Compression readings

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Mar 19, 2011 | 07:32 PM

rdweathers

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From: Florida

Compression readings

1974 390 with mild cam, 224 intake, 224 exhaust, 050 duration, cam lift .293 int & exh, valve lift .507 Int & Exh, lobe center 109 int & 115 exh. One cylinder at 82, one at 92, two at 93, three at 95, and one at 98. Are these good or bad readings? The engine has a flat spot off idle or a skip and will go away after you pass this point, when driving it is at 30. After the engine is hot the skip is noticable at idle, after you pass 30 it is gone and on the highway i do mot notice it. I have taken the plugs out and they look new just like i when i put them in. Any ideas would be greatly appreciated. I have tried another carb and it was still there.

I just installed this cam, carb, and intake. I did not have this problem before the changing of parts.

Mar 19, 2011 | 09:31 PM

HIO Silver

Fleet Owner

Joined: Dec 2009

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From: NorCal

Compression readings shouldn't be more 10% btwn adjacent cylinder and no more than 15% overall. Your readings however seem low in general Did you have the butterflies pinned at WOT while cranking like it is supposed to be while doing a compression check???

As far as the miss, here is a list of things to check:
Faulty spark plugs (check)
Ignition wires insulation breakdown and crossfiring
Crossed spark plug wires
Weak battery
Weak distributor ground (inside cap)
Faulty ignition coil
Poor distributor rotor continuity
Wet ignition system... sometimes condensation can form inside the cap
Intermitten short in primary or secondary ignition wiring
Blown head gasket
Fuel mixture extremely lean

Mar 19, 2011 | 11:32 PM

co425

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That's low, low compression. A new motor will be around 155 lbs used about 135. Less and your pushing a rebuild.

Mar 19, 2011 | 11:51 PM

BiltFordTuff

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From: Western Kentucky

those are not the best results but i think you should really start worrying if it gets any lower, the lowest on mine is 90.

Mar 20, 2011 | 01:43 AM

theNorthernFORDman

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Your compression numbers are very low. Ive had engines with just under 90 pounds of compression on a cylinder,and the spark plug wouldn't even fire that cylinder. Try shouting some oil in each cylinder and see if the numbers go up.
Most professional repair shops don't do compression tests they prefer do cylinder leak down tests.

Mar 20, 2011 | 06:25 AM

rdweathers

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From: Florida

My compression readings are with the butter flies in normal position closed. Is the carb supposed to be open in order to do this test? Would taking the carb off for this test be ok? Seems like if you left it on and pinned open by the time you are complete with the test the engine would be flooded?

Mar 20, 2011 | 07:14 AM

rdweathers

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From: Florida

I think i have answered my own question, it needs to be warm also for the test and throttle wide open. This will be my project this morning and will post what i find once completed.