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MG MGA - Backfiring

This problem started at the end of an outing (thankfully). It was misfiring as I pulled into my driveway.
Today - investigating, having purchased new points and rotor. It starts fine and idles normally. But as soon as I try to load it up and drive it backfires and I can only reach about 10 mph. Seems to backfire on all cylinders. Loooked at ignition wires in the dark for sparks but none seen.
Any thoughts before I start dismantling stuff?
Art Pearse

fuel pump sounded normal when starting from cold, clicking away until carbs full, then stopping.
Pulling choke out did not help the problem.
Art Pearse

Have you checked the actual fuel flow Art? If you pull off the fuel pipe from the rear carb and put the end into a container, you should get at least 1 pint per minute from it.

Cheers
Colyn
Colyn Firth

Compression test--sounds like the start of a blown headgasket
William Revit

May also be a vacuum leak, over lean mixture leads to back firing.
Cheers
Mike
Mike Ellsmore

Easy to check first- is the distributor still tight in the clamp and not moving
Dominic Clancy

A neighbor of mine had the same issue. It turned out to be the secondary coil wire went spare.

Cheers

Gary
79 MGB
Gary Hansen

i will check all these suggestions.
the backfiring is in the exhaust.
Art Pearse

Dominic - dizzie is a little slack in clamp, but is not out of place. It wiggles about 1.32 inch.
There is a bit of play in the dizzie bush, i'd say about 2-3 thou.
The rotor was stuck on the shaft and had to be prised off using a chisel.
The new rotor has a different shape than the old. Its from Echlin. See pic.
Any problems with the smaller arc?
Art


Art Pearse

This comment courtesey of MGAguru:
"A rotor may run normally for quite some time, but when it fails it dies very quickly. Sometimes you get intermittent misfire for a few seconds before it quits. Otherwise it just goes dead like switching off the key, and no way to restart leaving you stranded on the road. When a rotor fails in this manner it will often leave a telltale dusty black carbon smudge inside of the rotor socket where it fits on the shaft. It is a good idea to carry a spare rotor, points and condenser in the traveling tool kit. "
Well this fits my description of black smudge on the dizzie shaft as I had to prise it off with a chisel.
Art Pearse

Changed the rotor. No difference.
Art Pearse

Also some condensers have a habit of going bad. The car will run just fine and as the car warms up the condenser will fail intermittently causing backfire. until complete failure

Cheers

Gary
79 MGB
Gary Hansen

A few years ago I suffered an intermittently failing condenser in a new Moss distributor on its first trip after about 100 miles!!! But there was no backfiring, it just stopped the engine so doesn’t sound like you problem. I no longer have any of these components in my distributor (Peritronix) but I do carry my last working conventional dizzie and spares condenser and points. Colyn would be proud of belt and.braces approach

Paul
Paul Dean

No wonder we all hate a misfire, there are so many possible causes and so the only way to go is to eliminate them one by one until you find the problem. Looks like you are going to be busy Art!

Well done for having a spare dizzy Paul, I used to carry a spare points dizzy in the boot and, when the electronic one began to fail, I fitted it at the roadside.
Unfortunately, the engine wouldn't fire up at all with it and I had to refit the faulty electronic one to get us home.

It turned out that the new points dizzy had the timing dog on the distributor shaft set over 30 degrees differently to the electronic one and we just couldn't get it to run at the roadside.

After that, I bought a spare electronic distributor to carry in the boot, I have never used it on my own car but we did loan it to an owner whose own dizzy had failed at the end of an MGA Tour weekend.

In the event it did us both a favour, it got him safely back home from Oxford to Liverpool, avoiding the problem of arranging a tow truck, with the added bonus of proving to me that the previously untried dizzy definitely worked.

My most recent misfire experience was a couple of weeks ago when we were descending the Hardnott and Wrynose passes in Cumbria. I was using engine braking to control the speed of the car instead of using the brakes and I noticed that at the bottom of the descent, the engine wouldn't run cleanly. We were staying overnight at St Bees and so I decided to replace the spark plugs before breakfast in the morning. So I got up early, replaced all the plugs and went inside for breakfast.

When we had packed the car later ready to depart, on starting the engine, the misfire was very much worse, it was only running on 3 cylinders! Surely the head-gasket hadn't failed overnight!

When I lifted the bonnet it turned out that there was no spark at number 4 plug!
But it was soon sparking fine again, once the mechanic had re-attached the plug lead that he had somehow forgotten to put back on!!

Don't you hate those idiot mechanics!! :^)

Cheers
Colyn
Colyn Firth

Removed the points. Look quite rough on the faces. Will install new. Meanwhile, removed condenser, tested for leak through, seems Ok but will renew.
Problem is the minute screw holding the condenser is in a very awkward place. To avoind dropping the screw out as I put it back I am glueing the head of the screw to the condenser flange with gasket maker! Waiting for it to set. Might have been easier to remove the dizzy but didn't want to lose my timing.
Art Pearse

Art,
I ALWAYS remove my distributor if I have to work on it! It is just so much easier to work on when it is on the workbench.

The trick is to leave the clamp set-screw alone, don't loosen it at all, or you will lose the timing. Just take out the two set-screws that fix it to the block and the dizzy will lift right out.
I always take the dizzy cover off first and make a note of which direction the rotor arm is facing, this makes it easier to refit it when you have finished working on it.

The trick is NOT to turn the crankshaft whilst the dizzy is out. If you rotate the engine, you will probably lose the timing.

The reason you can safely remove the dizzy in this way is that the drive "dog" on the bottom of the dizzy drive shaft is offset and will only go back into its correct position. (So long as you don't spin the engine )

Cheers
Colyn
Colyn Firth

Great idea Colyn. I should have known that! I did all the engine work years ago while it was on the bench.
Art Pearse

All done! Runs great again. New points and condenser and rotor but I'm sure it was the points looking at the contact surfaces.
Colyn, a word of caution re removing the dizzie via the two set screws - the holes are elongated allowing for some timing movement. Lucky I used the paint marks.
Thanks everyone for comments.
I almost had a disaster - I had connected the post wires above the insulating sleeves instead of below, lucky I realized at the last moment.
Art Pearse

Sorry Art, I should have mentioned that you should mark the position of the dizzy with tippex or a small dot of white paint before you remove it.

Mine has had the white dot on it for so long that I just forgot about it.

Glad it it running ok now.

Cheers
Colyn


Colyn Firth

I'd put my money on the condenser. Faulty condensor results in excessive spark at the points which greatly accelerates points erosion (hence the points condition) and upsets the primary current decay rate. And creates exactly the misfire you desribed. It very common with cheap nasty condensers, which abound. Always make sure you fit a good quality one, they might cost several times as much but they do the job.
Paul Walbran

You're probably right Paul, but I did both at once.
New set on order for next time.
Art
Art Pearse

This thread was discussed between 17/10/2021 and 24/10/2021

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