>
> A little history is in order for those who haven't been following my car's
> saga this summer. This is a '74 MGB roadster with a '69 engine in it.
> After running flawlessly for over a year, problems started in the beginnning
> of June when I drove from here to Columbus, OH (~75 mi each way) for my
> first ever autocross. The car was running fine, but I hit a pothole on
> I-70 (hard to believe, I know) and my muffler got knocked off. I got it
> jury rigged back into place and drove the autox. After running fine for my
> first couple runs, it began to miss and backfire under hard acceleration.
> It ran fine on the drive home, though.
>
> It continued to run fine for about a week or two and in the meantime I
> replaced the two part muffler with a pipe and a cherry bomb. It still ran
> fine, though it occaissionally would miss and backfire when cold. Then
> I noticed I was losing coolant out the side of the head. Off came the head,
> on came a new gasket. Problem fixed, car ran great for about 10 miles,
> then the backfire problem got worse than ever.
>
> I swapped distributors. No real change. I swapped coils. No real change.
> I put in new plugs. Some improvement, but not a lot. I looked at the
> compression and it seemed okay three ~160psi, one around 170psi. I
> rebuilt the carb (a Weber 45 DCOE 9, jetted 130 main 170 air correction,
> I think.) This helped a little too, but the problem persisted. It was
> suggested that my timing chain might be bad and since you could see my
> timing bounce all over the place when the engine was revved, this seemed
> logical. So I took it all apart and last night got the new timing chain
> installed. The car started after a few tries, but ran poorly. I turned the
> distributor until it ran smoothly. With it advanced ALL THE WAY, it idles
> GREAT (if a little fast) and the timing is nice and steady, even when
> revving the engine (at least with it in neutral.) But when driven it
> runs the same as before, missing and backfiring through the carb under
> acceleration and backfiring through the tailpipe LOUDLY when deccelerating
> with engine braking.
>
> So SOL experts, what could my problem be? Tonight I plan to retorque the
> head (I know you're supposed to drive five hundred miles, but ...) and
> adjust the valves again and hope that solves my problem, but I have very
> little faith it will. Any ideas, thoughts, comments would be GREATLY
> appreciated, as I am at the end of my rope.
>
Your mixture is too rich when deaccellerating, you have an air
leak in the exaust system, or both.
Ideally, you should get a Colourtune and use it to check your mixture.
The '69 engine has an air injection pump and manifold. There is a
check valve in the air manifold that should only work one way. Also, the is
(or should be) a gulp valve connected to the intake manifold. If this valve
goes bad, it will cause backfiring. The original Smiths part is hard to find
and expensive when you do. You can use a Nissan valve in its place. I don't
have the part number here, but can get it if you need it. I have the
Nissan valve and it works fine.
If the air injection manifold has been removed, make sure the
injection ports are plugged tight. Allen head setscrews are best.
Since you have a Weber, it's likely the gulp valve has been removed.
If you still have the air injection manifold, a rich condition plus
extra air will backfire frequently.
Although less likely, a worn camshaft produced similar symptoms on
my '69. Mostly, I think, by upsetting the induction system and the gulp valve
system. Look at the lift on the #1 valve. If it is substantially below spec,
your cam needs replacement. [This seems to be the last part of the engine to
get oil]
BTW, if you have inverted canister oil filter, replace it right now
with the later type! They take some 10 seconds to fill and the wear
prematurely trashes the engine internals.
Good luck, and don't worry, this list can fix damn near anything :-)
Dave Ambrose
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