1980 Austin Princess mild custom - 1951 Lanchester LD10

A bit longer a post now I've finished work for today, so you've got a bit more details on what I've actually done.

I removed the head because of a failed head gasket. Before removing the head, I followed all the instructions to make sure the timing belt was in the correct location as per the manual, and then dismantled everything required to remove the head from the car. The head has not been dismantled or rebuilt, because it hasn't overheated and the issue was purely a bad gasket, there was no need. Therefore, crankshaft and camshaft have not been disturbed. I then rebuilt the engine, new gasket, reconnected everything I needed to, and fitted a brand new timing belt. The crankshaft and camshaft pulleys had not moved from where they were when I removed the head so the new timing belt went on in exactly the same location as the old one came off. Additionally, before doing all of this the car was running properly with no timing issues as you can see in the most recent Princess video (check back a couple of posts for that). Nothing I've done should have affected the timing in any way, I've been extremely careful about that.

I have checked all of these things too, to try and figure out what might be amiss, even things that might not be relevant to the issue:

Distributor setting - removed and refitted as per manual.
Points/Condensor - car is running hall effect electronic ignition, no points gap or bad condensor to worry about
Rotor arm/cap - clean, very few miles, no sign of damage
Plug leads - flexible, no arcing, good connection
Coil - appears to be working as it should, connections clean and good
Spark - Nice and healthy
Fuel - getting all the way to the cylinders
Spark plugs - correctly gapped, healthy spark
Firing order - exactly as the book describes it
Compression - yes, plenty
Head bolts - torqued correctly, and checked twice
Tensioner - roller type, no tensioner spring. Roller is free moving, tensioner isn't moving out of alignment once set, is putting a relevant amount of tension on the belt
Alternator - belt goes around the crankshaft pulley, no belt damage, alternator not seized, V profile rather than toothed
Timing belt - no sign of damage, correct profile teeth, correct number of teeth, brand new Gates 5024 toothed belt of appropriate size and application for this engine. Identical (as far as I can tell) to the belt that was removed which was the same model and design
Battery - in good health
Starter motor - in excellent health
Clutch and hydraulics - operating as they should, not dragging or sticking or leaking (though obviously can't test with the engine running yet)
Carburettor - not leaking, overflowing, or suffering from vacuum leaks
Cam and Crank pulleys - tight, not loose, haven't been disturbed

I will add that a long time ago I have had this problem with this car. When I first got the car in 2014, the timing belt was an unknown quantity (as was the rest of the car really) and while you could get it to be timed correctly, it wouldn't stay put and would drift off like it is doing now. Previous owner/s had adjusted the distributor, the carburettor, and I believe the plug firing order to compensate so the car ran badly rather than not at all. We replaced the timing belt, which stabilised that side of things, corrected the distributor timing, readjusted the carburettor, and corrected the plug firing order and then it ran fine.

That I'm back where I was when I first got the car in terms of running issues is quite frustrating. I know the carburettor is adjusted correctly, the firing order is correct, and the distributor is set correctly for where the timing belt was when initially set correctly. I feel like that elminates everything but the timing belt.

Things I haven't changed:
Head
Crankshaft pulley
Camshaft pulley
Tensioner

I know what it's doing is not something it should be able to do. I know that's not how it's supposed to work. All I can do is tell you that this is what it's doing and that I don't understand why. So far, general consensus from folks I've spoken to outside of the forum has been it's probably a bad belt or an incorrect belt. This sort of problem is precisely why I dislike working on engines.

Although any of those faults could alter the timing, since the marks are on the pulleys, it would not be evident unless the valves and pistons came into contact; the problem must lie with the belt itself.

It's too wet out to be fitting the new belt that arrived, hopefully the weekend will be better weather. The old belt I took off before removing the head felt stiffer than the new one that's one. The new belts I've not fitted yet both feel stiffer than the new one that's on and more like the old one I took off. Hopefully all this faff really is just down to a bad belt since I shouldn't really be able to tell a difference in the stiffness of the belts as there shouldn't really be one.

glenanderson: It can't be mismatched pulleys, as I've mentioned a couple of times, because the pulleys are the exact same ones the old belt was running on with the exact same make and model of belt as the new belt.

gtviva: If you got four revolutions to get back to where you started, the marks end up misaligned. Go another four and it's misaligned further.

My working theory is that the belt I've fitted is bad and has some stretchiness to it that it shouldn't have. I'm assuming that the belt itself is stretching a tiny bit every time it's rotated because of the tension it's under and more revolutions stretch the belt further. When I take the belt off it's relaxing back to its original size which is why it's going back on at the same marks but then when you rotate it, the belt is stretching out again. Now, I know the tensioner should take out the slack as it rotates but since it's not a sprung tensioner, it's not compensating for the loss of tension. Maybe there's one rounded off tooth on the belt that's causing it to slip unseen, maybe the belt is stretching enough to flatten a tooth allowing the belt to slip to a new position. After all, I can't see every single portion of the pulleys and belts at the same time as I rotate the engine, so it's possible. It's only a theory though. If it turns out the new belt doesn't do this then it's the theory I'll have to accept since I won't know either way. If it turns out the new belt does exactly the same thing then I have no idea how to fix it, so here's hoping the new belt fixes the problem.

With the plugs out, rotate the engine so number 1 cylinder is on compression. Look to see where the rotor arm is actually pointing, and make sure the lead is going to the right place. Repeat through the other cylinders.

Make sure you have a spark too. Get Pat to hold a plug lead in his teeth, and crank the engine over. If he dances up and down, all is good. 🤣

As above because the crank rotates twice for every rotation of the cam / distrubtor it is easy to mistakenly time it on cyl 4 rather that cyl 1, also check which way the distributor goes round and make sure you have the right firing order.

All of the above confirms the issue, and Pat and I had an epiphany while in the van earlier. You see, the diagram in the book shows the distributor cap and the 'firing order', but that's the firing order of the cap, not the engine.



So that means my firing order is currently 1-2-3-4 which is, er, wrong. What a silly mistake. I will correct the firing order tomorrow and I'm going to add some relevant marks on the distributor cap just to make it easier in the future so I know which socket goes to which plug at a glance.

Daft mistake, easily fixed at least.

Thank goodness for easy fixes, especially given how wet it is outside. Getting the plug leads in the proper order saw the car fire straight up like nothing had ever happened. Massive relief. I'll have to get a handful of miles under the tyres just to make sure all is well, check head bolt torques and so on. Joyful moment just having it fire up today, proper bit of mojo boost.

Great result, the MOT exemption is useful for this sort of thing, take it for an MOT in the spring when you've had a chance to make sure it not going to overheatand the weather is reasonable enough to roll around underneath it if you need to.

Or alternatively take it for a safety check not an MOT, nothing is recorded and you can assess what really needs fixing(ie unsafe things) rather than legal technicalities and fix it at your leisure, most garages will do one for you.