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MG MGB Technical - unusual miss etc

I have been reading through the archives looking for some answers to some annoying little problems that have recently surfaced. My car is a 1974 MGB GT chrome bumper with LC engine. The car has started missing recently but only occasionally, especially when cold (car not weather). It feels like it cuts out and then catches again, this is under no load throttle. In my roaming in the archives a couple of items caught my attention
- the vacuum advance should be connected to something but in this car it has never been connected in my ownership (10 years) and in searching around I can't find anywhere what it would connect to on the carbs or the manifold?????
- the antirun-on valve clicks when the ignition is turned off so I assume it is working but recently on a particularly hot day it was heard to click on and off as I was driving on the highway?

The car generally runs well with good power and good fuel mileage. I would like to get the miss sorted and feel that it is ignition related but nothing is obvious at this time.

I have not changed anything out yet.

Brian
Brian Smith

A North American 74 should have the vacuum pipe from the distributor connected to a nipple on the inlet manifold. If this pipe is not connected and the nipple is left open it will cause running problems.

If the pipe is not connected but the nipple not present or is sealed, the car should run OK but you won't be getting the best mpg when cruising.

On manifold vacuum cars the timing *must* be set with the pipe disconnected and the source (manifold nipple or carb nipple on earlier cars) *must* be sealed.

Generally speaking if the cutting-out is ignition *LT* related the tach will be dropping to zero at the same time. So if this isn't happening it is either ignition HT or fuel related.

If the anti-runon is momentarily operating when the ignition is on i.e. during normal running, then if the emissions and anti-runon plumbing is all present and correct it *will* cause momentary cutting-out as that is its job - to stop combustion.

The valve normally operates when the ignition is turned off and the engine is running, when the ignition switch puts out 12v on the slate/grey, via an in-line fuse and a slate/red to one side of the valve. Oil pressure from the running engine closes the contact in the oil pressure switch (i.e. it operates the reverse way to a normal oil pressure warning light switch) which puts out a ground to the other side of the valve, which operates. The valve causes manifold vacuum to be applied to the carb overflow vents, which sucks the fuel out of the jets, instantly stopping the engine from running-on (or from normal running, in fact). A few seconds after the engine stops, oil pressure dies away, the pressure switch opens, releasing the valve.

The next time the engine is started there is no 12v from the ignition switch, so even when the engine starts, oil pressure builds, and the pressure switch closes, the valve cannot operate.

As there *should* be a ground on one side of the valve when the engine is running, for it to be operating when it shouldn't 12v must be being applied to the slate or slate/red, either by a short to some other wire carrying 12v or from a faulty ignition switch contact.
Paul Hunt

Thanks Paul, I was hoping that you would respond. I have read your posts in the past and always get good information. After further investigation in the archives I saw a posting where someone included a picture of how the distributor was plumbed and checking on my car the nipple has been crimped and sealed. I should have indicated that the polution equipment was removed sometime in the past.

I will try to get the vacuum advance set up properly and see what that does.

The antirun-on valve normally only clicks when the car shuts down but it was clicking on and off with the extremelly hot weather, the only thing is that all of the plumbing appears correct except the small outlet at the bottom of the valve. It is not hooked up to anything....should this be connected somewhere?

During the periods of cutout the tach does not appear to zero. I had wondered if it might be the HT circuit.

Brian
Brian Smith

You may want to replace the distributor rotor, the replacements are known to fail thru the plastic to the shaft of the dizzy. Also depending on your weather you may be getting some condensation inside the capwhen cold.
J Heisenfeldt

On the valve I have (not on car, just in a drawer) there is quite a large port on the bottom and that is supposed to be open to atmosphere as it is the fresh air inlet for purging the canister and the crankcase. There are two smaller ports, one on either side of the body above the actual valve flap, that are connected to the canister and inlet manifold. It depends what pollution equipment has been removed, but if any of the plumbing associated with the valve or canister has been removed or sealed off the valve won't be functioning anyway so can be discounted as a cause of cutting-out.

