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MG MGB Technical - Compression and vacuum low

I had a motor built a couple years ago for my 1974 MGB, and just got it on the road it August. Had to gather up the following a 69 head, GB block,18v flat sided crank and 18v rods out of my 74. Bored .030 TRW pistons 8.75 and Iam sure its been so long ago probably had chrome molly rings. Used a Delta KB cam with double gears. Before I started it up I ran a compression check, came in at 70 lbs. on all of them.I called the builder and he stated the cam has a lot of overlap and the compression would build with time.I checked it at 500 miles 70,80,80,70. At 1000 the current mileage 80,90,90,80. Not to great right. I put a vacuum gauge on it ,at idle 10in and fickers rapidly to 8 ins.At cruise it only holds 10 to 12ins. at 55 mph. I have a weber DGV 2 BBL, 45D dist. with pertronix ignition and coil. Set the timig at 15 degrees at 950 rmp. Runs fair, so the big question is Could my compression and vacuum problems be the cam and crank gears be off a tooth or maybe indexed wrong. Ray
H Ruble

Ray: When the engine was built, was the static compression ratio checked? The 1969 head has a 43.5 cc combustion chamber, whereas, the stock head for your 1974 engine had a 39cc combustion chamber. The larger combustion chamber will lower your already low compression ratio. This can be exacerbated by after market pistons that have a short compression height. I normally test fit the crank with rods and pistons but no rings in order to check the distance below the deck of the block the piston is. If the engine is running well otherwise and holds good oil pressure and does not have any blow-by issues, I'd get a Moss Supercharger kit and use boost to build compression pressure. Unless you are running a brake booster, the low vacuum won't be a problem.
John Perkins

While John is going in the right direction, those numbers are too far off, likely because the cam timing is at least one tooth out. Or else that is a very drastic cam!

FR Millmore

Just got back from a high speed run. The engine goes flat at 80 MPH, at 4500 in overdrive if the tach is correct. Cant hear any more but sounds like its dropping a cylinder also. All the other gears it will pull 5000 rpm with no trouble. Ray
H Ruble

This could be piston crown to block deck height wrong resulting in low compression, I have seen engines built and sold with this schoolboy howler mistake.
Stan Best

Your vacuum is low because your compression is low. The piston moving downward is what creates the vacuum. Those numbers are very, very low. Even a tired engine in need of a rebuild will pull low 100s compression. Check the timing, and if that is correct (dots on the chain wheels lined up perfectly) I think the head would be the next thing to look at. I have a couple of 12H2923 heads (72-74.5), one fully rebuilt, and one bare head that's been magnafluxed and pressure tested. Shoot me an Email off-line, maybe we can come to terms or do a straight swap if your '69 head (12H2389) is good. mgbpaul at yahoo dot com


Paul K

This thread was discussed between 07/10/2007 and 13/10/2007

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