MKIII Distributor but no vacuum tube

[interceptor@joc.org.uk]

Hi,

I am looking at a MKIII which has no vacuum pipe from carb to distributor.

Now, there might be magic in the distributor but how can I tell. The Jensen drives well and owner sees no issues

How can I check if this distributor is able to work without carb vacuum please ?

cheers
Neil

[RockyUSA]

If the vacuum line is disconnected, the distributor will only get mechanical advance. If the owner reports good driving, then it’s likely the mechanical advance is working just fine.

He is giving up the ~3 mpg that the vacuum advance will provide when in a high vacuum, cruising condition… I.e. on the motorway (low engine load, light throttle, cruising along in top gear).

Race cars don’t use vacuum advance.

If he’s cruising from Calais to Istanbul (as all you guys on The Continent should do) in his Gentleman’s Express, without ever stopping, he could go an extra 50 miles per tankful if you get the vacuum advance working (probably a best-case estimate).

Rocky

PS. To answer your question about seeing if it works, put a hose on the distributor connection and suck on it while the engine is running. If the idle speed increases, the vacuum advance is working!

[slotcarone]

In a stock configuration you surely need the vacuum advance for the engine to run at it's best performance. That being said you need to know if any modifications have been made to the ignition system in the last 50 years. It is possible that it is not necessary but I doubt it. Don't let that stop you from buying the car!

[interceptor@joc.org.uk]

Thank you both, so it seems, maybe the vacuum advance is not mandatory but sort of optional.

yes, vacuum advance will improve running, but it seems from this owner you might not notice

I'll remove and check it all inside but it looked like a very normal dizzy

I know we normally do the timing of about 12 degrees BTDC with the vacuum removed, so I will check this first

cheers
Neil

[Steve Payne]

In my experience if an owner complains the car feels flat it is well worth checking if the cannister is working. I have seen cars with faulty cannisters running a lot of initial advance to get them to run better but they then take the chance of pinking.
Even cars highly modified with big cams and high compression still benefit from having a working vacuum cannister.
Check the total advance, if it is not at least 35 degrees they are not getting the best from there engine.

[slotcarone]

Neil on a typical American V8 from the '70's the vacuum advance is necessary for the engine to run correctly unless there have been modifications made to advance the timing another way.

[slotcarone]

Thank you both, so it seems, maybe the vacuum advance is not mandatory but sort of optional.

yes, vacuum advance will improve running, but it seems from this owner you might not notice

I'll remove and check it all inside but it looked like a very normal dizzy

I know we normally do the timing of about 12 degrees BTDC with the vacuum removed, so I will check this first

cheers
Neil

Very easy to check if the advance is working. Just hold the RPM's around a thousand and connect a hose with vacuum to the unit. You will hear a marked rise in RPM's.

[RockyUSA]

A quick way to see if the advance is likely to work is just to take the distributor cap off, and push on the stud where the advance mechanism connects.

You should be able to rotate the plate slightly (~15*) and it should return to the original position.

I think it’s easiest to just connect up a hose with the engine running, though…

Rocky