Increasing valve size and improving exhaust flow without changing carburettor? And a puzzle for you all.


1923 Gwynne 8
Hi All

Dom here from the UK. Thanks in advance for your expertise/input.

Ok so this is a bit of a wierd query but I need some help and all will become clear .... I Hope.

So I recently had a damaged (frozen - i.e. split head and block) engine rebuilt by a garage.
(A 1923 Gwynne 8 engine - ohv, cross flow 950 cc - very like an austin mini 'A' series engine but early)

I had a spare head with slightly larger valves so I had them use that. (It was skimmed so there might have been a very slight increase in compression - but not much)
I also had a better steel exhaust manifold that was going to be more free flowing than the original cast iron one so I had them swap that out too.

So my first question is this:
If we then fitted the original carburettor with no changes is there any way we could expect a reduction in performance?
(I would have expected a slight increase or none but I would not expect a decrease)

My second Question:
If we then changed the jet/needle am I correct in assuming thet the induction system would no longer be the limiting factor and we could expect an increase in performace?

Before you answer I think its only fair I give you the whole rather sad story .....

The garage broke the throttle lever on the carb.
It was cast bronze and slightly bent (apparently) and they tried to straighten it.
But because it was a casting it snapped off like a carrot (as my dad would say).
Schoolboy error - castings are brittle.

Anyhow I arrive shortly after this to discover that the boss is trying to solder the broken bit back on while it is still attached to the carb.
Imagine my horror to discover a blowtorch aimed at my lovely (and pretty much ireplacable) 1923 Sloper SU carb!

Despite my scepticism I am assured all will be well.

Well it isnt.

Once reassembled the car wont run properly. It will no longer tick over slowly and miss fires at slow speeds.
So the car now runs very badly and there is no increase in top speed.
(52mph .... yeah yeah I know .... well it feels fast when you dont have any front brakes and the brake pedal and the throtlle pedal are the wrong way round....... well maybe not fast ..... but ..... exciting.)

Anyway according to the proprietor the poor performance is because the engine has been tuned but the carb and inlet manifold have not been improved.
I am skeptical but what do I know I'm an architect not a mechanic.

But I take the lid off the carb barrel and discover that the piston unit no longer slides up and down smoothly.
It sticks at the lower end i.e when the car would be running slowly or at medium revs.
At this point the boss concedes that heating the carb with a blow torch may have warped it.

Anyway roll on a couple of weeks ......

They have stripped the carb and worked on it and now the piston moves freely. I dont know how they achieved that.

Still not running properly though.

Apparently It needs a new jet/needle I'm told ..... so one is ordered.

It still doesnt run right - to get it to run at all they have had to open the two valves that are built into the inlet manifold to let in additional air.
(NB possibly Question three ...... what could these be for? none of the owners of Gwynne 8 s have a clue and nobody ever opens them)

Anyway now the car ticks over so fast you cant get it into first (no syncros) without a horrible graunching noise and now it will only do 40pmh.

I give up and go and get the carb off my other car. Better manifold and slightly bigger carb.

The car now runs properly ..... if anything it feels like it pulls better than it did before the rebuild.

But ....... interestingly ..... no increase in top speed at all.
Question 4: Assuming we have an overall increasse in power (To be confirmed) why no increase in top speed? Valve float? (we didnt fit stiffer springs)

So anyway.....

I have tried to make it a mildly entertaining tale ......and would really appreciate your help.
NB . I havent paid the bill yet.

To recap.

Q1. If you upgrade valves and exhaust but not the carb will you get qa reduction in performance?
Q2. Is rejetting the original carb usualy enough to get some increase?
Q3. Why would a 1920s car manufacturer expereiment with fitting little valves you could open to let air in on the inlet manifold.
Q4. If you have increased valve size, improved inlet and exhaust what might tehn limit top speed/revs.
Q5. Do you think they broke my carburettor?

Thanks Chaps

Oh dear oh dear ...and I thought I had problems.! The first thing that I would suggest would be to reinstate the original exhaust manifold. I have a suspicion that your problems are related to back pressure. Incidentally, is it possible the extra valves are for decompression as an aid to starting?

As to the garage you have been using; I would go somewhere more sympathetic towards vintage cars. In my opinion, they should pay for your poor old SU sloper to be properly repaired. What they did is little short of criminal damage.!

Last edited:
Thanks Ray
sadly it looks like they are going to come up with all sorts of BS rather than acknowledge fault
Boring ... and fairly consistant with my experience (58 year old architect) . Most people dont know what they are doing.
ahhh well
MY apologies for not paying enough attention to what you were saying. So the extra air valves are in the inlet manifold and not the cylinder head. I don't know the reason for them. All I would say is your mixture may be weakened which could cause more problems.

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