MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - How important is the vacuum advance?

Just recently my vacuum advance stopped working. I'm not sure, but I'm guessing that the lines to it are clogged as opposed to some problem with the distributor. So, how important is the vacuum advance?

Jeremy
1974.5 MGB
photos.yahoo.com/jeremysmgb
J. Palgon

Jeremy, the main benefit of vacuum advance is to provide better economy under light throttle openings. Without this your vehicle will undoubtedly be heavier on fuel.
Iain MacKintosh

The vacuum advance mechanism is used to advance the timing with the decrease in engine load. Therefore, if you disconnect the vacuum advance tube, or it is blocked, you will only be advancing the ignition timing by the mechanical advance weights. This is not too much of a problem as your timing will now be for ‘full load’ conditions all of the time, ok if you always have your foot flat to the floor! (this is/was often done on ‘performance’ engines to improve the throttle response). Under partial load or no-load conditions, the manifold vacuum advances the timing to correct for the leaner mixture, this improves the combustion process and therefore your exhaust emissions, plug fouling, soot etc.

Any good garage (auto-shop?) can test the vacuum operation, If your engine is not ‘highly’ modified, fit a new vacuum unit (and pipe) and get the distributor set-up correctly. A correctly set-up ‘B’ is a great engine!
MG Mike

Thanks guys,
I knew that the mechanical advance alone would work fine, but burn more fuel. I haven't heard any pinging since the vacuum advance failed; so I was really just wondering about the repercussions of leaving it for a little while. My B is 99% original, my parents bought it new and I got it our of the garage for the first time in 15 years about six months ago. I plan on fixing the vacuum advance, but I don't have time just this second.

I do know that most of the guys in my local MG club, who have modified their cars, have taken the vacuum advance off. In fact, my local MG shop told me it's no big deal. I just wanted to check if there was something I didn't know.

Jeremy
J. Palgon

Jeremy, if you're going to run your car with a non-functional or leaking vac advance, plug the hose coming from the carbs or manifold. A bad vac advance can is a vacuum leak that can cause other driveability issues. Also, leave the vac advance in place as it holds the points plate from rotating.
Jeff Schlemmer

Disconnecting the vacuum advance on a distributor designed to use it has more detrimental effects than using a vacuumless distributor (i.e. a 43D4) where the curve is designed to take no vacuum advance into account. Unless the timing is tweaked to take account of no vacuum advance you will get *less* likelyhood of pinging/pinking (less total advance), poorer part-throttle performance and hotter running as well as poorer economy at cruising throttle openings. How much you can advance it will be determined by the onset of pinging/pinking. Vacuumless distributors are often marketed as 'performance' distributors, simply because that type are fitetd to cars used for competion, where cruising economy is hardly an issue. On a road car it makes no sense at all to do away with vacuum advance.
Paul Hunt 2

Guys,
Thanks for your input. I only noticed quite recently that the vacuum advance had stopped working--when I was tuning the car up to take it to the Cal Autumn Classic in San Juan Batista. After encountering a few problems out of my expertise, I took the car over to my local MG shop. They tuned the car so it purred. Unfortunately they didn't have time to adress the vacuum advance issue. So they just plugged the line and left the vacuum advance in place. Now that I've resolved some of my other issues and installed a new fuel pump, I'm ready to tackle the vac advance issue. So, how do I go about troubleshooting this problem?

Jeremy
J. Palgon

Jeremy - one of the best tools to work on Vacuum Advance units is the MityVac, which is a hand pumped vacuum device, with a gauge, often sold for brake bleeding purposes (some versions omit the gauge), but vacuum-by-mouth will also work for troubleshooting.

First step is to identify what component has failed. Is the vacuum line plugged? Take both ends off, and see if you can blow through it (hint: blow, don't suck first, and put one end in a glass of water to look for bubbles - sucking first may get you a dose of engine oil). Some systems have an orfice in line which can get easily plugged.

If the line is OK, then reconnect it to the distributor (or use a piece of new line), and see if you can maintain a suction on the advance unit. That is, draw a vacuum on the line, and stick your tongue over the end of the hose in your mouth. If it holds for five or six seconds, you're probably OK. If it lets go immediately, you have a leak either in the vacuum hose, its fittings, or internally to the VA unit. If it's internal, not much to do but look for another one.

Then, pop the cap off the distributor, and view the breaker plate, which should be of the "two-pieces-sandwiched-together" type. There is a spring arm from the VA unit that attaches to a tapered peg on the upper plate, and you should see a definite movement of the plate when you apply mouth suction. Sometimes the two plates stick together, and need to be cleaned and re-lubricated to work smoothly. Just make good notes or drawings so that you get everything back together correctly, especially the electrical connections to the points.

That's about it, except if you have to replace the VA unit, try to get one with the same specs as your existing one. There will be two numbers stamped on the vacuum chamber (maybe three if the date code is intact). One is the Lucas part number, and will be eight digits long, and the other will be of the form 5 - 13 - 10, where the first number indicates the amount of vacuum where movement begins, the second is where movement ceases, and the third is the actual advance in distributor degrees (crankshaft degrees are double this last figure).

