1972 Mach 1 wont go above 4k rpm

jimb0slicee

New Member
Mar 8, 2020
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Arkansas
So I’ve dreaded doing this for a while because I’ve searched far and wide for answers and I’m still left with the same problem.
I have a ‘72 Ford Mustang Mach 1, i bought it with a 2v headed Cleveland, FMX, and a 8in rear. It is a true Q-code car so it is supposed to be a 4V cleveland, with a C6 and 9in rear. Long story short, when I got the car, everything was great and peachy. Well one day I didn’t have an gear but reverse. So i took it to a high rep tranny shop to have the tranny looked at. What they told me was that the tranny was no good. That was okay with me because I wanted to convert it back to a C6 one day anyways.
I found a sbf C6 and had then rebuild that and cut the driveshaft so it would fit in the car. 7 months later and over 2k of money. The car finally got done. Well I went to pull it out of the shop and all of a sudden the car wouldn’t go past 4000 rpm. So the next day i had it towed back and told them the car wasn’t right and explained the rpm problem and they said that they couldn’t find anything wrong with it but the car was missing and running like :poo:. So they put the blame basically on me.
Over the last couple of months, I put a duraspark II conversion in it, so distributor, module, coil, plugs and wires, and a brand new quick fuel carb with a new throttle cable.
Although the car does run a lot better now, the car is still hitting 4000rpm and just falls flat on its face. Its does it in P, R, N, D, 2, 1. And it does it stationary and in motion. Its acting like its hitting a 2-step or rev limiter and just will not go any higher.
I have looked up everything and I cannot figure out what it is.. I have redone all of the ignition and fuel stuff. And I wanna blame the trans shop but I just dont know what would cause the engine to act like its hitting a limiter with or without a load. If anybody has some advice or wise on the topic, it would be much appreciated.
Thanks,
Jimmy!
 
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08GT500

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Jul 12, 2018
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Massachusetts
Hi, Exactly at 4K, no matter how much throttle is applied, and every gear selected.
That’s pretty exact for an older car. If someone didn’t add some type of launch, limiter, something’s acting as one.
May be coincidence, I.e. problem repaired with new part, another recreated a similar issue.
How many miles on the motor, any rebuilds. or performance parts added?
Initial thoughts, info above will help: Electrical disturbance dropping spark (Alt)
Distributor advance issue, component failure, Firing order wrong for cam run, plug wires crossed.
Low oil pressure or sticky/stuck lifters, not pumping solid. Pump unable to open valves at higher rpm’s, oil contaminated.
Carburetor tuning, jetting, low float lvl.
Exhaust restriction, lack of flow.
PCV System not functioning
Vacuum leak(s).
Fuel volume limited, interruption, filter, pump improperly functioning.
Cam lobes worn, low lift, low flow. Incorrectly set rocker preload.
Blown head gasket between cylinders.
(Check for cross contaminants in
oil/antifreeze).
Bad fuel, old fuel with H20 within.
Stuck lifters, oil forth/contaminated
- I’d look at voltage to the coil (running a ballast?) & check your voltage regulator, alternator output tested (ignition voltage drop @ RPM), battery tested, voltage. Wiring associated to the ignition, or ignition switch damage..
Hang up an inductive timing light on a few different cylinders while trying to rev past 4K.
Are your sparks disappearing, or remaining?
Are the plugs loading up, what color are the electrodes?
Check the Transmission related only issues, a failed vacuum modulator causing a large engine vacuum leak perhaps pulling it transmission oil into the airstream & being burned. Any blue smoke while going after the mid-high rpm’s?
Transmission shifting as it should be at lower rpm’s? Any squealing noise?
—As the motor is hitting 4K & not higher
ignition voltage cut out from Alternator failure.


There’s quite a few that may play into this, accurate seems most likely to be dirty lifters only allowing little flow, oil pressure, valve timing, advance set improperly
What’s your oil pressure at idle, at 4,000?
I’d pull a valve cover and using a dial indicator sitting on the head, dizzy marked & removed, pressurize the lifters with a cordless drill and rotate the motor by hand. Are the Hyd. lifters pumping solid enough to hold the valves off the seats to correct lift, if so. how long after priming, measure valve lift on the retainer.
I’d pull a valve cover to view the Valvetrain, see if you may have a few stuck lifters, or lash set too tight, a few bent valves.
Timing chain may have chain jumped time, use the TDC Mark & verify both valves close in #1 cylinder.
Run a compression test, and a leak down test.

What is your base timing, and what advance type are you running, vacuum or mechanical, both(?)


Be certain you are getting full throttle. (engine off). Also make sure the primaries & secondaries are fee to open when you are at full throttle, sometimes they bind on a gasket or the linkage hits the manifold

Second, I would look at the timing curve, put a light on it and make sure the advance is coming in. I'd probably run that setup at 12-14 initial - 36-38 total, all in by 3,000 (vac hose disconnected and plugged, after dialing in the curve I would hook the vac line to ported vacuum(Carburetor)).
Once a bit more is known about what you’ve confirmed as correct above, the mod’s are known, it will help to troubleshoot
Good luck!
-John
 
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