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MG MG Y Type - Backfiring

Gertie runs at idle like a dream. Very slow and gradual depression of the accelerator pedal sees a slow and even rise up the revs, but any sudden depression causes increasingly louder backfiring (vapour visible at carbs) with swifter gas pedal depression.
I feel that it might be the carbs needing cleaning out.
I took out the plugs and added Redex to the combustion chambers to ease anything at the valves, but no improvement.
Could the distributor timing be at fault?
Does anyone have any ideas about what might be going wrong here?
A J R Felthouse

Does this Backfire happen at the carbs or through the exhaust? Do the carb pistons drop freely in the bells or are they "hanging up"? Is there sufficient oil in the dashpots?

Does the backfire happen when the engine is totally warmed up, or when it is cold?

What is your engine timing (Static) and at idle, and at 2500 rpm? See lots of questions, but we will sort it.

The SU carbs depend on vacuum/damping to accelerate as they do not have an accelerator pump.

... CR
CR Tyrell

Busy day on Gertie yesterday.
Tuned up the carbs and they are hissing the same now. Dashpots were a little low, and I noticed that the rods to the dashpot pistons are of different sizes, which I thought odd.
Timing was too retarded. I loosened the whole distributor and advanced it just a shade and she ran better instantly without backfiring frequently, just occasionally when gunning the gas. Gun timing seems to be on with the marker on the camchain cover.
Just as I'd got it right, the neighbour next door came round and asked my to stop revving the car, so that was that and off she went.
So, looking forward to our third test drive tomorrow.
First test drive lasted 4 minutes. Obvious heavy and loud misfiring in motion, nasty moment on a bend where the brakes vanished and I said my prayers, detoured past an ambulance and lamely limped home in humiliation.
Second test drive yesterday last two minutes.
First adjustment of the distributor timing clearly not enough, with sluggish rev rise, lack of power, no go in her at all.
A J R Felthouse

A couple of ideas to try.

Check the condensor in the distributor. Fit another if possible.

Start the engine in a dark garage and look for sparking/tracking around plug leads & dizzy cap.

Use a timing light on each plug to see if you are getting constant and regular firing in each cylinder.

Examine the inside of the distributor and look for track marks/cracks.

Check there is no corrosion where the plugs leads fit into the dizzy cap.

Good Luck - try them one at a time, then when you find the problem is gone you will know which worked.

Tony
The Classic Workshop
A L SLATTERY

Hello gentlemen,
The third test drive didn't happen as the timing was quite badly out again. I'm trying to line up the timing light mark on the pulley with the pointer on the cam chain case, and finding it's over retarded, the I slowly advance until the engine runs quickest, then turn down 5he idle and then she runs slowly with low power.
The backfires are through both the carbs and are frequent, every five seconds, but sometimes with twenty second runs when warmer.
The carbs seem to drop properly.
The dashpot dampers are oiled. The pistons are different lengths.
She backfires both cold and warmed up, but runs better warm. No overheating issues.
The choke has been sticking, but I'll loosen that off.
There is a vacuum pipe, crimped off at the end, from the inlet manifold, doing nothing.
I shall replace the condenser, thank you Tony, and I'll follow your other advice too.
But I must plead to ignorance with using my ignition light gun. Is there anywhere on this site that might instruct me in it's use? Because I can't find out what I should be looking for on the pulley.
Many thanks for all your help lads.
Best wishes,
Andy
A J R Felthouse

What make and model of ignition light are you using? There are many different ones that have different capabilities.

Paul
Paul Barrow

Yer tiz




A J R Felthouse

Hi When you set the timing with the timing light at idle raise the revs to see if the mechanical advance is working,( the timing should advance ) a drop of oil under the rotor arm/button on a service helps it to stay working correctly
Bob Wood

Hi Andy,
It's good to have a "dial up advance" timing light. So here are the basics for you.

The timing light flashes whenever the spark plug fires.

First of all, insert your crank handle in the engine and turn the engine until you see a notch in the crankshaft pulley about 2mm wide. Clean the notch and put a dab of white paint in the notch. At the same time out a dab of white paint in the timing marker on the front cover.

Remove the crank handle & allow the paint to dry.

Connect the battery clips on the timing light to the car battery - red to positive, black to negative. Clip the third wire to the #1 plug lead (the one at the front of the engine). There should be a marker on one side or an arrow that should point to the spark plug, so make sure you get this right.

Turn the dial on the timing light to 5 degrees.

Start the engine making sure no wires on the timing light can get caught in the fan or dynamo pulley.

Now press the trigger on the timing light and you should see a flashing light. Point this at the crankshaft pulley - watch out for the fan !. You should see the white mark on the pulley lined up with the timing mark.

If the white marks do not align, you need to adjust the distributor until they do. Once you have done that you have set the timing to 5 degrees before top dead centre #1.

Now rev the engine and watch the timing mark on the pulley - it should rotate away from the timing mark, If not the timing advance is not working in the distributor - clean & fix as required.

Switch off the engine & remove the timing light. Take the car for a drive and report back.

Safety Fast

Tony
The Classic Workshop

A L SLATTERY

Andy

Here is a link to a PDF for your timing light:

http://www.gunson.co.uk/items/PDF/Technical/G4132_Timestobe_instructions.pdf

and a video here

https://www.youtube.com/watch?v=T0dU6ISZHng

Paul
Paul Barrow

This thread was discussed between 14/04/2020 and 19/04/2020

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