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MG MGA - Vacuum Advance

Problem with MGA 1500 Vacuum Advance:
1. Connected vacuum gauge and a spare distributor to vacuum pipe coming from rear carburetter.
2. The vacuum pipe is a piece of 1/8 OD tubing coming directly from vacuum port (no isolator/oil block thingy). Can blow compressed air through it, nicely.
3. Revving engine from idle to over 3000 r/min causes vacuum gauge to rise to maybe 3 in.Hg, but no movement from spare distributor.
4. SAME set-up and test on an MGB engine results in about 15 in.Hg and approx correct rotation of distributor plate.
5. Squirting oil on vacuum connections, throttle spindles, etc. makes no difference.
6. Replaced throttle spindle on rear carb (original was a bit loose), but no improvement. (Yes, I will do the front one, next, but the oil squirt made no difference there, either.)
Does anyone have any ideas? I can't see why the MGA should be appreciably different from the MGB. The A has poor acceleration, which is why I'm trying to track this down.
Thanks!
Fraser Cooper

It sounds like the diaphram in the advance mechanism is shot. It's the metal bell shaped unit that the vacuum line connects to. I believe the suppliers have new assemblies.
John

Fraser. What do you mean by item 2? Does this mean when the hose is attached to the carb you can blow through it? Or, does this mean that when you attach the hose to the dizzy you can blow through it?

The best method to test a vacuum advance for function is by the use of a hand operated vacuum pump such as the MightyVac. This allows you to test off the car or on the car with the engine turned off. If the points plate moves with vacuum applied, and the vacuum advance diaphram continues to hold the vacuum until the vacuum source is disconnected, the vacuum advance can is working properly. If not, replace or rebuild it. (I believe there is a fellow on the MGB board who recently posted that he can rebuild the cans.)

The comment about only 3" of vacuum concerns me. Most vacuum advance cans operate at above this pressure and if you are only pulling 3" from your carb vacuum source it would be insufficient to operate the vacuum advance system. Thus, no movement of the points plate would be the correct indication with only 3" of actual vacuum and the problem with the carb would have to be corrected before the problem (if there is one) with the distributor addressed.

Les
Les Bengtson

Or are you saying that your engine only produces 3" of vacuum at the carb? Or do you mean that when you apply vacuum to the vac advance that you can't get it to go over 3"s?

Theoretically the only difference between the A and the B is age. The diaphragm in the A could be deteriorated and leaks. Might be time for a rebuild or replacement.

Jeff Schlemmer

John, Les, Jeff: The vacuum advance tests ok with hand vacuum pump. If I pull the hose off at the carb end, I can pump up (down!!) to over 20 in and get lots of advance on either the fitted distributor or the test one. I can blow compressed air through the vacuum line into the carb throat.
The test (spare) distributor has only its vacuum line connected to carb. On the MGB, the advance works and I get about 15 in when engine is revved. Same test set up on the A gives no advance and only about 3 in. Seems to be a vacuum leak somewhere, but I can't find it. Replaced the connections to vacuum port. Can suck over 20 in of vacuum from the hose attached to carb, with the tiny port in the throat covered by tape and there's a very slow leak-down (takes about 30-40 seconds to lose vacuum). Both vacuum units are stamped 5-13-10 which means advance starts at 5 in and goes to 10 degrees at 13 in. It does this on the B but not on the MGA.
More ideas, please! Thanks!
Fraser Cooper

Can you hook up a vacuum gauge to the intake manifold and test for vacuum there? How about reving the throttle while you are testing the vacuum at the carbs? If you're using "ported" vacuum, there will be little or no vacuum available at idle, while the B vacuum probably comes off the manifold and has vacuum all the time.


