Tuneing SU carbs
Discussion
Anyone know a step by step pictoral guide to doing this?
I've set/ballanced the idle and it runs and drives, but power is well down so the mixture needs looking at.
Instructions seem to be (with engine warm)
Remove piston (the plunger out the top?)
Remove suction chamber (????????)
Turn the nut underneath the carb to set the mixture untill highest idle speed is achieved.
Lift the piston 1mm, the engine should speed up then slow back to idle, 1 flat richer for power, 1 flat leaner for economy.
Does anywhere have that in pictures?
Go for a drive and check the plugs afterwards to be sure.
I've set/ballanced the idle and it runs and drives, but power is well down so the mixture needs looking at.
Instructions seem to be (with engine warm)
Remove piston (the plunger out the top?)
Remove suction chamber (????????)
Turn the nut underneath the carb to set the mixture untill highest idle speed is achieved.
Lift the piston 1mm, the engine should speed up then slow back to idle, 1 flat richer for power, 1 flat leaner for economy.
Does anywhere have that in pictures?
Go for a drive and check the plugs afterwards to be sure.
Edited by thisisnotaspoon on Tuesday 24th August 12:16
Haynes do a really good guide/ manual but you might find some general ideas in this:
http://www.checkthatcar.com/tuning%20su%20carburet...
Have you tried all th other basic possiblities: damper oil level, any leaks from gaskets etc?
G
http://www.checkthatcar.com/tuning%20su%20carburet...
Have you tried all th other basic possiblities: damper oil level, any leaks from gaskets etc?
G
If you've uprated the engine in any way you'll need a different needle. If you post on one of the main mini forums there will probably be someone running a similar spec engine to yours to give you a starting point in your needle choice although a rolling road setup is the way to get it spot on.
Also for setting your mixture invest in a gunsons colourtune (about £5 on ebay), basically a spark plug with a window in it so you can see whats going on in the combustion chamber. Swear by mine but again this will only set your mixture at idle as the profile of the needle will dictate what happens under load. Is your engine standard?
Engine is a BMC 1500 lump, running near enough standard bits. Exhaust is a stainless one but not particularly racey, and head is converted for unleaded.
They are only out of tune because I took them off to get the engine out. I've ballanced them but the cars still woefully down on power and idle wobbles up and down. So I'm confident the fueling is all over the place.
Dissapointingly the car is much quieter since I put new exhaust gaskets in, old ones must have been leaking like a sieve
They are only out of tune because I took them off to get the engine out. I've ballanced them but the cars still woefully down on power and idle wobbles up and down. So I'm confident the fueling is all over the place.
Dissapointingly the car is much quieter since I put new exhaust gaskets in, old ones must have been leaking like a sieve
You have balanced the airflow. To balance the mixture correctly remove both dash pots/pistons look down and you will see the top of each jet, these must be at identical heights. One low, one high and you'll have an average mixture thats potentially bang on but each pair of cylinders will not thank you. Running the jet adjustment all the way home then back "x" amount of flats does not balance anything but will give you an average mixture to at least get started with. If you have a vernier check the heights from above and balance the jets that way. Re-assemble the dash pots onto the carbs. When balancing disconnect the linkage and then balance airflow with the idle adjustment (in combination with idle speed) Then lock off the linkage. Now at least you know that they are balanced and assuming that your ignition is in order and everything is timed correctly you can start considering mixture. You should also check for full throtle at the carbs with accelerator down and check the choke is free. But I still don't think it's just balance unless something other than remove/refit has happened. How do the plugs look? are they all the same appearence? Are you sure you don't have an air leak, particularly anywhere after the jets i.e.between the carb back, manifold or head face? Or have adjusted the carbs in the wrong direction and you are rich?
Hope that helps.......
Hope that helps.......
Edited by Tictac07 on Thursday 26th August 16:52
thisisnotaspoon said:
Wahts to check in the linkage? It all moves as it should, as far as I can see.
