Mileage Quest 2011!

[woods]

I remember the days of plug read the right way, before the wideband was a common thing. It was a real bear. For race cars, it was pretty easy, because you really cared about one thing, wide open throttle. So you would start out with some fairly fresh or new plugs, get it to operating temp, make a full throttle run and with you foot still smashed against the floor, shut it off and coast to a stop. Then pull the plugs out, cut the metal off of all of then and look at the base of it to read the "mixture ring" (as seen in these photos).

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rich plug.jpg

You want that ring to be at a certain place and a certain color. With today's pump gas, you want that ring to be pretty close to black and the rest of it white. If not, you rejet the way you want it to go, put in new plugs and run her again. Keep it up until you get it right or run out of money buying plugs.

This will tell you what your mixture is at full throttle. Well, it really won't tell you your air to fuel ratio, but it would get you in the ball-park.

But, then there are guy who drive on the street. Wide open is not a whole ton of use for us. Guys like Joe who are trying to squeeze the best mileage out of his truck (more for fun and something to do I think) are going to want to know how close their mixture is at pretty much all engine speeds and throttle positions. Well, you can run the truck at the exact speed and RPM you want to get a plug reading at, then shut it off, coast to a stop and start cutting up plugs to read the mixture ring for that load.

What ever rpm you shut your truck off at, that is what you're going to get a reading from. So if you go out for a drive, come back home, pull into your driveway and pull your plugs, you just found out what your mixture is at idle and nothing else.

The exceptions to this is if your truck is burning oil like stove or massive amounts of fuel. Then they will be fouled. But, if it's not really fat or having another problem, then a plug reading has to be done at the specific rpm you're interested in and look at the mixture ring way down in it.

Or...a guy could get a wideband and just glance at the gauge to see what his air fuel ratio is like he did.

This will tell him where he is at when at light cruise, where he is at under enough throttle to make the power valve snap open, where he is at under full throttle and everything in between. You just have to remember not to pull your hair out, because after all, it is just a carb. By getting it right on the money in one area, you may have another spot that is fat or lean. Adjustments can be made to pretty much every area, but some are not so easy to make. So it's sometimes best to just leave it as close as you can get it to what you want.

[fomocoguy]

Ok, so I've got some results from a nice test drive today and it's not quite what I expected! It seems to be running lean at just about everything except full throttle. Here's the breakdown:

Light cruise at 50-60mph: Hovering around 17:1

Moderate acceleration: 15:1

2nd gear to the floor: 12.4:1

3rd gear to the floor: 13.4:1

I also found that when I shift it shoots up to around 20:1, and when it stumbles going into second gear it's between 20:1 and 22:1 until the stumble clears. It feeling "unsmooth" when getting on it hard after my shift from first to second seems to be a lean condition until about 2500-3000 rpms or so. I am very surprised at the lean cruise though, not what I expected....

[woods]

I am a bit surprised and also a bit happy that your truck will cruise at 17 and not feel all "surgy". I am assuming that you don't have a header leak anywhere?

The shift is you lifting your foot, high vac. That's normal and something you should see.
The lean spot from the shift can be cleared up with a bigger volume squirter (which will be there soon).

The moderate acceleration at 15 is before your power valve opens, so if you open it a little sooner, that will go away.
Do you have the stock jetting and PV in there now? I don't remember.

Keep in mind, that the 600 (or 300 in the case of the primary) Is really on the verge of being too big for a low rpm 360. A carb that is too big does NOT give you too much fuel, it gives you too much air (contrary to popular belief).

So, what happens if a carb is on the big side is that you have to jet it up so that the fuel will cover up the lean spot from too much air. Most people are happy with doing this, but I have a feeling you're not going to be.

See, if you had a 390 cfm, you are running on those really small primaries. You get the right amount of air, so it's easier to get the right jetting to match the air.

This is why a well tuned quadrajet works so great (yes, they really do) the primaries are very small, which makes it easy to meter the right amount of fuel.

But, back to your 600. A bigger pump shot, a jet size up and PV a higher number. You will still get around the same MPG, but won't have any stumbles.

[fomocoguy]

I am a bit surprised and also a bit happy that your truck will cruise at 17 and not feel all "surgy". I am assuming that you don't have a header leak anywhere?

I am also surprised, but I have a feeling that if it were 90* outside it would be pinging on me. No header leaks, just one or two pin holes in welds near the mufflers.

The shift is you lifting your foot, high vac. That's normal and something you should see.
The lean spot from the shift can be cleared up with a bigger volume squirter (which will be there soon).

I figured the squirter would probably do the trick for that part of things.

The moderate acceleration at 15 is before your power valve opens, so if you open it a little sooner, that will go away.
Do you have the stock jetting and PV in there now? I don't remember.

As far as I know it has the stock jets and power valve, but it is an older carb so the jets could have been changed. The pv is what came with the kit when I rebuilt it.

Keep in mind, that the 600 (or 300 in the case of the primary) Is really on the verge of being too big for a low rpm 360. A carb that is too big does NOT give you too much fuel, it gives you too much air (contrary to popular belief).

So, what happens if a carb is on the big side is that you have to jet it up so that the fuel will cover up the lean spot from too much air. Most people are happy with doing this, but I have a feeling you're not going to be.

See, if you had a 390 cfm, you are running on those really small primaries. You get the right amount of air, so it's easier to get the right jetting to match the air.

This is why a well tuned quadrajet works so great (yes, they really do) the primaries are very small, which makes it easy to meter the right amount of fuel.

But, back to your 600. A bigger pump shot, a jet size up and PV a higher number. You will still get around the same MPG, but won't have any stumbles.

I feel like I'm catching on to this Holley tuning stuff pretty quick! Everything you said is basically what I was thinking. I'll get it all straightened out and go from there. If I can't eek out a little better mpg's I'll probably start looking for something smaller.

[woods]

Yeah, it probably would ping in hotter temps. When I was running a holley on my truck, I would REALLY lean it out during winter because I could get away with it.
That and it helped keep it running good when the intake would start to ice up.

[fomocoguy]

Well, no changes to the carb yet but yesterday I had a little more drive time and played with it a bit. Just for fun at 60mph I pulled the choke out a little to manually set the a/f ratio at cruise speed to ~14.7. I could immediately tell it was running smoother so I left it there while I was running around. It was running great! Even my lean stumble while shifting into second went away. I'm thinking I'll make changes one at a time and see what overall effect each one has. If bigger mains has the same effect as my choke adjustment I'll be in good shape. One thing I did notice though is that if I floored it at 60 mph it would pull fine, but as I let off it pulled harder. So basically at 3/4 throttle it pulls harder than at full throttle. So this must be either the secondaries opening too soon or the carb being just too darn big and letting too much air in and dropping velocity. I'll have to try a stiffer secondary spring and see what comes of it.