DK4A Timing Query

[John P]

I've got an Ulsteroid fitted with a lightly tuned 2 bearing engine, an HS2 SU carburetor and 3-speed box.   It has a later "D" type axle which I think came from a van so presumable is a lower ratio.

There is some wear in the distributor but I don't think it's that bad.

The car starts easily and runs OK, if a little rough, but seems to lack power at lower speeds.   I've just been out fot another test drive and as soon as I start to climb even a slight hill in 3rd at 30 mph the speed quickly starts to drop and I need to use 2nd to carry on.   In 2nd it climbs quite well and, if anything, the engine is somewhat smoother.

Presumably when I change down and the revs increase the auto-advance increases the advance so does this suggest the static advance need to be further increased.  At the moment static timing is set to around 6 degrees BTDC (possibly a little less) and the distributor has an 8 degree action plate.

I've only ever driven this car so don't have much to compare it with.

John.

[Howard Wright]

Hi John

You don’t mention the size of SU nor the needle and spring and I wonder if fuel supply is more of a problem than ignition?

Cheers

Howard

[John Cornforth]

Hi John P

I am assuming this is a low power problem and is not accompanied by any signs of misfire.

This doesn't have the characteristics i would associate with a timing error, unless it's really extreme. I suggest you buy or borrow a xenon timing light (about £25 new). If your car is 6 volts you will also need a small 12 volt battery for the light. Fix a pointer near the camshaft pulley and make a TDC mark on the pulley rim. You will then be able to see what the advance characteristic is, and how good or bad the timing scatter is. The internal mechanism of the DK4A is rather sensitive to small amounts of wear in the various pivot points. As you know, the 8 degrees by Lucas convention are distributor degrees, so the advance range is 16 crank degrees. With 6 degrees static advance that will give you 22 degrees maximum advance which should be more than enough.

In my experience, the handbook settings for advance are a bit too much for modern fuel and engines modified for better breathing. Too much advance gives harshness around 1800 to 2500 RPM. You won't get pinking with modern high octanes.

If your problem is fuel related, it's possible that you are running a little lean at low RPM/low airflow. A lean mix may not be so lean as to obviously misfire, but will still be down on power. As a diagnostic, try pulling out the "choke" a little and see if it improves matters.

My own car uses a 1 1/8 inch SU with a GG needle. The mixture has been checked on the road with an oxygen sensor in the downpipe and is close to ideal. As you have a larger SU it's likely that even at high RPM/high airflow the piston is not fully lifting. This isn't a problem in itself but can make the choice of needle more critical because a lot of the time you are operating near the "thick end".

The original 3 speed box was suited to an engine with fairly restricted breathing and maximum torque probably around 1800 RPM. With better breathing the torque peak moves up to nearer 2500 RPM so it's also possible that the widely spaced ratios are no longer ideal. You end up with a bit of a gap where 3rd is too high and 2nd is too low.

[Chris KC]

I set my timing (with auto-advance distributor) at TDC then fine tune by ear. I would not profess any great expertise at this. But beware of too much advance when labouring at low rpm - engines don't pink any more, which used to be a useful tell-tale that they are in distress.

Your back axle is most likely a 5.25 - which should be adequately lively. Vans were originally a lower ratio, but most had a hard life and have been replaced with available parts over the years. It's fairly easy to check by counting wheel vs propshaft revolutions.

You don't mention what cylinder head you are using; though on an Ulster rep I'd expect at least the standard high compression head.

Do you know what tappet radius/ camshaft it has? A "sporty" cam may fly at high rpm but lack torque for slogging up hills.

I'd encourage you to meet up with other owners and compare cars. Austin Sevens aren't known for sailing up gradients in top gear, it may just be a matter of adjusting expectations & driving style.