Inlet Backfire

[phart]

Bad I know, luckily blew off the intake hose, so I hope its not done any damage. Anyway, she was running rough after I fitted the new cam - put it down to timing and the blowing manifolds, but then yesterday morning this happened. Second time round she fired and ran and off I went. Came to restart later and same again, started second time after the backfire. Got home and thought enough is enough.
Wondered if I had put the distributor in wrong, but checked that this evening and seems ok. I then wondered if the tappet pre load was wrong, could I have cocked it up so much that the inlet valve is permanently open? Pretty stumped really, so any ideas welcome!

[wedg1e]

HT leads in the wrong order?

[jchase]

Didn't the 420 engine have such large overlap values that the vacuum advance hose was removed to avoid this problem? Also the distributor can be as much as 5 degrees out between all eight cylinders - you might want to set the timing to be the best compromise for maximum advance. ?

-jim

[danny hoffman]

Could the cam timing be out?

[phart]

HT leads are all present and correct! Cam timing i'm sure is correct, if nothing else works though it will all be apart again...

[danny hoffman]

I would do a compression test before taking it apart again, maybe you have a sticky valve

[phart]

Good thinking, got a compression tester somewhere. Thanks.

[rev-erend]

Well - was it running reasonably before the new cam.

Check the carbon electrode in the dizzy cap - if you removed it.

The symptom is called standoff - for some reason you have loads of
fuel in the inlet that is not getting into a cylinder.

Whether it's just one cylinder or all of they is what you need to find.

Maybe one of the hydraulic tappets is faulty.

When you fitted the new cam - did you rev it straight to 2K and leave it there for around 20 mins.. or let in idle ?

[phart]

The electrode looks ok, as do the bits around the edge of the cap. On first start it was run in for approx 20mins at 2000 rpm. The oil pressure came up almost straight away, and the temperature stayed within usual limits.

[adam quantrill]

Try and identify which cylinder it is. Pull off the injector leads one at a time until it goes away. If equal on all cylinders then it's probably timing of some sort.

If you find the culprit you could take off the rocker cover on that side and measure the valve lift as you turn it over. Exhaust valve seems likely - frontfiring cos it can't get the exhaust out of the cylinder.

You can run it with the injector off for quite some time, if you need to use the car.

[rev-erend]

adam quantrill said:

Try and identify which cylinder it is. Pull off the injector leads one at a time until it goes away. If equal on all cylinders then it's probably timing of some sort.

If you find the culprit you could take off the rocker cover on that side and measure the valve lift as you turn it over. Exhaust valve seems likely - frontfiring cos it can't get the exhaust out of the cylinder.

You can run it with the injector off for quite some time, if you need to use the car.

Paul - this is a neat idea.. If you have not removed the electric plug before - you push the wire down and pull off the plug. No need to try and lever off the spring clip.

[rev-erend]

phart said:

The electrode looks ok, as do the bits around the edge of the cap. On first start it was run in for approx 20mins at 2000 rpm. The oil pressure came up almost straight away, and the temperature stayed within usual limits.

Excellent.

What did the CR test show - you need to warm the engine up first and you are supposed to open the throttle wen doing the test - although it made no difference on mine. I removed the coil to dizzy lead too. As a reference - mine were all 12.1 bar / 175 lbs.

[GreenV8S]

Is it actually misfiring all the time, or just occasionally? If there's a persistent miss you should hear the exhaust banging itself to bits once it's warmed up, and you can tell which it is by the header temperatures. Assuming it really is a bad misfire, the backfire may simply be the result of the unburned air/fuel mixture sloshing around and not a sign of some catastrophic mechanical problem.

[phart]

Thanks everyone, growling like she used to. Turned out to be the cold start injector, or coolant temp sensor, not sure which because I removed both connectors at once and now shes running perfectly. Thats an investigation for another day, just very happy to be running again - I shall be off out now to see what a difference this new cam has really made!
Ps, neat idea for removing those plugs rev, I have always struggled with the little sods! Thanks again all

[rev-erend]

phart said:

Thanks everyone, growling like she used to. Turned out to be the cold start injector, or coolant temp sensor, not sure which because I removed both connectors at once and now shes running perfectly. Thats an investigation for another day, just very happy to be running again - I shall be off out now to see what a difference this new cam has really made!
Ps, neat idea for removing those plugs rev, I have always struggled with the little sods! Thanks again all

Dave Batty showed me how they worked after I confessed to levering them off with a screw driver and loosing one

[danny hoffman]

I leave the wire to my cold start injector off permanently, it doesn't seem to make cold starting difficult and I used to get flooding occasionally with it on even though I checked the thermotime switch and it was operating correctly

[adam quantrill]

adam quantrill said:

Try and identify which cylinder it is. Pull off the injector leads one at a time until it goes away. If equal on all cylinders then it's probably timing of some sort.

Of course I meant - starting with the cold start injector ;^)