.
Jeff Schlemmer

I just re-read Les' answer and also his answer to the other question on advance mapping:
Les, I thought it might be a vacuum leak around the throttle shaft. The throttle shaft did wobble a bit, but not the 10 degrees of wobble mentioned in Moss catalogue. However, I replaced the shaft, which had definite signs of wear, with an apparently good shaft (no measureable or visible wear). Added oil-soaked felt washers on the shaft at each side of the carb body.
Same frustrating result. I will re-shaft the front carb, but I don't see that a leak at the front would have such a devastating effect at the rear. At least, the problem should have improved slightly; but it didn't. And squirting oil on all possible leak points makes no difference.
Fraser Cooper

Jeff:
The vacuum port on the MGA's rear carb comes off the bottom at about 7 o'clock while on the MGB it comes off at about 12 o'clock. Both ports (on the MGA's H4 and the MGB's HS4) are at the point where the throttle plate touches the throat wall when closed.
I could drill, tap and fit a vacuum fitting on the manifold, but that would give me about 20 in at idle which would drop towards zero when throttle is blipped. That would be the reverse of what I need here, wouldn't it?
Thanks!
Fraser Cooper

Manifold vacuum would give you the opposite of what you want, but when are you testing for vacuum? At idle you should have no vacuum, or very little. Only the small amount of air passing the throttle plate will develop vacuum - so there will be almost none. As the throttle opens your vacuum should increase. Does that happen?

Jeff Schlemmer


Sorry Jeff I'm not in agreement with you here.

At tickover there is a high vacuum as the pistons are sucking on nearly closed throttles. Then with the more throttle you apply, the lower the vaccum. When you take your foot off the throttle at high revs or on the overrun, then you will also get a high vacuum.
Hope this makes more sense Fraser...

Pete
Pete Tipping

Jeff, Pete:
thanks for your input. It seems to depend upon where the vacuum is measured. In the manifold, at idle, the vacuum is high, about 20 in Hg. This drops to near zero when engine is revved. The vacuum port on the carb seems to produce zero vacuum at idle, but rises to 15 in Hg or more when engine is revved. At leat, the above is true on the properly-operating MGB engine. On the MGA, there's zero vacuum at the carb port at idle but it won't go over 5 in Hg even when revved to well over 3000 r/min. Can't measure in the manifold, yet.
Hence, my problem. I noticed that I can get more vacuum in the A (maybe as high as 8 in Hg) if I stick my hand over the throat when revving. This gets me a tiny movement from the test distributor. But, of course, that kills the engine quickly. I really must add a vacuum pick-off to the MGA's manifold as on the MGB so I can get an idea if the engine is producing enough vacuum at idle, as suggested by Jeff on the 13th.
The MGA engine was rebuilt about 4000 km ago and works just fine except for poor acceleration until over 3000 r/min (when the centrifugal advance probably kicks in).
The only big difference (other than 300 cc) between my MGB and MGA engines is that the B has a stock camshaft; the A has a Piper BP-270K (292 degrees)
Fraser Cooper

Pete, Fraser seems to understand what I meant. I know the engine develops high vacuum at idle, but he is attaching his vacuum line to ported vacuum. That ported vacuum opening should have limited or no vacuum until the throttle is open, due to its location. It sounds like the port is partially blocked, or there are airflow issues that need to be worked out.

The Piper cam will lower the amount of vacuum the engine produces due to its relatively high duration. I would still expect it to produce around 15"s though. Did you do anything to alter the ignition curve, since you entirely changed your valve timing?

Can you install a vacuum gauge so that you can watch it while you go for a drive?
Jeff Schlemmer

Fraser.

It seems we have two problems here. First is that we are not correctly defining the problem and second is we have a lot of data, but, only a little information.

The information we have is:

Car accelerates poorly below about 3K rpm

The vacuum advance can seems to work correctly

The rear carb is only showing about 3" HG when the throttle is opened.

Restatement of the Problem: The vacuum advance port on the rear carb is producing insufficient vacuum to operate a functional vacuum advance system connected to a piece of vacuum line which is making a correct seal.