Best of luck, and please let us know what you find - - Alec
Alec Darnall

Alec,
Thanks for your wonderful instructions. My line is good, but I bought a new one anyway. However, I don't feel any suction at all when I put my tongue on the end of the hose. Likewise when I remove the hose and cover the vacuum intake with my finger. The vac advance doesn't seem to move at all (no matter whether the line is off and the inlet open or if it's totally plugged).

I did notice that I have the 25D distributor. I tried to get the numbers off the vac advance, but they seem to be imprinted on the underside of the unit. I can't imagine how I'd be able to see those numbers without removing the vac advance unit. Not only am I not sure how to do that, I'm also afraid I'd break something in the process.

So, I guess I need a new vac advance unit. I'm going to check with my local MG shop and see if they have one on hand. If they don't, perhaps someone out here on the board has an extra.

Jeremy
J. Palgon

They are easy to remove on the 25D

You need to un-hook the spring arm inside the distributor. The one mentioned in the ealier post.
Then on the end of the fine adjustment screw thread you will find a little clip. Pull this of with some pliers. Then you need to turn the fine adjustment screw all the way off the end. The VA should then pull out. I think it has a spring inside so watch for how it comes out it will need to go back in the same way. You may need to use some penetrating oil if the fine adjustment screw is stiff to turn.

Refitting is the reverse of removal!

Andrew
andrew.horrocks

Hi, Jeremy - Andrew is dead-on about removing the nut and VA assembly. From your post, it certainly appears as if you've got a bad one. Notice that the adjustment nut bears against an extension of the distributor body, and a spring within the recess behind it applies tension to keep the unit from moving about. There's also a flat spring between the nut and the body that provides the "clicks" as you adjust the nut.

Check out another thread running on this forum about someone who broke off the nut trying to remove it, and be sure and use some penetrating oil when you do yours.

I've always found the numbers around the outer barrel, up close to where the hose attaches, bu they're really small. Best wishes - - Alec
Alec Darnall

If you don't feel any suction at the distributor end of the pipe when the other end is connected to the inlet manifold then either the pipe or the nipple it is connected to is blocked, and that is why you have no vacuum advance!

With the distributor end of the pipe connected to the distributor there won't be any vacuum at the inlet manifold end that you will be able to feel, it is you that has to apply the vacuum to confirm the points plate moves and there is no leakage through the diaphragm.

I'm not convinced that you have tested correctly.
Paul Hunt 2

Paul, you may be right. Jeremy, the test I described requires that you suck on the hose attached to the distributor to see if the points plate moves. Then, to make sure there's not a leak in the diphragm, you should feel residual suction when you put your tongue over the end of the hose while it's still in your mouth. With apologies to Johnnie Cochran, "If the suction don't stay, your vacuum's gone away".

As to the vacuum supply on the engine, didn't you say your garage had plugged it? Cheers - - Alec
Alec Darnall

Guys,
Thanks again for all the pointers. Just to insure that I had it right before, I took a new length of hose and connected it to the vacuum advance. Then I sucked on the other end. It seemed to me that there was no residual suction at all. I also didn't hear the plate move at all. So there's definitely a problem there.

I looked again and I still cannot see the numbers on the vacumm advance unit. Just the same, I started a preliminary search for a new one. It seems that they're rather expensive. Apparently the cheapest one is about $75...and it might not even be the right one for me. I learned that they made quite a variety of different vac advance units for the same few distributors over the years. Some local club members suggested looking for a new unit at swap meets (I always wondered why people were sucking on parts there). I also found out that autozone is still selling rebuilt MG distributors online for a meager $60 (with core return).

Since I wasn't sure how best to proceed, I called my trusty MG shop. They advised me to just leave the vac advance as it is (and to leave the line from the manifold plugged). According to them, and they've been working on MGs for a long time, the vac advance has negligible effect on most MGs. He said it ought to help a bit when cruising in fourth and slightly increase fuel economy; but little else. Mostly, he says that unless your car is pristine and your engine rebuilt recently, you'd never notice the difference. Moreover, he says that almost no one can actually tell the difference in performance between the vac advance working and not.

I must say, I can't tell the difference between how the car is driving now (without a working vacuum advance) and how it drove before (when it was working). When I last had the car into the shop for tuning, the vac advance wasn't working; so they set the timing with this in mind. Also, my fuel economy is still the same. So, I think I'm going to leave it as it is for now.

Thanks again for all your help.

Jeremy
J. Palgon

Not only cruising economy but also running temps - it will be hotter without vacuum advance adding to the total advance as the retarded ignition causes more fuel to be converted to waste heat instead of forward motion. If by 'performance' he means acceleration then he is correct, but simply because under acceleration vacuum advance is reduced to little or nothing. Timing adjustments are *supposed* to be done with the vacuum advance disconnected and plugged at the manifold.

Always best to have it working, if it didn't make any worthwhile difference the factory would never have continued fitting them throughout production, given the penny-pinching masters they had.
Paul Hunt 2

This thread was discussed between 24/10/2005 and 03/11/2005

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.