Single misfire

[Peter Naulls]

Half a dozen times I've had a single misfire from the Ruby, always at high revs under load. It runs very well between times. It can be weeks between incidents but I'd rather it wasn't there. Any ideas? I could start changing components but with winter approaching I'll most likely have forgotten what I've done by the spring!

[Reckless Rat]

Have a look under the bonnet at night to see if there are any tracking sparks on the HT leads or dizzy cap when the engine is running. Next, try a set of plugs, then check the points, condensor and rotor arm.

Then get ready for some backache because you'll have to see if there's a sticking valve...

[David Stepney]

My RP does exactly the same! Usually climbing hard in 3rd gear and just a single miss. At first I thought that it was due to the worn state of the distributor, so I bought a newly refurbished instrument on Ebay. Fitting it made a vast improvement to the general running of the car, but made not the slightest difference to the single miss when climbing. As the car was running on what looked like the original coil, I thought that might be the culprit, so I changed it for a new Remax. That made not the blindest difference, so I fitted the old coil back, as it looked the part and appeared to be good. I have changed the  the coil and distributor leads since, with good quality copper cored wire and new crimped tags at the plug ends. That has made a difference. The car will still do an occasional single misfire, especially if the engine is really hot, but it doesn't happen nearly so often as it did, almost to the point of rarity, in fact.

I don't suggest for one minute that changing the plug leads is a cure. After all, my car has been serviced and tuned several times over the past four years of ownership (The distributor, in particular, tends to 'drift' a touch, so the timing is checked and reset every 2500 miles), so any of my sundry ministrations could have had an effect. or none! However, the new plug leads seem to have reduced the occurrence to nothing more than the slightest of annoyances.

Yes I would like the problem to go away completely, but have come to accept it as part of the character of the car.

[Peter Naulls]

Very helpful, thank you both. I had wondered about a sticking valve; I do get the occasional click - click - click when it's idling which makes me think there may be a problem. A winter job.

[Peter Naulls]

I finally braved the cold garage and began looking for the cause of the misfire. The exhaust valve on number 3 cylinder was clearly sticking. If I rotate it there is one position where it sticks; turn it 90 degrees in either direction and it is perfectly free. There is a little bit a residue on the stem but nothing that looks odd to me. 

I will replace the valve and run a reamer through the guide but wonder if anyone has any idea what is going on? 

I did try the valve in other guides and it sticks. I also tried other valves in that guide and they don't stick. A bent valve perhaps? but how?

   

[David Stepney]

Peter, it looks as if you may have found the problem, which makes my diatribe in October concerning plug leads a bit redundant! It's amazing that such similar symptoms can have such widely differing causes.

[Colin Reed]

Hi Peter,
I am making an  assumption that the valve in the photo was (polished) before trying in the guides .
if not then do that first then mount in a bench drill  grip it with about 20mm in the chuck you will soon know if bent
make sure to clean under side of head including valve seat also seat in block  ,then using fine paste just lightly lap in
this will also register on the valve in one place only if bent

hope that is of some help and it wont cost anything maybe frost bite

Cheers Colin
NZ

[Ruairidh Dunford]

We suffered a sticky valve, just like this, in Ian’s Chummy about 9 years ago touring the West Coast. It was just enough to stop it sealing, every now and then.

We cleaned all the black sticky stuff off and popped it back in. It is still in there now and the problem hasn’t reoccurred.

We assumed it was petrol related but, as I said, we’ve had no issues with it or any of the other cars since.

[Peter Naulls]

Thank you all for the advice. Don't worry David, I'd much rather your suggestion of plug leads solved the problem!

I spun the valve in the lathe; it's not bent, though there is a bit or wear on the stem level with the bottom of the guide so I'll change it anyway and check all the others. That's re-asurring Ruairidh, that the problem didn't recur on Ian's car.

Thankfully we have warmer weather for the re-assembly.

[Peter Naulls]

My running problems persist. What started as an occasional misfire is now joined by an occasional backfire, normally on the overrun when double de-clutching. On a recent 300-mile weekend away it got progressively more frequent but disappeared if the car was driven very gently. Eventually on the journey home it began to fluff and lacked all power (not a distinct misfire); we were on a single carriageway A-road with no opportunity to stop so I slowed down to 30 ish and it gradually cleared. We struggled home on back roads. 

Now it is back to the odd backfire, sometimes 2 or 3 times on the way to the shops but sometimes it will do the 40-mile round trip to work without issue.

To recap what I have done so far. Head off, valve guides reamed, tappets re-set. New coil and condensor. cleaned and gapped the points, fitted an earth to the distributor base plate, visually checked the plugs, leads, distributor cap and base plate (all are newish but pre-date the problem). Checked the battery terminals, battery cut-off terminals and earth strap to the engine. New manifold gasket, checked the carb flange for leaks. Visually checked the float chamber level and the jets. cleaned out the "hidden" jet as well as I can in situ. Drained the tank, flushed it out and blew back through the pipe to clear the filter. All of this has taken several months because it is never immediately apparent if I have solved the problem!

Where should I look next? Beginning to run out of ideas.

Edit: I forgot to mention that when it backfires, it is often followed by it stalling at junctions.