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MG MGB Technical - #1 piston 20psi down: rebuild or not?

Hi there -

a quick question to all you folk who know more than me (almost everyone, that'll be).

The performance on my '81 GT has been down for a while, awful at low revs. I did a compression test:

#1 140
#2 160
#3 160
#4 150.

A squirt of oil had #1 back to 160: so it's rings/bores.

I phoned an mg specialist who I trust (Watford Classic Cars - nice people) who said that if the difference between the highest and the lowest is less that 20% of the whole pressure, then I'm OK..... said my results are not great, but OK. He added that this compression reading shouldn't be responsible for the poor performance.

The car starts fine, never uses oil, never overheats....

What do you guys think?? Should I look elsewhere, or remortgage for the rebuild?

Thanks!
Bruce

PS thanks also to Chris at Octarine for all his advice as well.
B. Mann

Whilst the compressions could be more evenly balanced Bruce I don't think they are responsible for the poor performance. The garage was pretty straight with you. I think there are a few miles left in the engine yet and if you are just using the car for occasional pleasure use it could last many years.

Give it a darned good tune up and see what hjappens then. You'll find all the info you need on this BBS

Iain
Iain MacKIntosh

Bruce, I agree with Ian. Not ideal compression reading but not bad enough to warrant spending great gobs of money on a complete engine rebuild. If it's not burning oil and runs good I'd say it has several more years before a rebuild is required. Save your money at the moment.

Andy
Andy Preston

I have found that a single compression test session is unreliable when differences on this order are found. A tiny flake of carbon on a valve seat can do it, and can be masked by oil. Run it for a few days and retest. Did you set rocker clearances? A few thou tight can give a low reading. I check compression first for condition "as received", set the valves, and recheck. These compression readings are certainly not the cause of the bad performance; but very tight valves can cause bad idling. I just had a midget with clearances of .001 to .006; idle improved drastically when valves were set, but this was an extreme case.
FRM
http://www.usachoice.net/gofanu
FR Millmore

Bruce,

I think you should do the following:

1. Replace the carbs with the new ones you have and set the mixture.

2. Adjust the valve clearances.

3. Check the valve lift.

3. Replace the points and the condensor.

4. Replace the plugs with NGK BP6ES.

5. Set the timing to 10 degrees at 1000 rpm.

6. Ensure the vacuum advance is working.

7. Ensure the centrifugal advance is working.

If you can't ( or don't want to ) do this yourself, get Tony to do it for you.

Then, if performance isn't restored, pull the cylinder head and have the valves reseated - at the same time a visual inspection of the bores will help identify any problems with rings etc.
Chris at Octarine Services

Bruce
Outstanding advice so far - I would only endorse it by emphasizing that the valves need to be in really good condition for a compression test to have any reliable meaning. I would be surprised if a head overhaul did not give you another 5psi and sort out most of the disparity with No 1 cylinder.

Roger
RMW

Many,many thanks for this, guys, much appreciated.

I'm happy to do all the work you suggest, Chris, and then if necesary take off the head: it's just the engine removal I could handle myself.....I'll see what the carbs etc do!

Cheers
Bruce
Bruce

All good advice!
If my memory serves me right, my mechanic's training taught me that you are OK if it's under a 20% difference.
You have just over a 10% difference.
Check valve adjustment!
I have found engine's with tight valves, that had ZERO compression, adjusted valves, and within a short time, recovered full compression.
Also had a Mercedes 450SL engine that was smoking/heavy oil consumption, extremely, that we changed oil, [20W50 Castrol GTX, from Pennzoil 10W40] replaced the plugged air filter, and within 1000 miles the problem disappeared.
See my SMOKING EXHAUST thread.
Safety Fast
Dwight McCullough
Dwight McCullough

Worked on a Spitfire once (ugh, I know) that was waaaay tight on the valves. "corrected" that for the owner (had it to insulate cockpit) and it wouldn't run. Tried everything. Compression was misreable... 70# or so.
Finally, in frustration, I checked the cam... flat. Every lobe was flat so there was no overlap at "proper" lash. Really, every single one was the same (or close enough).
Put the poor things back where I had found them and it ran just fine (cringe).
Owner sold car a few months later.

Mike!
mike!

This thread was discussed between 09/09/2004 and 17/09/2004

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