Car fastest at 3/4 throttle vs WOT. Lean?? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 06-03-2011, 08:26 PM
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Car fastest at 3/4 throttle vs WOT. Lean??

Well it's just the butt dyno talking here but it seems obvious on my street driven, carbed, SBC that my 0 -60 times are much lower if I don't go to the floor with the go pedal. 355 SBC roller cam motor, 10:1 CR, Dart Pro-1 heads, Edelbrock AVS series 650 on RPM "Gap", somewhat mild Crane cam, timing 10 deg. base 35 tot. all in at about 3000, headers, etc. Probably 350 plus HP. Car is light, a Mazda RX-7, 4:10 rear, 2004R auto. Carb is new with stock rods/jets. This is weird and I'm wondering if it may be an indication that I'm too lean at full throttle. 3/4 throttle pulls very well indeed. Full throttle and I'm clearly much longer in each gear (full manual valve body, so it has nothing to do with the TV). What gives? Leaning out at WOT? Fuel pressure is holding fine. Should I start playing with rods and jets? Is this certainly a carb issue? Thanks.

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Old 06-03-2011, 08:53 PM
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lack of fuel flow volume into the carb at high rpm. (fuel pump, pick up, lines, kinked- soft hoses, fittings tank vent, filter clock the banjo fitting etc)

Lean jetting, leaking booster cluster gasket, dirt in internal passages.
vacuum leak

Air cleaner lid too close to carb inlet.

Does adding a air cleaner height spacer (edelbrock) fix it?

Does adding a 1"+ carb spacer correct it?

Get a AFR gauge to help dial it in.
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Old 06-03-2011, 09:48 PM
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Quote:
Originally Posted by Zeeb
Well it's just the butt dyno talking here but it seems obvious on my street driven, carbed, SBC that my 0 -60 times are much lower if I don't go to the floor with the go pedal. 355 SBC roller cam motor, 10:1 CR, Dart Pro-1 heads, Edelbrock AVS series 650 on RPM "Gap", somewhat mild Crane cam, timing 10 deg. base 35 tot. all in at about 3000, headers, etc. Probably 350 plus HP. Car is light, a Mazda RX-7, 4:10 rear, 2004R auto. Carb is new with stock rods/jets. This is weird and I'm wondering if it may be an indication that I'm too lean at full throttle. 3/4 throttle pulls very well indeed. Full throttle and I'm clearly much longer in each gear (full manual valve body, so it has nothing to do with the TV). What gives? Leaning out at WOT? Fuel pressure is holding fine. Should I start playing with rods and jets? Is this certainly a carb issue? Thanks.
Could be too rich. When you partly close the throttle, the vacuum increases and that seats the rods against the step up spring pressure, leaning it out.

Use the info starting at Page 7- CALIBRATING THE WIDE-OPEN-THROTTLE (WOT). The Calibration Reference Charts that are mentioned start about Page 10.
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Old 06-03-2011, 10:26 PM
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Throttle opening the throttle plates too far?
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Old 06-04-2011, 01:45 AM
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Sounds Like You're Drownding It

Sounds like it's getting too much gas at wide open throttle. Might adjust secondary springs if its vaccum secondarys. If it was too lean it wouldnt run good at 3/4 throttle either. I try to think of the induction system as the damper on a woodstove. Open the damper up wide open before the fire gets going good and it doesn't do any good, sometimes even makes the flame smaller. Shut it down a bit and get it to drawing hard, it'll burn like crazy. Then when you get it to burning faster and hotter when it's drawing good, you can go ahead and open your damper on up and you can burn the house down if you don't watch out. I try to think of the whole carb, intake, head set up as the chimney on a woodstove. The smaller it is the harder it will pull(draft). But get it too small and it'll only burn so hot and to a certain point and then won't get any hotter. Get your chimney too big around inside though and it won't hardly draft at all till you get your fire cooking pretty good. That's just hillbilly thinking though.
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Old 06-04-2011, 05:33 AM
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Last year I put a new Edel 1806 on my SBC in my T-Bucket. My engine has some similarities to yours except I run a lower CR, FT cam, and iron heads. I put the carb on right out of the box and found the same thing about WOT vs something less than that.

