Vacuum distributor question

[fred98050]

I am trying to undertand how it works...

I have the original (25D?) distributor with vacuum installed in my 64 2.25 petrol. Today after doing some work on the carburetor I hooked up a vacuum guage on the line runing from the carb to the distributor. At Idle it reads zero, as I reved the engine the pressure went up to about 20psi.

With the engine off, I attached a vac pump to the distributor and it does not hold vac. I can go to about 15/ 20 psi and it goes back down to zero in about 6 seconds.

Is this normal?
Should I change the distributor?

Thanks


-Weber 32/36 dvg
-Pertronix ignition system
-Early lucas distributor.

[gudjeon]

The vacuum diaphragm should hold so no its not good. It is one of the very few sealed components on a series. It is a component that can be replaced on the distributor. As for availability?:confused:

[fred98050]

Thanks.

Looks like you are right. Might be hard to find...

Just curious here, how this vacuum leak affect the distributor? Is it more of problem at low or high rpm?

[J!m]

More problems at high RPM. It is a "Vacuum advance" which advances the timing as the RPMs increase.

It will run with out it, but be low(er) on power...

I replaced mine with the Dulciller one before Africa and it has worked fine ever since, even though it seems to advance a little bit faster than the book spec for 1971.

You need a timing advance light, and set the RPM precisely per the book and check degree of advance at each RPM.

Idle advance
1000 RPM
1500 RPM
2000 RPM etc to the top. The advance will stop working before red line so don't worry.

and make sure they are close to spec. Advancing too fast or too slow will not run correctly. Not many people check this either... Stretching the spring (if advancing too slow) or replacing with a new one (if too fast) is how you 'tune' this. Fiddly work no one wants to do, but it is how you get a Rover running well...

PS the vacuum leak caused by the bad diaphragm is not doing you any favors either!

[LaneRover]

I believe that the vacuum advance is also for a more immediate advance while the mechanical advance takes care of the advance needed at a steady rpm (or at least most of it).

[TedW]

Might be hard to find...

These are on ebay all the time. Also, our hosts might have them in stock.

[gudjeon]

I just did a tune up with a strobe. Pertronix and AC Delco plugs of R44xls persuasion. Gapped at .040" with a 7:1 engine. I advanced the timing to about 8-9 degrees BTDC. No pinking on hard accel or hills. Goes like stink it does.:thumb-up:

[jac04]

It is a "Vacuum advance" which advances the timing as the RPMs increase.

Centrifugal advance is based on engine RPM. Vacuum advance depends upon engine load and not necessarily engine RPM. High vacuum (= lots of advance) can happen at low RPM, low load conditions.

Here is an excellent article, written for Corvettes, but the same principles apply:
http://www.corvette-restoration.com/resources/technical_papers/Timing101.pdf

[J!m]

Good article!:thumb-up:

Note the change in 'vacuum advance' port location as noted in that article.

If you change your carburettor, this must be checked...

[bobzinak]

If you look at your vacuum unit it will have three numbers stamped on it as I recall these are : 1st number/ when vacuum unit begins to move in lbs vacuum. 2nd number when vacuum maximizes pull. 3rd number total number of degrees of vacuum advance.. something like 6-12-21 make sure you get one that is the same...many of the refurbed ones will not have the proper one for you dist. (I once got a "rebuilt" one that only had 6 degrees of total advance, the rover did not have any power at all.) to check on which you vehicle shoukd have had from new you can go to the web site "MG guru" this site has a listing of all the vehicles that used that dist. (100's). As well as advance amounts and the vacuum numbers for the vacuum unit. also of note is that on some carbs the throttle plate blocks the vacuum port. giving you no vacuum at idle, other carbs have the vacuum port away from the throttle plate..which may cause a slight miss at idle. sort of a putting sound. not really a problem..hope this helps..ROVE-ON...bobzinak

[bobzinak]

I looked up the proper vacuum unit for your rover( for all 25D dist.) on the mgguru web site. the right 3 number code for LR's is 4-18-12...this should be stamped on the vacuum unit...if at some time any one has used a 'rebuilt" dit. it may have not come with the proper one..the 25D was used on many vehicles..best of luck...ROVE-ON..bobzinak.