Robert Berning wrote:
>
> Ever since I bought my 1972 MGB it had a slight backfire when shifting with
> any slight load on the engine. In the time I have had my MGB, I have
> replace the manifold gaskets and exhaust(donut). Yesterday while drive I
> blew the manifold gaskets or exhaust(donut), no exhaust at the muffler and
> very loud. I am in the process of finding which one went, but this brings
> up a question of what is causing it. Talking to a non-MGB mechanic friend,
> he thought it was a restricted/blocked exhaust and the backfiring was from a
> leak into exhaust (air hitting the unburned fuel). With this does anyone
> have a suggestion of better gaskets or gaskets prep. The manifold gasket
> shouldn't be a problem but the donuts seem really cheap, are there any
> better one I could buy? Is there a way of checking the exhaust besides
> cutting it apart?
>
> Thank you,
Backfiring is nearly always air getting into the exhaust pipe, allowing
futher burning of the unburnt mixture...often runnung too rich as well.
The places it can get in are at the manifold to head joint, a good
gasket will cure this BUT both metal surfaces must be clean and flat.
The next area is those pipe to manifold joints, the most often 'bodged'
joint on the car. The pipes must line up and face the manifold square
on. Because of the air-leaking problem, ('out' on acceleration, and 'in'
on decelleration, hence the popping, or backfire,) exhaust CEMENT is a
good thing to use here. The gasket needs a film each side, and so does
the pipe and manifold faces. Do not be tempted to make it into a
'ketchup-sandwich' with the cement oozing out, as it will also ooze
inside and restrict the exhaust flow.
A sticking exhaust valve can cause backfiring, as can one that is just
beginning to burn out its seat in the head. This can only be checked
with a full head-off 'de-coke'.
Other areas of backfiring are too much retard on the ignition, allowing
the mixture to still be burning as it leaves the combustion chamber,
loose and worn timing 'centrifugal-weight' springs under the points in
the distributor cause this, giving false settings on gapping the points,
but it is rare.
One more place to check, is for ANY holes in the exhaust system. Get a
friend to block off the end of the exit pipe, with the engine running,
whilst you grub about underneath. The pressure build up will soon reveal
any holes, as it will 'hiss'. A good exhaust pipe will not have any
leaks, and you can stop the engine by blocking the exhaust exit pipe.
This was a trick of my youth, many of you will have done it....stick a
potatoe, or apple, into the pipe firmly, and watch the car conk out down
the road.
Neil.
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