Therefore: The problem lines not in the vacuum advance, or the connecting hose, but in either the carb vacuum take off port or in the carb system.

Discussion: On a properly set up system, the rear carb ported vacuum take off will show about 3" Hg at under 1K rpm as most of the hole to the take off nipple is covered. As the engine rpm increases, the hole is more exposed and will show 10"-15" Hg vacuum present. (Tests done this week on a Weber DGV and should be considered illustrative of a properly running system, but not definitive. My only SU equipped car is in the shop having new floors installed.)

This is not happening on the carb under discussion. Causes of low vacuum can be a plugged opening/nipple, a too large opening in the carb, or, an air leak in the carb system, particularly at the rear carb.

An air leak in the system would also have the added effect of causing the vehicle to accelerate poorly and not run well below about 3K rpm. A plugged hole/nipple or too large a hole would not have this effect because the vacuum advance system is a sealed system (unless the diaphram is torn or cracked).

Defined in this manner, the focus shifts from the distributors, which seem to be working properly, to the source of the vacuum--the engine/intake manifold/carbs and their associated gaskets and spacers.

Probably time to use some carb cleaner around the various mating surfaces to see how it affects the idle of the engine.

An aside. Yes, your cam will affect how much vacuum is produced, especially at idle. However, at higher engine rpms there should be more than sufficient vacuum available to operate the vacuum advance mechanism. Expect to see your vacuum gauge read low at idle and to pick up rapidly as the engine rpms increase and the system becomes more efficient.

Les
Les Bengtson

Won't intial base mechanical timing effect vacuum at idle too? That can be rules out as a source be setting it to specs with vacuum line blocked on the carb side.
brucep

There's something not right here Jeff. The vacuum take off on the MGA rear carbie is well after the butterfly so must pull the same vacuum as in the manifold.

When the engine is stationary, (no vacuum) the distributor will be fully retarded.

When accelarating, the vac timing needs to be retarded by the low vacuum occurring at this time.

When cruising with reduced throttle, (higher vacuum) the timing needs to advance for economy, in this situation the higher vacuum will advance the ignition.

I would assume poor vacuum is leakage in the vacuum pipe, at carbie gaskets or worn carburettors. If you have original tired out old olives on the end of the vacuum pipe these can still be loose and leaking even when the nuts are tight.

Anybody agree?

Pete
Pete Tipping

Sorry Pete, but no. The vacuum take off on the MGA is a small hole in the bottom of the throat immediately in front of the throttle plate when it is nearly closed (idle position). As such, it is exposed to atmospheric pressure and will have little or no measurable vacuum at idle.

As soon as the throttle starts to open the plate edge moves ahead of the vacuum port. The vacuum signal then goes high when the port is exposed to manifold vacuum, so you get almost instant vacuum advance as soon as you poke the throttle. This give nice throttle response with very low engine speed. The only thing limiting quickness of the advance is flow restriction in the small bore vacuum signal pipe.

As the throttle opens farther manifold vacuum declines. As a result, if you lug the car at relatively low speed with full throttle it will have less than the maximum vacuum advance, which is good to prevent ignition pinging when lugging.

As engine speed increases and air flow velocity through the throat increases (before the constant depression area), the vacuum will go higher again due to the venturi effect caused by air passing perpendicular to the signal port. Vacuum advance for the distributor will be at maximum whenever the vacuum signal matches or exceeds the high end number for the vacuum diaphragm unit.
Barney Gaylord

Barney

Thank you very much for that very clear description of the subtleties going on in our SU rear carb. I totally appreciate the venturi effect at the take-off position you describe and explains the reasons for the vacuum readings that others were saying above.
I have never checked out just where that little hole comes through into the throttle body. Next time its in bits...
So - apologies everybody - we learn something new every day!
Goes away abusing himself - muttering things about venturis...

Pete
Pete Tipping

Vacuum advance shouldn't effect power anyway. By 3000 rpm a stock MGA's mechanical advance is "all in" or fully advanced.

