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MG MGB Technical - MISSING, high speed

This spring after the winter lay up my 79 MGB with a ZS carb had a high speed miss. some times it started missing at 3300 rpm, other times the miss came in around 4000 rpm.
I have been working on this problem since April.
History first. The car was purchased 6 years ago in CA. The car passed the pollution test. It was driven back to BC Canada with no problems. Now has clocked 116,000 miles.
I then rebuilt the head, 60 thou removed, ported & polished, smog equip removed, flywheel lightened and balanced installed BMC Marina exhaust intake manifold which did away with the convoluted exhaust system, new Europe spec distributor. Compression 160, 165, 155, 155.The engine ran beautifully.
Then a few years later the MISS.
I have replaced, plugs, HT wires, tappets checked, plug gap set, timing at 12 degrees BTDC at 1000 rpm, vacuum advance is 20 degrees at 4000 rpm.
Vacuum is barely into the green at 18 units.Lower than spec.
I have checked for vacuum leaks with a propane torch till I was blue in the face, no change.
I have adjusted the ZS carb between full lean to full rich. No matter what I do the porcelain inside the plug remains white. We drove the B on a 1700 mile trip in June and the milage was 32 mpg. I have an over drive.
The miss used to occur under load, running down the hyw and just cranking it up to 4 to 5000 rpm sitting in the shop.
I have tried the timing lite on all wires but I can't "see the miss" with the light, it just flickers too fast.
I just removed the auto [water] choke from the carb and installed a manual choke. Applying the manual choke does not cure the "MISS".
It has been suggested that I have a tiny leak between cylinders 3 & 4 & very small spray of moisture is shorting out the plugs at speed. I do have low vacuum. Could it have anything to do with the vacuum assist brakes? Is there an easy test for this.
Today, the engine idles down to 500 rpm and runs smooth. It has lots of power until the miss hits. I can pull an 8% hyw grade in O/D and hold 60 mph.
On the hyw I can drop back into 3rd and drive at 4000 rpm without a miss but as soon as I hit the gas pedal it starts to miss and lose power.
50 years ago I had a similar problem with a racing MGA. I could work the engine slowly up 5500 rpm then lose power. We traced the problem to a dissolving seal in one carb that restricted the fuel flow. If I hit the gas the engine missed.
My gut tells me the problem is fuel related. Could it be caused by low vacuum?
Oh! fuel pressure is 5 lbs, free fuel flow is 72 Liters per hour. Fuel filter replaced. I also pulled the ZS carb apart and set the fuel level just slightly higher than spec. No rust in float bowl & fuel needle checked OK.
One last bit of info. Last year the car quit on the road but I was able to limp home. I found the fuel filter plugged with rust as well as the filters and connections in the pump, I replaced the tank over the winter, just prior to the "MISS"
HELP!!!!
Thanks
Morris
M Wadds

Morris,

I'm not too well schooled on the details, but when you say, "timing at 12 degrees BTDC at 1000 rpm, vacuum advance is 20 degrees at 4000 rpm" do you mean your maximum advance is 20 degrees? If so, it sounds a little short of ideal.

Given that you have a ZS carb, you might want to look into John Twist's video library for dealing with that.

Charley
C R Huff

I had a highway/track speed miss for a while that I couldnt eliminate. Eventually traced to a partly blocked breather for the fuel tank, its in the cap on my car. Not unreasonable to need to clear it, its the original and never done before. I soaked the cap in bleach for a few hours. I have to say your low vacumn and white plugs does sound like inlet manifold. However my car can return over 40 mpg Imperial so yuor 32 US is fine. Can you borrow a set of SUs and manifold and see what happens? You are reaching the point where you need to think big.
Stan Best

"the porcelain inside the plug remains white."

'the plug', does that mean just one plug? Or all of them? If one that does sound more like a vacuum leak. If all of them that sounds like the carb. Does pulling the choke make any difference to the running of the engine? Can you make the mixture obviously rich and weak by maladjusting it? The answer should be 'yes' in both cases, if not then it points to the carb.

A miss under load can be caused by ignition problems, marginal components breaking down under the higher voltages that are needed to jump the plug gaps when the cylinder pressures are higher. You don't mention cap or rotor.

I've heard of a case where an over-long screw had been used on the points or condenser and was being hit by the centrifugal weights at certain rpms.

If you suck on the vacuum pipe leading to the distributor can you pull air through? You shouldn't be able to. Can you move the points plate clockwise? You should be able to.

It could be over-advanced, just revving it, you have vacuum advance as well as centrifugal, but if it misses just the same at full throttle under load at those revs then there should only be centrifugal advance in use so less total advance. What happens if you deliberately retard the timing?

Vacuum should go higher than idle just revving the engine slightly, as well as on the overrun. 18, if that's at idle, doesn't seem at all low to me. If that's the maximum you get even on the overrun or closing the throttle having revved the engine then that is very low, but could be the gauge.

Brake servo is easily eliminated, simply disconnect the hose at the inlet manifold and plug the port. But note that if you have the later master with integral servo this will make the brakes significantly heavier to operate.

Mist spray onto the plugs is also easily eliminated by laying a dry cloth over them. But if the miss happens simply revving the engine it is unlikely to be that as cylinder pressure are relatively low then even though the revs might be high, wide throttle low revs are going to generate higher pressures.

