13 replies to this topic

[OldBetty1979]

Hi! I'm trying to set the mixture on my carb and I just can't seem to master it.. How do you know when it is too rich or lean. I know that when it is to rich the exhaust will be black and will smell of fuel and revs will be high but how do know when it is too lean I just keep turning the nut and seeing how it drives but I can't get it anywhere good! At the moment when my foot gets half way down or three quaters it doesn't pull and the exhaust pops like crazy! But when I put my foot to the floor it pulls? I reckon to rich? So am I doing this all correct and should I just keep twisting the nut or am I missing something? (Also I am right thinking clockwise richens it & Anti clockwise weakens?)

[jaydee]

Maybe too lean aswell, what colour are the plugs? How did you set the timing?
Yes clockwise for a rich mixture, but you need to tweak the needle to adjust fuelling properly

[Samwise777]

Popping sounds like too rich like you said. When mine was running lean it was noticeable in the car- the engine would often splutter and lag when I put my foot down

[ArneBrys]

First of all I would check ignition timing is correct and that you don't have any intake air leaks and don't forget the carb oil level.
There is a quite good guide on how to setup the su carb on there website somewhere (sucarb.co.uk) and it would be a good place to start however proper tuning is not done with the jet stop adjustment but by swapping/tweeking needles.

About the popping, this can be both rich and lean (or even ignition problems).
This happens when the mixture is to lean or rich to actual burn properly in the cylinder and it ignites when entering the hot exhaust.

[OldBetty1979]

Thanks for the replys! I have set the ignitions timing bout two weeks ago and set it to 8 degrees. The only thing I have done since then is noticed my crank breather was broken so I bloked the carb hole up and just left the crank breather this is when it started running worst hense why I'm messing with the mixture.I haven't checked the spark plugs but I will what colours should I be looking for? Also how do you tweak the needle?

[OldBetty1979]

I've just read online for a basic mixture turn the nut all the way lean then turn it two full circles and 1/16" of a turn? Is that correct?

[jaydee]

To set up idle mixture, raise the idle to a smooth point between 1000 and 1500 rpm then lean it out or en-richen it, see if the revs increase, you have to look for the highest idle point. Blip the throttle after any adjustment.
Now you can check by lifting the piston a few mm, the revs should hardly change, if fall down its too lean, if raise quite a bit its too rich.
You should really make sure you have no air-leaks and adjust the timing first, otherwise any adjustment will be pointless.
You can adjust the timing by turning the dizzy clockwise with engine at idle above 1k rpm, turn it until the revs increase, at the point revs fall down turn it counterclockwise a few degrees. Road test it flooring throttle from a little below 2000 rpm in 2nd, if you get pinking then advance it back a couple of degrees by turning anti-clockwise the dizzy and re-test.
Igntion timing have to be sorted out before mixture, then rechecked after you sorted the carburation.

Edited by jaydee, 28 February 2013 - 10:54 AM.

[OldBetty1979]

I don't have a rev counter? What would be a staring point, do I lean it all the way out then two full turns lean or the other way round then when I'm close ill just keep tweaking it until it good enough? Thank for all the help so far

[jaydee]

You dont need a rev counter, just use your ears. Raise the idle a bit to smooth it out at first.
Dont work 2 full turns, do half turn in one way and see if engine speed raise or drops, you're looking for the highest revs, you'll hear that with no need of a rev counter. Work half turn at a time, blip the throttle a couple of time after any adjustment.

[jaydee]

Also when you remove spark plugs, read them: they'll be white if lean and black-sooty if rich, air leak if sparks plugs are different to each other

[tiger99]

You first really need to fix your breather properly, as the carburettor needle was chosen to allow for the amount of air from the breather, and it will not run right, over the full speed range, unless it is replaced. Or, you can spend a lot of time and money on a rolling road to find a needle that will be correct, and then have to change it all again when you fix the breather.

[OldBetty1979]

Been messing with it for ages now & burnt my wrist on the exhaust so it can stay like that for now! I will replace the crank breather pipe when I mess with it again. Also I took the spark plugs out and they were white so I richend it up abit more. I'm just going to leave it for now cause its really annoying haha thanks for all the help though

[crock]

You can't go by the plug color since the days of unleaded petrol. The car can be very rich and give white plugs or very lean and give black plugs. A Colortune is a good investment for setting idle mixture for the home mechanic just starting out. The advice about lifting the piston and seeing if the engine speeds up or slows down is the classic technique for determining a rich or lean condition at part throttle.

[jaydee]

Deffo wrong, correct colour with modern fuel is GREY, either brown and black with unleaded means rich and lean is still white, colourtune is hopeless.
Liftling piston is for IDLE and again useless at part throttle hwere you need a gas tester.