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Excellent, thanks for that.
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Change the air filter [u]first[/u]....
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Not enough info, which SU what two ajustments do you mean tick over and mixture ie. on the throttle cam and the nut at the bottom of the carb. Is the card old/ done 60k miles and whay grade of oil have you been topping it up with. You have been topping it up haven't you. Regards Peter
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Hiya Peter, will tell you as much as I know as its hubbie & cousin who have been rebuildling her. Got so far, but cousin is used to fuel injection metro's & isnt too hot with carbs. You are correct I'm referring to the adjustment on the throttle cam & nut at the bottom. A 1275 engine was fitted with a HIF44 carb, it was old and started burning oil, so its now out and waiting rebuild. Thought it would be quicker to pop old engine & carb back in but keep the the rest of the set up which Mini Sport said would work fine but to change the needle which we have.
Bit more info - starts 1st time but bit rich even with choke fully pushed in. Revs seem quite high too.
1980,
998cc engine (A+ series),
HS4 carb,
LCB manifold (new),
pipercross cone filter (new),
inlet mainfold from a 1275 engine (new).
Carb has been topped up regularly, but has now been rebuilt - have not a clue what grade of oil though.
Hope this helps.
Daisy1
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Daisy1,
Don't get too worried about the grade of oil in the dashpot. It only makes a difference when the dashpot is rising, i.e., when you are accelerating. It restricts the rate of rise of the piston, and in effect works a bit like an accelerator pump. For setting up the basic idle condition of the engine, it is a red herring. Indeed you can tweak the viscosity of the dashpot oil to vary this accelerator pump effect if you were so inclined.
It is very important to make sure that:
the inside of the dashpot is scrupulously clean
the needle is fitted with its collar at the right height
the throttle spindle isn't excessively worn
the jet moves freely up and down when you adjust the nut
there are no inlet manifold / vacuum pipe air leaks
I tend to get an initial setting by lifting the dashpot a bit, and listening to the revs. You should aim to get a slight increase in engine speed for the first part of the motion, then a decay in speed as the dashpot rises.
You can always check for a weak mixture on the road (when the dashpot is not at its idle position) by pulling the choke a little to see if it goes better.
number_cruncher
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NC
Excellent advice to get the basics right first before tweaking. I'd add:
Make sure the dashpot moves up and down smoothly without any catching over its whole range of travel.
Check the condition of the float needle valve - in fact I'd replace it as a matter of course on an old carb, and then make sure the float level is correct - both are potential causes of over-rich mixture.
JS
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I agree, and some of the carbs including this one from memory have a small button to lisf the slide to check the revs increase. Screwing the adjuster under the carb in weakens the mixture and undoing it richens thew mix. Is you have a the correct new needle and jet it should set up fine. Grap an older manual from a library on astin cambridges, minis or A35 and metros for more info is this forum doesn't fix your problem. Regards Peter
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You are wrong to say the grade of oil does not matter, because it does by having the wrong grade will effect the mixture and engine response.
Where I worked for a BL garage, we used ATF oil which Rolls also used.
Don't what ever do use wire wool or similar to clean the inside of the dashpot or piston to remove the scum, use metal polish like brasso or you will effect your mixture etc. by having air leaks past the piston and dashpot. We coated the piston with a flim of the same oil ATF. Never had any problems with any SU carb, no matter what they where fitted to.
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Thanks for all the help everyone. My task for this weekend is to get her up & running properly - then its the dreaded MOT!!!
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The above posts have covered virtually everything. I would echo the advice, get everything else spot on first, before fiddling with the carb.This includes ignition, timing, HT, etc, and the condition of the carb itself..
The rule of thumb for checking the mixture is to run the engine at idling, warmed up, with the air cleaner off. Lift the piston a fraction with a small screwdriver. If the engine speed dies immediately, it is too weak. If it rises, it is too rich. If correct, it rises a tiny bit and then falls again.
You can get a good longer-term check by looking at freshly cleaned spark plugs after a good run. They should be a nice pale choclatey brown. Black = too rich, bleached = too weak.
At least you only have one carburettor - try it with 2 (or more!) and then balancing them, and then rechecking allowing for their interaction.