On rotors it seems one type of construction causes problems - the type that has the round-headed rivet securing the brass contact to the body of the rotor, where the rivet is very close to the hole in the body i.e. very close to the distributor shaft when fitted, like the one on the left in this image. This one actually has a set of magnets in the base for a Hall-effect trigger and so is deeper than normal, but some standard rotors are manufactured with the rivet like this and should be avoided.

Paul Hunt

Paul

Thanks for the info. I spent some time under the hood and noticed the plug wires were routed around the heater valve (by me!!!) and it looks like one of them might have chaffed a little. I re-routed the wires and the miss appears to have gone. I will still replace the plug wires but ensure that they have a clear run. The rotor I have is the old style without the rivet as shown.

I have recently been called out of retirement and have taken a contract in a town about 30 miles away and I use the MGB GT daily for the run down the highway. It is running better all the time and seems to be thriving on the daily commute.

Brian
Brian Smith

They do run, handle and feel better the more they are used, Roger Parker said as much last month when comparing various types of suspension systems on various cars. My V8 has had an annoying tick for some time since I stopped commuting in it. I have found before that a longer run (touring Cornwall) stopped it, and over the last couple of months I've done an 800-miler and several 200-milers and it's lovely having it so quiet. It'll be interesting to see how long it takes to start ticking again now it is back to shorter journeys.
Paul Hunt

Hi Brian:

Sounds as though you likely solved the 'missing' issue.
Regarding the vacuum advance, I think it might be worthwhile plumbing that correctly. I know that my MGB (1973) idles far better with the added advance that the vacuum capsule provides, and I was able to turn down the idle screws quite a bit compared to not having it connected. Also, as Paul says, there are further mileage advantages when cruising (IE light-ish throttle openings/low engine load situations which create higher intake vacuum therefore more advance). The vacuum capsule provides extra advance when the engine is under light load and under minimal throttle openings. This improves the efficiency and also reduces emissions. When you step on the gas, the intake vacuum is reduced, therefore the vacuum capsule reduces the advance that it provides, eliminating the pinking that would happen had the advance remained as it were for light loads/throttle openings. It can be connected to the carbs (as it was in earlier cars - called ported vacuum advance) but I feel that the manifold vacuum advance is better (gives you advance at idle, not just when cruising).

FWIW,

Erick
Erick Vesterback

The advance at idle increases the idle rpm for a given throttle opening, so the idle screws can be reduced, which reduces emissions for a given idle speed. This was the reason for changing from carb to manifold vacuum and nothing else. Manifold vacuum can make starting a bit harder as it applies some vacuum advance during cranking, unlike carb vacuum, unless you open the throttle as well, but MGBs don't normally need throttle when the choke is open for cold starts at least. Manifold vacuum also results in the vacuum capsule being exercised from max to min and back again more often, which may contribute to earlier failure of the diaphragm than for carb vacuum cars. You have to say that if manifold vacuum was better other than for emissions reasons, all cars would have had it anyway. The fact they didn't, until emissions demands came to the fore, says to me it is somewhat less desirable (emissions aside) than carb vacuum. But I reckon the differences (again emissions aside) are marginal.
Paul Hunt

After reporting that all was well, on my drive to work yesterday it started to act up again. So, as I am not working today, I pulled the distributor and found that it was a 45d not the 25d it was supposed to come with. That would explain why dealers try and sell me different points and cap. Anyway, after removing the distributor and opening it up, the points had closed up. I have now reset them and also checked the vacuum unit and it appears to be healthy (holds vacuum and plate moves) so now I have to get some tubing and install a new nipple on the manifold and the vacuum advance should work as well.

I can report that it starts and idles better already without doing anything to the vacuum advance. The vacuum canister has the numbers 4 - 18 - 12.

Brian
Brian Smith

Brian,

I was having a problem with my points requiring adjustment too frequently. As much as I hate to admit it, others may learn from my oversight. I wasn’t paying attention to which way the distributor was turning and I was putting the grease on the wrong side of the rubbing block.

