Sluggish

[Telemuhcaster]

Hi everyone, I'm getting towards the end of my rebuild... however, it's quite sluggish. I've had them before and so I know this is under powered.

Things I've done so far:
Checked ignition timing is at 6 degrees.
Bought new intake pipe (as old was quite nasty.
Ordered new points, condenser, arm and cap.
Checked brakes aren't adjusted too tight and wheels spin freely.
I rebuilt the carb a while ago, but there's no saying It could have problems since then.

I have noticed when revved high and released it will backfire a small amount.
Engine is rebuilt and has good compression evenly over all cylinders.

Perhaps somebody could point me in some direction, perhaps one of you have chased this sort of problem and it was something small.

All the best

[Andrew]

I assume this is a static ignition timing test with a bulb since you talk about points and 6 deg BTDC?
It might be worth putting a strobe gun on it and seeing what the advance is when it's running, with and without vacuum connected.
Its just possible the weights in the distributor are not free, and aren't advancing the timing when the engine is running, not static, with some RPM on it. Timing should be in the region of 10 degrees more advanced, i.e. 6+10=16 deg BTDC at ~1000 rpm, and ~14 degrees more advanced at ~2000 rpm, i.e. (6+14)=20 deg BTDC.

If the timing isn't advancing with RPM, maybe the distributor weights are seized/rusty or something.

Also check that the vacuum is connected, and working. You can check this by unplugging it and plugging it back in, while watching the strobe timing, with the engine just above idle so that the carb is pulling a bit of vacuum (they tend to pull very little vacuum at full idle, you have to open the throttle just a wee bit to see any vacuum on the port of the Zenith and Weber).

[Alan Drover]

To check the distributor mechanical advance you should be able to turn the rotor arm anticlockwise a little and it should spring back. It could also be the vacuum advance. What distributor is fitted?

[Wittsend]

With what passes as "petrol" these days, E5 or E10 etc. you'll find 3°BTDC a better starting point for the static timing.

But I would have thought a broken vacuum advance or sticking bob weights the more likely.

[Telemuhcaster]

To check the distributor mechanical advance you should be able to turn the rotor arm anticlockwise a little and it should spring back. It could also be the vacuum advance. What distributor is fitted?

I'm not sure, all I know is that it's the original so it has the twisty vacuum thingy majig. I've got the workshop book so I can have a look through that and make sure it's correct

[Telemuhcaster]

With what passes as "petrol" these days, E5 or E10 etc. you'll find 3°BTDC a better starting point for the static timing.

But I would have thought a broken vacuum advance or sticking bob weights the more likely.

Ok that's super helpful, I'll change it perhaps to between 3 and 6 and see how we go. I do chuck e5 in all my series and tbh the fuel tank isn't the cleanest. It had as good a rinse out as I could manage.

I'll look through my workshop diagram about servicing the vacuum advance. It does have the twisty thingy on it to adjust it (I think)

[Alan Drover]

If you suck on the vacuum advance pipe the baseplate in the distributor will move. If not then it's had it.
I'm not sure if the Distributor Doctor will sell vacuum advance units separately but he might be worth a try if it is faulty.
It sounds like you've got the correct 25D4 distributor.

[w3526602]

Hi AD.

On a couple of occasions (since 1958) I have met immovable " breaker plates" that had been clamped by POs using overlong 2BA (?) screws, thereby jamming (jambing?) their ability to slide.

I have also met presumably punctured  ADVANCE diaphragm units  .... nothing moved when sucked, so they remained in "at rest" RETARD position. (Somebody check my logic, please)

Less often, I have met "dangling" rotor arms, where the rotor could be twisted manual andly, by  hand, with zero spring resistance. AKA "floppy".

602

PS. Do modern dizzies still use a thin braided earth cable between the breaker plate and the the distributor case? Many (50?) years ago, somebody was able to demonstrate that EARTHING the breaker plate increased the tick-over speed by a measurable number of RPM.

I suppose that electronic ignition has obviated that need?