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Author Topic: Checking vaccum advance  (Read 6255 times)

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Checking vaccum advance
« on: August 28, 2008, 08:53:24 PM »
You responded to my question about a 1980 MGB with some hesitation and having to pull the choke out a little. You thought the vacuum advance was the problem.
How do I check this? It is original.
Thank you.


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Re: Checking vaccum advance
« Reply #1 on: August 28, 2008, 08:56:44 PM »
The easiest way to check if the advance is working properly is to set-up a timing light and watch the timing mark move under various conditions.

If you have not used a timing light on your car or don't own one, this may help.  Otherwise, set up the light as if setting the timing, which is not a bad idea to check first anyway.

If you are buying or borrowing a timing light, try to get one with a clip-on (induction) coil for the spark plug wires. 

Older types required an in line adpater that meant removing the wire from the distributor cap or spark plug and the connections were often not as good and sometimes fell off during use.

Clip the light to the #1 spark plug wire (though for this purpose, any wire will do as long as you cann see the timing mark with the light on).

With the battery behind the sets and under the floor, it is easier to connect your timing light to the cigarette lighter on the console. 

The adapter is cheap and avaiable at any auto parts store or Radio Shack.  Make sure the adpater has fairly long wire leads.  Then just connect the 2 leads from the light to those on the adapter.

Withe the engine off, clean and paint the timing mark on the engine damper pulley (crankshaft belt pulley) with light paint or even "white-out".  The mark should be a slot in the pulley outer ring.

Also paint the mark on the engine, usually on the timing cover just above the pulley.  It looks like a saw-tooth metal tab sticking out from the cover at about 10 o'clock facing the engine or a bit closer to the passenger side of the car.  Finding the correct tooth to mark is often difficult, but you should paint the right one to avoid any confusion later.

Start the engine as normal and warm it up.  Remove the vacuum advance hose from the distributor and plug it.  If the idle drops, the diaphragm of the advance unit is not blown, though at low idle there is often no difference.

Aim the timing light at the marks watching if the the mark moves at idle.  Too much motion may be indication of mechanical wear in the distributor, but hold that thought for a momnt.

Slowly raise the idle and then race the engine a bit.  Gradually raising the idle allows the mechanical advance to work.  Quickly racing the engine is like acceleration without the advance unit and you can see how the engine reacts. 

I there is no motion of the timing mark on the pulley (relative to the casing), there is a mechanical problem in the distributor, usually a stuck advance plate.  I don't think this likely as you say when you open the choke, the car runs better.  Raising the rpm will mechanically advance the timing, if jammed, even if the vacuum system works, nothing would happen.

Next, reconnect the vacuum line to the distributor and repeat the above.  If the engine stumbles and the timing mark does not move any differently than with the line removed, the advance pot is probably leaking and the vacuum advance unit needs to be replaced (about $70 from Moss). 

The original 1980 system is a Lucas electronic unit on a unique distributor.  If you are OK with some modification to your car, I would highly reccomend going to an earlier model 45D points-type distributor (still available new) and equipping that with an aftermarket electronic conversion, such as Pertronix.  The installation of both the distributor to the engine and the conversion kit to the distributor are bolt-in with no modifications to either the car or distributor, save possibly some wiring connections.  The 45D is easier to maintain and the electronic conversions are more reliable than the Lucas CEI or Opus units.  Just a suggestion to consider, especially if you find both mechanical wear and the advance unit faulty.   

If the performance of the advance unit is good, you may need to move on to the carb for fuel problems, but the advance unit is such a common issue on these cars, I think you'll find your answer here.

I hope that helps you.  Please keep and let me know how you make out, as well as anything else that comes up. 

Good luck.


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