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Compression test doesn't look good

Posted by Sinewave 
Sinewave Avatar
T. Keith Vezina
Kenner, LA, USA   usa

I did a compression test on my '76 low compression engine with 133K miles on it. I'm getting about 175 psi on 1 and 4 and 125 psi on 2 and 3 (I don't have the exact numbers with me). A wet test brought the numbers up 10-15 psi across the board.

Is this an indication of a cracked head? If so, can a cast iron head be welded?



T. Keith Vezina
British Motoring Club New Orleans
1976 MGB, 1976 MGB Trailer & 1967 MGB MK. I
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rcedward Avatar
Rob Edwards
Raleigh, North Carolina, USA   usa
1951 MG TD MkII "TDC6000"
1969 MG MGB "182608"
1969 MG MGC GT "8806"
1969 MG MGC GT "8651"
1970 MG MGB GT "Donor"   → more

Different wet vs dry usually indicates worn cylinders and/or rings.

You probably don't have a cracked head, but cast iron can be welded -- it's very difficult, and it's hard to find someone who can do it. As I understand it, the piece needs to be heated to red hot, welded, then brought back down to room temperature over the course of days.



--
Rob Edwards
Raleigh, NC
http://www.mgcarz.com

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175 sounds high for a stock low-comp engine....but 125 sounds low - I think something like 135 -140 was the factory spec. If so, 125 might be normal for one with that mileage - but I can't explain why the compression on 1 & 4 is that high....maybe someone else can...

At any rate, I think low compression between 2 adjacent cylinders is much more likely to be a head gasket than a cracked head....



Joe Reed, Cordova TN

'02 Subaru.....'95 Miata.....'78 MGB
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Sinewave Avatar
T. Keith Vezina
Kenner, LA, USA   usa

I did replace the head about 12 years ago with one that had been freshly rebuilt but did not fit the seller's car. Is possible that it is an earlier, high compression head and if so, how do I tell?



T. Keith Vezina
British Motoring Club New Orleans
1976 MGB, 1976 MGB Trailer & 1967 MGB MK. I
Rod H. Avatar
Amity, Oregon, USA   usa
1964 MG MGB
1968 MG MGB GT

Before drawing compression test conclusions I always replace the plugs and run the engine briefly, then pull them again and retest. True, this won't lower the compression on the high cylinders...and the two low adjacent ones are a bad sign. Never-the-less, often carbon gets on the valve seats from the first plug removal and running it briefly will help clear that out and may give you more accurate readings.



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'63 MGB
'68 MGBGT
'80 VW Vanagon Kombi
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Jerry Haugaard
Fremont Nebraska, USA   usa

Could there be sugnificant carbon build up on 1&4 causing high numbers? Seem that compression tests can be dificult to get right the first time, especially if your not used to doing then all the time. I've seen people here report #s then rerun the test with a whole new set of #s and over again until they get the procedure down pat.
Rerun the tests a couple more times and see what you get. Someone will be along soom with the exact steps to take.



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David Kiehna Avatar
Memphis, TN, USA   usa

Heads from 1972-1980 have the same combustion chamber size. The earlier 1962-1971 heads would yeild a lower compression ratio as the combustion chamber sizes are larger.



David Kiehna- 27 year old British Car Enthusiast!
'74 MGB - My driver
'74.5 MGB - Dad's driver
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rcedward Avatar
Rob Edwards
Raleigh, North Carolina, USA   usa
1951 MG TD MkII "TDC6000"
1969 MG MGB "182608"
1969 MG MGC GT "8806"
1969 MG MGC GT "8651"
1970 MG MGB GT "Donor"   → more

I just reread the original post. Are you saying that the wet test added 10-15 psi to each cylinder, or that it brought them all to within 10-15 psi of each other?



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Rob Edwards
Raleigh, NC
http://www.mgcarz.com

If you're in NC, check out the North Carolina MG Car Club!
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JNickell Avatar
Jay Nickell
Plano, Texas, USA   usa
1980 MG MGB

How does the car run? Are you burning any significant amount of oil?



A foolish consistency is the hobgoblin of little minds, adored by little statesmen and philosophers and divines.
-Ralph Waldo Emerson

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1980 MGB



Edited 1 time(s). Last edit at 02/15/2006 12:47PM by JNickell.
Jim Smith
CASSIDY, BC, Canada   can

Are the valves adjusted properly?



Jim...Nanaimo, BC
1971 MGB collector
1975 MGB driver
1975 MGB in pieces waiting for V6
2005 Mini Cooper

A wise man overhauls his brakes before his engine.
BumbleB74 Avatar
William Milholen
Tidewater VA, USA   usa

David -

Didn't compression ratio go down on the later cars, because they added the dished out pistons? Seems like you logic was backwards.



1974-1/2 Roadster, "Bumble Bee", Corvette Yellow - in shambles, wire wheels
1976 Roadster, un-named, Sandglow - "driver" condition (stock + 32/36 Weber DGEV, cast iron header, 25D distributor), bolt on wheels
Caruthers Avatar
Jim Cardamone
Baltimore MD, USA   usa

I'd bet that your exhaust valves in the middle cylndrs are just beginning to burn/leak. I watched mine get worse and worse until I finally lapped a few fresh valves in. Runs like a top again. Oh yea, leaned out the carb a touch too.
Gerry Avatar
Gerry Masterman
Prairieville, Louisiana, USA   usa

If you put a 18V (small chamber) head on an early block you will get an increase in pressures. Put an early head(large chamber) on a 18V block and the numbers will go down

The very first thing I'd check is tappet clearences. Set them all to spec or just slightly wide and retorque the head. If no better, I'd consider pulling the head for a look-see



Signature removed at Skye's request
mgbjockey Avatar
Frank Patton
Eastern Shore area of Maryland, USA   usa
1969 MG MGB "Sapphire"
2010 Mini Cooper S "Hot Pepper"

The FIRST thing to check is a visual inspection of the plugs. If they all look consistent, the test results are questionable. Should that be the case, have a new compression check completed by someone else. Don't waste time and energy tracing a problem until a second opinion confirms that it exists.




*************

Frank
MGBJockey

Sinewave Avatar
T. Keith Vezina
Kenner, LA, USA   usa

Thanks for all the good input.

The wet test just increased each one 10-15 psi over their dry reading.

The runs fine (for a 65 HP RB) although it does burn a little oil. I did get the Exxon Valdez award at our unCar Show, but I have replaced the rear seal since then. It doesn't burn enough to see it at idle or to see it in the rearview mirror. If I had to guess, I would say about 1 qt every 750 miles.

The plugs look good, a nice light tan color, and there is no difference between them.

I am beginning to doubt my test results. I think I will re-do it. I did not adjust the valves just before the test. I did prop open the carb slide and butterfly. Maybe the carb ports weren't open enough and the low readings are because of the shared port of 2 & 3.




T. Keith Vezina
British Motoring Club New Orleans
1976 MGB, 1976 MGB Trailer & 1967 MGB MK. I
jgbowman Avatar
Greg B
Greeneville Tn, USA   usa
1973 MG MGB

I think that is a good plan Keith. I once had uneven readings too. I retorqued the head, and adjusted the valves again, and they evened right out.



Greg
'73 B Mallard Green
18GH with Brittek 270 cam, Balanced
SU HS6
Peco header and exhaust
Eurospec dizzy, Lucas Sport Coil
O/D Black label
Kumho Solus 195/60-14 on Miniators
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2&3 share the exhaust port...not the intake port. That shouldn't be a factor in your compression readings.



Joe Reed, Cordova TN

'02 Subaru.....'95 Miata.....'78 MGB
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