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Old 12-28-2019, 01:56 PM   #1
V4F
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Default Wierd compression

CONFUSING QUESTION : I have asked this before but still need ideas .
stock model "A" motor . rebuilt recently . after a back fire , compression : #1-4 … 75# #2-3 45#
pulled head & resurfaced , magna fluxed , all new ignition (stopped backfire) , set valves , tried different carb …………. no change in weird compression readings . any ideas appreciated … steve
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Old 12-28-2019, 02:56 PM   #2
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Default Re: Wierd compression

Sounds like you checked all of the normal things. You don't say but did you check the pistons and rings? Maybe stuck rings or collapsed ring lands from the back fire? Hard to believe it would happen to two cylinders but...

Just thinkin' outside the box.

Have you done a leakdown test?
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Old 12-28-2019, 04:19 PM   #3
Chuck Sea/Tac
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Default Re: Wierd compression

Was the compression even before the backfire?
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Old 12-28-2019, 04:27 PM   #4
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Default Re: Wierd compression

It sounds like you have a weird engine rebuild. It is hard for me to connect 2 low compression readings with a backfire. Maybe you should explain what the rebuild included.
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Old 12-28-2019, 05:04 PM   #5
David R.
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Default Re: Wierd compression

My guess would be head gasket between 2-3 but looks like you already been there.
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Old 12-28-2019, 05:41 PM   #6
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Do a leak down test to see if you have a couple of valves that aren't seating well.
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Old 12-28-2019, 11:02 PM   #7
Mike V. Florida
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Default Re: Wierd compression

If the motor runs, try a vacuum meter to see what is going on.
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Old 12-29-2019, 12:03 PM   #8
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Default Re: Wierd compression

great ideas , all . thank you & I will try the 2 I haven't done ………… steve
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Old 12-29-2019, 12:21 PM   #9
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Default Re: Wierd compression

Was the compression even before the backfire?


*** do not know as not my motor . but ill check ! …….. thanks
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Old 12-29-2019, 12:32 PM   #10
Patrick L.
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Default Re: Wierd compression

I agree with the leak test. Could be

crack
leaking valves [ which I would doubt, but, possible]
Rings [ not lined up correctly]

First thought of course was head gasket, but, you did all that related stuff.
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Old 12-29-2019, 01:14 PM   #11
Jim Brierley
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Default Re: Wierd compression

Could be the backfie bent 2 valves? That's kinda far out but worth checking. A leak-down test would reveal that.
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Old 12-29-2019, 02:15 PM   #12
The Master Cylinder
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Default Re: Wierd compression

Quote:
Originally Posted by Y-Blockhead View Post

Have you done a leakdown test?
Quote:
Originally Posted by 1crosscut View Post
Do a leak down test...
Quote:
Originally Posted by Patrick L. View Post
I agree with the leak test.
Quote:
Originally Posted by Jim Brierley View Post
A leak-down test would reveal that.
Maybe you should consider doing a leakdown test??
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Last edited by The Master Cylinder; 12-29-2019 at 02:20 PM.
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Old 12-29-2019, 02:37 PM   #13
Patrick L.
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Default Re: Wierd compression

Quote:
Originally Posted by the master cylinder View Post
maybe you should consider doing a leakdown test??






:d:d:d
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Old 12-29-2019, 04:13 PM   #14
SteveB31
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Default Re: Wierd compression

Exhaust valves not seating or burned.
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Old 12-29-2019, 05:30 PM   #15
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Default Re: Wierd compression

For the leakdown test take apart an old sparkplug or buy a adaptor from Brattons or Snyders thread in a male adaptor from an airline into the sparkplug base.Hook up your airhose and pump 30 to 35 lbs into the cylinder which is at tdc with the valves closed,make sure the car is in neutral or hand brake on in case the engine turns over from the pressure.Listen for air escaping from carb,exhaust or into the crankcase.
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Old 12-29-2019, 08:23 PM   #16
The Master Cylinder
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Default Re: Wierd compression

Quote:
Originally Posted by rocket1 View Post
Listen for air escaping from carb, exhaust or into the crankcase.
And radiator...
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Old 12-30-2019, 01:40 PM   #17
Jim Brierley
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Default Re: Wierd compression

rocket1, I put the car on high gear, and the e-brake on while doing a leakdown test, thus preventing the engine from turning if it is not at exactly TDC.
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