Edel's seem to run rich out of the box, and I confirmed that was true with mine with a recording AF meter. After setting idle mixture, I found light throttle cruising too rich, power mode was a tad too lean, and WOT was too rich. Also, stabbing the throttle open from about 2000 rpm caused a momentary lean stumble which was caused by the AVS opening too early and the step up springs a little too stiff.

An Edel rod and jet kit resulted in a much improved setup. Keep in mind that this kit provides the most common rods and jets, but not all that Edel makes. The tuning charts indicate which ones are not in the kit.
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Old 06-04-2011, 11:49 AM
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Edelbrock.....That's A Horse Of A Different Color Then

I didn't realize you were running an Edelbrock. Guess I didn't pay attention very well. I've got two and I've only worked on one. Gona make a lot of people mad, BUT FOR ME, I don't like them cause I can't get them tuned FINE enough. And, those darn jets and rods can get expensive, especially when you don't know what you're doing, like the first time I set mine up. I stick my foot in my mouth alot, but it seems like with Edelbrocks, there's a small gap between running and running really good, I can't seem to hit. I can take a Holley, install a trick kit, rejet it, get my squirters changed over to the right size with the tubes on it, and get the right accelerator pump cam installed, and the right secondary spring installed (the kit takes a short amount of time to install...the tuning takes me 2-4 hours but I'm crippled up) and get performance that I can't come anywhere where near doing with an Edelbrock. I can fire the motor up, stick the pedal to the floor and it doesn't cough, bog, fart or anything....it just goes and goes hard too. That's just me though. I about went crazy with that Edelbrock when I first bought it and was trying to figure out how to tune it. It weren't like a Holley is what threw me.
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Old 06-04-2011, 02:41 PM
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Make sure your air cleaner is not to restrictive. I just had a similar problem with a friends car.
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Old 06-05-2011, 12:15 PM
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Found a problem

Thanks for the responses. I discovered that in an effort to tune out a bog I was having when my secondaries opened, I went way beyond the factory recommended number of turns ccw on the AVS adjustment (this is a "Thunder Series" AVS carb). I returned the carb to factory settings (one and a half additional turns ccw after the air valve closes). My bog is back, but WOT is much better. I think the air valve may not have been opening all the way under load because I had so much spring tension on it. Should have spent more time with the instructions. I ordered a calibration kit and will experiment to see if I can smooth the trasnsition to the secondaries without having to get too agreesive with the air valve adjustment. Could someone recommend a low cost AFR instrument that would work great for this type tuning? Thanks again!
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Old 06-05-2011, 06:31 PM
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Might Try A Vaccum Guage

I always hook up a vaccum guage and adjust till I get my highest vaccum readings instead of going by a certain amount of turns. More exacting with a vaccum guage and they are pretty cheap. I include one in my meter cluster on my ride. You can watch your vaccum guage and tell when somethings going wrong with various things, just like you can watch your tac and tell when you're having ignition problems when the needle goes to dancing or fluxuating. A vaccum guage will do the same thing when different problems arise. Also, you can train yourself to drive with a vaccum guage and get better mileage by trying to keep it at it's highest reading all the time, while you're driving. You'd be surprised how much improvement that a person usually needs on maintaining a steady throttle. Just a thought.
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Old 06-05-2011, 08:26 PM
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As far as setting idle mixture, the vacuum gauge will go a long way. As far as WOT operations, you'd have to have a very critical eye for small differences using just a vacuum gauge to get where you want to go.

I bought an Innovate Motorsports recording AFR system. The LM-1 was on sale on Amazon for around $210. I also bought the companion LMA-2 device so I could also record RPM and vacuum. All together about $300. Seems expensive until you realize that you can record your runs, download to a PC, and look at the graphs and the plots on the software that comes with the system. This is a moderately complex system that will take you some time to get familiar with but when you do, it is a gold mine of information.

A more basic and cheaper setup would be a simple AFR gauge and the corresponding wideband O2 sensor. You can go down the road and just watch it. Chances are with just the gauge you will miss that lean spot bog when you stomp on the pedal - this is where the recording system will help you out because it sees all the transitions.
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Old 06-06-2011, 01:45 AM
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Absolutely Right

You're right about the vaccum guage not doing much for the WOT situation. I'm a disabled vet and I can't even dream about hookin up $200 toys to my ride livin on a disability check. It'd be neat though to have one of those gadgets to play with. Maybe some day when I grow up lol.
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