Look at all 8 plugs especially those of 3 and 7 and compare them and see what the differences are.
Hope you can stand another "compression test results" thread
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Does the engine have adjustable rockers on it?
jcolman
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The compression test was done with all the plugs out, the ignition and fuel disabled, and I cranked the motor four or five times with the starter. This is how a compression test is supposed to be done according to everything I know.
jcolman
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jcolman
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Most all of the plugs show some soot on them. Hard to see here but this is all the plugs. I broke the insulation on # 5 & 7 getting them out.
#7 is very sooty.
# 3 isn't as bad
Best way to take photos of plugs is to hold each and in the palm of your hand and take a close up looking down into it. Is 7 shiny at all? because if it is that's most likely oil also if its a dual plane manifold check the plugs that share the same side plenum and pull the carb and see if either plenum is darker than the other.
jcolman
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jcolman
Well-Known Member
Your plug looks shiner than mine. I suppose I could have some oil leakage as well, perhaps from the valve seals.
Rat Bastid
Dunamis Metron
You are dead rich and it needs more total timing. You need to fix that before you do anything else.
What plug and heat range is that?
jcolman
Well-Known Member
Champion RC12YC
Rat Bastid
Dunamis Metron
Thats about right. You just need to get some fuel out of it. And add some timing. You’ll see the compression numbers get better once all the fuel isn’t washing the oil off the cylinder walls.
Then the difference in compression could easily be in a crappy valve adjustment.
jcolman
Well-Known Member
Ignition *should* be fine. MSD Blaster SS coil. MSD 6AL ignition is what I have in the car.
I've seen a pulley change on an alternator drop the voltage .5 causing the engine to run rough and the plugs appear rich when the mixture wasn't changed.
Check to make sure that the alternator you are using is putting out enough voltage to run the ignitions requirements.
I may have missed it but had this car been sitting or driven often.
jcolman
Well-Known Member
It’s driven usually once a week
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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front to back, Drivers on the left, is
186------------183
155------------195
170------------180
150------------180
195 is an anomaly so throw it out
150/155 on non-adjacent cylinders are indicating a problem.
the average of the best four, not including the anomaly is 182.25. Therefore, 150/182= 82.4%, equals 17.6 % low, and all on the same side.
Couple that with the thousand rpm idle and the pull-back, this is all pointing to one of two things; A bad driverside head or a bad cam. The LD will tell you which it is. If you got rocker-clatter on that side then you already know the answer.
jcolman
Well-Known Member
Thanks for the info! Much appreciated. Question...how exactly does a leak down test tell you if a cam or head is bad? I understand that if you have a poor seal on an exhaust valve you'll hear a "hissing" sound but that's about all I know.
I wouldn't be runnin any leak down test until I had the tune on that thing 100% including zeroing SLAM in on a good valve adjustment. Otherwise, your results might tell you a lie. You don't want to end up tearing into it for nothing. Just food for thought.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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An LD test will tell you what is, or is not, leaking. By itself it cannot tell you if your cam is bad. But,
if your LD test shows no obvious leakage, then together with your compression test results; the low pressure cylinders by default, have to be caused by either; 1) the valves were not opening for as long as the others , or 2) the compression test was not done quite right.
Too often the compression test is run for an arbitrary number of cycles, instead of until the highest pressure is recorded at least twice in a row.
On a good engine, peak pressure will be observed within about three, mayber four cycles. But if the cylinders are dry, it may take many cycles. Or if the valves are sooted up, the results will be erroneous. Sometimes, in the past, I have had to remove the rocker arms/shafts and smash the soot off the seats by tapping the valves with the cylinders pressurized, which will; 1) get the pistons out of the way by blowing them down to the bottom of the cylinder, where, hopefully, the best part of the cylinder is, and 2) blow the soot off the seats. When tapped in this way, sooty seats sound different when the valves land, than clean seats do. It will be obvious.
Rat Bastid
Dunamis Metron
Said the goat. lol
jcolman
Well-Known Member
My timing at idle is 30 degrees. At 2000 rpm it drops to around 28. I'm beginning to think that I may have an exhaust leak that is causing the O2 sensor to dump fuel into the throttle body since my AF/R between idle and 2000 rpm fluctuates between around 11.9 and 12.4 as shown by the Holley Sniper wizard display.
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