer than shown so as to get below the exhaust manifold.

Whilst nice to have the carbs will give little if any improvement in performance.

Hi Phil,

Thanks for another thorough answer! You really are a fountain of Moggy knowledge.

From previous research on the forums, I've seen the little effect that dual carbs have, I can imagine less so on my engine, but for the sake of having 2 now, though I might aswell have some fun!

Stirrup and bracket have been ordered (the bracket is also missing) and I'll have to fashion together the fittings that hold them together because God are they expensive and/or on backorder.

Thanks again

[alanworland]

If you search back through my posts I fitted a pair to my sidevalve. Constructed inlet manifolds etc and those overflow pipes were long enough to get to top of sump level.
All together it was a lot of work and as Phil says, improvement was minimal. I did measure (it was measureable!) the times taken on a local road before and after the mod over a distance and yes it was improved.
But would I do it again? No!

Alan

[Andymoor94]

If you search back through my posts I fitted a pair to my sidevalve. Constructed inlet manifolds etc and those overflow pipes were long enough to get to top of sump level.
All together it was a lot of work and as Phil says, improvement was minimal. I did measure (it was measureable!) the times taken on a local road before and after the mod over a distance and yes it was improved.
But would I do it again? No!

Alan

Thankfully, it's not about improvement for me! I have said that a few times now

[alexmcguffie]

I fitted twin 1 1/4" many years ago to my 1000 because a par were available. The other thing I found at the time was a 12G295 head. The pair together gave a noticeable performance improvement (by 1990s standards!). The main improvement was fuel consumption. On long steady runs I was managing 40-50 mpg!

[philthehill]

The 12G295 head is the best small block head available. Even in standard form will give a good increase in performance.
If fitting the 12G295 head to a small bore block it is best to give the head a 0.060" skim to get the combustion chamber to the right size.
Because of their performance benefit they do now command a hefty price of around £300.
A 12G295 head and twin 1.25 HS2 carbs will provide a noticeable improvement in performance.
Adding a LCB exhaust manifold will improve things even more.

[Andymoor94]

The 12G295 head is the best small block head available. Even in standard form will give a good increase in performance.
If fitting the 12G295 head to a small bore block it is best to give the head a 0.060" skim to get the combustion chamber to the right size.
Because of their performance benefit they do now command a hefty price of around £300.
A 12G295 head and twin 1.25 HS2 carbs will provide a noticeable improvement in performance.
Adding a LCB exhaust manifold will improve things even more.

Thanks Phil. I'd have thought for an 803cc though, this wouldn't be advised? I been ushered away from doing many performance upgrades purely for the fact that the bottom end is weak and fragile

[liammonty]

The 12G295 head is the best small block head available. Even in standard form will give a good increase in performance.
If fitting the 12G295 head to a small bore block it is best to give the head a 0.060" skim to get the combustion chamber to the right size.
Because of their performance benefit they do now command a hefty price of around £300.
A 12G295 head and twin 1.25 HS2 carbs will provide a noticeable improvement in performance.
Adding a LCB exhaust manifold will improve things even more.

Thanks Phil. I'd have thought for an 803cc though, this wouldn't be advised? I been ushered away from doing many performance upgrades purely for the fact that the bottom end is weak and fragile

Absolutely right, Andy. A 12G295 head, LCB or indeed twin HS2s would be bonkers on an 803 and would destroy it very quickly. These mods work very nicely on 948s and 1098s though.

[Andymoor94]

The 12G295 head is the best small block head available. Even in standard form will give a good increase in performance.
If fitting the 12G295 head to a small bore block it is best to give the head a 0.060" skim to get the combustion chamber to the right size.
Because of their performance benefit they do now command a hefty price of around £300.
A 12G295 head and twin 1.25 HS2 carbs will provide a noticeable improvement in performance.
Adding a LCB exhaust manifold will improve things even more.

Thanks Phil. I'd have thought for an 803cc though, this wouldn't be advised? I been ushered away from doing many performance upgrades purely for the fact that the bottom end is weak and fragile

Absolutely right, Andy. A 12G295 head, LCB or indeed twin HS2s would be bonkers on an 803 and would destroy it very quickly. These mods work very nicely on 948s and 1098s though.

Thanks Liam - I thought I wasn't going bonkers!

