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Compression test results!!!

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dbd74mgb Avatar
dbd74mgb Don D
Frederick, Frederick,MD, USA   USA
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Dry #1 = 110 #2 = 105 #3 = 90 #4 = 120
Wet #1 = 130 #2 = 120 #3 = 120 #4 = 150

I did the test twice each cylinder dry and wet.
Diagnosis???
Comments???

Time to rebuild her???

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mgbanthony Avatar
mgbanthony Platinum Member Anthony Henderson
Eastern Thousand Islands, ON, Canada   CAN
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1962 MG MGB
1974 MG MGB
Your test indicates potential ring problems in a couple of cylinders, as well as perhaps general wear and tear in the valve train as well.

A cylinder leak down test would give you more data.

If engine is running OK with decent oil pressure, not burning large amounts of oil and has fair power I would likely continue to drive it.

Your results sound like those of an average tired low compression engine, which may well run a while yet.

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Rod H. Avatar
Amity, OR, USA   USA
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1964 MG MGB
1968 MG MGB GT
If I get questionable compression readings I put the plugs back in, check the valve clearances, warm the engine up, then retest. Often the readings improve greatly. Assuming the engine runs fine and has good oil pressure, don't necessarily think rebuild yet.

OTOH, if your readings don't improve and/or oil pressure and running are also issues, start thinking about a new engine.



Friends talking around a fire is the history of mankind.



Edited 1 time(s). Last edit at 2014-10-25 12:53 PM by Rod H..

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tourtelot Avatar
tourtelot Gold Member Douglas Tourtelot
Seattle, WA, USA   USA
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1976 MG MGB "Heathcoat"
Defiantly a warm engine for a compression test.

D.



Douglas Tourtelot
Seattle, WA

"Every education has a tuition"

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HiPowerShooter Avatar
HiPowerShooter James Booker
Lake Winneconne, WI, USA   USA
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1973 MG MGB
Why the aggressive posture for starting your engine Douglas??

I would Definitely start and warm it...and leave the contempt issue for another day!lol!

Sorry...it's Saturday.

But yes, start and warm it for a few minutes. Doesn't have to be operating temp but just warm to the touch. Also...how do your plugs look? You can get an overall idea of individual cylinder health by reading them.

In reply to # 2833633 by tourtelot Defiantly a warm engine for a compression test.

D.



"One test is worth a thousand expert opinions"--Alvin "Tex" Johnston...Boeing test pilot.

"Who do you think you are? I am."...Pete Weber

73 MGB. Tires: Round, black, hold air. Oil: Sometimes old, sometimes new...always slippery. Oil filter: Yellow, usually full of oil. Carbs: 2 SU HIF. Distributor: Yes. Headlights: Not that bright but bright enough. A bunch of other stuff most cars have but not really important enough to itemize. Oh, wait...it has a cool sounding exhaust with stickers on the chrome tips. Really slays the ladies...



Edited 1 time(s). Last edit at 2014-10-25 01:09 PM by HiPowerShooter.

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tourtelot Avatar
tourtelot Gold Member Douglas Tourtelot
Seattle, WA, USA   USA
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1976 MG MGB "Heathcoat"
In reply to # 2833640 by HiPowerShooter Why the aggressive posture for starting your engine Douglas??

Because 1) it's Saturday, 2) it's raining and 3) I have a bad clutch master cylinder. And because I have trouble spelling "definitely" (and the computer doesn't REALLY help.)

smiling smiley

D.



Douglas Tourtelot
Seattle, WA

"Every education has a tuition"

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ghnl Avatar
ghnl Eric Russell
Mebane, NC, USA   USA
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1961 MG MGA "Calvin"
In reply to # 2833711 by tourtelot it's raining

You're in Seattle...



