Stoic AFR or Good Steady Vacuum Pressure at idle?

[taimurahmad91]

Should I aim for good vacuum pressure or AFR at idle while adjusting / tuning carburetor of a car? I get the most stable vacuum pressure of 18inHg at 12.5:1 AFR. At stoic, my vacuum pressure gauge needle is unstable at 17-18inHg.

Its a non turbocharged engine. Emission standards are not important for me. The engine has non-stock carburetor, I am using both AFR & Vacuum Pressure gauges for its tuning. Throttle response at load & cruise is tuned to be acceptable and close to stoich. But the matter to resolve is tuning at idle as mentioned in the question; high & stable vacuum pressure at richer afr or otherwise?

Ignition timing is already advanced but not enough to cause pinging or knocking.

I read online that for idle, give engine what it wants to achieve higher & stable vacuum pressure in manifold, stoic afr should not get priority. But this needs confirmation.

[Hassan1991]

Share a graph it will help us understand better what are your target afrs with respect to rpm and pressure.

[taimurahmad91]

Unfortunately I dont have access to dyno, want to do tuning for daily driving just by afr and vacuum gauges

[Hassan1991]

A rough graph on paper will work.

[Xulfiqar]

What cylinder head temperature are you running? And how much spark advance are you running?

[taimurahmad91]

Cant get cylinder head temp, no OBD.
Spark advance is 20 deg BTDC against factory spec (1990s) of 8 deg.
Cant comment on how local ustaads assembled the engine. So just aiming for higher rpm at idle through ignition advance and validating no pinging / knocking over wide rpm range.

[Xulfiqar]

20 degrees idle advance? Is engine fullly warmed up above 80c? Is the carburettors hot idle compensator opening up?

[taimurahmad91]

No hot idle compensator valve in carburetor. It has manual choke.
Yes, 20 deg advance at idle.

[Xulfiqar]

What engine is this and what carburettor is installed, hot idle compensators have been on asian carbys since the 60s.

You are running way too much advance and are stabilizing at 12 afr with that much advance and no knock leads me to believe you have other mechanical issues like bad valves or bad valve timing even.

The carburetor is the very last object to adjust on an engine, you first have to make sure everyrhing else is top notch, check the

Intake for leaks
Engine operating temperature (in ancient days we checked with multimeter or even heat marker)
Correct valve timing and clearance - make sure the damn thing actually is on tdc when it says it is (sometimes the pointer mark/scale is also wrong)
Correct pcv working, including no leaks on engine
Correct distributor working (centrifugal and vacuum advance both)
Airbox correctly seated to carb
Carby with correctly levelled float and correct, idle, low and high jets, working and good sealing powervalve etc and clean emulsion passages.

Good luck - carb engine tuning used to be standard work 3 decades ago, now its fading off.

[taimurahmad91]

I stand corrected. Carburetor has hot idle compensator. Valve timing, leaks etc have been checked and found in correct. I have all my doubts on timing marks therefore adjusting idle ignition timing at highest rpm achieved without pinging at light to medium loads. Will check proper functioning of HIC, may be it will lean the mixture a bit and solve the issue.
Thanks.

[Xulfiqar]

Check valve timing by the oldschool method of making a dead stop through the spark plug hole and then using some tailors inch tape on the crank pulley,

Once you are sure the crank is good, check the cam timing as to where the lobes are to determine if you are near or off true tdc, then verify with the timing marks (if multiple)

What engine is this?

Once you determine that the timing of the crank is also off, then make a new mark or make a note of where its actually on tdc compared to the provided scale/point/mark

[taimurahmad91]

Thanks for the great advice. The valves clearance was a little bit tight. Loosening it upto fsm specsin hot condition improved afr at idle. I had got the clearance adjusted few months back, but mechanics always prefer their own jugars & experience, not paying heed to customer or factory specs.
Engine is G16a 8 valve.
Will run exhaust analyzer to check HC ppm, to see if their is any difference. Earlier it was less than 300 ppm with carbureted engine. Hopefully it will be lesser now.

[Xulfiqar]

The g16a engine has a timing belt pulley that can be installed in more than position,

Make sure its at the right position, to detemine that you will need to find crank tdc first and then find when the cam circle is centered.