Join Our Ford Truck Forum Today

Document your Ford truck project here and inspire others! Login/Register to view the site with fewer ads.

1979 F250 backfire issue

I have a 1979 F250 with a swapped in 460. The engine has been in this truck and worked fine for 15+ years. last month is started to backfire thru the exhaust an also the carb when at half throttle or higher only. Idles fine.

I replaced the plugs, roter, cap, wires, "brain box" on the firewall.
Removed the valve cover pan and while running and back firing nothing seems amiss, no leakage past the valve guides, no bent push rods, everything seems to be moving as intended, leaving me to rule out a bad valve, lifter or bent push rod. Although I could be wrong about the lifters or cam area as I did not yet remove the intake.

With true dual exhaust I can tell that it is deffintly only the passenger side that is backfiring.

Any ideas on what to look at next?
 

heavydoc

Instigator
Staff member
1,994
70
WNC
Most backfire issues I've seen have always been carb related. Although I am not that knowledgeable on gas engines. Maybe @dustybumpers will be by soon and can answer it more clearly or better.

Sent from my SM-N975U using Tapatalk
 

Blue-Truck-Nut97

crank polisher
551
18
Check it over for vacuum leaks.
Which carburetor are you running?

Could be in the carb or starving for fuel for another reason, check over the fuel filters and pump.

There are other possible problems, but let's start simple.
Sent from my E6810 using Tapatalk
 
Last edited:

heavydoc

Instigator
Staff member
1,994
70
WNC
I guess or Blue may swing by

Boop[emoji1787][emoji1787]

Sent from my SM-N975U using Tapatalk
 
Holly double pump, not sure of the number will have to check for tags as I did not install it. Huge carb and new fuel filter this year so can prolly rule out starvation for fuel. The back firing only on the passenger side baffles me. Thats why I was leaning towards a stuck lifter, bent push rod even a cam lobe issue.I also did a simple compression test not a bleed down and it came out good on all cylinders. Also the reason I replaced the plugs wires and cap rotor.

Will check for vac leaks and get back.

Easy enuff one vacum line, to the brake booster thats it.
 
Last edited:

dustybumpers

don't play well w others
As stated above, probably a vacuum leak.
Could be a timing issue. I would think a carb related issue though because of ethanol gas.spray carb cleaner around everywhere could be a leak, if the idle speed rises, you found your leak. Check throttle shaft into sides of carb too, they are a known problem on the older holleys

Sent from my SM-N960U using Tapatalk
 

Blue-Truck-Nut97

crank polisher
551
18
One vacuum line to the booster. Yes. Then the vacuum line to the advance on the distributor, the gasket between the carb body and the base plate, the gasket(s) between the base plate and the intake, the metering block gaskets, the throttle shaft, the diaphragm for the secondaries, and the intake gaskets where they meet the heads. All potential leak points.

Sent from my E6810 using Tapatalk
 

dustybumpers

don't play well w others
I he has a stock intake, could have burned out egr tube in the intake in the center, passenger side. That would give a miss

Sent from my SM-N960U using Tapatalk
 
Not a stock intake, alluminum after market, No vacum advance on distributor, linkages seem ok, but will need to check for leaks around the intake/ carb .

Why would it backfire only on one side of the engine if it was a carb issue and not backfire on both sides all the time or randomly? All worthwhile suggestions , will check more into those.
 

dustybumpers

don't play well w others
Could be valves are Carboned up, and not seating properly

Sent from my SM-N960U using Tapatalk
 

Blue-Truck-Nut97

crank polisher
551
18
With your aluminum intake be very careful when you check the gaskets at the rear, I've had several leak there.

In my experience, valve problems usually start warning you at idle, you'll get a miss at idle, and a mild pop out the exhaust or intake, usually clears up around 1200 or 1500 rpm. Now a sticking valve would be higher RPM, which could be your problem, but it's usually only an intake, or exhaust, not randomly one then the other.

A collapsed lifter, bent push tube, broken rocker arm, stuff of that nature is usually not RPM dependent, it'll miss, rattle, pop, whatever, whenever.

A flat cam usually shows itself at higher RPM, 2000 plus in my experience. It'll wind up fine, then just fall on its face. Just my 2 cents, I've been wrong before.

Sent from my E6810 using Tapatalk
 

dustybumpers

don't play well w others
Had forgot, don't know what year your 460 is, the newer efi 460's had a run of bad lifters

Sent from my SM-N960U using Tapatalk
 

Blue-Truck-Nut97

crank polisher
551
18
Another 460 fun thing was the fiber timing gears. They had a bad habit of jumping time and if they ran at all it wasn't good. But, if it's a built engine hopefully they put solid gears back in.

Sent from my E6810 using Tapatalk
 

Ford Truck Articles

Top