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MG MGB Technical - Vacuum Advance differences

I diagnosed that my vacuum advance is useless, the diaphragm is gone. So, I've been searching for a new one. I contacted Lucas who told me that the one I have was supplied only to MG so it is most likely original and as a result they didn't have a replacement. So I have bought one from MGOC taking their recommendation.

Spec: 74mgb roadster 25D4 dizzy High Compresssion

Part no of dizzy: 41391
Partno of advance removed: 54420500
Partno of new advance: 54411985

Two questions:

1. Has anyone else had this combination of part numbers and got it set up OK?

2. Will the new advance give a different curve to the original one? If so what should I expect. (I have the curve for 41391 from Paul H's site).

Thanks as ever, Richard
Richard Evans

Richard,

Look on the units - there should be a group of three numbers like 4 12 8 or 3 11 10.

These are vacuum start in " hg, vac finish in " hg and total advance.

If these are the same on both units then don't worry!
Chris Betson

Thanks Chris that is what I call specialist knowledge.

My numbers are slightly different.

Old numbers are 4 12 8
New numbers are 5 12 10

So what does that mean? What effect will it have? Would it explain why the engine is missing at low revs but OK at higher revs or is that something else like the dwell angle?

Cheers Richard
Richard Evans

how do u know if your vacume adance is working properly or not?
jim

Richard,

The difference in the new unit from the old is not material. It simply means that with the new unit the vacuum advance is starting at a slightly higher vacuum, (5" hg instead of 4),,is achieving maximum advance same as the old one, and the maximum advance is 10 degrees rather than 8.

Cheers, Doug
Doug Keene

Jim-
Simply remove the distributor cap and disconnect the vaccum line from it's source on the carburetor or the intake manifold, then suck on the end of the tube. If the advance plates moves back and forth when you apply and release suction, then the vacuum advance unit is OK.
Steve S.

Thanks all, I feel fully informed now. After reading more archive stuff I think my mis-firing on acceleration is either an HT lead or the timing might need to be advance another 2 to 3 degrees. We shall see.

Richard
Richard Evans

You might find that with the higher maximum advance of the new unit you may have to *retard* the ignition a fraction if you find you are getting pinking on light throttle acceleration which goes away when you floor the throttle. Incorrect timing, especially retarded, should not cause an intermittent missfire, although it will cause pinking if sufficiently over-advanced. If the tach also flickers during the missfire it is an ignition LT problem, which for example can be caused by a damaged ground wire inside the distributor or points wire where it exits (they bend back and fore as the vacuum capsule alters the timing). A marginal cap and/or rotor can cause an HT missfire under acceleration as the voltage is higher which may be causing them to break down (BT, DT on two different cars).
Paul Hunt

Richard,

Can also be caused by a weak fuel mixture or lack of oil in the carb dampers.

You will just have to work through the system methodically.

As an aside, I had a car in yesterday that was misbehaving, not starting, then starting, running oddly.

Went through my usual diagnostic routine and found three things missed on the last service (25 miles ago by another garage)

1: a massive air leak on the front carb - wrong gasket had been used,

2: a broken plug insulator (the carbon resistor inside was worn down to 3/4 length by the arcing!

3: the ignition timing was out - went to adjust only to find the dizzy corroded solid into the drive housing which in turn was loose on its screw - managed to get screw out and smash off the housing before refitting with a replacement housing.

Engine now running OK again, but what quality service from the other garage!!!
Chris Betson

Steve S.,Richard,
You also want to see if it will hold the vacuum by holding it for a bit after it moves(stick yer tongue on the opening). Mine would get there (plates would move) initially but wouldn't hold the vacuum and the plates would move back on their own which they shouldn't. My replacement vacuum unit holds it solidly. Course I was using my MityVac and it was much easier to hold vac and see what was happening in the dizzy.
J.T. Bamford

Thanks guys. I tried tuning it again tonight but still mis-firing on acceleration. I tried the timing advanced and retarded from specification and still mis-firing. So as Paul says that's probably not it.

Re-set carbs, after balancing tried a richer mixture still the same. So aving exhausted all timing and carb options I'm starting to look elsewhere. I've done the points, happy with the dwell angle.

So I started on the HT leads, checking for resistance. Pulled the first one off the cap and low an behold I was facing a bare wire, its plug still in the dizzy cap. Cheepo Halfords crap methinks. I'll try to get some decent ones tomorrow.

Cap and rotor are only 500 miles old, as are points and condensor, might try new ones of those just in case, always do as spares for next big service.

One other thought..when i rebuilt the dizzy I put a little grease on the weight spindles, would that cause them to be too slow in spinning out causing the ignition to advance too slowly?

Richard
Richard Evans

Richard. Timing advance will affect performance, but not cause mis-firing unless it is so far off that the car is unlikely to run at all. A quality set of spark plug leads is an excellent place to start. I have good luck with Robert Bosch wires which are made for the MGB. Les
Les Bengtson

I think I've fixed it.

I meticulously rebuilt the dizzy. Removed small amount of grease from weights. Set up points as accurately as possible (14thou freely 16thou some resisitance), used paper to removed oil deposits from face of points - thanks Paul. Fitted new HT leads (could only get Champion today Les). Set carbs to 12 flats down. Checked damper oil levels, yes they were low - thanks Chris. Set the timing with variable dial strobe. It now revs up without the mis-fire. Road test to go tomorrow. Thanks for all your help.


