Vacuum advance with Weber carb, some tuning issues

[Guest]

I have a Weber DCV on this thing with a Mallory Unilite distributor that uses vacuum to advance. What I've discovered in my preliminary tuning is that the car runs better with the vacuum hose disconnected and plugged off at the carb. It runs horrible at idle with the vacuum hose connected. I also found that setting the timing and the idle by the book (10BTDC and 900 rpm)and the Weber mixture 1 turn out from bottoming yield the smoothest running.

I haven't been able to put the car out on the road yet, but I am wondering if I need to use the vacuum advance at all, or go to a mechanical advance.

Also, I'm a little concerned that I am too close into the stops on the mixture screw. Should I be concerned?

 

[jlaird]

I suspect and remember that it is only a guess but that your dizzy needs rebuilt or replaced. I think your vacumn advance is not working or not working correctly.

 

[tosoutherncars]

Hi Michael,

https://www.britishcarforum.com/bcforum/u...ge=8&Main=29736

See my post about halfway down the page, with a number of DGV links.

The Redline guides in particular address your question. Inter alia, "When the mixture screw is less than 11/2 then the Idle jet is too rich (too large)." But that's assuming that you set up the carb in the correct sequence. Have a look...

As to the advance, most of the advance curve is taken care of by the weights attached to the shaft in the bottom of the distributor body. The vacuum advance is supplemental. As I understand it, distributors can be recurved, but I don't know whether it's completely necessary. An effect on smooth running, particularly in a low-vacuum situation like idling, suggests to me that something's off.

Put together, the two issues make me think that the carb might not be set up correctly. I would suggest going through the setup one step at a time as per Weber / Redline's instructions. My two cents...

 

[piman]

Hello GM,

I take it that the Mallory has a mechanical advance?

I get the feeling that the vacuum advance's purpose is misunderstood in many cases.
All its function is to enable the carburattion to be set a bit leaner at cruise so help with economy. Yes you can run without it. Some Lucas distributors come without a vacuum advance at all.

You have the enviable task of tuning an engine with both a non standard carburettor and a non standard distributor. If both the suppliers\manufacturers of those items can give you the correct setting that's fine otherwise it is a rolling road session to get them tuned in properly.

Alec

 

[tosoutherncars]

YIKES! Please ignore most of my previous comment... I was thinking you were still on the stock distributor! I have no knowledge of the Mallory system. Blame the heat, and maybe the Corona. Thank you Alec, for the on-point answer.

 

[racingenglishcars]

Complete agreement with Alec on the vacuum advance. A problem might come with the centrifugal advance curve. If it is setup similar to the original distributor it would just need static timing adjustment. I doubt it is set up that way. The carb is probably not correct either, but that's easily (relatively) done with a CO meter.

 

[Morris]

Actually, vacuum is very high at idle. Many carbs are designed with advance circuits that prevent the vacuum advance from being applied at idle. I would suspect that you Mallory dizzy is set up with this in mind...

 

[piman]

Re: Vacuum advance with Weber carb, some tuning is

Hello Donn,

"but that's easily (relatively) done with a CO meter."

up to a point, certainly idle can be set with a meter but to get the fuelling correct far all revs and under load is a different kettle of fish. You can use a lambda sensor and a voltmeter driving on the road to get some idea. But then you need to set the distributor curve? Very tedious if you do it by driving the car, hence my suggestion to do a rolling road set up.
Alternatively down grade to the standard carburettor and distributor that the manufacturer spend a long time on a dynamometer getting it right.

Alec

 

[JerryB]

Re: Vacuum advance with Weber carb, some tuning is

the Mallory dist has vacuum and adjustable centrifugial advance.

The cent advance is probably 14* distributor. It is adjustable. Read your instruction sheet or the Mallory website. You might shorten it to about 12* dist so as to give 24 crank degrees and set the timing to no more than 32* at 3500 rpm (when the cent is fully advanced). Let the idle advance fall as it may..but it will be about 8* BTDC but set it at full advance (with the vac disconnected and blocked on the carb of course) and not at idle.

Mark your pulley or dampener at 32* with white paint thin line. (circumference divided by 360 x 32).

Then connect vac line form dist to PORTED vacuum on the carb. NOT to manifold vac. Ported will have no vac at idle (or very little).

Make sure you have no vacuum leaks.