Engine Back Firing

[Markken2008]

Hi There
I have a 1968 280SE Automatic Saloon petrol version , 6 cylinders , Engine M130 ,Chassis Number 108.018.22 018549 . Engine Number 130980.22 013780 ,Registered February 1969

I had new spark plugs fitted and changed the points but this resulted in the car back firing

To eliminate the points I had a 123 Ignition system fitted

I had the compression checked and the 4th Valve has low - zero compression

Question is what is the course of action to correct the compression problem and also stop the back firing ?
Is this linked directly to the compression issue or is their something more obvious causing this ?

All advice would be appreciated
Mark

[mpolli]

You might post in the vintage forum. As for zero compression I will guess burned valve. What were the symptoms before the plugs and points were put in? This sequence of events seems somewhat strange. At what point did the electronic ignition go in? Who did what? Was it all done by a shop?

[t walgamuth]

Backfiring is generally caused by retarded ignition timing. Zero compression about has to be a mechanical problem. The simplist you can hope for is a valve stuck open.

[mpolli]

Originally Posted by t walgamuth

Backfiring is generally caused by retarded ignition timing. Zero compression about has to be a mechanical problem. The simplist you can hope for is a valve stuck open.

Yes. Could the flame travel through the open valve and cause the backfiring that way also?

[Markken2008]

Symptoms were sluggush performance but no real back firing
The electric ignition was the last upgrade made
It was leads and sparks plugs
then points
At this point the back firing appeared and with the new distributor it is worse than ever

I had a friend from the classic car scene do the work for me , he is a qualified mechanic who primarily works on a Citroen DS that I have but he works on all models and is very competant
He said the same thing the valve may be stuck but he was surprised that the back firing was worse now with the new parts
Having said that he linked it back to the valve rather than the ignotion which he sees as a part of the issue but not the cause.

would correcting the valve resolve the problem or is it possible that it is a pistons problem ?

[PanzerSD]

the reason it's backfiring, raw fuel is being injected into the exhaust system. does it backfire regularly? or intermittantly? under load? or during a free rev of the engine?
a mis in the ignition system could load the exhaust up with fuel, but it less likely than a burnt valve or valve seat. or your injector could be still spraying when the exhaust is pumped out of the cylinder (which ever it is) very likely to be the one with low compression. Does the car see top end RPM very often?

[mpolli]

I think only valve problems can give zero compression but I am not positive. I think bad rings will always give "some" compression. I had a cylinder with bad rings on my old Toyota and it burned a ton of oil and the compression was somewhat low but nowhere near zero. I suppose a hole in the cylinder could do that also.

[cliffmac]

I would think that 0 compression is virtually impossible with the engine still crankable. The engine should lock I would surmise...how can you have 0 compression on an interferance engine and still be able to turn it over? hmmmm...I'd like to hear more about this one

[mpolli]

If it was compression locked then it would have high compression. Zero compression means a hole somewhere so no pressure can build up. We are talking gauge pressure here.

[Markken2008]

thanks All , got some tests that I can run advice in the Vintage section of the forums and will see if that gets to the bottom of the problem

[Arthur Dalton]

Originally Posted by cliffmac

I would think that 0 compression is virtually impossible with the engine still crankable. The engine should lock I would surmise...how can you have 0 compression on an interferance engine and still be able to turn it over? hmmmm...

How do you come up with this stuff ???????????????????????