• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

MGB 1972 MGB, Head Gasket replaced...runs very poorly.

Share this page

B

borjis

Senior Member
Offline
Engine rebuilt a few years back, less than 10,000 miles on it.

A few weeks ago was out driving then bam, runs really bad,
compression is 150psi in 1 & 2, 25psi in 3 & 4.

Got the head off, copper head gasket blown between 3 & 4.

I check the block and head surfaces with a machinist square, everything is good.

put in a new payen head gasket, new exhaust/intake gasket get it all back together
and now it runs very poorly/erratic. especially under load.

Everything was torqued to spec and valves adjusted to .015 cold. Distributor had been rebuilt recently, but not seeing any
issues with that.

Compression tests same as before but 3 & 4 are back up to 140 psi now.

Spark plug wires are on and in correct firing order. Checked coil to distributor wire,
checked coil wires.

Any ideas? Engine was perfectly running / in tune prior to blowing the head gasket.
 
Last edited by a moderator:
Tried new plugs, same results.

Also checked the lucas gold sport coil. 3ohms on the low side which seems fine.
I checked the points with a light probe, they are doing their job well.
I am running a weber dgv, though the car did run perfectly prior to the gasket
blowing out.

This is what happens:

When I start the car and hold my pedal down just a bit, it is difficult to keep it running,
it will randomly go up and down in rpm with the throttle position maintained.

sometimes it makes it's way up to 2 or 3,000 rpm and runs very smooth for a second or
two then it bogs back down again....all with the throttle in the same position.
sometimes when it goes back down it dies.

I have a knack for having the fun weird problems. :cool:
 
I'd go through that Weber with a fine-tooth comb. Ethanol may be a contributing factor.
 
Check, double check and triple check that everything got hooked up in the correct location when you re-built.
 
In the spirit of throwing a few more ideas out, I'd look at anywhere you might have caused a vacuum leak when you disturbed stuff pulling the head off. Don't know anything about Webers, but is it possible that something got jiggled/knocked out of the proper whack when you pulled it off?

Nothing like strange problems to get your mind working to full capacity. Keep us in the loop, I'd love to know what you finally discover. Good luck!
 
I would check and re-check firing order. If all is well, would pull wires one-by-one, looking for a worsening condition. It's not unusual toward the end of a long job to quickly install wires in wrong position. If you're certain that all is correctly connected on carb, would then be looking at ignition - points, rotor, condensor, cap, wires, and plugs.
 
What you're describing sounds like an air leak to me. Either vacuum or in the intake manifold joints.

If the car was running well previously, it's not likely to be something that was OK before the head removal, and is most likely to be something that was disturbed when you removed the head, perhaps inadvertently.
 
Thanks for the suggestions, I've thoroughly gone through every part of the ignition system at this point.

I'm leaning towards an intake manifold leak as some suggested.

I'm going to confirm the issue with the trick of spraying carb cleaner in the area and see if it affects engine running.
Will post what I find.

I bought that fancy new type of intake manifold gasket moss is selling. looks like bright silver with micro dot stamping all over it.
Are those junk that causes leaks?
The one previously in the car was the standard type.

I have the brass nuts just a little tighter than when I torqued them and it
made no difference.

Are those gaskets not thick enough? (I do have a pico header with some welding work done on it to flush fit intake manifold but it's been fine previously)

Would it be foolish to put just a slight ring of high temp RTV red gasket maker around the very outer
edge of the intake manifold? (as far far away from the actual intake ports of course)
 
I had a similar problem on a small truck I had done a head job on. Turns out when I put the intake manifold on, I kinked it and hence a bad leak occurred.
 
Even though I own a AH I used to have a MGB. I remember a similar situation with the head gasket. 2 things I'd check again. One, are you sure you have the gasket located properly on the head? I remember reading that it's good to ream the holes out especially around the head bolts. Two, did you remove and check the push rods? On my car the pushrods (2) broke when the "BAM" occurred.
 
yes to both.

the headgasket can only go on 1 way.
and all the pushrods are perfectly straight and functioning as I did a valve adjustment.
 
Will be interested to hear what you find with respect to vac leaks.
 
Today I pulled off the top of the carb, removed and inspected, blew out all jets.
Verified fuel levels in float chamber and gas is squirting into venturi.

The main nuts for the carb were not as snug, so tightened those up.

Tried spraying carb cleaner at intake manifold while it barely ran...no difference.

Weather is perfect for driving and I'm at my wits end with this issue.

I'm going to test it with a vacuum gauge and rule it out.

Then go over ALL ignition components again if vacuum test passes.

It has to be one or the other.
 
Had a friend come over with a vacuum gauge.

Hooked it up to where the distributor vacuum line goes into the manifold.

Vacuum gauge reads: 5
It should be reading just over: 20

Looks like it is that intake manifold gasket.
I'm going to find and order the regular intake gasket, not that fancy new moss one
(same as I had before)

hopefully that should do it. Will report back.
 
You ~may~ find a Fel-Pro gasket thru a local NAPA.
 
Hope that clears up the issue for you.
 
I am having EXACTLY the same problem. '73 B, newly rebuilt head, new intake/exhaust and carb gaskets. Everything is as it was before previous head cracked. BUT, it runs very poorly. I'm certain, as it sounds like yours its too, that it's a vacuum leak. I can only get it to run very poorly if I turn the mixture up to full rich. Got me stumped too!
 
Back
Top