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MG MGB Technical - ENGINE WITH BAD CYLINER

I have a 18v with a Judson Supercharger with about 4 lb boost. All cyliners show 150 pressure except #4 which shows 90 lb. I put oil in and it went up to 115 lb. I put it on a leak down detector and I could hear air comming out of the valve cover. When the engine was running, I also saw smoke comming out of the cover. The cyliner #4 was on compression stroke and I used a screwdriver to see that it was close to TDC. This is when I could hear the leaking from the valve cover. I cranked the engine and the leaking STOPPED. The position of the piston was now near the bottom of the stroke. This is where I am confused. At TDC both valves should be closed and I should have 0 % on the leak down and I now have 80% with air comming out of the valve cover. With the piston down I have 0% leak with no air leaking. Any thoughts as to why pressure leak at TDC which would be normal and no leaking with the piston down at near bottom of stroke.
JEFF BECKER

Jeff - I don't understand the term "valve cover" but it sounds as though the compressed mixture is escaping through the valve guides. Is it possible that you are hearing the mixture escaping from a leaking intake manifold ? .... turning to the compression reading, the oil test shows that there is a major escape down the side of the piston - that would produce noticeable pressure inside the rocker cover. Since the other cylinder readings are even, I would guess this is caused by a broken ring rather than just wear. I guess you also have a valve problem with this cylinder. It should be less than two hours to find out just how serious and whether you need new guides. Maybe the right time to get it cleaned up to breath properly.

Sorry to be so cheerful.
Roger
Roger

My suspicion is that there is a score in the #4 bore high in the stroke, which is allowing the air by when the piston is near TDC. The reason you are hearing the air from the valve cover area is that the air is blowing by the piston rings near TDC and travelling up the pushrod holes (also explains why smoke comes from the valve cover). If it were a valve issue, then the air would be heard either through the exhaust pipe or the carb throats, depending on if it is an exhaust or intake valve affected. Also, the reason the compression rose with oil added is because the oil filled some of the gap of the score in the bore. A valve problem would not show any increase in compression with oil added. Could be a broken piston ring. Was the engine fairly fresh when the supercharger went on, or was the boost too much for a somewhat tired engine?
Sorry for the bad news, but it doesn't sound good, I think its time for a rebore.
Erick Vesterback

This is a fresh engine with about 8000 miles on it. The boost is low at 4 lbs. The head was completley redone by Sean Brown ported and polished. I would expect to hear air leaking from either the carbs or the exhaust but most of the noise is thru the valve cover(rocker cover) filler hole where the oil goes. The engine is also redone and rebored. Is there a way to determine a broken ring. I might have two problems. I am going to do another leak down test. I might have read 100 as not leaking vs leaking a hell of a lot. The extra air might be going thru the exhaust which I did not check. Before I pull the head or the engine, I want to know what I have to do so it will come out right and so it will go

ZZZZZZZZZZOOOOOOOOOOOOOOOOOOOOOOMMMMMMMMMMMMMMMM

again.
JEFF BECKER

Jeff
With comp. reading of 90 for no.4 and the others at 150 there is something big going on. Your leakdown reading and blowby through the tappet cover tell me you have a ring/piston problem. The reason you have a zero reading(no pressure retained) at the bottom of the stroke is because the exhaust valve has already opened at 20-30 deg.(depending on cam grind) before bdc.and all your testing air is going out the exhaust. Cheers Willy
WilliamRevit

Jeff - 8000 is nothing for one of these engines unless pushed very hard. I don't think there is much doubt you have a ring prob - let's hope the bore is not much scored and that a new piston and rings (without removing the engine from the car) will solve - I still think you should be prepared for a valve prob too - easily sorted.

Pse keep us all posted.

Roger
Roger

This thread was discussed between 24/04/2009 and 25/04/2009

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