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MG MGB Technical - missing at speed

1979 MB B, U.S. Spec, no air pump but with EGR valve and evaporative loss system intact.

The car runs well but misses out consistently when running at speed. The miss is not noticeable under a load or when decelerating, it just will not run smoothly at 60 - 75 mph without a surging or missing sensation.

I have made various adjustments starting with valves. Valve lash was quite excessive and is now to spec. The car runs without ticking which was an excessive clatter. The valve lash was probably in the .030 range. I found the Stromberg rich and have leaned it out to the base line setting (washer level with piston base). The timing remains very advanced, about 30°. I have tried setting at it at the nominal 10° BTDC but the car suffers noticeable power loss and the miss persists although perhaps in a lower RPM range.

Compression is fine, about 125 PSI per cylinder. Plugs were white, even before leaning out the mixture. New plugs are too white, showing insulator wear and no sign of deposits.

I have worked with 1275 engines extensively but do not have much experience with the 1798. Could this be an EGR valve that is stuck open?

The car has an adapted Pertronix and it might be misfiring. I have used the Pertronix in the Midget without failures but have had inconsistent firing in 1st generation adapted electronic ignitions.

Any suggestions would be appreciated.
Glenn Mallory

I forgot to mention that I replaced the diaphragm in the Stromberg. It was mildly cracked. The carb seems to be in order.
Glenn Mallory

Is the tach steady or flickering when the misfire occurs? Anything to do with the ignition LT - bar the condenser (not present with a Pertronix presumably) - will cause that. If it's steady then the implication is that it's HT or fuel.

One fairly common problem with replacement triggers is that they can alter the rotor phasing, which is the relationship between the rotor and the cap contact at the moment the spark occurs. This varies with vacuum advance, and is why there is an arc of burning along the edge of the rotor contact. Ideally it should be across the middle, with clean bit each end, which would show that the rotor was always adjacent to the cap contact on firing. In practice this doesn't always seem to be the case and the burning can extend right to one edge. In which case there is nothing to say that the rotor isn't moving *further* than that, making the spark gap under the cap larger and larger, and possibly causing misfiring or the cap or rotor to break down under the increased voltage. I've seen an under-cap trigger that was so bad the spark was occurring between cap contacts, and the engine ran very badly indeed. If you have access to an oscilloscope you might be able to see this increase in HT voltage if you apply maximum and minimum vacuum advance.

There is no change in phasing with centrifugal advance.

If there is a fuel delivery problem it would be more noticeable when accelerating hard in any gear, particularly at higher rpms, than steady speed cruising in 4th (or OD 4th) at 60-70 mph.
Paul Hunt

The tach, in fact, is not steady when the misfire occurs. The flickering may or may not coincide as the needle movement is irregular anyway. It has occurred to me that the tach might be the problem but it is not a series tach (like the one on the 71 Midget). It must be picking up the irregular beat at the coil.

The car cannot be timed to 10° BTDC. The distributor reaches its maximum rotation within the base plate and is still at around 20° BTDC. The rotor and cap seem to be fairly new and do not show much. But I think that irregular trigger fire is probably at the root of the problem. The spark is occurring so late that the timing must be advanced to get it within range. Should I replace the Pertronix with a new one? Is there a better system for the late model B?
Glenn Mallory

if the Pertronix is an igniter head on an old dissy then you could have a newish part teamed with an old and worn part

I've read that back in the day the dissy were worn enough to give variances after two or three years from new

I'm a big fan of the 123 fully electronic replacement dissy, I've got one on my Midget after having an Aldon igniter head in an original dissy and the difference is very marked, well worth the money to me

the Distributor Doctor does Pertronics so he may advise - http://www.distributordoctor.com/lucas-distributor-electronic-ignition.html

I don't know if this would be of any help to you - Lucas dissys - http://www.youtube.com/watch?v=3IFluuwJ1S0

as for rotor caps some are so loose fitting that even when clasped you can twist them quite a bit either way



Nigel Atkins

If the EGR valve were the problem, the engine wouldn't idle below 2,000 rpms. I've also encountered the same problem where the Pertronix unit altered where the rotor and cap line up when spark is initiated. On my car, with a new distributor, the alignment was so far off that it blew a hole in my distributor cap! One of the most common reasons that your timing results are so far off is that the harmonic damper, on your crankshaft, delaminates over time and the result is that your timing marks no longer line up. This will give you a false reading when trying to set the timing with a timing light. Bring #1 cylinder to TDC and check where the timing marks are on your damper. If the pointer isn't aligning with O degrees, your damper has sshifted. Also, try disconnecting your vacuum advance unit and plug the hose. Take the car out for a drive and observe the engine's behavior at the speed where you're haveing problems. It might be that the vacuum signal is causing your timing to advance and retard, resulting in a rough running engine. RAY
rjm RAY

If the tach needle is irregular anyway who's to say you are not getting misfiring that you are *not* aware of some of the time?

