popping out exhaust

flatmotor40

flatmotor40

Well-known member
Just started.Went to store and came out and seems to run a little rough but then when I stepped on it from red light a pop from the exhaust,Did it a couple times as i went heavy on throttle.but it seem to run a little rough.It has done this before but it was moisture in cap.Frt cap 59ab.Checked cap when I got home no moisture.Cleaned it and put it back on.Cranked up and at high idle ok but rev up a little more and a pop.Removed plugs and looked good.Compression check 135 -150.But my gauge leaked down as i held it.Old gauge.Need a little help.Have 2 97's for Clive and 1 of his electronic dist.Thanks
 
If all cylinders are working the reading on a vacuum gauge should be steady. If the needle momentarily drops that means a cylinder did not hold, it's compression during the stroke. If the gauge has a smooth reading than it's probably an ignition problem. A compression test would normally show a weak cylinder but like Ken said it could be an intermittent sticking valve. Without a scope to check the firing it's a little harder to pinpoint a bad cylinder, what we used to do at work was pull off one plug wire at a time with it running and see if it made a difference. Do not do this with electronic ignition or you could damage the module. You can still do it but first shut off the engine and ground one plug wire at a time to check it. If you do not ground the wire the voltage can back track up the plug wire and damage the module. If it just old school points ignition there is no problem with yanking wires one at a time, except for getting the crap shocked out of you. You can at least narrow it down to one cylinder by doing this and then do further testing.
 
key symptoms for me to be looking at would be 1) is running ok until warmed up, then 2) after stopped, sat for a few minutes and restarted problems arose and 3) and symptoms become more severe when load is applied.

those three point to spark being compromised due to excessive heat.
 
Thanks will put vacuum gauge on it then Marvel oil.Thanks It has just happened.Always something else to crop up.
 
Found a small crack in the distributor cap.Put another one on a fixed.Thanks but will still staart adding a small amount of MMO to the tank or lead additive
 
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