[Blazin72]

My brother in law has a Ford dump truck with a 429 and he is having some problems that neither my dad or I have been able to figure out. Sometimes the engine will run very well and run to ~3500-4000 rpm under load and others it won't go above 1000 rpm in gear or in park without backfiring and cutting out. One day it was running so poorly I had to hook it up to my truck and tow him home. It was fully loaded and my measly gas 6.0 and clutch didn't like that very much. Anyway, we started with checking that the timing was set to the factory spec of 10 degrees BTDC and varied it as far as the distributor would go in either direction. We cleaned the carburetor, fuel lines and fuel tank and changed filters because the tank had a lot of rust. We started it and it ran fine with good power while driving it up and down the road but the next day my dad called me and said it was running poorly again. It would sit at idle for several minutes before the idle would dip down momentarily before returning to its normal idle rpm. After that we installed a temporary fuel pressure gauge and have been observing the fuel pressure and it remains a constant 6 psi regardless of how the engine runs. We even spliced in a MSD coil I had laying around without any change. Today my dad disconnected the entire mess of vacuum lines attached to the engine to see if that would change anything but it didn't.

At this point we know it is getting fuel and air and think it has to be ignition related but can't figure out what. Sometimes we can start it up and it runs just fine but others it starts backfiring and stumbling as soon as you step on the gas. Is there anything else we need to look at?

[LATECH]

I have seen the TACH give those exact problems. Its in the dash, as I presume it is the factory one.
Pull the instrument cluster and go drive it, if it clears up, then unhook the tach on the cluster panel.
If the issue is still there It could be a pickup coil in the dizzy, but I would lean towards the ECM. IIRC it has an EEC III type ignition, minimum function computer, and a module with a brown grommet. Somewhere in there is a governor,not sure if it is built in to the ECM or if it is another box under the dash.
I would start with the tach, as I have definitley seen those cause issues just as you describe.
Have a look inside the distributor, if there is no pickup coil, then it has the Duraspark III ignition system.Those have a crankshaft sensor in the back of the engine, not a pickup coil.

[Blazin72]

Originally Posted by LATECH

I have seen the TACH give those exact problems. Its in the dash, as I presume it is the factory one.
Pull the instrument cluster and go drive it, if it clears up, then unhook the tach on the cluster panel.
If the issue is still there It could be a pickup coil in the dizzy, but I would lean towards the ECM. IIRC it has an EEC III type ignition, minimum function computer, and a module with a brown grommet. Somewhere in there is a governor,not sure if it is built in to the ECM or if it is another box under the dash.
I would start with the tach, as I have definitley seen those cause issues just as you describe.
Have a look inside the distributor, if there is no pickup coil, then it has the Duraspark III ignition system.Those have a crankshaft sensor in the back of the engine, not a pickup coil.

I'll have my dad look at the tach and see what happens. I haven't looked inside the distributor but my dad is saying there is a magnetic pickup under the rotor. As far as the governor goes, it's on the side of the carburetor.

[LATECH]

Originally Posted by Blazin72

I haven't looked inside the distributor but my dad is saying there is a magnetic pickup under the rotor.

That is a plus. Must be a duraspark II (very good) .probably no computer of any sort on board. I would be looking real hard at the tach.
You could probably just unplug the Instrument panel to test it out.
Unplug it and drive to see if the trouble clears up.
Those tachs are fed right from the - neg side of the coil,one of the two green wires on the neg side,goes to the tach

[lmsport]

Are you still getting rust into the carb? A regular carb engine fuel filter will bypass with very little dirt, whereas FI engine filter has no bypass. I use the Ford 5.0 filter from the 88 Mustang on cars with rust in the system.

[Blazin72]

The truck has a screw on and inline fuel filter. Both were replaced when the tank, carb and lines were cleaned.

LATECH, will the tach show any abnormal operation when the truck is running poorly? What about the pickup coil, is there a way to test it?

[LATECH]

I have seen tachs go both ways, but usually it wont act crazy,itll just kill of the spark at the coil.
Testing a pickup coil, it should be between 400 and 1300 ohms, and also, hook a DVOM to the two leads of the pickup coil, put it on A C volts , have someone crank the engine, and see if it makes about .5 volts AC
The only thing is, intermittents are difficult to test as when they happen , by the time you get setup on it, it may not provide conclusive results as the condition may not be present by then.
Check the pickup coil, then run the truck, and do the tach disconnect test.