You should be able to rotate the distributor 360 degrees in the clamp plate, it's just a friction clamp around a smooth cylinder. Or should be smooth, there could be damage to either or both surfaces preventing full rotation. Or other parts of the distributor are fouling something else, in which case turn the distributor the other way and move the plug leads one step round in the appropriate direction.

You should also be able to time the engine back to zero BTDC, if you can't even get it back to 10 then that is definitely the first thing to address. I take it you are disconnecting the vacuum advance when timing? Cars with inlet manifold vacuum like the 79 (and many years earlier for North America) have full vacuum advance at idle.
Paul Hunt

>If the tach needle is irregular anyway who's to say you are not getting misfiring that you are *not* aware of some of the time?

Exactly. I am going to disassemble the distributor, clean everything up and install a new Pertronix. The drive gear may be off a notch too as the engine was recently reassembled. The irregular tach is probably a manifestation of an irregular firing of the coil. It seems like the logical starting point.

I thank Ray for the comments regarding the EGR valve. My Midget (1971) is too old for me to have experienced problems with this device. The newer Midgets had a distance counter and warning light that came on periodically so that the owner would replace the valve. This argues for the device being unreliable or at least short lived so I considered this a potential fault.
Glenn Mallory

Whilst the drive gear being off will result in the rotor pointing at somewhere other than 2 o'clock (or thereabouts) at TDC on No.1 compression stroke you should still be able to get the timing anywhere between 0 degrees TDC and 20 degrees TDC by a combination of positioning the leads in the cap and turning the distributor appropriately.

Where does the rotor point, out of interest?

The EGR valve was a serviceable item, hence the service counter. It wasn't necessarily unreliable, but of course anything could be faulty by now.
Paul Hunt

Paul, I will be dismantling the distributor on Thursday and will let you know where the rotor currently lands at TDC. I have had drive gear positioning issues a couple of times after the 1275 was torn down and you are correct that as long there is clearance for the vacuum advance, it does not really matter where notch in the drive gear lands. In the case of the B, I did not loosen the backing plate and assume that the stops I encountered were related to the range through which the distributor has been moved in the clamp over the years. I will remove the clamp, clean everything up and install a replacement Pertronix.

I have had similar misfiring situations with early generation retrofit electronic ignition units and assume that this may be related to a defective unit.

Regarding the EGR valve, for now I will let the sleeping dog lie.
Glenn Mallory

If nothing else, disconnect and plug the vacuum line going to the EGR valve. These valve tends to get plugged up with carbon, over the years, and eventually gets hung up in the open position where it will allow exhaust gasses to enter the intake manifold all of the time. This is why it was considered a "service item". RAY
rjm RAY

Glenn,
just re(found) a JT vid that might be of use to you - http://www.youtube.com/watch?v=dZFCRf_2c1E
Nigel Atkins

I installed a replacement Pertronix yesterday. The backing plate had the module in a different position and I suspect that the Pertronix in place was not specific or correct for the distributor.

I cleaned and lubricated the weights and checked the harmonic balancer. The timing was spot on at 0° with piston n° 1 at TDC.

The new Pertronix cured the problem completely. There is no miss, the tach is stable and the car runs smoothly and with plenty of power with the timing set at 10° BTDC (vacuum disconnected). The Stromberg is set with mixture at the baseline setting (much leaner than before, valves are properly adjusted and new plugs properly gapped. The EGR, as much as I worry that it may be stuck, is going to be left alone for now.

So... thanks to all who contributed, especially to Paul for the insightful comments.
Glenn Mallory

Had the same problem years ago and it turned out to be the EGR valve was no good. Eliminated it and the car ran good

Cheers

Gary

79 MGB
gary hansen

This thread was discussed between 17/08/2013 and 09/09/2013

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