Re: 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?
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
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.
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!