Hi, I am new to posting in the forum, however I have used it quite a bit to solve problems I have encountered with my truck.

I have a 1977 F350 with a 460, hedman headers, edelbrock carb and intake manifold.

I just installed edelbrock 60669 heads with harland sharp rockers and chromoly pushrods. I continue to have a backfire that occurs at idle and it sounds like it comes out of both of exhaust pipes. I set the timing to TDC @ 1. Whether I advance or retard the timing it continues to backfire. The engine seems to run smoothly otherwise. I also noticed that when the engine is under load the backfire goes away.

Had no problems with engine temp and vacuum gauge on PVC shows 18.

I don't have a timing light but the engine runs good on the road.

Any help would be greatly appreciated.

Thanks,
Matt

 

That's a timing and AF mixture issue... either static or dynamic timing. A backfire back up the carb is fuel igniting when an intake valve is open. A backfire through the exhaust is extra fuel (rich!) being ignited.

GET A TIMING LIGHT... THERE IS NO EXCUSE. Autozone will even loan you one for free.

Okay there is an excuse... only if ya live out in the sticks.

 

How did you set the timing to 1° without a light?

 

Sorry, I've been MIA. I only make occasional trips up to where the truck is kept. I bought a timing light and set the timing to 15 BTDC and total timing is 32 at 2700 rpm. I am still getting some backfiring at high RPMs. Ive tried advancing the timing and retarding it with no luck. It idles around 750 rams.

Ive also made attempts to adjust the air fuel ratio on the carb. Even with the screws all the way in I am still getting the backfire at high rpms of when I stomp on the gas on the street.

Any help is appreciated.

 

I would listen closely to each exhaust pipe outlet. If you hear a spitting sound you may have valves that are too tight. Same thing with the carb listen closely with the air cleaner off.

 

When installing the heads and rockers, I recall my buddy tightening them down 1/2 a turn while the truck was running. And i noticed the backfiring got worse when he tightened either the 6 or 7 rockers.

Sounds like i may may need to pop the valve covers and take a look.

 

I believe the way to set them is to get the particular lifter on the base of the lobe and then tighten until all the slack is out plus about 3/4 turn. You can turn the push rod with your fingers until you feel a little resistance.

Running too tight will burn your valves in a New York minute.

 

After I adjusted all the rockers on the drivers side, I was still getting a steady backfire. I removed the #7 spark plug wire and the backfiring went away. Made several other attempts to readjust the rocker with no luck. Any ideas?

Matt

 

valve stuck or burned. compression check. Bad cam lobe can do it too. Did you check geometry on the rocker arms as you installed? Pushrods on 460 engines are notoriously short anyway, and with different heads, push rod length needs to be addressed.

 

I had a friend of mine who is a ford mechanic install the heads, so I never personally checked the pushrod length. Also when he changed the heads he took a look at the valves and said all of them look good.

Any chance this could be a distributor problem. i noticed that the backfiring stops anytime The engine has a load on it or when I unplug the #7 spark plug. It's still factory distributor and I've had a couple other friends suggest that could be my problem. I've ordered new ford racing spark plug wires, just wasn't sure if I the distributor would cause only one cylinder to backfire.

 

I think you've already found the problem, and it's on #7

Run a compression and/or leakdown test. Sounds like a bad valve

 

Inspect that plug, wire, and the distributor cap as well for good measure.

 

as papa bear said, do a leakdown test on #7....

 

Ok, so after a week of reading and ordering some parts I'm back in this desolate town to do some work on the truck.

I performed a compression test results for plugs are as follows:

1 - 120
2 - 130
3 - 120
4 - 130
5 - 130
6 - 130
7 - 130
8 - 130

I also put on new MSD distributor, ignition module, ignition coil, and ford racing wires.

Still have the backfire from plug 7 at idle. When i remove the #7 plug wire, the backfire goes away at idle but I get backfire with high RPM's. I'm lost at this moment.

Any other suggestions?

Matt

 

Make sure there is not carbon tracking in the distributor cap. Pull that plug and check the gap. Recheck the firing order. Inspect the plug wires. Your fuel is not burning in the cylinder and any of those things can cause a misfire. A misfire means fuel and air in the exhaust stream which can be ignited by the other cylinders exhaust. Also check you idle mixture adjustment just for giggles.