Not the average misfire - SOLVED

[simmitc]

After 30+ years of minoring, I thought that there wasn’t anything that I couldn’t solve, but now I have to admit defeat and ask if anyone can spot what I’ve missed:

Standard 1098cc Minor with servo brakes. It starts perfectly, idles smoothly, has plenty of torque to pull away in second gear, and up to around 50 mph performs as one would expect. Depending on which gauge is used, compression is either a straight 140 psi or 160 psi across all four cylinders; and there is no significant difference between a dry and a wet reading. However, a while ago we started to notice a lack of power and a misfire under load. Investigation found a failed vacuum unit which was replaced, but the misfire continued once reaching around 50 mph in fourth. On fast runs, fuel consumption has increased to about 18 mpg, but at lower speeds is still around 35 (the car is heavily loaded).

As we have three Minors and loads of spares, testing parts is easy, as they can just be swapped between cars. Whenever I have done this, the fault has remained with the original car. To cure the misfire, I have tried:

New condenser, points, rotor arm, cap, leads, plugs, coil. Complete distributor (twice) swapped with working good car.

Fuel pump and cap swapped with good car.

Bypassed fuel tank and pipe work by fitting temporary tank under bonnet and feeding direct from that, using new petrol.

Isolated servo to ensure no air leak.

Checked carburettor for jet, piston, and float operation. Swapped carburettor with good car.

Removed plug leads one by one and run on three cylinders. As expected, power right down, an obvious non-firing cylinder, but still a misfire as speed built up.

Swapped manifold.

Run low tension direct from battery to coil, bypassing normal ignition circuit.

Removed head and checked gasket. No problems. Stripped all valves and confirmed no excessive wear in guides, no sticking valves, all springs correct length, Inspected bores, all smooth, no scoring or ridges. No movement in piston to suggest ring or bearing problem.

No overheating, no loss or contamination of coolant or oil. No clouds of smoke from the exhaust.

The only things that I have not changed are (a) the exhaust system and (b) I have not swapped the head, merely inspected it and replaced the same one.

Two other experts have been consulted, but to no avail. One observed that it seemed that the engine was hesitating across all cylinders, rather than missing on a particular pot.

Throughout all of this, the misfire at around 50 mph has persisted. Frankly I’m stumped. Any suggestions will be welcome. It might be that I’ve already tried it, and just forgotten to mention it; but I would rather discount it than miss it! When it is eventually solved, I’ll report back as to what it was, but in the meantime, ideas please.

[Minornut59]

Hello.
I had something very similar which i reported to Rob Thomasson the tech chap from the club and was in Minor Matters recently.

My 1967 Minor, 1098cc, used and driven hard, frequently at high speeds of 60-70 mph and 15000 miles a year works hard for its living.
I had a 'running on 3' type of misfire that seemed to be valves leaking but having the head done with new guides and seats etc cured it... for six weeks and then back it came again!!
I took the head off three times, swapped heads, swapped everything and it still persisted. Compression tests all showed healthy compressions on all four pistons.

In the end i rang Rob Thomasson and he suggested removing the head again and pouring a measured quantity of thin oil (i used running in oil) into each cylinder with the pistons all at half mast; ie; mid way between TDC and BDC.

I left it over night and in the morning all the ol had gone from one piston.. No 4.

When i removed said piston i found that although the rings were more of less intact, so giving a good compression reading on the gauge, the piston itself had disintegrated where the ring grooves were, allowing the rings to move up and down as the piston moved, so letting compression escape when the engine was running.

I took the piston to show an engineer friend, (he rebuilds engines for a living) and he consulted his book which gave the cause as poor quality fuel.......

After replacing that piston and fitting all four with new rings, honing the bores etc the car was fine but i now chuck in a tank full of named brand unleaded every fourth tankful and don't solely rely on my local supermarket with a yellow sign...!!!

Good luck but i reckon you may have a similar problem here.

[simmitc]

An interesting idea, and one that I shall pursue unless anyone else comes up with anything before the weekend. This car also works for its living!

[bmcecosse]

I suppose the exhaust could be choked and causing excessive back pressure - you seem to have covered pretty much everything else. And yes - it could be a piston as above I suppose - although I would expect to see 'very heavy breathing' from the vents if that was so, and hefty oil consumption. You could try rigging up an extra earth to the block directly to the earth terminal of the battery, I have heard of 'bad earth' problems like this on Minis. And I suppose it's just possible it could be the coil.....are the connections right way round on it? Other possibilities are a badly worn camshaft - not lifting the valves to the normal height - you would probably need a dial gauge to measure accurately, although a good approx could maybe be done with a vernier caliper I suppose. Also - just possible the cam timing is wrong, perhaps jumped a tooth -although very very unusual.... If everything else fails - I can run you through the method to check the timing.

[Minornut59]

When mine gave trouble there was no increase in oil consumption (it never uses a drop) and there was no 'heavy breathing' either. Everything seemed normal. I wouldn't have thought it was the cam; its very rare for them to wear like that even after huge mileages. Not impossible of course.
If the cam timing was out the car would be very sluggish all the time. I got it wrong once- one tooth out and when i drove that car it was hopeless!

If it was a bad earth then it would be hard to start and i wouldn't expect a noticeable difference from having the coil terminals the wrong way around; there are a lot of cars where they are wrong anyway.

