73 Brave Dodge 318 Backfires

Frank M · April 17, 2016, 05:09 PM

My 73 Brave Dodge 318 has a rough idle and backfires when throttled. I changed out the coil, spark plugs, spark plug wires, distributor cap, rotor and points/condenser. Set the timing. I even ran the gas right from a gas can to eliminate the possibility of stale gas. Got it running a little better. I have been spending so much time in the motor home it is like I am full timing! LOL
Anyone have any thoughts on what I can check next? Carb rebuild? I had this thing running like a champ a few years ago but had to park it for a couple of years. Thanks for any suggestions someone may have.

Rickf1985 · April 17, 2016, 06:37 PM

Timing chain? Have you checked the position of the rotor when the timing mark is at TDC?

Frank M · April 18, 2016, 08:08 AM

Have not done that yet. Will check it tonight. Thanks

LJ-TJ · April 18, 2016, 09:08 AM

Well right off the top I would have done everything you've don't."Ben there done that so's I have to chuckle a little bit". So now I'd be frustrated and pissed. Backfireing I believe is a timing issue. Sooooo like Rick say's I'd be tempted to think timing. If you've all ready set the timing I'd be inclined to check it again and if it show right then like Rick say's check to see if the timing chain has jumped.

(Ignition Timing at Idle (+ - 2 1/2 Degrees BTC) Hm?

Frank M · April 18, 2016, 01:30 PM

I'll check the timing again tonight. I was also thinking about a vacuum leak. I only have 3 lines. One goes to the PVC valve, one to the brake booster and the other to the vacuum advance. I did notice that the PVC vavle was rattling allot. Is that normal? Also a completely different question. What is the tube that comes out of the manifold looking from the back of the engine it is on the right side of the carb? It goes up above the water pump. There are all kinds of brackets and a pulley up there. Was this an air pump? Air conditioning? Would that have vacuum in it?

Rickf1985 · April 18, 2016, 02:19 PM

Checking the timing with a timing light may tell you if the timing chain has jumped as long as you have not reset the timing recently. You can set the timing so that it lines up on the mark but still have the valve timing off. So if you check your timing and it is substantially off then suspect a timing chain as long as you are sure you do not have any major vacuum leaks. A PCV valve rattling means it is working, it is when they do not rattle that their is a problem. I see that in your first post that you set the timing so checking it now is moot, how far off was it when you set it?

Frank M · April 18, 2016, 02:28 PM

The timing was not that far off after I changed everything. The distributor is pretty much in the same positions as it has allways been for the last 10 years or so. I am going to plug the vacuum lines tonight just to see if that makes any difference. It seems to be idling much better than before I changed everything. Still has a backfire though when I accelerate.

legomybago · April 18, 2016, 04:33 PM

vacuum advance is toast/leaky or froze up and not moving in the dizzy is my bet...
When at idle, take the vacuum hose coming from the dizzy and suck on it and see if your idle increases. Or with the engine off, pull the cap and see if your advance plate is working by sucking on the vacuum advance line.

Rickf1985 · April 18, 2016, 07:00 PM

I know this is a really BAD cliche but you guys must be able to suck really hard! :)rotflmao :)rotflmao :)rotflmao Seriously, you need more than that to move the advance. I always plug it into a constant vacuum source or use a hand pump. A vacuum advance that is not advancing will leave the timing retarded which is not likely to create backfires. It is starting to sound more like a burnt valve. Is it a POP, POP, POP, POP. or just a random blowback through the carb?

Frank M · April 18, 2016, 07:07 PM

Its actually a random back fire through the exhaust that is why I was wondering if a carb rebuild would help.

Elandan2 · April 19, 2016, 07:15 AM

You did say that it had sat for a couple of years, could it be a sticking exhaust valve?

Frank M · April 19, 2016, 10:09 AM

A sticking valve is possible. At this point I would check to see if a clogged toilet is the result. LOL I did start it periodically and even changed the oil last summer. I am not to excited about pulling the heads off so I am trying to find any other possibility. Funny thing is that 30 years ago I would pull the motor out of my 70 Challenger just for fun and clean and paint it. Drag raced a 70 Dart Swinger so played with that for years. Now I just don't feel it anymore. Ha! I guess the next steps I may take are a carb rebuild and maybe pull the valve covers off and see what it looks like in there.

Rickf1985 · April 19, 2016, 10:25 AM

A misfire and running rich will cause an exhaust backfire as the excess gas from the misfire is ignited in the exhaust. This usually has to be accompanied by an exhaust leak fairly far upstream in the exhaust system like the donut at the manifold or a manifold gasket so the rich mixture can get air for the bang.

The misfire could be from running to rich, bad plug wire, bad condensor (that's a big cause) Dirty points, bad coil, accelerator pump going bad or a vacuum leak.
Needless to say, this list needs to be pared down a bit.

Frank M · April 19, 2016, 09:23 PM

Thanks to everyone for all of the suggestions. I think the next and cheapest step is to rebuild the carb. I'll do that and check the exhaust for leaks. I know there is one but have not really crawled around to find it. Years ago I had a hell of time sealing the exhaust manifolds. I finally spent some cash on a high performance set from a local speed shop and the did the trick. The cheap ones just blew out. I'll re-post as I go along here. I'll have to find a rebuild kit for the carb. Anyone buy one? I think it is a carter 2 barrel BBD 6225S with electric choke. There is no tag on the carb so I am making somewhat of a guess here.

brians1969 · April 20, 2016, 06:09 AM

While you are rebuilding the carb, check for looseness in the throttle shaft. Carter bbd's are known to have that problem. I got a (holley rebuild) BBD carb at Advance Auto and had to reject the first two because of excessive play. Holley should never have sent them out without rebushing them.

What happens is, air gets sucked in thru the gap between the shaft and the body and you get a lean condition, making for a rough idle. Maybe other problems too? I don't know. I was replacing the original Stromberg WW carb.

Once I had a good one though, the thing ran so much better! And the electric choke was a plus!

Frank M · May 30, 2016, 03:21 PM

She's running like its 1973 again! Changed the points & condenser, rotor, cap, fuel filter, coil, plug wires and rebuilt the carb. Still did not run right. The spark plugs did not have many miles on them but I figured why not change them. Thought maybe a bad plug out of the box the last time I changed them. Also took off the valve covers and the rocker arms. Sprayed the valves twice a day with a 50/50 mix of acetone and trans fluid for about a week. Tap on the tops of the valves with a rubber mallet after each soaking. Put the rocker arms back on and the valve covers back on. Threw in a fresh set of plugs and gave her a crank. She started up. Ran a little rough until I set the idle screws and the timing. Running great now. No backfire, hesitation or miss. I am thinking either a bad plug or a sticky valve was the problem. Now I just need to get it inspected and I can hit the road for some R&R before summer ends. Thanks for all the suggestions. Patience and tenacity pay off!