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Low compression on left no.4

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CHRIS J
Posts: 53
Joined: Sun Aug 14, 2016 7:01 pm

Low compression on left no.4

Post by CHRIS J »

Have started running in my V8 250 after engine rebuild, yesterday the engine was not running very well after checking compression no.4 has only 60psi. Small amount of oil into plug hole this is up to 75psi.

What is the most likely cause?

If I have to remove piston for inspection can this be done by removing sump?


Any advice welcome

Phillmore
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Location: Worcestershire Herefordshire border

Re: Low compression on left no.4

Post by Phillmore »

What were the readings on the other cylinders?
Andy

1954 Conquest Mk1, 1956 Conquest Mk2, 1957 Conquest Century Mk2, 1955 Austin A90 Westminster

CHRIS J
Posts: 53
Joined: Sun Aug 14, 2016 7:01 pm

Re: Low compression on left no.4

Post by CHRIS J »

Hi Andy
All other cylinders were around 125psi Engine warm but not at full operating temp.


Chris

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JohnM
Posts: 206
Joined: Thu Feb 11, 2016 1:31 pm
Location: N W London

Re: Low compression on left no.4

Post by JohnM »

Hi Chris,
it sounds like you may have a valve on that cylinder not closing properly. Try re-checking your valve clearances and see if that's your problem before getting any deeper!
Pistons have to come out through the top, I'm afraid, but let's hope it's just a sticky valve or wrong valve clearance.
Hope that helps,
John
John M in Middlesex, NW London
1962 SP250 "B" Spec
DLOC Herts Region Sec

Ian Hastings
Posts: 163
Joined: Tue Mar 15, 2016 3:00 pm

Re: Low compression on left no.4

Post by Ian Hastings »

Take an old spark plug and break out the ceramic bit then Isopon in a long thin tube, about 7 or 8 inches long.
Screw into the plug hole of the suspect cylinder with that cylinder at TDC on its firing stroke and lock the engine by putting it in gear.
Apply an airline to the tube and listen for escaping air - oil filler cap, carb intake or exhaust pipe. You should find the leak fairly quickly.

tjt77
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Re: Low compression on left no.4

Post by tjt77 »

good advice given by Ian . I will add ..the slight increase when adding oil indicates its a piston/ring/bore wear issue.. but the exhaust valve may be pitted on its face too..60lbs is a rather low reading and barely enough to fire the fuel/air mixture regardless its going to be minimally a cylinder head off situation to rectify the problem.. 125 psi on the other cylinders seems to be on the lower end of the spectrum.. should be closer to 150-160lbs.. you may have general engine wear and in need of a full rebuild..
The pistons in these engine have just 2 compression rings and they often have sealing problems .. softer compound rings do help when doing a 'freshen up' to compensate for light wear (ALWAYS hone or 'glaze bust' the bores with either bottle brush or stone cylinder hone when replacing rings) ..both 'hastings' and 'grant' brand rings tend to be the softer end of the spectrum and hence will bed in faster than most.... 'deves' and the old 'Hepolite' brand are made from harder meterial.. fine in fresh bores.. but they do require tougher 'break in' period which goes against standard running in practice.. they need to be run on wider throttle and more load in order to 'seat' properly.. BTW..for 'cost effective' replacement rings you can use those designated for the BMC 1622cc engines (MGA mk2.. Austin A60, Sherpa and J Series vans etc etc)..in Uk there are still good number of the old 'Cords' brand rings..which are often available on ebay. These are designed for worn bores and work very well .. you do of course need two engine sets but will end up with 'extra' rings as the BMC pistons have 3 compression rings fitted..

CHRIS J
Posts: 53
Joined: Sun Aug 14, 2016 7:01 pm

Re: Low compression on left no.4

Post by CHRIS J »

Thanks for the advice I have checked and adjusted tappets, I will let you know how I get on.

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