Replace faulty vacuum advance or complete distributor?

[carl350i]

Good morning.

Now that I’ve been using the griffith a lot more recently after an 8 year layup.

I’ve noticed a bit of hesitation under light throttle, and I don’t remember having it before.

I’ve looks at loads of past threads and thought out of interest to check the timing, it hasn’t been touched in 15 years.

But before I went to check it, I checked the vacuum advance to see if it would hold pressure and move the the base plate, it did not, probably due old age and not being used.

So question is, just replace the vacuum advance and see it helps or replace the whole distributor?

Cheers,

Carl.

[Penelope Stopit]

If base plate and centrifugal advance bob weights/springs are good, replace vacuum unit

[O mage]

If it's a 500 i wouldn't run vac advance, when the shunting became annoying they left the factory with a "modified distributor" basically the vac advance units were disabled but with the vac pipe work still in place which was a quick fix.

[Beverley Grove]

carl350i said:

Good morning.

Now that I’ve been using the griffith a lot more recently after an 8 year layup.

I’ve noticed a bit of hesitation under light throttle, and I don’t remember having it before.

I’ve looks at loads of past threads and thought out of interest to check the timing, it hasn’t been touched in 15 years.

But before I went to check it, I checked the vacuum advance to see if it would hold pressure and move the the base plate, it did not, probably due old age and not being used.

So question is, just replace the vacuum advance and see it helps or replace the whole distributor?

Cheers,

Carl.

The vac unit is easy to replace, just undo the two screws holding the unit in place and unhook the arm from the dizzy base-plate pin, then withdraw. It can be a little tricky to relocate the arm on the new vac unit on the pin but it's straightforward. Base timing shouldn't be affected but worth rechecking.

Your hesitation could be cause by your leaking vac unit diaphragm allowing air into the plenum. I had exactly this when I forgot to reconnect my fuel pressure regulator vac line. It could also be caused by your vac advance unit not working, part throttle is exactly where it gives the additional advance required.

Timing, >4000rpm, 28deg, vac disconnected

500's should have their vac connected. They actually use a ported vac system which is why the vac take off is on the plenum right above the butterfly, so vac is not provide at idle hence giving a low ignition advance at idle speed. This is done to light the pre cats early after startup for emissions reasons at the expense of smooth idle.

At the risk of opening a can of worms, I would't recommend disabling your vac advance, far better to make it work correctly. By removing the vacuum (load related) component of ignition advance, you only have the correct ignition advance at full throttle.

Edited by Beverley Grove on Thursday 16th July 13:03

[carl350i]

Ordered a vac advance, should turn up tomorrow, I’ll let you know if it’s worked

[carl350i]

FIXED

New vac advance did the job nicely.

Easy to fit, The inside of the distributor was lovely and clean and it all moved freely.

£20 well spent.

For anyone searching for hesitation at light throttle, check yr vac advance.

Cheers.