Richard Evans

Richard. Hope it works. If not, let us know and we will keep guessing. But, it sounds like you have covered the most frequent trouble areas. Les
Les Bengtson

Don't forget 12 flats down is only a generic starting point, the ideal for your installation maybe a flat or so either side.
Paul Hunt

I'm still not quite there. The test drive wasn't a success. After much trial and error I have found that if I set the timing at 22 BTDC it is running smoothly again and very little missing on acceleration. What I don't understand is that the timing spec is 11 BTDC (18V, HC, 25D4-41391). This seems quite a difference.

I am using a (new) gunson timing light with an adjustable dial so I can read off the degrees accurately.

The other effect is that the idle speed is quite high around 1400. I have run out of screw on the idle adjust so can't lower it any further, the rear carb more so. The idle speed goes up with the advance setting.

I've set the carbs 12 flats down and about another 1/4 turn each to what seems the right setting, front carb richer, rear carb leaner.

Any more ideas? Thanks again, Richard
Richard Evans

Richard,

Have you gone through the setup of the carb linkages to the throttle shafts, etc. I had the same problem of high idle with my 74 1/2 with HIF-4's, and found that the linkages were totally out of adjustment. Both the Hayes and Bentley manuals have a good description of the procedure. I'm sure you have checked for vacuum leaks, but checking all vacuum connections one more time may be helpful as well.

Cheers, Doug
Doug Keene

Richard,

Sounds like an air leak somwhere to me - the carbs should be the same mixture, not one lean one rich.

Check the manifold and carbs nuts are all tight.

I would be inclined to take them all off and replace the manifold and carb gaskets.

The fact you cant wind the idle down also points to an air leak and over advanced timing.

Put the timing back to 11 degrees at 1000 rpm.
Chris Betson

I've sprayed some carb cleaner around the carb linkages and inlet meanifold but can't detect anything.

The reason I started all of the this work was a racing idle. Then I discovered the Vaccumm was caput so though the air was getting in there. I'll go round the linkages, gaskets again.

One other thing I have done is solder up the poppet valves. I've also checked the seating of the throttle plates and at best I can see a slither of daylight around the edges - would this be enough to let too much air in?

Thanks again, I will crack it. Richard.
Richard Evans

I couldn't slow my idle down enough after rebuilding both SU's. Found that one butterfly wasn't quite centered - corrected this and bingo, problem solved.

Good luck.

BarryQ
73B
B.J. Quartermaine

Barry, It is carb rebuild time tonight. I think there is soemthing to do with the butterflies, either letting air buy or I suspect the seals on the ends of the butterfly spindles are not seated properly. I noticed that if I opened the throttle by hand at the carbs and put a bit of leteral pressure on it then it revved up better, so I'll investigate that more. I did a carb rebuild earlier this year and the carbs do not appear worn (46K miles from new). Thans again, Richard
Richard Evans

Was this 22 with the vacuum connected? Disconnect and plug it for dynamic timing setup. The 4-cylinder engine usually starts pinking just a few degrees more advanced than spec.

Chris - have to disagree with you there. It's true that with an air leak the idle will be higher at a given throttle opening than without, but you should still be able to stall the engine with the idle screws, an air leak only supplies air, the fuel to make it run has to come from the carbs, and whatever the timing. If you can't stall it with the screws, then either the linkages are misaligned as has been said, or one or both of the butterflies aren't seating in the throat (as Richard can see daylight this is almost certainly the cause - letting in mixture not just air), or a defective poppet valve in a butterfly (now soldered, I note), or the throttle is hanging on the cable, or the fast idle screws are way too far in. Neither will worn throttle seals cause a high idle, they cause mixture imbalance across the range.
Paul Hunt

Paul working throught your comments....yes 22 BTDC with advance disconnected! The timing marks on the pulley are aligned inner and outer. I wish I had read the timing before I took the dizzy out, live and learn. I'm confident the linkages are aligned, the throttle actuating lever will only sit in one place, with the throttle plate installed, leaving about 1/4 inch for the throttle adjusting screw to fill.

I've fitted some new throttle plates and they seem to sit better, no daylight now except at the bottom where the fuel outlet is for the bypass idle system. I've ensured the seals on the throttle spindle are well seated. The throttle cable and lever has play in it 0.012. Fast idle screws not touching cold start lever.

I've inspected all sufaces on the inlet manifold, heat shield and spacers - all cleant and flat. Gaskets were new last Jan. Just wondering whther gasket sealer is recommended - will search the archives.

I think I will verify the TDC marks and set the valve clearences tomorrow aswell.

Aim to run it tommorow evening, will report back. Richard (still not giving up).

Richard Evans

I can close this off now, for the record. I concluded that the throttle spindles were too worn so bought some new ones. I am certain that is what caused this problem. Car now ticksover at 900 with spare turns on the idle adjustment screw. Revs freely up through the range. Timing set at 11BTDC which is spec.

Thanks for all your help, the BBBs helps me solve yet another problem. I don't know how I would have solved so many problems so quickly if it wasn't for this board.

Richard
Richard Evans

This thread was discussed between 14/07/2003 and 23/07/2003

MG MGB Technical index

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