Technical Carb Fuel Mixture Screw Help please?

Currently reading:
Technical Carb Fuel Mixture Screw Help please?

Joined
Mar 6, 2013
Messages
3,029
Points
859
Location
Ayrshire
so i have done a search and read through some threads, but I am not clear on where exactly (a picture would be really helpful) the fuel mixture adjustment screw is, and how exactly to make the car idle easily without cutting out. I have a 750L.

a few weeks ago i was really happy with running the car. choke lever all the way out to start, halfway once out the driveway and then gradually pushing all the way in as it heats up. but recently (not sure if its because of warmer weather) it only starts with the lever halfway out and then struggles to run. sometimes it likes the lever all the way out once its started.

once its warmed up its fine. can someone please give me a guide? thank you
 
Mixture screw only affect idle mixture.
Generally carbs do not go out of adjustment, so adjusting this will introduce another problem, not fix the real problem. If it was running fine until recently, leave the carb alone.

First checks should be plugs and points, then ignition timing.
Also check that there are no air leaks and air filter is clean.
 
Cam have run the 750 low on fuel? If so could be debris in the jets or filter slowing down flow also check pump is deliverig fuel properly generally unless it been adjusted the setting should be fine if it worked it still should think about outside causes 1st don't just start changing the mixture or idle speed
Look at the cable is it worn does it move freely and have an imesdiate reaction or not !
 
Mixture screw only affect idle mixture.
Generally carbs do not go out of adjustment, so adjusting this will introduce another problem, not fix the real problem. If it was running fine until recently, leave the carb alone.

First checks should be plugs and points, then ignition timing.
Also check that there are no air leaks and air filter is clean.

Cam have run the 750 low on fuel? If so could be debris in the jets or filter slowing down flow also check pump is deliverig fuel properly generally unless it been adjusted the setting should be fine if it worked it still should think about outside causes 1st don't just start changing the mixture or idle speed
Look at the cable is it worn does it move freely and have an imesdiate reaction or not !

I will check into all of this thanks fellas

havent run it low on fuel
 
Also bear in mind the temperatures we are up to now you should need a lot less choke - if any at all - to start and run the car from cold. I'd say start it with no choke at all,catch on the throttle, then give it a little tweak for the first couple of minutes just to bring the idle up a touch. Starting with the choke halfway out now it's struggling more likely because it's flooding than because it needs more.
Choke use is just a dying art..
 
hmm to tune an engine you really need a CO2 meter. sadly i cant remember the values its needs to be set to :( but i do remember how to do it. you have to adjust the fuel mixture and the idle screw till you get it to the number or PPM of CO2
i think it was 1.200 ppm??? could have been 1000ppm... i just cant remember.. last time i had it done was on Talon before i had him on the road.
 
hmm to tune an engine you really need a CO2 meter. sadly i cant remember the values its needs to be set to :( but i do remember how to do it. you have to adjust the fuel mixture and the idle screw till you get it to the number or PPM of CO2
i think it was 1.200 ppm??? could have been 1000ppm... i just cant remember.. last time i had it done was on Talon before i had him on the road.

But with a fixed jet carburettor, you are only adjusting the idle mixture. It has a very small effect as the throttle is opened, and if too weak can cause hesitation, but once the throttle is opened more than a tiny amount fuelling is set by the jet sizes and carburettor design. If too rich, jets are worn, so replace with identical new ones. Setting with CO meter only really worthwhile with variable jet carbs, like SU or Stromberg.
 
Also bear in mind the temperatures we are up to now you should need a lot less choke - if any at all - to start and run the car from cold. I'd say start it with no choke at all,catch on the throttle, then give it a little tweak for the first couple of minutes just to bring the idle up a touch. Starting with the choke halfway out now it's struggling more likely because it's flooding than because it needs more.

Choke use is just a dying art..


Agree. If you have an automated air valve like on a punto. At this time of the year it will be open to cold input only changing only to exhaust heated air it gets colder
Like wise choke should be off then gradually pull until engine picks up then gradually open again. "O the joy of the open road driving will kangaroo juice in the tank "as gramps used to say retro car love [emoji209]❤️[emoji209][emoji51][emoji36]
 
And just for a final "leave that screw alone" - you said it idles fine when it is warmed up, well that is the idle mixture that screw controls so it's probably fine. The mixture at idle when you've got the choke out is controlled by you via the choke knob. If it's wrong, that's what you use to adjust it.
 
And just for a final "leave that screw alone" - you said it idles fine when it is warmed up, well that is the idle mixture that screw controls so it's probably fine. The mixture at idle when you've got the choke out is controlled by you via the choke knob. If it's wrong, that's what you use to adjust it.
ok dokey chaps :)
 
Back
Top