I've been tinkering around trying to get my new motor nicely tuned up.
I think I've got the timing pretty much spot on, but found it would die when I went to take off. To get it to run properly I've had to unscrew the mixture on the 1.25 SU, about 21 flat turns, is this too much? I've read that it should be unscrewed between 12 and 15 flats.

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If the problem arises when you go to take off then it could be a problem with the accelerator pump effect provided by the damper. have you checked your damper oil recently?

Running rich will tend to mask this. My car was running very rich when i got it and when i leaned it out i had to change carby spring. i also had to install a choke which it never needed before because it was so rich

edit: sorry, just realised that i haven't answered your question If the rest of the carby is functioning correctly then it should be ok, i had adjust mine a lot towards the rich side as well. It will depend on what needle you are using.

Fuel economy is a good way to get an idea if the car is running rich or not

I'm just running 20-50w motor oil in the damper.
The engine would rev normally when in neutral but would stall once I tried taking off,
but it would run with the choke out.
It sounded like it was running out of fuel, I checked the bowl and it was full so I thought the pump was working ok.
It's a bit hard to check feul economy as it isn't registered just yet, although I think my fuel tank/line is leaking as I found a wet patch underneath the car.

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try running auto trans flid in the damper a lot of people here me included swear by it

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I originally put Auto trans fluid in it but when I discovered this problem I changed the oil to 20W-50 motor oil. I might change it back and see how it performs.

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try lighting the wet patch under the car on fire - you will soon work out if it is petrol
(don't blow anything up though) you should be able to smell it anyway.

It sounds like a problem with the dashpot or spring if it will idle ok. try revving it a bit (holding it at a certain speed above idle) and then turning off the choke and see if it stalls. this will indicate whether the needle is too lean or whether it is just because the "accelerator pump effect" is not working properly. I can explain what i mean by "accelerator pump effect" too if you want

I just took it for a squirt up the road under the cover of darkness, but it's still doing it. When I stopped to do a u-turn it died, I started it ran ok for a bit then wouldn't accelerate. I think it might be the fuel pump.

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try smelling the oil on your dipstick or lighting it on fire using a match. If it smells of petrol or burns then you have fuel in your oil and it is the fuel pump. If it doesn't smell or burn then it could still be the fuel pump

Can you get a fuel pump kit?

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Richarde,

Please explain the "accelerator pump effect"

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yep. couldn't explain before because i was working (in theory)

When you put your foot down on the accelerator reasonably hard the engine needs a richer mixture because a whole lot more air rushes into the engine all at once. Many carbies have an accelerator pump or accelerator jet that gives an extra squirt of fuel when you put your foot down so the engine gets enough petrol.

In an SU carby, on idle, the piston/plunger sits in the way of the incoming air so that only a small amount of air can pass underneath the piston. The piston is held up by the vacuum that the engine is producing so higher rpm lifts the piston higher. The spring helps hold the piston down too. The air picks up fuel as it flows over the jet due to the venturi effect (you should be able to find this on wikipedia if you don't know what it is).

The way that the SU achieves the accelerator pump effect is by having a dashpot on the carby. This stops the piston from moving in a hurry. So when you put your foot down, more air starts rushing into the engine, but the piston will only move up slowly. You then have more air rushing through the gap under the piston, which will enhance the venturi effect and more fuel will be picked up, richening the mixture.

If the consistency of the oil in the dashpot is wrong or there isn't any oil in there and the piston goes up too quickly then you will get a flat spot in acceleration when putting your foot down. If the oil in the dashpot is too thick then the piston will fall slower and won't have enough time to fall back down when you change up gears, so you will get a flat spot on each gearchange.

The spring effectively sets the position of the piston on idle - the weaker the spring, the higher the piston will sit, and vice versa. If the spring is too weak (as it was on my car) then the accelerator pump effect may be too weak because the piston may be sitting too high up and the sudden rush of air when you put your foot down may not provide a large enough venturi effect.

The accelerator pump also depends a bit on needle choice so everything is nicely interrelated and it can take a bit of messing around to get it right. It isn't all that hard though - just change bits and pieces and see what happens

sorry if I didn't explain well. if you have pulled apart an SU carby before you should be able to work out what i am talking about, otherwise you should be able to work it out if you can find an exploded diagram

And now you see why i didn't want to explain while i was at work

I don't know about the fuel pump kits either because i have never needed one, but i don't know if they are repairable anyway - would probably just be cheaper to find a second hand one

Thanks Richarde, that all makes good sense. I have pulled down an SU so I'm pretty familiar with its parts, but now I understand what most of the parts do. When I bought the car I replaced the carby as the old one was leaking through the throttle linkage.
I've still got the old one and I'm wondering if I can try using some of the parts off it, like the spring to see if it makes any difference.

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Yeah, it is probably a good idea to compare springs and needles. the dashpot oil shouldn't be too critical - it should run ok with 20W50 although this is probably a tad thick. If the engine was running with the old carby you may want to put that back on to determine whether it is a carby problem or not.

You can compare the springs just by looking at them and testing their strength (don't mix them up though ).

To compare needles you need to get the code that is written on the fat end of the needle (it is actually on the part that is inside the piston). To remove the needle undo the screw that is holding it in then just pull it out. If it is hard to get out (normally the case if the carb is a bit dirty) then i have found that the best way to get them out is to get a piece of insulation off some electrical wire and slip this over the needle and push it up to the end near the base somewhere. You can then pull the needle out with pliers without damaging the needle.

The needle is precision machined and it is very important that you do not damage it or bend it. you may want to examine it for bends too, as this could cause the car to act strangely as you have described. You can also check the needle to see if there is evidence of filing. some people file needles to adjust the profile and if someone has done this then the profile won't match the number stamped on the needle anymore.

I guess it is probably not that good practice to change needles without changing jets as well because they are wearing components but i don't think it really matters that much

As said above,you should use transmission fluid in the damper.
I use it in both my minis and my mgb and never had any problems.
Felix

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1966 mustang coupe 289 auto
1970 mgb roadster auto
1970 cooper S mk2
1976 mini S
1964 wolseley 24/80
http://s160.photobucket.com/albums/t173/1966mini/

I discovered that the jet was sticking open sometimes after using the choke which was causing the engine to run rough.

I changed the dashpot needle and jet over to the ones from my old carby and put transmission fluid in the dashpot.

While I was at it I decided to inspect the fuel pump and found a chunk of rubber almost completely blocking the line. This must have been causing a lack of fuel when I took it for a drive.

I disconnected all the fuel lines and blew them out and discovered the hose clamps were dodgy causing a slight leak.

Then I removed the fuel tank to see if there was any crap in there and found a piece of cardboard about 75mm x 250mm in the bottom

Car is running really well now

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