PETER76

Hi All,

I just installed an air/fuel guage to help me tune my carb a little. The engine is a home-made zz430. It doesn't have the steel crank or 4-bolt mains, but everything else is the same (ie fast burn heads, lt4 hot cam).

Nothing has been done to the 750 (vaccuum secondary) except idle mixture and speed adjustments.

Has anyone used one of these guages? Mine is an autometer. Under load, the engine runs two bars into the rich side. I'm wondering how much leaner it will run if I step down two jet sizes. If you've used a guage like this, where in the guage have you gotten your best power while under heavy load?

Also, I seem to run VERY lean at extremely light cruise. If I'm stepping ever so slightly on the gas, and pulling anywhere from 14 to 20 inches of vaccuum, the mixture guage will drop off the scale. Is this the transition circuit? How difficult is this to tune? Do I even have to - since it only happens under the lightest of loads? Is this intentional for fuel economy?

Thanks for any input,
Peter

Fevre

Use a vac gauge in conjunction with the a/f meter to figure out your power valve rating and pump shot needs. I am running a 750 double pumper that has 2 pumps shots and 2 power valves. I am running lean on my mains and using a pv rated 2 to 3 lbs below my vac at idle so when I take off under light acc or go up a hill the pv gives fuel. Then I put in a pv in sec block that is rated 2 lbs below my reading when taking off under light acc so as not to flood it with fuel. Sounds like your main jets are pretty good you just need to play with the pv and pump shot.

zwede

You can't tune load with one of the cheap gauges. You need a wideband sensor. The cheap ones will only tell you if you are running richer or leaner than stoich, which is 14.7:1. Under heavy load you should run 12.5-13.0, well outside the range of a narrowband O2 sensor.

When you see "off the scale lean" all you're really seeing is leaner than 14.7:1. Running 15:1 during light cruise is fine, so you see that again the narrowband sensor doesn't tell you much.

Fevre

I disagree that you can't 'tune load' using one, you can't get it wideband FI accurate but you can get it better than guessing and reading plugs alone. Have you used one in conjuntion with a vac gauge? I was able to 'see' when my pv opened on both the vac gauge and the a/f meter. Before I did that I had a hesitation and once I could see what was happening (went way lean after the pump shot was spent and the vac did not go below the pv rating) I was able to choose a pv that corrected it. Not disagreeing that the a/f meter is not as accurate as a wideband but it is a useful tool.

Matt Gruber

. Sorry zwede,
somebody told me to always talk in the first person. That is your experience and i accept that.
my experience: http://community.webtv.net/MATTGRU/AFtesting :seeya

zwede

And here pro shops are wasting thousands of dollars on those stupid wideband sensors when all they had to do was listen to you and get a $20 narrowband... Whatever floats your boat.