More of a POP then a backfire (73.5T)

[Sunroof]

I am trying like hell to get my 1973.5T running right. I changed out the rotor and distributor cap and added new ignition wires, drained my fuel tank to replace the internal filter, set the points, installed a new ignition coil, BUT when it starts up every minute or so I get a loud pop coming off the right bank of cylinders (4,5,6). It does not come through the air box pop off valve!

I have only one theory....................

When I drained the fuel tank to replace the screw in filter, I only filled the tank up with one gallon of gas. I am thinking with the car in the garage during these very hot humid Atlanta summer days, I might be getting condensation in the fuel-tank causing the intermittent misfire. But I am also thinking that the fuel filter would scrub any water out.

There must be dozens of reasons for the misfiring and I am ready to tackle whatever you folks might recommend. Appreciate any advice.

By the way, its the 2.4L with CIS (no modifications).

Thanks

Bob
1973.5T

[Steve F]

Firing order?

[Sunroof]

The popping is intermittent. I was certain that the new ignition wires matched the distributor order. I would think that if they were not in the right order, all hell would break loose! The popping is coming from the right side (cylinders 4, 5 and 6), but I cannot isolate which cylinder.

[Dave Kost]

If the pop is coming from intake you have an air leak in the intake system- lean running pop. If it coming from the exhaust air leak in the exhaust, rich running, unburnt fuel in the exhaust- rich running.

So, lean it out it from exhaust pops and fatten it up if from intake. Small adjustments. Use an AF meter to set the mixture. ( fix the air leaks first of course). You can lightly push on the air box and runners the 4-5-6 side to test for leaks- rpm will change or it will pop some..

My guess is the mixture was good until you swapped out new parts.

[Sunroof]

AF Meter? I thought you have to use a CO meter, which are pretty expensive. Over the years I have just used a click at a time adjustment. and of course one sign is the up and down idle after warm up.

[Dave Kost]

sorry meant co meter

[dicklague]

AFR instrument is very valuable in tuning. One with memory is best.

You can convert AFR to CO directly

[db_cooper]

Air leak would be the first guess. If the engine has been spitting into the intake..then you may have a crack in the airbox..very common. They usually split/crack where they are glued together. Take a close look as best you can with a flashlight and mirror.

[techweenie]

Originally Posted by db_cooper

Air leak would be the first guess. If the engine has been spitting into the intake..then you may have a crack in the airbox..very common. They usually split/crack where they are glued together. Take a close look as best you can with a flashlight and mirror.

++++

If a lot of the combustion product is getting out through a seam crack, the pop-up valve may not need to open all that much. Might be hard to see. Can you hold it down while replicating the backfire? I might want to hang some tape in strips around the airbox to see if the backfire disturbs it.

[Sunroof]

Thanks everyone.................................

Bob

[boyt911sc]

Pressure test the system by supplying continuous supply of low pressure air. Air pressure more than 5 psi. would open the pop-off valve so apply slight pressure to POV to keep it closed during test. Spray plenty of soapy water to locate the hard to find leak sources/s. A smoke machine is a better choice but not commonly available in most homes. Keep us posted.

Tony

[LakeCleElum]

Bob - I have a 73.5 2.4 CIS engine sitting on a dolly in garage. From my experience running this engine:

1) Vacuum leak, or
2) Debris in an injector messing with your flow pattern. (Could have dislodged the debris when working on your fuel system)

I would pull each injector one at a time on the side of the engine. Put the injector (still connected) into a small glass jar and lift the plate of the CIS and check for a nice even spray pattern.

It's likely you will find one injector where the fuel just dribbles out. I was able to disconnect my bad one and clear it out by blowing compressed air both ways. Fixed my problem in short order once I isolated which injector.

You probably have a local shop that will clean injectors. I also have 5 brand new injectors for that engine stashed away somewhere around here.

[Sunroof]

Thanks Bob.
Replaced injectors several years ago and on the 73.5T its a real pain! Injectors are set into the cylinders and tough to pry out but can be done with bleeding knuckles as the end result. Unlike the 1974 - on with the injectors set inside the manifolds, its the location on the 73.5T that makes it challenging. I am going to first add some concentrated cleaner to the gas and do my best to let it run through first before messin with the injectors. If it stops the backfiring it was a miracle $10.00 fix! Best to try the simple things first before tearing it all up.