Help having exhaust /engine issues

[BMF2012]

I have true dual exhaust and on the passenger side the exhaust periodically will want to suck in. I did a compression test and the results were 115, 115, 125, and 120 on that side. Short of taking the heads off to inspect the valves, is there anything else I should look for? Nothing looks bent or broken under the valve covers at a quick glance. It's a 1970 302 with headers. Dave

 

[Powerbyford]

How did you determine it is doing that? Paper against the tailpipe? Does your exhaust have a crossover pipe or anything else where the banks mix together? What caused you to look for this in the first place, other symptoms like backfires or popping in exhaust? When does it happen?

 

[BMF2012]

No crossover . It sounds almost like a popping noise I noticed just from that side while idling. So I tried the paper against the tailpipe and sure enough it would want to suck it in periodically. Drivers side is fine.

 

[BMF2012]

BTW it's true dual exhaust.

 

[Broncobowsher]

What's compression on the other side of the engine?

 

[BMF2012]

If I can remember the readings were starting with #5 120, 117, 115, 115 on the drivers side.

 

[B RON CO]

Hi, if it is running good I wouldn't worry about it.
The basic explanation is your exhaust system has pulses of exhaust that will expand and dilute as they pass through the pipe. Say an exhaust valve is open for a short time every 90* of crankshaft revolution. So you have a short exhaust pressure pulse (high pressure) followed by a low pressure pulse, over and over.
Here is what happens.
Atmospheric pressure, @14psi is greater than the low pressure in the pipe, so guess what. The higher atmospheric pressure overcomes the lower exhaust pipe pressure and you have exhaust coming out and atmosphere going in and this will cycle.
It is probably happening all the time but at high RPM s the exhaust pulse is so quick you will never notice it.
Good luck

 

[Powerbyford]

You can listen with mechanics stethoscope to maybe isolate which cylinder is doing it. Put a vacuum gauge on it and see what that shows.

 

[DirtDonk]

You can also get those odd sounds out of one side when you have a slight misfire from an improperly adjusted carburetor, a weak spark, jumping sparks from old wires and plugs and caps and such, along with other stuff as well. Including lean-misfire, from headers and dual exhaust without making the proper jetting changes to the carb.
What altitude are you at?
No reason to pull a head at this point, since your compression numbers are so good as they are.

Try messing with the carburetor idle air mixture screws to see if the sound goes away or changes. Change your timing, looking for the same result.

Are you still running points ignition?

 

[BMF2012]

Points are gone. Converted to Pertronix and a flame thrower coil. I will tinker with the carb and possibly the timing. Don't want to pull the heads unless absolutely necessary. I'm in south Jersey not sure my altitude. Lol.

 

[BMF2012]

Thank you all for the help. Dave

 

[No Hay]

You can also get those odd sounds out of one side when you have a slight misfire from an improperly adjusted carburetor, a weak spark, jumping sparks from old wires and plugs and caps and such, along with other stuff as well. Including lean-misfire, from headers and dual exhaust without making the proper jetting changes to the carb.
What altitude are you at?
No reason to pull a head at this point, since your compression numbers are so good as they are.

Try messing with the carburetor idle air mixture screws to see if the sound goes away or changes. Change your timing, looking for the same result.

Are you still running points ignition?

Yep, mine sounded like this when it was set for emissions in Portland. Reset timing and carb and it's perfect.

 

[DirtDonk]

Being near sea level then, you may need to richen up your main jets a size or three.
But for now you can test things with different tuning methods. Changes in timing, changes in idle mixtures, changes in anything that's old (maybe the plugs need cleaning and gapping?) and stuff like that. All simple adjustments.

But headers and dual exhausts are well known for making an overall mixture a little leaner at the intake end. Putting a lot more pressure on the ignition/spark than normal. So a stronger ignition is a must at least (which you already have with the Pertronix), and re-gapping plugs can actually make a big difference.
But in the end, you may just need to richen up the mix a bit with larger main jets.

However, almost up there with cylinder heads, doing main jets would be near the end of my list even if I thought it was the most likely. Nothing wrong with making sure things like vacuum leaks and ignition tweaks are done first. Not like changing jets is the end of the world of course. Just takes a bit more hassling around than the other stuff.

Paul