idle/carb tuning.......

[ben739]

I need help. I think I may be running to rich on my carb, whats the easiest way to tune it/idle. I know there are experts on here who could do it with their eyes closed and drunk so help me out please.

[Packedup]

Check the linkages are as they should be - The jet should return fully upwards to teh carrier/ bearing with the choke off and not have slop or stickiness (if you can push the jet up at all with the choke off chances are the choke return spring is tired or the linkage is worn).

Make sure you've got oil in the dashpot (I use the same 10/40 or 20/50 as is in the engine, others use a variety of types).

Make sure the piston feels smooth when lifted by hand, and drops smoothly back down with a dull clunk when it reaches the bottom.

Then, get the engine warmed up. Go for a proper drive out to get it all up to proper temp.

With the choke off and the engine running, wind the nut on the bottom of the carb (it's held against a spring that runs around the jet) up until the engine sounds pretty rough. Wind it back down until the engine sdounds equally rough. Remember the number of flats from rough to rough, divide by two, and wind the nut back up that many turns. It helps to rev the engine from time to time to clear the inlet of any fuel pooling. That should have the carb pretty much right, though I tend to add another flat up if I want slightly better (fractional) mpg, and down if I want (not noticeable) a little more power.

You can mess with the lifting pin but I've never got the hang of that, and some of my carbs don't have it anyway. You could also invest in a Colortune, which gives you a reading by eye rather than by ear.

When you think you've got it right, go for a proper blast, then check the colour of the plugs. A light tan/ brown is what you're after, and hopefully what you'll see

[ben739]

cheers mate, you made it sound simple. thanks alot, if you want some cornish pasty's in the post for xmas, let me know.

[Packedup]

So long as the carb isn't worn out in one way or another (most bits are cheap and easy to replace, but if the spindle holes are worn it's pricey or replace time), tuning an SU is one of the easiest things in the world

A Colortune lets you do it by seeing the colour of the flame in the cylinder, but if your ears aren't totally shot you'll get it pretty much spot on using the above up and down divide by two method. A few miles of Cornish lanes should colour the plugs nicely too, which means even if you do get it wrong you'll be able to tell from them and fine tune accordingly. Black (soot not oil - If oily then you've got more things to worry about than teh carb...) is rich, white is lean. A sort of nasty 70s BL beige interior colour is what you're aiming for

[Packedup]

I've just realised, was I supposed to get back to you about a fuel pump?

Sorry, had a rather high background level of crud to deal with recently, totally forgot

I take it from the workingness of your car you don't need it anymore, but if you do I'll try and kick myself into gear and see what I've got...

[ben739]

if you have a fuel pump, i am looking for a spare one. cheers.
pm me if you do.

[bmcecosse]

Colour tune is not needed with SU. Check the idle by slightly lifting the little pin until it engages with the piston - then up a further 1/8" . If engine speed increases then it was too rich - if it slows down it was too weak - if nothing much happens it was just right! Turn nut down to enrichen, and up to weaken. Note - this is ONLY the idle mixture adjustment and has little direct bearing on the running mixture which is more to do with the calibrated needle in the carb/state of wear of the fuel jet/float bowl fuel level/spring in the dashpot and to a lesser extent the grade of oil in the dashpot. As pup says - wear on the spindles can lead to a 'weak' idle - but was virtually no effect on the running mixture - except you may richen up the idle to compensate for the air seeping in the spindles, and then the running mix will be slightly rich throughout. There are so many carbs out there - it's rarely worthwhile the hassle of overhauling - just pick up a better one !

[Packedup]

So long as the carb is in good condition and the needle and jet are both correct for the car and not worn, then setting the mixture at idle will have a direct impact on the mixture throughout the rest of the range.

Think about it - A tapered needle in a fixed size hole. The needle can move up or down, but the height of the hole presets a level of up or down which will obviously affect the area fuel can flow from.

So while it's true setting the mixture at idle on a tired carb doesn't mean it'll be right (or even near) throughout the rev range, setting it correctly on a healthy carb does.

And the lifting pin method is great if the pin isn't seized (and is fitted - I do have carbs where it isn't), you can judge the right amount to lift it, know what to listen for etc. Personally I find the up down divide by two far easier and just as accurate, but for those with cloth ears neither way works as well as a Colortune!

[bmcecosse]

Oh I agree - with experience you can indeed adjust the mixture 'by ear' - but the lift pin works well too - and all standard Minor engines have em. In fact ALL my SUs have the lift pin - including HIF 38 and 44. The idle mixture has very little to do with the overall running mix - yes you have moved the hole down slightly thereby microscopically increasing the annular cross-section available to the fuel - but changes in the needle shape are really quite severe to give richer running for modified engines - the tiny downwards movement of the jet for one or two 'flats' of the nut has very little influence - sure - if you turn it right round several times, that will affect the running mixture - more so at light throttle and no significant change at full throttle.