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MG MGB Technical - Cyl. no. 3 does not fire

Anybody there who has had the same problem?
A week ago I had a sudden breakdown of cyl. no. 3 on my 1868ccm B, fittetd with new dizzy, sparks, wires etc. nothing changed.
Head gasked and compression seem to be ok, temp. is quite normal. Rear carb (HIF6) seems ok as no. 4 fires quite normal.
Any recomandations or ideas or personal experience with this ignition mystery?

Engine details:
18 V, bored to 1868, flat top pistons, rally head, Piper 270 cam profile, Special Tuning dizzy (run 2000 KM sice new), everything balanced, 2 HIF 6 carbs with BDR needles and K&N filters


Could be something as simple as a bad wire (or its connection). Swap #3 and #4 wires on your cap and at the plugs. If your problem moves to #4 cylinder - it's the wire (or cap position). Could also crank engine and pull wire #3 (wear some heavy gloves), fit a spare plug to it and ground plug threads to head stud. If you get spark from the grounded plug - installed plug in #3 cylinder is probably bad. You say that compression seems to be OK. Have you actually checked it with a gauge?
Steve Buchina

Ralph. Have only had this situation once. Since I own a Sun engine analyzer, I put used it to see what was happening. All cylinders were firing (i.e. the spark plug was working properly). Could not find the problem. All cylinders showed good compression, all of the spark plugs were firing. Took it to a friend who used his engine analyzer on the car. Same indications that mine showed. He diagnosed the problem as an intake manifold leak on the port for cylinders three and four. As you know, cylinder four tends to run slightly rich, cylinder three slightly weak due to the siamesed exhaust port between cylinders two and three. With the minor leak on the intake manifold between cylinders three and four, cylinder four was still strong enough to fire, but cylinder three was too lean to fire properly. If you have checked for everything else, check for a problem in the gasket, or the manifold itself, feeding the port for cyliders three and four.

Les Bengtson

Ralph if you have a points type dizzy check the gap. All cams vary a tiny amount in shape and no.3 maybe the first lobe to close enough for the points to arc and not break contact. It happens more often then people think. Denis

Hi Ralph.

Does No.3 plug smell of fuel ?.

Remove the rocker cover, do the valves appear to be opening normally ?.



It could also be that a lobe on your cam deteriorated. As Don stated, check that the rockers are all moving as would be expected. Also make sure the lifters aren't binding in their bores.
Jeff Schlemmer

I was told by a well-known MG guru many years ago that the lobes on the later model MGB (especially #3 cylinder)were more prone to wear than earlier cars. This indeed was one of the faults with my B at the time. If the wear is pronounced enough, you can actually observe the insufficient movement of the rocker arms of the effected cylinder at idle with the rocker cover removed.

rick ingram

A simple way to check that dist would be to set the rotor on number 3 position and make cetain that it is on TDC. Then set the points to that lobe. If it works then you know what yopu must do. (replace cam)
Sandy Sanders

Ralph Another way of putting my earlier thread is that your points may have closed up and no 3 was the first to run out of gap. Denis

I can't see a sudden failure to fire on No.3 to be caused by cam wear. If the compressions are equal or within limits then the valves must be opening and closing as they should. Despite the bits you have changed I'd go back to first principles. Are you sure it isn't just firing on two cylinders because you have two plug leads reversed? With an inductive pick-up timing light does each plug lead flash? On 1 and 4 is it about 8 degrees BTDC and on 2 and 3 is it 180 degrees away from that? Make another mark on the other side to check. Have you (regardless of a change) swapped the plugs around? On cold cranking with the ignition coil disconnected for 3 or 4 seconds does No.3 smell the same as all the other plugs? They should smell quite strongly of fuel.
Paul Hunt 2

Paul, maybe I am missing the obvious here but how would worn cam lobes affect a compression reading? Worn cams would just mean the valves don't open enough I would have thought.

On my old GT when I had it my number 3 plug was always fouling up. I never did any work on the engine in the car but it must have been time for a rebuild of that engine or at least a valve job. When I sold it the new owner didn't seem concerned. I think he was looking forward to having something to play with and fix up.
Simon Jansen

You may be asking the wrong person, but as far as I can see if the intake valve doesn't open enough you may not suck in enough air and so get a low compression reading, which is why we are supposed to test compressions with the throttle wide open. If an exhaust valve isn't opening I can imagine the results would be different i.e. compressions might be much the same as with correct opening, but then I'd expect to hear some exhaust noises in the intake. At the end of the day valve movement is very simply measured and compared (since only one cylinder seems to be affected) after removal of the valve cover.
Paul Hunt 2

Ah, as soon as you mentioned the throttle being open then that make perfect sense. When I was building my engine I bought myself a dial indicator. I should measure the valve lift I get now then in 10 years time someone remind me to do it again and we can see what my cam wear is like :)
Simon Jansen

This thread was discussed between 20/01/2006 and 24/01/2006

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