At wits end...
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1st Gear
Joined: Jul 2024
Posts: 1
Likes: 2
At wits end...
I bought a 76 Highboy, with a 68 390 4V. I keep having problems with cylinders not firing. I've replaced the coil, cap, rotor, wires and plugs. The plugs are Autolite 45s, gapped at .44. Two cylinders quit. I changed both of those out for new spark plugs. Ran fine for a couple days. Then I had three different cylinders that quit. I replaced all the plugs with a new set, gapped at .38...ran fine for a couple days, 3 more cylinders went down...I check each wire with my timing light to see if they are firing...Should I try a different brand of plug? This is beyond me...and starting to get pricey. I haven't done an ohms check on the plugs that came out, nor have I done a compression check. Fuel is 91oct, non ethanol. Timing is 8 before. Carb is a 780 vaccuum secondary Holley, set up for high altitude. I live at 8700 feet. When all the cylinders are working, the idle is good, zero hesitation, and zero popping back on a down hill coast. What am I missing? HELP!!!
FTE Legend
Joined: Jul 2005
Posts: 65,938
Likes: 1,432
From: Millersville, MD
Go here to ask your question, You posted into a Forum that is reserved for photos of trucks.
https://www.ford-trucks.com/forums/index.php
https://www.ford-trucks.com/forums/index.php
Fleet Owner
Joined: Mar 2012
Posts: 26,116
Likes: 4,148
From: Near Cal. Avenue
Odds are you have a faulty distributor and advance unit.
Valves and Camshaft + lifters would be a secondary thought.
Bad intake valves lots of carbon on the intake side are always a concern for popping back.
If your advance shows well with the timing light I would pull the worst headfirst and clean up those black valves.
Carbon build up holds them up off the seat causing popping back thru the Carb. The problem can be problematic.
Usually one cylinder starts it out and then it can jump around. Intake valve guides are probably junk loose.
There are Gumout products, but it sounds like your valves are clogged with hard carbon. It's a common problem
With dry gas fuels like propane or CNG. Also improperly maintained direct injection motors. Use only pure synthetic
motor oils is a good choice.
Direct injection has been accompanied with port injection in the near past because of it.
Valves and Camshaft + lifters would be a secondary thought.
Bad intake valves lots of carbon on the intake side are always a concern for popping back.
If your advance shows well with the timing light I would pull the worst headfirst and clean up those black valves.
Carbon build up holds them up off the seat causing popping back thru the Carb. The problem can be problematic.
Usually one cylinder starts it out and then it can jump around. Intake valve guides are probably junk loose.
There are Gumout products, but it sounds like your valves are clogged with hard carbon. It's a common problem
With dry gas fuels like propane or CNG. Also improperly maintained direct injection motors. Use only pure synthetic
motor oils is a good choice.
Direct injection has been accompanied with port injection in the near past because of it.
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