Reading AF/R gauge

Zeke...PA

Zeke...PA

Well-known member
I installed an Innovate MTX-L digital air/fuel ratio gauge. I'm running a 8BA bored to 3 5/16, a Mercury manifold with a Rochester 2GC carb for a 265-283 Chevy engine Main jets at #51, which I cleaned again blowing out all passages before test. Also a PVC valve., and crab distributor.
At idle it reads a steady 11.2, running at 40MPH it dances in the 13-14 range not steady. Is it normal for the gauge to ready 13.4- 13.9/14 range ? Anyone with an AF/R meter ?
Thanks Zeke
 
The gauge will read whatever way your carb is set up.
The readings you are getting seem to indicate some changes needed to the idle circuit and smaller main jets. The dancing around is common.
 
Don't sound to bad on cruise, At idle, far to rich/fat. I'd like to see in the 13:1-13.5:1 at idle.
Martin.
 
I know that the "ECM/PCM" fuel injected guys shoot for 14:1 at WOT, and try to tune cruise mixture to around 16:1.

Don't recall what they shoot for at idle.
 
I have used 4 different brands of A/F meter. They are all different. Those which are extremely nervous indicate more frequent information updates. Those with minor nervous readings are more common and perfectly suitable for getting you in the game. But, you still have to tune for what is best for YOUR engine. The meter can only give a clue as to which side of the line you are on. You cannot tune to a specific set of gauge numbers. The best cruise reading can vary significantly, from engine to engine, based on turbulence and ignition control.
 
Zeke, I have the same AFM, and the same carb, jetted #52, no PCV at the moment. I see about the same numbers you do. I don't get into the mid-14's until above about 45, which is when I assume it is really on the mains. What's interesting to me is that I had a different Innovate AFM on it last year, and it consistently read higher numbers, and with #53 jets. I have to wonder how accurate they are??

I find the only real way to use them is to save output to a laptop over a good long drive with different conditions. Then open the output in the Innovate program, and click on "Channels" at the top, then select "Smooth Channel", then click on the MTX. It will give you a slide bar at the bottom of the log chart that lets you smooth out the output, which makes things much clearer.

As pointed out above, the idle mix screws affect medium-speed cruise readings quite a bit. I jacked up my idle speed a little to be able to lean them out and still get a decent idle.
 
I was able to adjust the idle screws to get 13.9-14 on the gauge at idle ,was not able to get the car on the road was blocked in my garage will try tomorrow and add to this post.
7/19 ran great on test run was in the high 13's to low 14's at cruise speeds. The idle adjust made a big difference at cruising speeds also.

God willing and the creek don't rise, calling for bad storms tonight. Zeke
 
Last edited:
Carburetors aren't nearly as presice at metering fuel as a computer controled injection system can be. The readings, even if a bit jumpy, are consistant with the capability of the old carbs. These A/F meters could have as much as 10% error in indication anyway and still be within manufacturers specs.
 
Have a A/F on my 8BA (80 over Max1 390 Holley etc). Be sure the engine is at operating temp before putting any stock in the idle reading. IMO 13.9-14 at idle is pretty good, 14.7 being perfect. If you are tuning for power 13-14 is OK. 14 up works for economy. My meter dances all the time, and leans off the scale when coasting. Remember it's better to foul plugs than to burn valves..
 
Having run two AF meters in Richards engine, Idle is around 13.2/13.5. Hard acceloration goes intp hi 12's low 13's. Cruse Hi 15's low 16's Edelbrock 500 CFM. Running along at 35.40 mph in 5 th (4.1 rear) wanders around in the low 15's. And yes the two meters have differant readings and one won't real over 16.
Very tight combustion chamber.
 
You must log in or register to reply here.
Back
Top