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Discussion Starter #1
I'm spinning this thread off of my old thread "correct air/fuel ratio" in the hopes of getting some advice from the Demom tech guy who visits this board. So demon tech guy please refer to that thread for more background since I haven't gotten anyone on the phone yet.

My problem is that I have my idle and WOT tuned in fairly well but my cruise is way too rich, like 11:1 A/F. Here's some tuning and spec background info:

Engine/car specs: 408 BBC, 9.2:1 CR, 248/254 @0.050, single plane intake, rect port heads, 2" headers, 3600 stall, 4.10 gear in a 3600 lb car.

Current cart setting: 18 deg timing, increased main jets from box 76/83 to 78/89, dropped to a 4.5 power valve in primary and left secondary plugged as from factory, idle mixture screws ~3/4 out, 8" of Hg vacuum in Park idling @ 900rpm.

Currently idles around 14.5 A/F and 12.5 @ WOT but rich as 10.8 A/F at cruise. I've been advised to jet according to cruise A/F and modify the PVCR to get the proper WOT ratio but I don' t want to drill on the metering block until I get a suggestion from Demon. Thanks in advance.
 

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Technical Support Barry Grant
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Tuning based on A/F ratio

To get a leaner A/F ratio at part throttle, while still maintaing your proper WOT A/F increasing your PVCR, and decreasing your main jet size to get the proper overall volume.

Before we get into that let's look at a couple of things first.
Other than a rich number on your A/F gauge how is the engine performing?
What type of A/F ratio meter are you using?
How much vacuum is the engine producing at your cruise RPM?
What type of ignition, and advance system are you using?
How much timing are you running when you're seeing this situation?
What tuning have you done to this point?
What were the results?
 

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Discussion Starter #4
The engine runs well but consumes more gas than even a BB should. Every once in a while it has a hard hot start but I think that might be that the plugs are a bit fouled from running rich at cruise. WOT is smooth and cruise sounds ok but the car does not feel smooth, kind of a surge then relax over and over...if that makes sense. It doesn't backfire or miss.

The A/F meter is the LM-1 which is a digital wide band unit.

The engine puts out right around 15" Hg @ cruise which is about 40mph and probably 2800rpm.

I have a MSD distributor and MSD AL box w/ no vacuum advance just the mechanical system in the distributor.

I'm running 18deg initial timing and the total is 33deg. I would guess that the timing at cruise would be anywhere from 25 to 33 depending on how fast I'm going.

As far as tuning I have set the idle mixture screws based on manifold vacuum mostly which turns out to be 3/4 out. I replaced the factory power valve 6.5 to 4.5. The transfer slots are as close to squares as I can get them. I then performed the following jet changes which had these effects on A/F. The main reason I use the A/F meter is to test WOT and cruise so I don't have to be pulling the plugs so much. Also the vacuum at WOT is zero.

76/83 Box Jetting
cruise 10.8-11
WOT 13.8-14.1

76/86 - since in was so lean at WOT
cruise 10.8-11
WOT 13.1

73/89 - to see if that would improve the cruise A/F
cruise 10.8-11.2
WOT 12.8-13.2

78/89 - current jetting
cruise 10.5
WOT 12.5

Thanks for the help!
 

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Technical Support Barry Grant
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68chevelle_ss

The LM-1 is a very good quality unit, we've had very good results with these units.

A few more questions for you:

1) What intake manifold are you using?

2) Are you running any type of spacer between the carburetor, and intake manifold?

3) Are you testing in a the same location each time when you down to a laptop?

4) Have you checked to see how the spark plugs are burning?

5) Are the plugs burning evenly from cylinder to cylinder?
 

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Discussion Starter #6
tech @ bg,

I'm running a single plane Holley Strip Dominator w/ no spacer.

So far I haven't downloaded to a computer. I can watch the idle and cruise and get a good idea on average A/F and I have a passenger watch the meter for me during WOT passes. This may not be as good as downloading to a computer but I think it is close enough.

Yes, WOT throttle testing is done in the same location. Long stretch of flat road which I am in no shortage of here in Illinois.

I was checking the plugs before I got the LM-1 and they were getting black very quickly. I don't think the cylinders on the corners were AS rich as the middle ones but still getting black.

Also, I realize this combo is not perfect for a small inch
BB but it is what I have since I got a good deal on it. So I'll just do the best I can and go on.

Thanks!
 

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Technical Support Barry Grant
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When tunning with the Innovate unit, it is best to set a course where you can run the same area each time while tuning. This will give you the most repeatable results while testing.

Before doing anything else I would install a 4-hole spacer between the carburetor, and intake manifold. This will increase the air velocity going through the carburetor, and intake manifold, and should help with this situation.

If it does not let me know, then we can walk you through changing the PVCR sizing.
 

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Discussion Starter #8
Just happened to have a 1" 4 hole spacer hanging on the wall in the garage. It seems to have improved throttle response but made no difference to the rich condition @ cruise. The WOT is still around 12.5 A/F. I guess it's time for some PVCR tuning. Let me know what I need to do first. Thanks.
 

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Technical Support Barry Grant
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If you'll send me an e-mail at the office with your return e-mail address I can walk you through this. [email protected]
 
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