My money is still on a piston. I had discounted it on mine due to a good compression reading. There was no difference between any of them; all about 150 PSI as i recall.

Its certainly a mystery!!

[beero]

Valves sticking, weak valve springs? You have just about covered everything else.

[bmcecosse]

@nut59 - if the piston/rings was allowing blow-by to the extent it affected power - then by definition there MUST have been heavy breathing!!!
Coil connections wrong DOES make a difference- especially when under load, with reduced spark intensity. And cams CAN wear badly - I have one in my garage where they are seriously worn. But at least that is easily checked. And timing retarded by one tooth due to a very loose chain is 'possible' - and would run fine at low revs but less so at high revs. If you assembled timing wrongly - it may have been advanced one tooth which would not run well at all. But all these things are grasping at straws - but any one of them could explain the problem.

[MarkyB]

How much would 30 mins on a rolling road cost?
Might prove a good investment.

[Minornut59]

Well BMc, i can assure you it WAN'T breathing heavily!! It was my car after all!! Evrything appeared normal; no blue smoke from the exhaust, no crank case pressurisation as far as i could detect but the fault was as described and was in Minor matters a while back as i said. It had me completely stumped.

[simmitc]

I'm taking on board all comments, but would add:

Sticking valves / weak springs discounted after stripping the head and inspecting bits. Confirmed that coil is connected the correct way round.
Timing chain is a bit rattly, BUT it is a duplex unit, so both cogs would have to have jumped ta the same time.
The engine has about 30,000 miles on it, and was running perfectly from "new" until this problem developed a couple of months ago. I would hope that the cam would last longer than that.
I seem to be faced with three definite things to try: Extra earth strap, swap the exhaust, and fill the pots with oil to do the soak test. The latter two will have to wait a week or so, but I will report back, and still accept any other ideas! Thanks all so far.

[IslipMinor]

Is it a 'hard' misfire, like removing a plug lead, or 'soft' like running out of fuel?

Had you done anything between the time that it ran normally and when it first started to misfire?

A rolling road would be able to identify the basic source of the problem - fuel, electrical or mechanical. Once the source is identified, it might be easier to get at the cause?

[lambrettalad]

also turn the head upside down and pour thin oil on to the valve seat depressions leave over night, to see if any valve seal is leaking?

[Minornut59]

Definitely worth doing the fill test as described on the head but it may be worth getting the head crack tested (about £30?) but then if you are taking the head off you need only do this if the oil down the bores test reveals nothing.
If could be a crack between a valve seat and the exhaust port or something but then again your compression test would have revealed that.

Looking forward to finding out!!

[izzi]

Simmitc, you have not mentioned a battery change, I might be coming from a wrong angle, but the battery, voltage regulator,polarity solenoid connections and igniiton switch etc; may easily be interchanged with the spare parts you have.
It is just that this misfire sounds electrical.(I know you have by passed a few things)
Good luck with this one
Roy.

[simmitc]

Well, as it was a nice day, I took some time off to potter around the house and fiddle with the car. I have added an extra (thick!) earth wire direct from battery to block. No difference

It is probably best described as a soft misfire as if it was a fuel problem - it occurs only under load, idles fine, and is OK at lower speeds. However, it "sounds" electrical, but then that probably just means that my hearing is defective too Agreed, I have not changed the battery, so I shall do that shortly. My reasoning for not bothering so far was that it starts on the key, and the alternator is kicking out a good charge, so there would be plenty of power whilst running (measured with volt & amp meters). However, as I said, I will try anything (sensible) to cure this. The head removal will probably be over the bank holiday weekend, so "keep tuned".

[MarkyB]

I have a feeling that a restricted exhaust would show similar symptoms, maybe the wadding has come loose or it's taken a whack from road debris?

[bmcecosse]

It can't be the battery if it starts up well.

[kennatt]

I once had a similar fault on an mbg,after just about checking everything ,like you,at a loss,same misfire but at about 60mph.then the alternator failed and i fitted a new one.fault gone. Can only think that there was a fault causing a surge in the volts at 60 which must have been overloading the coil,ignition system,somehow, would this be a possibility

[izzi]

It can't be the battery if it starts up well.

It is just "Process of elimination"

[simmitc]

Latest update: Changed the battery "just in case". No difference. Removed the head, pistons at half-mast and filled the bores with oil. After 24 hours, no oil loss. At the same time, covered the head in talcum powder, placed with ports upwards, and filled all ports with paraffin. After about 12 hours, level of paraffin had dropped noticeably in the inlet ports and and significantly in the exhaust ports. However, no sign of leakage around the valves, other than the merest hint on one exhaust valve, but nothing to which one would point and say "That's it" - it seems to be leaking more along the stems and out of the seals.

So, my thinking now is to replace the head, just in case there is something that manifests only when it is hot and under load. If that doesn't work, then it will be replace the exhaust. Although the latter should be an easy option, the other Traveller blew an exhaust over the weekend. Despite the "lifetime guarantee", the company from whom it was purchased is no longer trading. When I opened the new replacement (a spare kept in stock) I found that it lacked the b3nd to go over the rear axle, the down pipe was shaped so that no matter how it was fitted, it touched the body; and the flange, instead of being formed from the pipe, was a messy collar welded on. It was only after some serious engineering that the thing was joined to the remains of the old one and made to fit. Don't talk to me about the quality of new parts

I'll let you know how it all progresses.