Yes, I've ballanced the airflow at idle so it was driveable down the road without stalling, but nothing else.
Not a lot to check- I agree, but because the whole essence of a pair of SUs is that both need to be doing the same thing at the same time it IS possible that if the linkage is worn or loose at any point, there is not synchronisation.Yes, I've ballanced the airflow at idle so it was driveable down the road without stalling, but nothing else.
However, as you say, if it's all moving as it should then the problem will be elsehwere
A couple other things to check. Holding the dashpot upside down with the dashpot plunger removed, fit the piston into the dashpot and check that it slides down smoothly. Any stickiness can be removed by gently polishing with FINE grinding paste the dashpot inside and the outer of the piston. Do ensure to wash the residue off with some petrol and dry carefully.
Next, check that the needle seating in the piston is correct, ie, the shank should be flush with the piston bottom, not protruding or below the piston base. Also check that the needles aren't bent, roll them on a flat surface to check.
Then, check the butterfly bushes for wear. Disconnect the linkage and try to rock the spindle sideways. Any wear in the bushes will result in erratic air / fuel mixture when running on road.
A quick check for basic set up is ; with the linkages disconnected lift the pistons with the wee lifter about an 1/8th" the revs should momentarily drop by the same amount.
Good luck
Next, check that the needle seating in the piston is correct, ie, the shank should be flush with the piston bottom, not protruding or below the piston base. Also check that the needles aren't bent, roll them on a flat surface to check.
Then, check the butterfly bushes for wear. Disconnect the linkage and try to rock the spindle sideways. Any wear in the bushes will result in erratic air / fuel mixture when running on road.
A quick check for basic set up is ; with the linkages disconnected lift the pistons with the wee lifter about an 1/8th" the revs should momentarily drop by the same amount.
Good luck
Ignore all of the above.
Seriously, ignore all of the above.
Your problem is not in the carburettors, certainly not anywhere that you can adjust the fault out with the standard adjustments.
The first thing to do is to check whether the ignition system is set up exactly as per the manufacturer's specification, and that all of the parts are working properly. It helps if you have another known good motor to try the parts on. 99% of carburettor faults are ignition related.
After you have done that, the next thing to do is make sure you bolted everything back together properly, so that there are no air leaks. Grab a can of WD40, and then when the engine is running spray the WD40 at every joint between the carburettor and the cylinder head. If the idle speed increases you have an air leak and this is your problem.
If neither of the above help, you can diagnose whether the issue is mixture related by going to a nearby "test track", and flooring the accelerator in 3rd. Turn around and do the same again, but this time pull the choke out. Does the car go quicker with the choke out? if it does the mixture is too weak. Does it bog down massively as soon as the choke is pulled out even a little bit? mixture is too rich. If it bogs down a bit when the choke is fully out it's about right.
My bet is that you have an air leak.
Seriously, ignore all of the above.
Your problem is not in the carburettors, certainly not anywhere that you can adjust the fault out with the standard adjustments.
The first thing to do is to check whether the ignition system is set up exactly as per the manufacturer's specification, and that all of the parts are working properly. It helps if you have another known good motor to try the parts on. 99% of carburettor faults are ignition related.
After you have done that, the next thing to do is make sure you bolted everything back together properly, so that there are no air leaks. Grab a can of WD40, and then when the engine is running spray the WD40 at every joint between the carburettor and the cylinder head. If the idle speed increases you have an air leak and this is your problem.
If neither of the above help, you can diagnose whether the issue is mixture related by going to a nearby "test track", and flooring the accelerator in 3rd. Turn around and do the same again, but this time pull the choke out. Does the car go quicker with the choke out? if it does the mixture is too weak. Does it bog down massively as soon as the choke is pulled out even a little bit? mixture is too rich. If it bogs down a bit when the choke is fully out it's about right.
My bet is that you have an air leak.
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