Vacuum Advance

[conrod]

Guys,

I have a problem with the vacuum advance on my HEI dizzy. With it connected, I get pinking between about 2100 and 2600 rpm on any type of acceleration - but it's OK when cruising in that range. Disconnecting the vacuum pipe from the advance/retard diaphragm and blocking the pipe off cures it. The only problem is, my fuel consumption on long journeys has increased quite a bit since! And it "hunts" much more on idle too. Neither of which are surprising, as those are the very things the unit is there for!

DB and I played around with this the other week, trying different weights, springs and timing, but connecting up the vacuum pipe always caused it to pink - although changes to the mechanical advance moved the pinking rev range slightly. We also tried connecting it to the alternative vacuum take-off point on the Holley carb.

As I understand it, the way the unit works is that it advances the timing on high inlet manifold vacuum (i.e. when the engine is at idle or cruising), then goes back to zero advance on low vacuum (i.e. under acceleration). So, it advances the timing when you're cruising (to improve fuel economy), and retards it on acceleration (to avoid pinking).

Under acceleration, it should go back to the non-advance position anyway - in which case, why should disconnecting it make any difference (given that pinking means the ignition is too far advanced)? The only explanation I can see is that it's not returning to the non-advance position. If you suck on the pipe, the advance rod moves, then returns as it should when you release the vacuum. So it's certainly not jammed open or sticking.

My thinking is that maybe it's too sensitive, and is advancing the ignition even on very low vacuum. In which case, I'm thinking of trying a dizzy with adjustable vacuum advance. You can buy these in the US for $50 to $75 (for example, see http://speedcitymotors.com/product_i...0b1d3c9df19bf6), but is there anyone in the UK who sells these? They work by using an Allen key to increase or decrease the amount of vacuum requird to move the diaphragm - so if my diagnosis is correct, I should be able to stop it kicking in at too low a vacuum. Has anyone had similar experiences, and/or tried one of these adjustable dizzies?

BTW, it's a 383 Chevy stroker with a 600cfm carb. Timing is set at 12-14 degrees at idle, and 36 degrees at 3500 rpm. Without the vacuum advance it runs fine, and with it connected it's also fine apart from the above pinking.

As I've only had it for just over two weeks, and have already done over 1000 miles in it (I want to drive it, not polish it :thumb, I'd rather not burn fuel unnecessarily!

Any ideas guys?

[kev]

2 things cause pinking, too much advance and too lean. Under acceleration it could be running too lean. Is it pinking when you first stab the accelerator, ie is the accelerator pump working, or does it do it all the time under acceleration, ie you may need to run it a bit richer.

Kev

[conrod]

Kev,

That's a possibility - but having said that it's strange it's just in that narrow rev range. And also why would disconnecting the vacuum advance affect the mixture?

However, straight after posting this, inspiration struck me. All I need to do to test my theory is to put some kind of adjustable retrictor in the vacuum pipe. I've got an old set of vacuum gauges with inline plastic restrictors. It's a needle valve that you screw in or out to restrict the air flow. I've fitted it, and have tried sucking on the manifold end of the pipe to see if screwing in the valve causes more suction to be required to move the rod - and it does!

So it's out for a test drive later to check it out - I'll update the post with the results!

[Clive]

Alan, I think you have got things the wrong way round. When the engine rpm is increased gradually say to cruising RPM the centrifugal weights in the distributor advance the timing, so the ignition is more advanced at higher revs than at idle.

Under acceleration the suction through the carb acts on the diagphram of the distributor which also advances the ignition, but only for a short time while the carb is wide open. This is why most engines "pink" under heavy acceleration especially in high gears. This is why the pinking is cured when you remove the vacuum pipe.

It sounds to me that the ignition is too far advanced to start with.

Cheers, Clive

[bitsilly]

That was what I had understood too.

I'd add that I do not think a restriction in a vac tube will not reduce its effect/strength, perhaps an adjustable orifice or hoses of different squigyness but its a long shot!

[dave]

Originally Posted by Clive

Under acceleration the suction through the carb acts on the diagphram of the distributor which also advances the ignition, but only for a short time while the carb is wide open. This is why most engines "pink" under heavy acceleration especially in high gears. This is why the pinking is cured when you remove the vacuum pipe.

It sounds to me that the ignition is too far advanced to start with.

Cheers, Clive

Hi Clive.
We went with this theory but in reality the engine produces most vac. (Other than on over run with throttle off) at idle when the engine is sucking and the carb is closed (preventing air from entering the engine). As soon as you press the throttle this instantly allows air into the intake thus losing the vacuum. And in effect and for a short period the timing becomes less advanced. The static timing is within the correct range.:thumb:

[kev]

Clive
I suspect you've got it the wrong way round with regard to vaccuum advance. When the throttle is opened the vacuum reduces and the ignition retards. Under light throttle the vacuum is high and the ignition advances. The ignition retards because the fuelling is (should be) richer and the combustion flame travels faster.

Conrod
I guess you use that old fashioned anachronism called a carburettor in which case the fuelling could be weak over some parts of the rev/load range.
Ignition advance doesn't affect the mixture as such, just the way the mixture burns, I'm suggesting that under load the mixture may be too weak, it's just as feasible though that the timing is out. The most accurate way to test is to fit a wide band lambda sensor in the exhaust and datalog it.
By restricting the vacuum pipe what you're doing is altering the timing without knowing what you're altering it from and to, i.e you're in the dark. Take it to someone who knows how to use a rolling road to setup mixture and timing, that way you'll know it's done right.

[conrod]

Sorted!

I had to screw the restricor in to the almost closed position, but it's now cured the pinking and improved the idle. And it's also heavily reduced the popping I was getting from the exhausts on the overrun - forgot to mention that one!

Kev - re the timing. What I've done here is to delay when the vacuum advance kicks in, and possibly I've also restricted the maximum advance it applies. So you're right in that I've changed the advance curve. I'm intending to put it on a rolling road dyno fairly soon - as you say, that's the only way to check this for sure.

I attach a photo of the restrictor - I think you can buy these in pet shops to adjust the flow of air for fish tank aerator pumps! In fact, I think that's where I bought them originally for the vacuum gauges!

Thanks for all the advice.

[bitsilly]

have filed this post for when I'm setting up my 383. Bloody good solution! Mind you, I still say it looks like an adjustable orifice device and the black hose looks squidgy:thumb:

[craggle]

When I brought my engine from Re-Power the HEI distributor was fitted with one of these....

http://www.roadcraftuk.co.uk/index.a...ubCategory=HEI

That rather long link, should take you to Re-Powers web page for an adjustable advance unit for the HEI distributor. Not sure how you use it though as I dont seem to have the instructions for it.

Craig.