[philthehill]

The reply I gave was a generalised response to the post by alexmcguffie on the 17th April re his fitting a 12G295 head to his Minor and for those that are not aware of the benefits of the 12G295 head on a small bore block.
There was no reference given to fitting the 12G295 head to a 803cc engine only to a small bore block, though it will fit on the 803cc engine as will twin HS2 carbs.
Whilst the 803cc engine is not suitable for major tuning it can be tuned to give extra performance if done properly. See links below:-

https://www.oldclassiccar.co.uk/a_series_803cc.htm

https://www.oldclassiccar.co.uk/aquaplane.htm

Phil

[liammonty]

The reply I gave was a generalised response to the post by alexmcguffie on the 17th April re his fitting a 12G295 head to his Minor and for those that are not aware of the benefits of the 12G295 head on a small bore block.
There was no reference given to fitting the 12G295 head to a 803cc engine only to a small bore block, though it will fit on the 803cc engine as will twin HS2 carbs.
Whilst the 803cc engine is not suitable for major tuning it can be tuned to give extra performance if done properly. See links below:-

https://www.oldclassiccar.co.uk/a_series_803cc.htm

https://www.oldclassiccar.co.uk/aquaplane.htm

Phil

I appreciate that more power can be had from the 803 - fundamentally, the design of the cylinder head is as on the 948 and 1098 (though hampered somewhat by the small bore necessitating tiny valves). The issue, though, is the bottom end - white metal bearings, bypass oil filtration, and a crankshaft described by David Vizard as "A crank that resembles a bent piece of wire". I see that the Aquaplane links (very interesting!) don't make any mention of work to the bottom end of the engine, yet offer up to 40 bhp at 6000 rpm! The poor old 803 wouldn't hold up for long in that state of tune!

[philthehill]

If you were going to extract 40bhp from a 803cc engine you would make sure that the 'bent bit of wire' was appropriately worked on. For example Tuftrided and balanced in line with general tuning principles.
The bent bit of wire in my 1380cc was reworked, Tuftrided and balanced and has been known to rev to in excess of 10,000rpm and give 135 bhp plus - nearly twice what the crankshaft was designed for.
Doing the basics will make the 803cc engine reliable and allow for an increase in power. It is no good doing only part of the job if you want reliability and power.
The ethos of Aquaplane was performance with reliability.

[Andymoor94]

Well the good news is I was able to fashion a blanking plate for the exhaust manifold and mount everything up. Set the correct float valve clearance and checked floats, centred the needles and I THINK have successfully balanced them, albeit with tubing to my ear.

Definitely running rich, even at maximum adjustment so I presume my next step is needle selection. The research I've done is rather confusing and seems almost reliant on knowing what needles are in my carbs to begin with!

If I'm right and need to change needles, how can I identify the needles in my carbs currently and what the best course of action to choose their successors?

Or am I totally on the wrong track and should be attempting something else to help the mixture?

I must say, I'm having a lot of fun with this though

[philthehill]

The needles are stamped with a letter and/or number for identification on the part/shank that fits into the damper.

Here are three needles suitable for your set up.

Weak needle............................MOW.

Standard..................................GG.

Rich.........................................EB.

[Andymoor94]

The needles are stamped with a letter and/or number for identification on the part that fits into the damper.

Here are three needles suitable for your set up.

Weak needle............................MOW.

Standard..................................GG.

Rich.........................................EB.

Thanks Phil.
I've had confirmation from the seller that GG's were fitted to these carbs (come from a 948 Sprite).

I was just about to edit my post actually before I saw your reply, to amend my diagnosis of running rich;

The sound of the exhaust is very rhythmic as opposed to smooth, but it seems that the desired RPM effect is given when lifting the pistons on each carb, so I'm not sure what to think. One symptom says running rich, the other says mixture is fine!

[philthehill]

If when you lift the carb piston the rpm rises and the engine does not stall the mixture is about right. Road testing will confirm either way if the mixture is correct or not.
I would use and enjoy.

[Andymoor94]

If when you lift the carb piston the rpm rises and the engine does not stall the mixture is about right. Road testing will confirm either way if the mixture is correct or not.
I would use and enjoy.

I aim to, now it's all on and plumbed it - The weather seems to be calling for it too