Distributor behaviour has left me scratching my head

[jazzdude]

Following my previous posts on this subject, I fit this distributor that I found (sorted the cable to the amp), connect it up and set the timing. At this point, all I am aiming to achieve is standard timing, ie 28 deg at 4k rpm.

Before the distributor was sent out to me it was kindly tested by a well known TVR specialist and he confirmed that it indeed had 12 deg of movement in the rotor, which translates to 24 deg on the crank pulley, which is exactly what I was looking for, for this particular experiment.

So set it at 28 deg, vacuum off of course, and it settles at idle to 14 deg, which is 14 deg of mech advance only, which is the same difference as the slightly worn distributor I took off the car (which was supposed to have 16 deg of mech advance). Comparing the two side by side, the 'new' distributor definitely has more movement in the rotor than the old one. At higher than 4k rpm with the vacuum still off, the timing does not change from 28 deg.

So just to double check, I connect the vacuum again and run the tests again, this time the idle is still at 14 deg, but at 4k, the advance has moved all the way round to 50-52 deg.

I tested this set up with two different strobe lights, one with a manual advance adjustment on the back, the other full digital that also showed the rpm, both confirmed the same readings.

So the question is, I thought there was little to no vacuum at higher rpms so what is going on here?

I did go round and make sure any possible vacuum leaks had been sealed earlier this year, including plenum, crank vents, AFM hose and stepper motor so this is now baffling me.

I drove around the last few days like this, car feels smooth enough, no shunting, but as I was not sure about this setup, I wouldnt dare take it over 3k rpm. If anything, even though it drove quite well the car was a little lumpy in the mornings, and occasionally at the lights, but I am putting that down to 38-40 deg heat we are currently 'enjoying'.

One thing is for sure, I am going to put the other one back on, at least that is safer. It would be interesting to see if that one does the same, in which case I will be truly stumped.

Edited by jazzdude on Saturday 9th June 11:19

[Penelope Stopit]

There is plenty of vacum advance at higher RPM, this is meant to be
Read this from start to finish as it is a very good explanation and will save me hours of typing
http://www.superchevy.com/how-to/additional-tech/1...

[Sardonicus]

You have no load on the engine so the port is seeing plenty of vacuum post throttle blade this does not happen when driving, and those high numbers on very light load/cruise is what your after

[SuperApeInGoodShape]

Sardonicus is right vis the vac advance. As for the mech advance, sounds like your advance springs aren't strong enough and are already giving you the 1st 14 degrees of advance at whatever idle rpm you are testing at.

Most Rv8 (non TVR fettled) dizzys are designed to give mech advance from 600rpm

Edited by SuperApeInGoodShape on Saturday 9th June 13:55

[jazzdude]

As I said, the original dizzy I had on there had an '8' stamped on to one of the weights, meaning it should have 16 deg mech advance but it only gave me 14.

This one, the rotor moves more than the old one if you push it, and it has been measured at 12 deg, ie 24 mech advance at the crank pulley.

Whatever it is doing post 4k rpm with vacuum, why am I not seeing the full range of mech advance?

In my shoes what would you do? Leave this new one on, set up at 28 deg at 4k rpm, or put the other one back on, set up the same way?

In both cases, I will have a car idling at 14 deg.

[Sardonicus]

Whats wrong with 14 Steve? the RV8 is happier at 18/20 BTDC anyways idle notice your running a 4.0 I would try running max advance at 32 not 28 at 4k+ RPM 28 better suited to the bigger 5.0 higher comp motor , your 4.0 is low spec and likely less than 9.1 CR anyway

[SuperApeInGoodShape]

jazzdude said:

As I said, the original dizzy I had on there had an '8' stamped on to one of the weights, meaning it should have 16 deg mech advance but it only gave me 14.

.

Because it's already giving you that missing 2 degrees at your idle speed.

[jazzdude]

Sardonicus said:

Whats wrong with 14 Steve? the RV8 is happier at 18/20 BTDC anyways idle notice your running a 4.0 I would try running max advance at 32 not 28 at 4k+ RPM 28 better suited to the bigger 5.0 higher comp motor , your 4.0 is low spec and likely less than 9.1 CR anyway

I know Simon but I thought as a starting point I'd set it at the Bible's recommended 28 deg @ 4k rpm.

So you think at 32 deg @ 4k - 18 deg @ 950 rpm then? Not that it makes any difference if it's a 400 HC.

Where abouts is it where I would get detonation? I'm just worried that the mysterious missing extra mechanical advance appears at some point and lunches my engine.