How sensitive is a motor for idle mixture screw changes

[Peter]

I am trying to tune my motor in the low speed range,
after I got it to idle well.
can somebody please tell me what the correct way is...

do you set the idle mixture screws for best lean idle and change the jets (if necessary) for driving (until it runs well),
or do you choose a jet and adjust screws until it runs well?

I have played around with it for some time now, and I am a bit lost...
I found with 65 idles the engine pops out of the carbs while driving, so I stepped up to 70's.

with these jets installed, I tried some different settings of the mixture screws. I found out that with 3/4 turn, it idles to lean, 1 turn still popping,
seems to be running better with 1.5 turn but too fat at idle.
It think I might be making too big changes on the mixture screws but I don't really know. seems nothing I do is really helping...

[Jim Ratto]

Hi Peter

Are you running Weber 48 IDA?

[kever65]

Hello,

better give us some specs of motor and carbs please.

i have a street driven 1776cc with 48 ida's and 42 vents in them, the idle mixture screws are very sensitive, not even a quarter of a turn between way too rich or way too lean.
on my 2000cc with 44 idf and 36 vents it is not so delicate.

Davy

[Bruce]

I found with 65 idles the engine pops out of the carbs while driving, so I stepped up to 70's.

Nobody needs a 70, it's too big.  Go back to your 65.

 I found out that with 3/4 turn, it idles to lean, 1 turn still popping,
seems to be running better with 1.5 turn but too fat at idle.

Stop counting turns.  You adjust each cylinder until it's mixture is right.  When you're right, the screws will be all different number of turns from seated.

[Peter]

Hello,
Yes, running IDA's
motor is a 2276 , 11/1 , JPM cam, MS230 heads
40 vents.

So Bruce you suggest putting the 65's in and play around with the screws until it runs right?
Problem for me is to know when the mixture right. (and do you mean at idle or when driving in the low speed range?)
 I ve read so much about it but cant seem to find it by myself.

[Zach Gomulka]

The idle mixture really effects drivability when you first dip into the throttle. Don't give much attention to a/f ratios at idle, if you have it too lean it will fall on its face when you start from a stop. If the idle jet is too small you will have to have the screws backed out very far in order to get it to run right, that's how you'll know you need a larger jet.

[Jim Ratto]

Part of getting the idle circuit correct is adjusting not only idle fuel jet. The idle jet holder is the air corrector for the idle and transition circuit and can be changed or soldered/re-drilled to help with getting your idle and transition running right.

But getting this set up where it works means a few other things HAVE to be correct as well, number one and two, without exception are float/needle valve settings and fuel pressure. You have to make sure your float level and needle valve height are exactly as Weber indicates. Don't second guess their settings unless you have re-designed where the ducts are in the booster venturies and how the emulsion tube works. The float level is what Weber says it needs to be because of these factors. If you run it too low or too high, you will chase your tail until the end of time trying to mask problems created by incorrect settings. Same with fuel pressure, though problems usually come from too high, rarely too low of pressure.

Second, the correct emulsion tube and main jet need to be set up for whatever size venturies you're running. 40mm is a good size venturi for you engine, I would suggest either:
165main F4 emulsion and 195 air
or
160 main, F2 emulsion and 155 air

If the main jet is too big or too small, the idle circuit will have to get set up to compensate, creating a problem at off-idle or in transition (or both). If the main jet is too small, you will try to correct a lean hole by increasing idle jet size and then having the idle pig rich and limping and loading up. If the main jet is too big, the problem won't be as obvious but the engine might "hunt" and surge at steady state throttle in 3rd and 4th gears.

I would, like Bruce suggested, start with 65 idle fuel jet. Make sure the engine is completely warmed up. What I do is go drive the car normally (NOT hard, because if your jetting is lean, that's not good for the engine) for 20-30 min (so oil is up to temp and you hear valve lash) and then bring it back to shop and let it sit for 15 minutes. Then fire it up and make sure carbs are synced and opening at same time (linkage adjustment). Then proceed to adjust each mixture screw. The number of turns out doesn't really indicate if your ide fuel jet is too big or small when the engine is idling. I have read there is a procedure to using the mixture screw to gauge idle jet size, but it's done at rpm where main jet begins to admit fuel spray into booster venturi. I've never tried this so I can't tell you how to do it or if it works. What I do is slowly close (clockwise) the screw until you hear (and FEEL through screwdriver- use your sense of feel too) when the cylinder gets rough. Leave the scre at that position for 10-15 seconds, then slowly begin opening it until you hear and feel the cylinder come back to life and smooth out. Go slow. If you go too far, the idle won't drop as dramatically but it will get unsteady and lumpy sounding. And if you go back clockwise it will smooth out and sound more steady and urgent. Once it's there, leave it and repeat with all 3 other cylinders. If your valve lash is good and CR is same in all cylinders and carbs are clean and no vacuum leaks, then all 4 mixture screws should be about exactly turned out the same amount of turns. When the idle is correct, it should idle nice and even, with a hard, hollow beat from tailpipe(s), not a limping or lumpy idle, even with a big cam. I like to set my idle rpm @ 1200 with Web 86C (warm) and make sure you have some initial advance timing set too, at least 10-12 degrees if your cam is radical.
The idle air corrector will affect idle quality but more so it affects how the idle circuit "runs" out as the engine comes into main circuit. You need it small enough that the idle circuit won't go lean too soon, but not too small as it then forces the main and idle circuit to fight each other. What I learned to do was remove emulsion tube stack altogether (no mains / airs / tubes) and go drive car (parking lot, not street or highway) and adjust idle air corrector size until the engine will take load and not fall on its face up to 2900-3200 rpm. If it dies under this rpm you need smaller idle air jet. If it loads up and bumbles and stumbles under steady cruise at 2200-2800 and then cleans up and runs to 32-3500 then you need bigger. Be care going too big.

Iridium NGK plugs help with setting idle as will a good CD ignition and coil.

Good luck,

Jim

[Peter]

Hello,
Jim, thanks a million for your extensive explanation!



Now I have to get back in the garage and make this thing run like hell!!

first, I ll put back the 65 idles and try to solder the idle jet holders and run it without the main jets.

[fish]

What Jim said, plus make sure you have no intake or exhaust leaks and correct valve adjustment. How many progression holes in your IDAs? 2 holes 60/65 idle, 3 holes 55 idle. While you are adjusting idle speed/mixture, disconnect your linkage. Once you have your idle speed/mixture, syncronize your carburettors using a sync tool, re-install your linkage and make sure both open the same amount at the same time without binding.