Eric Russell ~ Mebane, NC
1961 MGA #61, 1981 Alfa Romeo GTV6, 1984 Alfa Romeo Spider, 1991 Honda ST1100

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Donthuis Avatar
Donthuis Don van Riet
Rijswijk, ZH, Netherlands   NLD
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As usual cylinder #3 is the weakest, both on dry as well as wet it is more than 10% below the others. Still for a LC engine the overall level is OK.
Strange that cylinder #4 is so high when wet, maybe too much oil added? Anyhow you may contine driving if the crank is not rumbling in its gears

Maybe cylinder #4 has slightly more carbon buildup inside the head, LC is normally 130psi max, not 150

PS Sometimes cylinder #3 is lower because of headgasket problems, so if the bearings are OK, a new headgasket may be a gamble worth taking smileys with beer
Once the head is off checking the valves with petroleum, see JT video is easy enough. When new ones are necessary going to a leadfree head is the most logical step



Edited 1 time(s). Last edit at 2014-10-26 06:52 AM by Donthuis.

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ohlord Avatar
ohlord Platinum Member Rob C
A tiny Island off the coast of Washington State, N.W., USA   USA
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1957 Land Rover Series I "EYEYIYI"
1957 Land Rover Series I "OVRLND"
1971 MG MGB
1971 MG MGB "Bedouin 2"    & more
Do a proper leakdown test by cylinder it will tell you way more than a wet or dry compression test.
If that is what you want to know about your engine.



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barry s Avatar
barry s Barry Stoll
Alexandria, VA, USA   USA
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1972 MG MGB GT
1974 MG MGB
1976 Triumph TR6
1980 MG MGB
As Anthony said, if its running OK, just drive it! If you are looking for something more than a 'fun' occasional drive, then by all means consider rebuilding to bring back to 'original' or 'MORE'. If each of mine idles nice and 'seem' like they are moving 'fast' especially on twisty roads, I'm happy.

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pro4art Art Hughes
OH, USA   USA
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I have a desk top PC, so don't know how a laptop does this.
Hold the left click down, as you move the cursor across the word. Let off after the ending letter. Right click on the "blued" word. Drop down menu will fix your problem.

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Basil Adams Avatar
About 12 miles from Sears Point, CA, USA   USA
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Don't do anything yet. There is nothing diagnostic about a compression test or even a leakdown test if the engine is dirty. You may want to Seafoam it and then adjust the valves and test again. Or spend an afternoon and pull the head, lap the valves and re-install and test again. Compression numbers like that are probably valve leakage, not rings. You may be a candidate for a minor rebuild - lap valves and change the rings and rod bearings but that's the worst I can imagine with those initial compression numbers. Holler if I can help. Basil 707.762.0974 basiladams@yahoo.com



Basil C. Adams
1956 MGA Coupe (Show Car)
1957 MGA Roadster (Driver)
1958 MGA Coupe (Racecar)
1959 MGA Coupe (unrestored)
1960 MGA Coupe (unrestored)
1960 MGA Roadster (Driver)
MKIII Elva Courier (E1056)
1967 427 Cobra
1972 Alfa Romeo Montreal
A coupla late MGBs
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More Cars than Brains

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dbd74mgb Avatar
dbd74mgb Don D
Frederick, Frederick,MD, USA   USA
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OK there were a lot of answers and a few questions, so here goes -
- I did on a warm/hot engine,
- oil pressure is about 60 pounds driven, falls to about 45-50 at idle,
- car runs fine, just seems to lose power on steep hills,
- not burning oil but it does tend to leak, about a cup after a long run 150+ miles,
- the plugs look great, here are pics, they are a light grey, and this was after town driving.


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Jim K Avatar
Jim K James A. Krasnansky
Liberty, KY, USA   USA
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1970 MG MGB GT "Chloe"
1971 MG MGB GT "Roscoe"
1972 MG MGB "Camilla"
I am following this thread as the engine I put in a couple months ago (wanting the original re-bored with OS pistons this winter) is getting oil pressure like Don Deans.



Jim K is a grease-stained wretch

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barry s Avatar
barry s Barry Stoll
Alexandria, VA, USA   USA
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1972 MG MGB GT
1974 MG MGB
1976 Triumph TR6
1980 MG MGB
Cure to loss of power on steep hills, downshift.

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