Ignition advance and retard

Garry · Post by **Garry** » Mon Jul 01, 2013 7:01 pm

Hi there

I have a Rover 3.5 V8 fitted in my MGB ... it has an Edelbrock 500 carb. I have fitted a Range Rover distributor, which was originally points but has been converted to electronic.

I have read on the forum that the Edelbrock carb doesn't mix too well with the Lucas distributor ... my problem is the distributor has 2 vacuum take-offs ... one pointing away to the outside of the distributor and the other on the opposite side pointing towards the distributor. The one on the inside seems to operate the advance and retard better ... but is this correct?

I am also a bit confused on what connection I should use on the carb for the vacuum pipe. The MGB book that I have states that you should use the take-off on the left side (viewing the carb from the front) but in the Edelbrock handbook it says that the left hand take-off is for emission controlled engines and the right one is for non-emission controlled engines.

Anyone got any thoughts on this??

Thanks for any help

Garry

sidecar · Post by **sidecar** » Mon Jul 01, 2013 7:10 pm

The carb does not work too well with the vac advance system on a Lucas dizzy. In my humble you are better off not connecting the vac sytem up at all. If you run the idle timing at around 12 degrees with an all in figure of 36 degrees at 2750-3000 rpm then your engine will perform well. You may have to do a few modifications to the dizzy to obtain the above figures.

NorthernBloke · Post by **NorthernBloke** » Fri Jul 05, 2013 9:12 pm

I have to agree - when my car was set up on the rolling road the vac advance ended up being disconnected.

mgbv8 · Post by **mgbv8** » Fri Jul 05, 2013 9:38 pm

Garry
If you look at the height of the two vac connestions you will see that they are not level.

The lower one will give max vac at idle with the flaps shut. This vacuum will go away as you open the throttle.

The higher one will see no vac at idle, but will give a progressive vac signal as you open the throttle.

The lower one is the one you should use for a dissy. The higher one is the one I use to connect to the vac modulator on my autogearbox for the kickdown system. If the kickdown cable is fully out but the vac modulator does not see WOT it wont kickdown.

I disconnected my dissy vac yonks ago even though I use a Mallory unilite dissy. Even when the car was on the road the dissy worked better on its own.

DaveEFI · Post by **DaveEFI** » Fri Jul 05, 2013 10:28 pm

The main purpose of the vacuum unit is to advance the ignition at cruise - to improve fuel economy. It shouldn't have any effect on the max power or torque of the engine, where vacuum will be low at high throttle opening. It does need the take-off carefully positioned so you don't get high vacuum at idle, though.

On the GM box, manifold vacuum is the main thing which controls the gearbox change up speeds, and provides the part throttle change down. The kickdown cable only does anything at max throttle. Try driving the car with the vacuum line to the box blanked off if you don't believe me. It won't change up until near the max speed in each gear.

sidecar · Post by **sidecar** » Sat Jul 06, 2013 5:02 am

DaveEFI wrote:The main purpose of the vacuum unit is to advance the ignition at cruise - to improve fuel economy. It shouldn't have any effect on the max power or torque of the engine, where vacuum will be low at high throttle opening. It does need the take-off carefully positioned so you don't get high vacuum at idle, though.

Your statement is quite correct and it is unfortunate that the Eddy carb just does not work well with the Lucas dizzy with regards to the vac advance.

Believe me when I say that I spent weeks trying to get it to work, I tried restricting the movement of the vac system and an adjustable vac 'bleed off' system but nothing would allow it to work AND at the same time run a decent mechanical advance curve. If you use the vac system you end up with a low static advance which gives poorer performance overall.

Robrover · Post by **Robrover** » Sat Jul 06, 2013 8:00 am

Vac advance provides better fuel economy, but for performance cars that are not driven everyday that may not be a priority. I specified my Scorcher dissy to be built by Performance Ignitions without a vac advance machanism at all. Some performance cars never had them fitted in the first place - eg Mini Cooper S, Cortina GT500, Torana XU1....

Garry · Post by **Garry** » Sun Jul 07, 2013 5:36 pm

Thanks for all the info

I think the best solution for me will be to blank off the vacuum!

Regards

Garryt

sidecar · Post by **sidecar** » Sun Jul 07, 2013 5:46 pm

Garry wrote:Thanks for all the info

I think the best solution for me will be to blank off the vacuum!

Regards

Garryt

If you set your dizzy up to give the figures that I posted earlier your engine will run very well without the vac system. You don't even need to blank the holes in the vac canister, you just need to blank the ones in the carb. Don't use those crappy blanking plugs from Real Steel, they break up and leak air. Use some decent tubing and a bolt or blind rivet or something similar.

Here is some gumph i wrote on modifying these dizzies..

http://how-to-build-a-pilgrim-sumo.wiki ... by-members