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Old 10-10-2005, 09:01   #1 (permalink)
Joe
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Missing at 3,000 rpm

I have a 1972 Ford Mustang, 351C-4v. It has an Edelbrock Performer
Intake, and Edelbrock Performer 750cfm carb, with vacuum secondaries.
I just swapped the stock dizzy, with a MSD pro-billet (magnetic
pickups), MSD-6A CD ignition and a Pertronix Flamethrower II coil. I
also installed new Autolite plugs and NAPA sparkplug wires.

I have the dizzy curved to have all advance come in at about 2,800. It
is timed for 16 degress initial, with 25 certifugal, for 41 total
degrees at about 2,800 RPM. The dizzy also has a vacuum advance, which
is hooked up just above the butterfly plates in the carb.

The car idles beautifully (much better than it ever did). However, at
about 3,000 RPM (cruising), it is missing pretty badly. There is no
knock or pinging though, and I know that 41 degrees total advance is a
lot. Any recommendations for what's causing the missing? What could I
do? Should I back off the initial advance to about 13 degrees, so I
total out at 38 degrees?

Thanx...

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Old 10-10-2005, 11:01   #2 (permalink)
Jim Warman
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Re: Missing at 3,000 rpm

At the very least, I'd back the timing up until you get stuff sorted out.
Did you regap the plugs to take advantage of the MSD primary circuit?
Also... did you get the 6A or the 6AL? If it's the 6AL, don't overlook the
limiter chip... if it's the 6A, make sure it's PN 6200 (not 6246).

I can't see required voltage at idle being very high (even if the plugs are
gapped in the 0.060" range) but even so, I would have likely opted for
something a little better than "chain store" wires. We've been using the MSD
wires for a few seasons and they are very good.

One afterthought... is this a case of dropping cylinders randomly or a
steady miss?


"Joe" <lanser1996@joltmail.net> wrote in message
news:1128952100.534663.67990@g14g2000cwa.googlegroups.com...
> I have a 1972 Ford Mustang, 351C-4v. It has an Edelbrock Performer
> Intake, and Edelbrock Performer 750cfm carb, with vacuum secondaries.
> I just swapped the stock dizzy, with a MSD pro-billet (magnetic
> pickups), MSD-6A CD ignition and a Pertronix Flamethrower II coil. I
> also installed new Autolite plugs and NAPA sparkplug wires.
>
> I have the dizzy curved to have all advance come in at about 2,800. It
> is timed for 16 degress initial, with 25 certifugal, for 41 total
> degrees at about 2,800 RPM. The dizzy also has a vacuum advance, which
> is hooked up just above the butterfly plates in the carb.
>
> The car idles beautifully (much better than it ever did). However, at
> about 3,000 RPM (cruising), it is missing pretty badly. There is no
> knock or pinging though, and I know that 41 degrees total advance is a
> lot. Any recommendations for what's causing the missing? What could I
> do? Should I back off the initial advance to about 13 degrees, so I
> total out at 38 degrees?
>
> Thanx...
>



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Old 10-10-2005, 12:01   #3 (permalink)
Joe
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Re: Missing at 3,000 rpm

I'm going to swap the NAPA wires for MSD's super conductors.

Its the 6A box, PN 6200. I have the plugs gapped to .035 (stock calls
for .034).

Also, it is a steady miss, not random. I can't pinpoint the cylinder,
but it always happens.

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Old 10-10-2005, 15:01   #4 (permalink)
Jim Warman
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Re: Missing at 3,000 rpm

With the MSD setup, you could probably go as wide as 0.070".... We runn
0,060 on our 9.5:1 B block Dodge with nary a hiccup. Check your fuel
pressure and flow to make sure the carb bowl isn't going down, too.


"Joe" <lanser1996@joltmail.net> wrote in message
news:1128963594.945550.129980@g14g2000cwa.googlegroups.com...
> I'm going to swap the NAPA wires for MSD's super conductors.
>
> Its the 6A box, PN 6200. I have the plugs gapped to .035 (stock calls
> for .034).
>
> Also, it is a steady miss, not random. I can't pinpoint the cylinder,
> but it always happens.
>



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Old 10-10-2005, 17:01   #5 (permalink)
John
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Re: Missing at 3,000 rpm

"Joe" <lanser1996@joltmail.net> wrote in message
news:1128963594.945550.129980@g14g2000cwa.googlegroups.com...
>
> Also, it is a steady miss, not random. I can't pinpoint the cylinder,
> but it always happens.


With an ice pick type electrical tester, hook the alligator clip to ground
and poke the ice pick end into the spark plug wire at the spark plug one at
a time. If it's always missing on one cylinder, the one that makes no
change in the idle/sound is the one thats missing. I'd do this before you
take the cheap wires off and put the expensive ones on.

