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MG MGB Technical - No spark #3/4

I have a 5 main 1800 with Petronix and Weber. While checking for the cause of powerloss, I was not able to detect a spark on cylinder 3 & 4 plug wires. Disconnecting both wires single or together from the plugs didn't make any difference. I have never heard of an engine running only on 1 & 2, but this one did (poorly). I searched the archives and found info on other dead cylinders, but not this combo. Any ideas? Thanks
Glenn Johnson (Georgia)

Glenn. My idea is that you should trouble shoot the problem. Start by seeing if you have spark at the end of the wire. If so, it may be a plug problem. If no sparks at the end of the wires, check the connections in the distributor cap. If the connections are good, inspect the inside of the distributor cap for broken contacts and grounding at the terminals for cylinders three and four. Do you have a points type ignition system? If so, are the points opening on those lobes of the points cam?

There is information on how to check out the ignition system and the spark plug wires on my website, www.custompistols.com/ which might provide some guidance. But right now, the only information we have is that you seem to have removed the wires from the spark plugs, not noticed any difference in the operation of your engine, and have assumed that you are not getting a spark to those two cylinders. Not a valid assumption based on the information posted. Could also be a fuel problem (they share a common port), broken valve, or several other possible causes. You need to start your trouble shooting by checking for actual spark at the ends of the spark plug wires, then deciding whether you need to investigate the ignition system or the cylinder head/valve/fuel area. And, a compression check is always a good idea in the case of a dead cylinder.

Post again when you have been better able to define the problem and we can provide some focused guidance. Right now, that is impossible.

Les
Les Bengtson

Aside from Les' excellent website, and suggestion to confirm spark at the ends of the wires, go ahead and switch the plug wires from the distributor to the spark plugs (i.e. disconnect 1 and 2 and replace at both the distribuor and spark plugs with 3 and 4) and see if the problem stays with the wires. It isn't uncommon for spark plug wires to fail, as the conventional ones on the B are fiberglass core impregnated with latex graphite (for radio and EMI supression), and not as long lasting as the copper strand type found on early cars. Conventional wires tend to age with temperature and fail. The original resistance of this type of wire is 3000 to 12000 ohms per foot of length, but what you are looking for is that each wire has about the same resistance. Finally, the metal clips in the wires can loose contact with the internal strands. Cutting off a 1/4 inch and reseating the clips can solve problems.

warmly,
dave
Dave Braun

A timing light is best to diagnose this. If clipped onto the coil lead it shows irregular flashes then it is probably the points, posibly too small a gap not opening on two lobes. If it shows regular and consistent flashes i.e. for all four cylinders, but it only shows flashes on plug leads 1 and 2, then almost certainly the cap is breaking down, but could be the two leads. If it flashes on all four leads but only two plugs, then it is the plugs.

As far as leads gp the original carbon string type were increasing their resistance almost from day one and eventually caused weak sparking, but I've been using silicone-cored for over 40 years and never had one fail, the ones on my V8 have been on 15 years and 80k miles. If this suddenly happened then it is unlikely to be two leads suddenly failing together. It could also be that the rotor isn't in the correct position when the spark is produced (points phasing) and is marginal on two cylinders but not working at all on the other two. This can happen with after-market electronic triggers.
P Hunt

Paul. I have seen one example of a Lucas 25D4 distributor which has both a bent distributor shaft and a very poorly made points cam. (Normal, in my investigations, is for the height of the cam lobes to very between .001" and .002" from the lowest lobe. All of which affects exact ignition timing by some small amount.) With that combination you had drastic variations in timing, and whether the points would open at all, based on which lobe of the cam the points were set on. But, in the case under discussion I would like the owner to do a little more research before getting into an depth analysis.

But, I agree with you, the timing light, if he has one, will make it much easier to do the initial definition of the problem.

Les
Les Bengtson

I've never had a pertronix so I don't know anything about them, but Glenn says he has one. I think that means the points and lobe issues aren't relavent doesn't it? Maybe noting that it is Pertronix would bring in some more ideas?

After checking the ignition issues, maybe run a compression test?

Charley
C R Huff

Thanks for all the ideas. I should have added that I had tried my spark detector which fits between the plug wire and plug. It showed spark on 1&2,none on 3&4. Yesterday my timing light indicated same. I switched out dist cap,no change, different wires, no change different coil,no change. plugs on 3/4 looked blackened otherwise normal,cleaned all of them and reversed locations with 1/2. From here, I plan to swap in a points type dist, then a compression check. If I remove the valve cover Could I tell by observing the rockers if I have a valve problem? Les...Great articles! Glenn
Glenn Johnson (Georgia)

Glenn. Good job so far. Compression check is definitely in order. That will help check out the valves to see if they are leaking. Removing the valve cover will tell you whether any problem discovered by the compression check are related to the movement of the valves (e.g. flattened cam lobe does not allow the intake valve to move sufficiently to move air and fuel into the cylinder). But, while a compression check is a basic part of any trouble shooting process when looking at a dead cylinder, your trouble shooting indicates that the first, known problem is the failure of the distributor to fire on two cylinders.

