Car is a 1974 smallblock, 4 speed. Pertronix ignition.
Symptoms are as follows:
Used an air/fuel ratio gauge to set the Qjet up as good as I could get it. After a year and several months of running like a champ, the car began to missfire at times. A/F gauge shows richer than I had it before, especially when it sneezes. Put in slightly smaller primary jets. No improvement..
Several weeks ago, the Pertronix coil failed. Replaced it with a new (Standard) one from the local auto supply house.
When cold, the engine starts right up and runs normally. After the engine is fully warm, it backfires through the carb. It can sneeze hard enough to chirp the tires and make the whole car jerk. Seems to be worse under load. Sometimes it just seems to be random. Gets worse the longer the car runs. Happens at highway speeds and around town.
Parts changed while attempting to fix it include:
Plugs(which looked fine. A little lean, perhaps), plug wires, cap, rotor, fuel filter. Swapped the Pertronix out for a Mallory E spark. No improvement. Put the Pertronix back in.
Checked Qjet float level, primary jets, primary and secondary rods.
Everything looks normal.
Except the #$% car still runs like %^&*.
Now what?

 

i had/hate problems like that, if it were me i would look at timing chain stretch, compression check, (burnt / sticking valve), valve train rockers, pushrods.....

 

Engine was rebuilt about 7000 miles ago. Adjusted the hydraulic lifters to zero lash plus 1/2 turn preload over the weekend.
Might even be worse now.

 

I would check timing after pushing my 74 pretty good on the hwy my distributor moved a bit had backfire in carb did my timing all good now

 

Timing checked three times in the past few weeks. As before.

 

just a wild *** guess-could the heat riser be stuck causing the carb to overheat and boil the gas?

 

Heat riser has been removed.

 

The things that cause backfiring through the carb in my experience are:

Spark plug wires not in the correct firing order
Timing way off
Spark jumping across wires
Exhaust valve not opening

 

just a wild thought.....what gasket(s) are you using between the carb and the intake????

 

Yes, 76 Ray, I am also worried about adjacent plug wires picking up the spark energy from each other. Ideally, all the plug wires should be at least 1/2" apart from each other and anything else, says Smokey Yunick. Hard to do with the factory wire routing, inside the shields and under the ram's horn exhaust manifolds. Don't know how to address that, really.
Plug wires are correctly routed. Timing is 12 degrees BTDC initial.
Ran the car in the driveway with the valve covers off. (Put cardboard around the heads, but it made a mess anyway.) All the valves are apparently operating normally, as far as I could tell. Then I adjusted the valves per Edelbrock's directions. (Cam is Edelbrock 2102).
SS Roadster, I am running the recommended 1/4" thick gasket under the carb.

 

Will it backfire if you rev the engine while in neutral? If it does then you could disconnect the plug wires (one at a time) at the distributor. If removing a particular plug wire makes the backfiring stop, then you will have it isolated to one cylinder. If you can use one of the plug wires you removed to hook up that cylinder by routing the wire over the valve cover (temporarily) and it doesn't backfire then you have nailed down the cause as spark jumping. If it still backfires then there is something wrong in that cylinder. I suppose a small leak at the intake valve could allow enough compression for the cylinder to fire but still allow the explosion to come back out of the cylinder and through the carb. The other thing that could have happened is the outer ring on the damper may have slipped and the 12 degrees that you are showing with the timing light could be incorrect.

 

Thank you. I will try that next.

 

Well, excuuuse meeee.
Pulled the Pertronix out, and installed a set of breaker points and a condenser this afternoon. Ran like a champ. Not a hiccup.
Dunno why the Mallory ran like @#$% too, when I swapped it in last week.
Anyway, now it runs fine with 100(!) year old technology.
Whew. Time for an adult beverage to celebrate.

 

Celebrated too soon.
Went out for a drive early this morning. It ran like an absolute champ for 45 minutes. Then I stopped for gas (Shell), and when I got back on the highway, I began to run it through first and then second gears at WOT, adolescent that I am, and I could feel the power was "off". Could the ethanol in the gas be messing things up?
Popped and spit at me the whole way home. Backfiring out of the carburetor. As bad as it was before. Couple of times I didn't think I would make it.
Now what?

 

just a wild early morning guess......

from what you have switched out, it sounds like you are experiencing some type of short in the system. the fact that one system works and the other does, then doesn't just doesn't seem right.
could the following be possible???
1-your coil, are they are wires or connections that need to be rechecked?
2-have you tried a new or old coil that you have just have around?
3-even though your plug wires appear to be ok, have you checked out each one individually?
4-I think that if the fuel was boiling, you would not be able to keep the engine running for any length of time, or do any restarts until it entirely cooled down.

On a side note on my 82, I know it has the HEI, but anyway my coil wire under the cap had broken completely off and left me stranded to get towed. The point I am trying to make is that the wire must have been hanging by a thread before it broke, and was causing some strange engine problems which are now corrected with the new coil.
Don't give up, it has to be something really simple, but just hard to locate and repair. Just start from the begining and go over everything once again. Good luck.......Tom

The only way that the gas would affect your engine is that you were pumping from the bottom of the gas station tank and sucked up a lot of water in the process. You can always add "Heet" or some other type of fuel additive "Lucas" cleaners, etc. and see if it clears up.

 

reread your original post, on your intake manifold, is there a slight depression on the intake forward of the carb that runs from side to side.
if there is, that 1/4" gasket that you mentioned is not sufficient to prevent the heat from the manifold from entering the carb and therefore boiling the fuel. If you look at some of "Lars" papers, he recommends 3 different types of gaskets to prevent this from happening. I know this is the set-up I will be using on my 74 BB. I would imagine the same could or should be used for the SB too.
Just trying to think of all possibilities...........Tom

 

Thanks, guys.
Plug wires, coil, plugs are all new.
I noted (with an infrared thermometer) that , as long as the engine has been running for more than a minute or so, the carb does not get too hot. Between the fresh fuel entering the bowl, and the liquid fuel vaporizing in the venturis, it doesn't get much hotter than 130F, even on a hot summer drive. When you shut it down, especially with the hood closed, it gets plenty hot, as the heat from the rest of the engine rises. So on a hot restart, it runs rather lean for a short while. Only a problem if it needs to idle. If you can drive it right away, it's not an issue.

Just bypassed the primary wiring. Ran a new wire from the fusebox to an aftermarket ignition resistor and then to the coil positive terminal. Removed and taped off the origional wires.
Still backfiring through the carb.
Maybe tomorrow I'll get another coil, and see if that helps. I seem to feel the main problem is electrical, not fuel.

Anybody ever mount the coil on the firewall just to get it somewhat away from the hot engine?

 

Cam lobe going flat.....

 

I guess that would show up on a compression test as a rogue pressure reading?

 

bump