PaulH Solihull

Thanks for the responses. To clarify I will start with Charley. The total advance is 32 degrees at 4000 rpm. I charted the advance at various rpms and the results matched the euro spec distributor.
Stan, good idea I will try your idea today. One change at a time these days.
Paul,all plugs are identical in colour, white.
I can apply my new manual choke and it makes no difference to the missing. A great disappointment as I was sure the enrichment would eliminate the miss.
The vacuum advance is working properly under various rpms
I have retarded the advance in increments all the way back to zero advance, the vacuum drops and the engine loses power
Vacuum rises to 23 on over run.I have tuned and retuned for some time and the vacuum is now at it's highest at just 18 at idle. I have even tested my gauge by using several others and they all read about the same.
Stan, I don't understand this sentence.
"Mist spray onto the plugs is also easily eliminated by laying a dry cloth over them. But if the miss happens simply revving the engine it is unlikely to be that as cylinder pressure are relatively low then even though the revs might be high, wide throttle low revs are going to generate higher pressures."
Could you explain the dry cloth , when & how and "wide throttle low revs", I don't have a miss under this situation. I don't have a miss under high revs low throttle but high revs wide throttle
Thanks everybody, today the gas cap, tomorrow I will borrow a distributor cap, coil and rotor and one by one try an exchange
Thanks
Morris
M Wadds

I wasn't thinking too clearly last night. I don't have a vented gas cap. my tank vents thru the charcoal filter. Just for the heck of it I left off the cap and no change.
I may have more than one thing wrong so I am going to first try to concentrate my efforts on the carb. The white plug porcelain has got to mean a very lean mixture.
Toward that end I borrowed 6 zs carbs taken from Canadian cars and was able to pull needles from three of them. I will mic them tomorrow to see if mine is a lean needle.
My needle, which I was able to remove for the first time after practicing on the other three, looks a bit beat up.
I will install the best of the other three & see what happens. I will keep you posted.
Cheers
Morris
M Wadds

You still don't say whether or not pulling the choke makes any difference to the running of the engine. Forget the miss for the moment, pulling the choke fully out, either at idle or running at any speed, should make a very definite difference. If it doesn't then the choke is doing nothing, which points to carb.

A weak needle IME is more likely to cause 'bogging-down' i.e. a failure to accelerate at a certain point rather than a misfire.

The mist spray comment is mine not Stan's. The cloth will at the very least stop any mist spray getting on the plugs, and at worst would be wet after running with the misfire. Mist spray would be worst under the highest cylinder pressures. Therefore relatively low revs but full throttle are more likely to cause it than high revs with a relatively small throttle.
PaulH Solihull

The miss problem has been solved, I hope and pray. I have had four days of driving without the miss returning.
One by one I replaced every ignition component with no success. I then installed a borrowed distributor no change.Timing was carefully checked repeatedly with each distributor. Using the timing light I checked each HT wire at 3000 rpm.A miss would appear every few moments. I then checked the HT lead from the coil and the miss was there. No rhythm, just a random miss
I contacted Bob at Brit-Tek & he was very helpful with ideas and suggestions. Bob supplied my euro spec distributor in 2005. One suggestion was that I do a test by supplying an alternate source of 12V to the coil.
To do this easily, I pulled off the radio suppressor capacitor which was attached to the + side of the coil.
The miss disappeared. I reattached the radio suppressor and the miss came back. I asked a "ham" buddy who has been in radio communications for 50 years & back when communication radios were AM. The techs had to install radio interference suppressors [read capacitors] everywhere to eliminate radio interference. In his experience capacitors did fail. They could also interfere with another capacitor, like in the distributor, fail completely or just start to short to ground by slowly deteriorating.
I still have a lean fuel mixture. Amazingly, I located a local mechanic who rebuilds ZS Stronberg carbs for a Volvo dealership. He told me he could very easily move the jet down to enrich the range of carb adjustment which is an adjustable needle.
To answer Paul's question, pulling the choke on at speed did not help the miss. I could tell the choke was on but no improvement.I am very pleased with the conversion to a manual choke from the water choke. The manual choke is only needed for a short time where the water choke remained on much longer and fouled a plug or two until the car was run for a few Kms.
Thanks to everyone for all the help. Bob at Brit-Tek gave freely of his time to answer my interminable questions.
Thanks
Morris
M Wadds

Thanks for the come back, Morris.

I'll file that radio supressor capacitor info into the back of my mind. It might come in handy some day.

Charley
C R Huff

Morris
Congrats on finding the fault in your car

With you ZS Stromberg there is a trap to fall into.
I got caught out on my Lotus Elan years ago and burnt out a few exhaust valves before I realized what was happening -- Havn't had a single valve problem since

With an SU carb the needle is fitted into the piston till the shoulder of the needle is flush with the bottom of the piston and then secured with the lockscrew from the side - and all is well

With some (most) Strombergs the needle is in a little holder and if you push it right in as far as it will go and secure it with the side screw it will still be too lean ---- This spot hasn't even reached the adjusting section of the needle -- The needle holder has a fine thread on the outside of it and after you push it right in THEN you have to start screwing it further up into the piston to richen the mixture and then secure with the sidescrew
From memory, mind you this is a long time ago I think it was about four turns in to get the mixture correct
The worst thing about it is the fact you have to check the mixture with a gas analiser or by lifting the piston with the engine running method and then pull the pistons out each time to adjust At least though it is a deterent to stop fiddling once you get it sweet
Hope this helps Willy
WilliamRevit

This thread was discussed between 21/10/2010 and 07/11/2010

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