I have heard of Petronix failing completely, but have never heard of a partial failure of the system. Thus, a physical inspection of the internals of the distributor might be in order. Since you will have to remove the distributor to install another one, take the time to examine the existing distributor and see if you can find a damaged part or inconsistent movement (in relationship to the ring movement for cylinders 1 and 2), excessive shaft wobble, or anything else which seems out of place. That might give you a clue as to what is happening.

Glad you found the website of value.

Les
Les Bengtson

While spinning engine with starter, rockers looked fine. Went back to dist, as I felt that's where my problem was. I removed petronics stuff and replaced with points and condenser. wouldn't fire at all. I switched the distributor for the one out of my midget project, Static set timing, fired it right up. Dynamic timing smoothed it out! Appears that a dist. rebuild is in order. I still don't see why the failure was two cylinders, but it's OK now. I really appreciate the advice! Glenn
Glenn Johnson (Georgia)

The Pertronix units often don't alighn properly with the points base plate. This will cause the spark to fire when the cap and rotor terminals are not adjacent to one another. The end result is a burned out cap and poor running engine. RAY
rjm RAY

Glenn. Just for closure, put the dizzy cap from your Pertronix dizzy on the new one. Then see if the same problem arises. Just wondering if it's a bad dizzy cap and not the entire set up.

cheers

Gary
79 mgb
gary hansen

Glenn It could be a crook distributor cap. Nos3 and 4 are next to each other in the cap and a cracked cap can cause cross firing cutting out both plugs, or even tracking to earth, usually to a cap clip. Denis
Denis4

OK, so Glenn sees no flashing on a timing light on 3 and 4, with two caps. But what about flashes on the *coil* lead, as asked before? If the flashes are missing there too then the problem is the points not opening for those cylinders, i.e. distributor shaft and/or dwell too high i.e. gap too small. If all four flashes occur on the coil lead then either two caps are bad, or the phasing is so far out that when the spark occurs on 3 and 4 it is going to the wrong place, regardless of cap. That will probably be accentuated by any shaft wobble problems.

Pertronix can get round some distributor problems, but not neccessarily all.
P Hunt

As I pondered the possibilities, it suddenly occured to me that I should be able to replicate on the bench what was happening to the dist in the car. I connected a wire from coil + to + on a battery,coil wire between coil and dist. Then the Pertronics red lead to + and the black lead to -on the coil. Then a ground jumper to the dist body. Then I connected a spark detector (about $7 at Advance Auto Parts) to the #1 plug wire socket in the dist cap, and the detector's lead to the battery ground. I then moved the detector around to each wire socket. As I rotated the Dist shaft from the bottom, I could see a healthy spark from 1&2 as well as hearing an arc. There was no spark on 3&4, and the arc sound was very weak. Could have been trying to fire, or maybe was weakly firing somewhere, but not to the plug socket. This occured with both dist caps. Maybe phasing, maybe alignment, maybe bent shaft...It's Broke and outta here. The above should work to check plug wires too. Simply connect wires at dist and move spark detector to end of wires. Thanks again to all. Glenn
Glenn Johnson (Georgia)

As I pondered the possibilities, it suddenly occured to me that I should be able to replicate on the bench what was happening to the dist in the car. I connected a wire from coil + to + on a battery,coil wire between coil and dist. Then the Pertronics red lead to + and the black lead to -on the coil. Then a ground jumper to the dist bodyfrom battery -. Then I connected a spark detector (about $7 at Advance Auto Parts) to the #1 plug wire socket in the dist cap, and the detector's lead to the battery ground. I then moved the detector around to each wire socket. As I rotated the Dist shaft from the bottom, I could see a healthy spark from 1&2 as well as hearing an arc. There was no spark on 3&4, and the arc sound was very weak. Could have been trying to fire, or maybe was weakly firing somewhere, but not to the plug socket. This occured with both dist caps. Maybe phasing, maybe alignment, maybe bent shaft...It's Broke and outta here. The above should work to check plug wires too. Simply connect wires at dist and move spark detector to end of wires. Thanks again to all. Glenn
Glenn Johnson (Georgia)

Glad you figured it out Glenn.

Paul, I had a silicone core wire fail two summers ago on the B after only a year's use. I agree it is unlikely to have two fail at once.

warmly,
dave
Dave Braun

This thread was discussed between 25/11/2009 and 28/11/2009

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