I can think of four things that would cause your issues:

Distributor mechanical advance worn or spring/parts broken- timing wrong and/or wandering until full advance. If your B is running well, you may want to pull it's distributor and coil and put them in the A. Plug the vacuum line. Only caveat is if your B is a rubber bumpered chicken choker- they have too much high rpm advance so don't take your A autocrossing.

A constant vacuum leak- very lean at low rpm/load, has less effect at high rpm since leak is smaller percentage of total flow. Put a wrench to all the intake manifold nuts and bolts including any linkage mounts, etc. Some new studs are not strong enough and may have stretched and/or broken. Mounting holes may be drilled through and leak if they aren't sealed. No gorilla action- you are looking for grossly loose, not slightly undertorqued.

Valve timing off or an exhaust on 3/4 not closing. This would also give the motor a slight "chuffle wheese" at low rpm. Pull the valve cover, check clearances, and verify that the center of overlap (both valve open the same amount) happens at top dead center.

Fault in the "constant depression" system. Either the dashpot is not coming all the way down or the spring is gone/missing. Again, less effect at high rpm since the dashpot is all the way up anyway. If this were the problem, the car should run OK full throttle at all rpms but have trouble at low speed, low load.

Other than that, I'd do a full tune-up including a compression check and timing up to 3k with an adjustable timing light. I chased an ignition/carb problem for nearly a year only to discover the breather had contaminated the front air filter, killing air flow.
Bill Eastman

Hello, I'm back.
Len, you've re-stated the problem exactly. I'm in the process of putting in a good throttle spindle in front carb; adding a vacuum port to the manifold so I can read vacuum there.
Barney, I was considering e-mailing you with the problem--thanks for your input. The carb vacuum port is, indeed, at the throttle plate and is mostly covered by the edge of the plate when the throttle's closed. So, minimum carb port vacuum at idle.
Bill E., the acceleration above 3000 r/min is fine, so, as you say, the mechanical advance has taken over. It's the lack of response below 3000 that I'm trying to track down.
I've already put a new distributor on the 'A' with no improvement, so I agree that the problem's at the carb end.
I'll continue this when I've got more data to share including manifold vacuum. I'll also re-check rocker clearances, ignition timing, etc.
Thanks for all the help, so far.
Fraser
Fraser Cooper

Why not follow the MG factory modification for the mga twin cam and throw the vacuum advance into the trash can?
I never had any trouble driving a Twin cam in heavy traffic and pulling away from idle without vacuum advance.
(Just trying to draw some flak).

Mick
Mick Anderson

Fraser While I believe the vacuum advance is a good thing to have on a road car (it can improve throttle response a tiny bit, will improve the economy, helps keep the plugs clean and temperature under control)it will make little or no difference to the acceleration, top speed, or feel of the car as far as power is concerned and your performance problem it elsewhere. What Jeff said about the throttle plate is correct, at idle the plate almost covers the port, at cruise or fast idle the plate acts as a venturi with the air squeezing past and drops the pressure in the vac port, but when the throttle plate is wide open the venturi effect it had over the port is lost and the vac port pressure returns to the overall pressure in the carby throat. Being a constant vacuum carby this wont alter above part throttle. At no stage does vac "take over" from mechanical advance, vacuum works under load conditions and mechanical at all times up to Max Adv, usually about 3-3.5k rpm Cars that are raced or flogged all the time are better off without a vac advance but a road car is improved with a good vac adv.
Denis
DENIS H

Mick - for some people your method is the way to go. For anyone who doesn't believe that the vacuum advance actually improves performance and throttle response, disconnect yours and see how she runs!

Fraser, which distributor do you have in the car? Presumebly its not a 40510? A good rebuild/recurve can bring throttle response that the car never had when it was new!
Jeff Schlemmer

This thread was discussed between 12/04/2006 and 23/04/2006

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