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autumnboy,
I said that oil grade doesn't matter for idle settings. It does matter when the dashpot is moving - when it provides damping, and an accelerator pump effect. By this I meant that as long as there is some oil in there, and the damper works, then you won't be *far* wrong. Playing with the grade of oil comes later. As the engine, in this case, has been modified, it may not be the original spec of oil in the dashpot which will give the best results.
I have used a variety of fluids in the dashpot to vary this effect, and indeed have used ATF in some.
I also agree with the suggestion of coating the inside of the dashpot - I usually use brake fluid.
number_cruncher
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The dashpot is not like an accelerator pump. It is designed to stop the needle rising to quickly. If this happens you get to much fuel before the engine has sped up enough to suck in the corrosponding quantity of air. In contrast, an accelerator pump is designed to squirt in extra fuel when the loud pedal is floored.
Another problem with worn SU carbs is that the dashpot oil dissapears rapidly! This is why some people stick higher viscosity oil in.
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"Nothing less than 8 cylinders will do"
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As you accelerate, the damper slows the rise of the needle *and* the dashpot. The dashpot acts as a restriction to the increasing airflow. More vacuum is felt at the needle. The fuel flow through the orifice is dependant on the orifice area *and* the pressure drop across it. The extra vacuum at the orifice draws out more fuel. A temporary enrichment ensues - an accelerator pump effect.
number_cruncher
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Your probably a cleverer man than me Number_Cruncher but I still disagree. If the damper did not work,the needle would fly up when accelerating hard. This would allow more fuel to flow thus richening the mixture.
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"Nothing less than 8 cylinders will do"
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There is, I suspect truth in what each of us are saying - two opposing effects competing, a richening effect, and a weakening effect.
Taking an extreme - if you held the piston at the normal idling height, and opened the throttle, what would happen? The jet would behave like one in a 'simple' fixed jet carburettor. More fuel would flow as the engine speed rises because there would be more vacuum above it. This would work for a limited speed rise, but, eventually, the mixture would become too rich.
In a fixed jet carburettor, this richening effect is overcome by putting an emulsion tube in series with the main jet - admitting an air/fuel emulsion to weaken and correct the mixture.
In an SU carburettor, the piston is allowed to rise, allowing more air in to provide the necessary correction. Restricting the speed at whih the piston rises, via the damper, allows for temporary enrichment. Viscous oil in the damper gives more temporary enrichment than thin oil.
I think there is a section in the Haynes manual for SU carburettors which describes this enrichment action. As I haven't needed it for some time it may take some finding...
number_cruncher
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Er, no, number cruncher is right. The throttle butterfly opens alowing more air into the engine, which accelerates. The increased airflow through the SU increases the vacuum at the main jet, and the dashpot rises allowing more area of jet to open. This is the principle of the SU - it operates with a constant vacuum at the jet - that's why the dashpot rises in response to air flow. The damper reduces the speed at which the needle rises and this creates a greater vacuum at the jet for a short period (the air flow is coming through a smaller orifice), temporarily increasing richness during acceleration. If the damper doesn't work the dashpot rises too fast and doesn't produce the temporarily increased depression at the jet - it moves quickly to the 'normal' fuel flow for the increased engine speed, which will cause a hiccup.
JS
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To add to my own post (I was 40 last week you know and exiting the prime of life and a little absent minded), the adjuster nut at the base of the carb only adjusts the idle mixture. The taper of the needle determines the mixture during running. To this end a number of different needles are available to alter the mixture from Burlen Fuel Systems. The idle adjustment needs to altered with the aid of a gas emissions tester. Once this is set and assuming everything else is in good order the mixture should then be ok
--
"Nothing less than 8 cylinders will do"
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Assuming the needle is correct for the car, I agree. The height of the jet on an SU, however, affects the mixture under all operating conditions, not just idle. As this car has been modified, there is no guarantee that the needle fitted is appropriate anymore.
Conversley, in a fixed jet carb, there is, generally, a seperate idle circuit - this means that the idle settings have little/no effect on the engine operation at other speeds, when the main jet/emulsion tube dominates.
number_cruncher
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Blimey! This is good revision for me as it is has been a few years since I had a SU carbed car although bike carbs work on the same principle. It is even more fun balancing 4 or even 6 of them!
--
"Nothing less than 8 cylinders will do"
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I agree about the revision! Makes a refreshing change from thinking about computer controlled fuelling, etc.
Cheers,
number_cruncher
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