1969 2a Distributor Vacuum Advance

[GFelicio]

Hi all, looking for some help with a vacuum advance issue. I bought a 1969 2a 88" over the winter, got it running, registered, and back on the road this summer. Last week while out for a drive, it started running erratically. It was idling fine but whenever I opened the throttle, it would start to lurch with the engine revving high-low, high-low in a regular pattern despite steady application of the throttle. I was able to get it home lurching and backfiring the whole way. I checked leads, plugs, timing...everything looked fine. The next morning it was behaving the same way. It fired right up, idled smooth, but continued with the high-lo revving under throttle. I tested the distributor vacuum advance unit by disconnecting it at the intake manifold and applying suction to the open end. The distributor base plate moved as expected but the diaphragm seemed to leak as the baseplate quickly returned to its original position. With the vacuum tube plugged, the truck was back to running like a champ with just a hint of a stutter here and there. I ordered and installed a new vacuum advance unit and with it connected, I'm right back to where I was before...smooth idle with high-low revving under steady throttle application. With the vacuum advance disconnected I was able to take a 4+ hour ride through central and western MA this weekend that included both high-speed highway stretches and low-speed bumpy, rutted dirt roads. It ran great in all situations. Have you seen this before/got any advice for what to check next? Thanks, Glen.

[roverp480]

First ,is it a 25D4 LUCAS distributer ? On a 2A LR the vacuum advance is basically an economy device to advance the timing during part throttle opening such as medium cruising speed on a level road . .It hardly does anything at idle and nothing at full throttle . The fact it ran well with the vacuum advance inoperative would point to something wrong with its action, and not fuelling or basic ignition. . Allowing the fact the diaphragm actuator has been replaced I wonder if there is a leak in the vacuum pipe and or the rubber connecters either end , allowing air in thus making the whole oscillate. Also is the advance movement of the base plate smooth in its action and not sticking? As a matter of interest the Military FFR 24 volt versions don't have any vacuum advance, only centrifugal.

[GFelicio]

First ,is it a 25D4 LUCAS distributer ? On a 2A LR the vacuum advance is basically an economy device to advance the timing during part throttle opening such as medium cruising speed on a level road . .It hardly does anything at idle and nothing at full throttle . The fact it ran well with the vacuum advance inoperative would point to something wrong with its action, and not fuelling or basic ignition. . Allowing the fact the diaphragm actuator has been replaced I wonder if there is a leak in the vacuum pipe and or the rubber connecters either end , allowing air in thus making the whole oscillate. Also is the advance movement of the base plate smooth in its action and not sticking? As a matter of interest the Military FFR 24 volt versions don't have any vacuum advance, only centrifugal.

Thanks for the response. It is a Lucas 25D4 distributor and movement of the baseplate did appear to be smooth though I really don't have much of a basis for comparison...very limited experience. I will have to look more closely at the pipe and connectors. It's a rubber boot at the manifold and a threaded fitting at the vacuum advance unit. I had read that there was no harm in driving with it disconnected. That is interesting about the military model. I guess having one less point of potential failure outweighs an increase in fuel efficiency in a military application.

[charles1943]

Your motor mounts might be either soft or broken. Look for the gearsticks to rock back & forth when your engine lurches. Also, at what point is your vacuum advance connected? What carb are you running?

[GFelicio]

Motor mounts look good and no movement of gearsticks noted. Zenith carb. Not sure if I understand the question about the point of connection for the vacuum advance...

[mearstrae]

Your hose might be attached to manifold vacuum or carb port vacuum. When in doubt use manifold direct vacuum.

[GFelicio]

Thanks, I’ll take a closer look at that. I wrongly assumed there was only one place to connect.

[ep7360]

I replaced my distributer with an HEI system years ago and have never looked back. Runs smoother and starts everytime.