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MG MGB Technical - Bad valve ??

Hello MG experts

I hope you have a great "top down" summer.

The 78B ran great so far but on my way to work this afternoon the engine suddenly lost power. I stopped on a parking lot to investigate and parked right beside a CAA tow truck so I decided to have it towed back home.
Here is what I found:

Compression on cyl. #3 is zero PSI (dry or wet).
Compression on other 3 cyl. is OK
Valve clearance is OK and all the valves, rockers, etc is working fine.

I suspect a broken valve, I had the valves redone a couple of years ago (I know, it means nothing) and strangely it was one on the 3rd cylinder that was faulty.

p.s. when I cranked the engine for the compression test I could hear what looks like a "pulsed air leak" as the engine turned. Since I had nobody to crank the car for me I can't tell with precision where it came from.

Here is my question (finaly): is there a way to check for a bad valve without removing the head?

Thanks in advance

MG
M Gaudreault

A cylinder leak down test should help determine where the leak is. You know it's bad and I don't know of any way to repair it without removing the head.

Clifton
Clifton Gordon

M-Gaudreault... If you have zero compression on one cylinder please do not waste any time on guessing. Pull the cylinder head and you will see the problem right away.
Sandy Sanders
Florida
Bagotville
Sandy Sanders

Agree totally with Mr. Sanders. You can have the head off in no time. Here is hoping it is a valve and not a hole in the piston. Even if you broke a ring, I would still expect some compression.

Since you had a problem with this valve in the past. If it is the valve, I would really want to know what was the underlying cause. Hardly ever see a single valve go.
BEC Cunha

Hello

I think you're right, I will go the easiest way (and the fastest) and have a look inside. Like Clifton said, there is no other way to repair it anyway, valve, piston ring, piston hole, etc.

Thanks to you; I' keep you informed of what I've found.

p.s. S. Sanders: is it Bagotville, Quebec or Florida ?
M Gaudreault

Hello again

As per your advice I removed the head tonight and found: A broken valve on cyl #3.
Curiously (or mysteriously ?) it's the same valve that passed away 3 years ago (and about the same period of the year, hence mysteriously !). Is there something odd with cyl #3 on these engines and specially with that valve. (it's the small one just next to the cyl #2; exhaust I think).

Waiting for your comments on this

Thanks
M Gaudreault

M Gaudreault,

I got the same misadventure on cylinder #3 exhaust valve. On this occasion I had noticed to re-attach the leaking exhaust collector/tube connexion, not good for exhaust valves. Provided your car is a stock one, you may have a look in this area, the low cost seal is much much cheaper than a valve repair.
Regards.
Renou

Hello Renou

What do you mean by "I had noticed to re-attach the leaking exhaust collector/tube connexion". Sounds interresting. My engine is quite stock (only air pump is missing).
Maybe you can put it in french if you prefer.

Thanks
M Gaudreault

M Gaudreault,

I mean that I had previously noticed a leak between m exhaust manifole/tube connexion level and I had planned to fix it. I only had to replace the seal and the *@#! screws and bolts onto the manifold,..., you guess what I mean. So close to exhausts valves, this situation may result in valve burning by fresh air inducted onto really hot valves. I hope that tip to help you.
Regards
Renou

M Gaudreault..
I was at the RCAF station Bagotville Quebec. Lived in Chicoutimi.
Contact me using my email address.
Sandy
Sandy Sanders

Hello

Thanks Renou for the clarification. The manifold gasket and nuts were brand new since the valve incident in 2004 but, who knows, a nut may have turn a bit loose and some cold air enterred the cylinder. There's a torque spécification for these nuts but it's not easy to torque these *@#! where they are.

Sandy, until 1992 I lived in St-Ambroise, a small village which is about 1/2 hour drive from Chicoutimi ..... what a small world !
M Gaudreault

This thread was discussed between 04/09/2007 and 07/09/2007

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