The perfect fuel/air ratio

[Dirk]

Since we now have a little daughter and only one income the prospect of aquiring a shiny new Weber Carb is a distant dream for me. So for now I settle for the enjoyment of taking my little mechanic apprentice to the breakers yards in search of a carb in better nick than my own.

 

I am on my third carb now but to be honest it is just as worn out as the first. But what has interested me in my quest is this: Although each carb appears to be the same hitachi body, the jet sizes seem to vary some what. Now without proper measuring equipment I cant say for sure if the barrels of these carbs are exactly the same size but they do come from the same model engine (EA 71) of varying years.

 

So how is it then that the jet sizes to differ from the same capacity engine? Is there a variable (other than human intervetion) that I am missing?

 

The jets I am currently using are mostly the origional sizes since they seem to work best (don't fix it unless its broken) but I have noticed on other threads that some of you have altered jet sizes to achieve slightly different driving characteristics.

 

I myself have upped the size of my primery idle jet with pleasant results but I wonder if the rest of the jets are optimally set.

 

And finally the question:

 

Is some one able to tell me what the physical and driving characteristics are of an engine that runs either too lean or too rich? I think I have some of the answers but for now I will save myself potential embarrassement by keeping quiet.

 

I guess that someone will tell me to get the engine professionally tuned but lack of cash and a carb that leaks air from every orifice tell me not to at this stage. So what I am asking for is advice on tuning the agricultural way.

[GeneralDisorder]

Fuel/air ratio is a BIG topic. It changes thoughout the RPM band. In general:

 

Best economy: 15.5:1 - 16:1

Part throttle cruise and light acceleration: 13.5:1 - 14.5:1

Full throttle: 12:1 or less

 

Stoich is 14.7:1 and that's where you will get the cleanest burn for emissions purposes.

 

As for why there are different jets on the EA71's - they jetted the auto's different from the manuals, jetting changed over the years to match gearing, and the EA71 went through a couple different revisions including a "fat case" side-starter version and even some with hydro lifter's like the EA81's. Intake valve sizes changed, etc, etc, etc.

 

Jetting a carb is all about tuning them for specific F/A ratio at a given speed, gear, and RPM. Unlike FI, carbs can't run perfect at every point in the RPM curve. There are trade-off's that have to be made.

 

That said, the BEST way to tune a carb is with a wide-band O2 sensor. Unfortunately they start at about $250. Great tool though.

 

You will always notice a lean condition more than a rich one. It will buck and jerk if it's lean, and it will bog and have a nagging power loss if it's rich. It takes a lot of experience to get a feel for tuning a carb with your butt - but it can be done. I've always done them this way and eventually I got a good feel for it - picked up a wide-band a couple years ago and after hooking it to my EA81 I didn't make a single change. Mostly I bought it for turbocharged EJ projects where the butt tuning doesn't work at all.

 

GD