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Old 08-29-2006, 08:56 AM
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750 Demon - lean from factory?

Hey all,

I just put a new 750 mighty demon w/annular boosters on my SBC 406 big dart heads, hyd roller 240/248 at .050, 10.5:1. When I first started running it around I noticed that as soon as I would hit WOT, the A/F gage would head toward 17:1 immediately, and above 5K, the motor didn't seem to pull very well. But cruise was pretty rich, around 12.5:1 (of course my narrow band O2 doesn’t do a great job of telling the exact a/f). But when I would rev it in the garage and hold at 3K, I could hear popping out the exhaust (seems lean, right?)

I was thinking my mechanical ebrock pump couldn’t keep up with the higher demand at WOT (although with my holley 750 it did great). Starting looking at electric pumps and all that jazz to perhaps fix the perceived fuel delivery problem.

I decided to jet it up just to see – went up 3 jets (holley jets though)– now it stays in the 12.5 range at WOT, but will hardly get past 12:1 at cruise. I was pretty surprised to find that I had to upsize the jets. Seems like something else is wrong to me.

Any ideas?

Havent’ spent a lot of time reading the plugs – but did see after the initial runs that the ground strap had some tan deposits that looked to me like a lean condition. Haven’t checked them after I rejetted though.

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Old 08-29-2006, 02:01 PM
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NO carb is right out of the box. Never be surprised.
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Old 08-29-2006, 03:02 PM
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750 Demon-lean from factory?

I just put a new Mighty Demon annular 800 on my big block and I went up 4 sizes all around in the jetting as it was running a bit lean,now it seems to be pretty near perfect,I'm really pleased with the performance.Works way better than my old Holley did!As far as I was concerned,re-jetting the carb a bit was a very minor issue.
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Old 08-29-2006, 05:02 PM
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I am curious as to whose lean/rich indicator you are using. I put an Autometer one in my street rod and it doesn't work at all. I called Autometer and the guy in tech I talked to said it isn't designed to work on a carbureted engine, only on an EFI engine. Of course it doesn't say that on their web site or in their catalog nor is there any mention in the instructions that it won't work on a carbureted engine. So I am not sure if the guy knows what he's talking about or not. I have a lean/rich gauge on another car of mine that seems to work fine but unfortunately, the company who made it, Camden Superchargers, is no longer making it.

In the instructions it does say that if the vehicle is running too rich, it can foul up the O2 sensor. When I first got the car finished it was running extremely rich so I may have permanently fouled the O2 sensor.
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Old 08-29-2006, 05:08 PM
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750 Demon-lean from factory?

I just do it the old fashioned way and pull the plugs and have a look at them!
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Old 08-30-2006, 08:23 AM
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I am using the Nordskog A/F gage. It has a little black box as well that helps to average out the readings. Considering that it is the narrow band type, I think it only tells rich/lean, even though the dial indicates a number. I know that narrow band sensors are really only accurate at 14.7:1, above or below that and the voltage vs. A/F is very non-linear. Couldn't say whether the gage adjusts for that or not. I am guessing that's why the Autometer guy said EFI only - because they are much better at running in the neighborhood of 14.7:1. A carb has a hard time accomplishing that successfully it seems.

Guess I need to suck it up and get a wideband system with datalog like Innovate or FAST. I woudl think that would provide some actual useful information. Anyone ever used one?

I think I probably will have to just do it the old fashioned way as well for now. Getting married this weekend. not much money at the moment.

One question - do you have to use new plugs to get a good reading? After I check them, do I have to install new plugs to see if any changes I make have made the combustion conditions better or worse?

thanks for the replies.
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Old 08-30-2006, 11:25 AM
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Hotrodf1,

Now that you’ve gone up in jetting how does the engine pull above 5000 RPM?

Using NBO2 is pretty much worthless when tuning a performance engine. They are not accurate enough to give you a proper reading, and most won’t react quickly enough for the type of testing/tuning you’re doing.

The Innovate Units are really nice, they have excellent software so you can download data, and even expand with other instrumentation.

You don’t have to use “New” plugs, but it is best. The simplest thing to do with plugs, is find one cylinder (that’s easy to get to) that is in the middle of your burn pattern, then just put a fresh plug in that hole for inspection.
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Old 08-30-2006, 12:23 PM
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thanks for the info!

would love to have an LM-1.

maybe santa will bring one
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Old 08-30-2006, 12:26 PM
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Guess I should have answered the first Q

It seems to pull more consistently. I may try one size down again and also one up. I do have G tech that helps, but it's hard to find a place to be able to use and not get in trouble.


I drove it the other night and was not all that impressed as a whole. Probably due to the humidity and heat, though. Havne't had much time to tune cause im getting married on sat. yikes. Can't wait till i have "more" time to play.
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Old 08-30-2006, 12:50 PM
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If you're just getting married you've got enough on your plate!

Try going down one size when you get a chance.
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Old 08-30-2006, 01:09 PM
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Tech @ BG - how much difference is there between BG and HOlley jets/???

I've heard different stories. i have a whole set of holleys- are the numbers pretty comparable??
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Old 08-30-2006, 01:15 PM
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Watch the Holley jet's. I measure mine so I know what they really are. Some of em are way off. You can put a 82 in and so no difference than your 86. Because they both measure 86. Seen it happen a bunch.

I think the BG jet's have a different thread or something like that. I just pay to convert all my carbs to holley jet's so I'm not sure on that.
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Old 08-30-2006, 01:16 PM
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Watch the Holley jet's. I measure mine so I know what they really are. Some of em are way off. You can put a 82 in and so no difference than your 86. Because they both measure 84. Seen it happen a bunch.

I think the BG jet's have a different thread or something like that. I just pay to convert all my carbs to holley jet's so I'm not sure on that.
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Old 08-30-2006, 01:57 PM
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Quote:
Originally Posted by hotrodf1
Tech @ BG - how much difference is there between BG and HOlley jets/???

I've heard different stories. i have a whole set of holleys- are the numbers pretty comparable??

The flows are comparable, we try to hold a tighter tolerence on the flow rating on ours, so they should be closer.
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