I'm trying to get my special running sweetly, and have an issue with acceleration.
I have an 1 1/8 carb from a morris 8, which I've replaced cork seals and gaskets on, and fitted AN needle. Bunch of bananas manifold, Cambridge finned head. ticking over nicely.
It goes ok, but the acceleration isn't fantastic, it pulls, but then as I open the throttle more at the top end it stutters and there's no progressive acceleration. It also seems that if I drop off revs, and slowly open the throttle, it goes better.
It doesn't like hills much- the hill out of my house is a steep one on to the Mendips, but it won't go up in top, which I thought it should, even if its at a steady rate rather than ascending it very fast. Anyway, third is necessary.
When I got the carb, it had a damper and no spring, but with the damper in place I could hardly lift the piston it was so tight, so I removed it, and put in a blue spring.
Probably not the ideal thing to do, but it seemed like a good idea.
The mixture screw is 9 flats out, the timing seems ok.
It's not awful, but not quite right.
Help needed
Thanks
14-08-2022, 04:09 PM (This post was last modified: 14-08-2022, 04:11 PM by Reckless Rat.)
Steve, this sounds like the same carb I have on my RP - it's an SU H1. As an experiment, take the dashpot off and drain out the oil from the piston's reservoir. Remove the blue spring, you don't need it on this model. The weight of the piston is sufficient
Replace the dashpot and make sure that the jet is centred and the piston can rise and fall without sticking.
Try it dry of oil ( 3 in 1 is OK, nothing thicker) and if the piston rises too quickly add a drop or two. Don't use too much as the low vacuum of an A7 may not be enough to fully lift the piston and the oil damper will not help.
14-08-2022, 04:12 PM (This post was last modified: 14-08-2022, 04:15 PM by Steve Jones.)
As original, your carb would have had either a bronze piston or an alloy piston with a steel weight. Neither would have a spring. What piston does yours have? Put the carb back as you got it and fill the damper with a light oil and see how it goes.
Steve
Edit. Crossed with Bruce's post above but we're saying the same thing.
Thanks for this, it's an aluminium piston rather than brass.
I will have a go as you suggest and see how it goes.
Why do you think the damper makes it so difficult to lift the piston? Without it, it moves nicely and drops back with a clean clunk.
Hopefully removing the spring and limiting the oil will sort things out.
14-08-2022, 04:36 PM (This post was last modified: 14-08-2022, 04:40 PM by Alan.)
Depends what oil you have in, probably. If it’s engine oil the piston will be stiff to use. Specialists sell special damper oil but I’ve always used 3in1. You could try cleaning the piston and damper in petrol and trying them dry. They should move quite easily then. But 3in1 is what you use.
But does the alloy piston have a steel weight on top? If it doesn't, it might be you have the wrong piston for the carb. I've 6 Morris 8 type 1 1/8 SUs and have had them fitted to cars for many years. None have a spring, all have weighted bronze or alloy/steel pistons.
The damper makes it difficult to raise the piston because that's its job. The piston shouldn't be easy to lift as that means it will rise too quickly under acceleration leading to the symptoms you describe.
I see- thanks a lot for these responses.
I was confused by how easily the piston lifts with just the spring, and the great difference in resistance with the damper in place, but it seems that is what it should do. You live and learn.
I will have a look at the piston and check that it it has a weight.
I am using 3 in 1, so that should be ok.
Will give it a go tomorrow.
I use an SU H1 on a reproduction "Nippy style" manifold, rest of the engine is standard. My initial needle choice was GG, and as it seems to run very well I have never experimented further. From your description it sounds like something is seriously amiss.
These early carbs rely on piston weight alone (no spring), and have an oil damper. The piston/dashpot assembly is built with precise clearances, and the original damper oil recommendation was SAE20. I can see how over-thick oil could delay the rise of the piston (causing temporary richness) but not how it could affect its steady state position. Pushing the piston up with the fingers should require a reasonable effort.
I recommend you clean things out and try thin oil as a first step. If this brings no improvement, it's possible that someone has fitted a piston from another model
On the road, the airflow needed for full piston lift equates to something over 20 BHP so this will only happen on a fairly tuned engine. Partial lift doesn't seem to matter, but does mean that the needle profile at the lower stages is the more important.
And as usual when discussing fuelling, check the ignition is OK !
Like John, I have run a GG needle in my inch and an eighth SU for years. Did so as a result of early advice and never felt the need to experiment further.