If the expensive wires are on and it's still missing, pull the spark plug
wires off the spark plugs one at a time WITH insulated gloves on. Again, if
it's always missing on one cylinder, the one that makes no change in the
idle/sound is the one thats missing. With a '72 351 you should have room to
do this.

--
John
'69 Mach 1 390 Toploader Acapulco Blue
ThunderSnake #59



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Old 10-11-2005, 22:01   #6 (permalink)
.boB
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Re: Missing at 3,000 rpm

Joe wrote:

> I have a 1972 Ford Mustang, 351C-4v. It has an Edelbrock Performer
> Intake, and Edelbrock Performer 750cfm carb, with vacuum secondaries.
> I just swapped the stock dizzy, with a MSD pro-billet (magnetic
> pickups), MSD-6A CD ignition and a Pertronix Flamethrower II coil. I
> also installed new Autolite plugs and NAPA sparkplug wires.
>
> I have the dizzy curved to have all advance come in at about 2,800. It
> is timed for 16 degress initial, with 25 certifugal, for 41 total
> degrees at about 2,800 RPM. The dizzy also has a vacuum advance, which
> is hooked up just above the butterfly plates in the carb.
>
> The car idles beautifully (much better than it ever did). However, at
> about 3,000 RPM (cruising), it is missing pretty badly. There is no
> knock or pinging though, and I know that 41 degrees total advance is a
> lot. Any recommendations for what's causing the missing? What could I
> do? Should I back off the initial advance to about 13 degrees, so I
> total out at 38 degrees?
>
> Thanx...
>


Too much timing. To set base timing, get rid of the timing light. I know, I
know! That flys in the face of common wisdom and known mechanical rules. But just
listen.
The timing light is used to set the base timing of a stock car with stock parts
to a setting that the factory engineers determined to be the best based on current
fuels, technology, parts, expected use, yada, yada, yada. How many of these
standards apply today? Answer - None! So forget that standard and come up with a
new one. Also remember that base timing has absolutely no bearing on power output or
efficiency - unless you measure those standards at idle, and who does that?
What you need is a vacume gauge and a half a glass of water. Unplug the vacume
advance line at the distributor, and plug the line in to the vacume gauge. With the
engine full warmed up, turn the distributor until vacume is highest at the same idle
setting. Now watch the glass of water. Fine tune the setting until the water
ripples are the smoothest. Now shut the engine off and restart. If it starts
easily, that's your best base timing. If it's seems to be hard to start, back off
1-2* and try again.
Use the timing light to check and see what your base timing really is. It' will
probably be around 10* or 12*. Save that info for later, you'll need it.

Now it's time to set total mechanical timing. This has a direct impact on your
total power output. Most SBF's like about 34-36* total advance, all in by
2,800-3,200 rpm's. More than that rarely adds more power, and often causes
detonation that you can't hear. So I would start with 34* all in by 3,000 rpm's. Do
a few hard acceleration runs in 3rd or 4th gear (not first). If it's knocking, take
out 2*. If not, add 2* until it does knock. Then back off 2-3*. That's your
maximum total advance under power. Now you can change the curve to come in sooner or
later as you need to for best power and driveability. That's your best mechanical
advance. Again, using the timing light to find out exactly what it is, it will come
in handy later.

Finally, you can set vacume advance. All that does is provide a little fuel
economy at light/partial throttle, like cruising. It doesn't add any power at all.
If not adjusted properly, it will cost you some power.
Start at about 8*. Drive around in your normal cruise mode and see how it does.
If it starts to ping, back off. If not, add a little.

Also, check your plug gaps. Stock for '72 was about .032 or so (if I remember
right). But with your high powered ignition system I'd open that up to at least
..045". You NAPA wires may be the bottle neck keeping you from opening them up even
further. But once you got the rest tuned correctly, you could give it a try.


--
..boB
1997 HD FXDWG - Turbocharged!
2001 Dodge Dakota QC 5.9/4x4/3.92
1966 Mustang Coupe - Daily Driver
1965 FFR Cobra - Finally on the Road!

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Old 10-12-2005, 08:01   #7 (permalink)
Joe
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Re: Missing at 3,000 rpm

Thanks! I'll give it a try.

To verify, though, using your vacuum gauge method, it is measuring
ported vacuum just above the butterfly plates in the carb - it's not
measuring intake vacuum. Right?

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Old 10-12-2005, 17:02   #8 (permalink)
.boB
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Re: Missing at 3,000 rpm

Joe wrote:
> Thanks! I'll give it a try.
>
> To verify, though, using your vacuum gauge method, it is measuring
> ported vacuum just above the butterfly plates in the carb - it's not
> measuring intake vacuum. Right?
>

Since you're at idle, you can use either one. If you use manifold, be sure and
unplug and block the vacume line to the distributor.

--
..boB
1997 HD FXDWG - Turbocharged!
2001 Dodge Dakota QC 5.9/4x4/3.92
1966 Mustang Coupe - Daily Driver
1965 FFR Cobra - Finally on the Road!

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