Charley
C R Huff

Charley (and others)

I have to admit that this car was my everyday driver for almost 10 years but when I retired I stripped it to bare metal and rebuilt it totally (almost). Lots of new metal, changed the running gear to wire wheels, painted it red, installed and OD transmission, new interior etc. but the engine had always been a strong runner so all I did was install some new gaskets and then reinstall in the car.

I finished it this Spring after 3 years and it fired right up and ran great so I have basically just run it until it started having a few minor problems. It was at this point that I remembered that I hadn't really done anything other than change the oil....it was one of those things that I was going to do....one day.

You can see what I did on my web site at:

http://www.island.net/~btsmith/

Just follow the link to the MGB GT.

This afternoon I hooked up the Vacuum advance mechanism and the only thing noticed so far is a faster idle that I set back down to just under 1000. The test will be driving it this weekend.

Brian
Brian Smith

Oh yeah, Brian, that's pretty. All three of them for that matter.

Charley
C R Huff

There *should* be a felt block as part of the points for 45D distributors, which should be greased. For 25Ds the grease should be applied to the cam - sparingly.
Paul Hunt

Paul

I have the cap from a 45D but the points are similar to the old 25D so I greased the cam a little. It is really a testament to these cars that they keep on running as well as they do even when things under the hood are marginal at best.

The fellow I bought the car from many years ago had owned it since 1976 and gave me boxes of old parts that he had replaced as they failed and also gave a complete history of the car in his ownership.....funy though, he never mentioned the distributor or the changes inside. I never paid too much attention to the model number but had certainly had parts sold to me in the past that didn't appear the same as those I took off. Always good to check out the details.

Once again, thanks to all for the wisdom and help given! Great cars but even better people!!!!

Brian
Brian Smith

Just to update, I reset the points and drove the car....al was well with no miss at all. I then connected up the vacuum advance that had been removed sometime prior to my ownership. Had to make a fitting for the intake manifold but no big deal. I have now had it out and about for a few days and once I adjusted the idle speed down the car actually seems to run "stronger". I don't think that it is imagination on my part but the car actually seems to have more power from the takeoff and is smoother over the rev range. Back to work tomorrow so it will get a good long run at highway speeds so that should be the final test.

Thanks to all who have contributed, the car is running a whole lot better.

Brian
Brian Smith

Vacuum advance does contribute to sharper part-throttle acceleration as well as cruising economy. When fiddling with vacuum capsules I changed a low advance to a higher, and my wife noticed an improvement even with that, and even when I was driving.
Paul Hunt

Paul
I have just obtained a 1970 MGB GT that has been off the road for 8 years and I am going through the motions to wake up the engine. I have systematically replaced the ignition service bits and checked the SU's for fuel flow etc. I have managed to get it sort of get it running but very weakly and sounding like only on two cylinders-very poorly and lumpy.
I am intending to reset the timing etc but have noticed that although it has a vacuum advmce there is no pipe connected and that despite loosening the clamp the dissy seems stuck fast. Can you shed any light? It has a replacement engine (no:48G 755)
Tim
Tim P

With no pipe between the distributor and rear carb unless the carb port is sealed you will be getting a weak mixture off-idle.

It's not uncommon to have a distributor stuck in the block if it hasn't been run or touched for a long time. Remove the two bolts securing the clamp plate to the block and slacken the clamp-plate bolt, then it is a matter of using progressively more force until it does shift, like a drift and hammer on the body of the distributor so as to try and twist it in the block. Dribbling a releasing fluid such as Plus Gas above where the distributor goes into the block in the hope that some will find its way down may help. Also try doing it when the engine is hot as well as cold to make use of the different expansion rates of cast iron and alloy.

You need to get the ignition right before tackling the carbs, then set those up from first principles for both air-flow and mixture balance.

48G 755 is the Gold Seal replacement for an 18GD high compression engine, which would be correct for a UK 1970 car. Originally this has a static timing of 10 degrees BTDC and strobe of 14 at 600 rpm, although that can only be a guide with today's fuels, especially with 95 octane you may need to retard it from that to prevent pinking.
Paul Hunt

This thread was discussed between 11/08/2008 and 29/08/2008

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