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MG MGB Technical - Why won't #3 fire?

This isn't an MG question, but rather a triumph question... but seeing as I get the best response from you folks, I thought I'd test your mettle.

I'm working on a friends TR2. It has a 2.4 liter out of a TR4 (I think), and the #3 cylinder is not firing. I've replaced the points, cap, wires and plugs. I've checked for spark and the #3 wire has big time spark. I've checked compression and it is pretty good(about 160). I've checked valve clearances, and the seem to be in order, no sticking valves or anything. When the motor is running you can pull the #3 wire off and there is no noticeable difference in the sound of the motor (both at idle and at high rpm). While pulling the wires off the other cylinders makes a big difference. We've also futzed with timing to no avail. My only guess is an intake leak, however both 3 and 4 draw from the same carb, so why would an intake leak only affect #3.
So any ideas on this one?
Bill
Bill Mertz

The only thing I can think of, other than what you've checked already, is a worn cam lobe on #3 intake. A long shot, but you've already checked the obvious stuff. You didn't mention if this problem started suddenly or gradually.
Derek Nicholson

Could it be an exhaust valve sticking open causing you to lose compression while running, although you should hear the air/mix escaping. You might want to buy or borrow a gunson colour test kit which will let you see the ignition firing - or not - in the cylinder.
K.G. Martin

take a running compression check. Connect your gauge to cylinder #1 while grounding its wire to block. start engine and read compression. Press the valve on gauge to release pressure reading and see how long it takes to get compression again- it should be fairly fast, under 3 seconds. Reconnect plug and wire for #1 and repeat for remaining cylinders. Low or no reading indicates valve-train problems including camlobe as Derek states. I've never seen a TR4 engine, but in general, intake leaks can affect only one cylinder - even on shared runners How do the plugs look?
Joaquin

Sounds like a valve not opening to me too - I'd also guess the intake valve.

Try running it a while with #3 plug wire removed. If after removal the plug isn't wet with unburned gas the valve may not be opening.
Glenn Kendall

Just coming from a different angle - if there is carbon or a glowing point in the 3 cyl, then it may still be firing even though there is no spark.

Does the engine tend to run on after normal switch off?

The Colour tune will also reveal this.
Chris at Octarine Services

Thanks for all the guesses. I just tried a running compression test, and #3 only gets up to 90 psi when the engine is running even though it will read up to 160 psi without the engine running.
I'm affraid I don't know what to make of this. Why would it make a difference whether the motor is running on its own power or being spun by the starter. I know its spinning a lot faster with the motor running, but it seems that if the valves close with it spinning on the starter, they would close with it running under its own power.
I'm going to try backing off the valve adjustment a little bit.
But as usual, your advice is greatly appreciated.
Bill
Bill Mertz

Bill,
The compression difference running vs starter would be the throttle plates. I presume you followed the standard procedure for doing the test -- all the plugs out, throttle wedged wide open. Doing it that way ensure you get maximal cylinder filling whilst doing the test, and thus an accurate pressure reading. If you do it with the engine running (and I must say I've never heard it done that way) then the closed throttle plates will mean the cylinder never completely fills and thus you get lower cylinder pressure...

HTH!
Rob
Rob Edwards

What were the other three reading on the running compression test?
K.G. Martin

Bill. Rob makes a very timely comment. I, too, have never heard of a "running" compression check. As Rob notes, standard compression checks are made with the engine warm and throttle(s) fully open. Thus, a "running compression check" would only be valid if you performed a similar test on each of the other cylinders as per K.G.'s post.

You need three things to have a cylinder spark--fuel, compression and spark at the correct time. If the plug is wet, you are getting fuel. You have done a "correct" compression check and show, at 160 psi, very good compression. That leaves spark at the correct time as the most probably suspect.

Spark plugs can fire outside of the engine, but not under compression. Replace the number three spark plug with the number one spark plug and vice versa. If the problem moves to cylinder number one, you have a plug problem. Replace the number three spark plug wire with the number one spark plug wire and vice versa. If the problem moves to cylinder number one, it is a spark plug wire problem. Check the inside of the distributor cap for shorting in the number three position or excessive erosion on the terminal inside the cap. It is perfectly possible to have a spark plug wire that will, in air, produce a weak spark, but cannot carry sufficient current to induce a spark in a plug. Information on checking out spark and coil leads is in an article on my website, www.custompistols.com/. Check the MG section, then articles to see if any can provide you with guidance. Les
Les Bengtson

Crack in distributor cap, burned rotor --- ?
Dan Robinson

running compression test is as common as a static compression- only gives you more information. This was a common way to diagnose valvetrain problems before fancy oscilloscopes became the norm. Infact, compression checks are hardly necessary today as an electronic relative compression check together with a vacuum transducer synchronized with ignition can reveal the state of engine integrity. You can actually "see" the idividual valves open and close- all without pulling a single plug.
Sticking valves, lifters, broken springs, bent rocker arms/pushrods, even piston rings in allignment will easly be masked if you rely on the starter to spin the engine. This is because the components are spinning so slow that, for example, a sticking valve caused by gummed up valve stem has enough time to open and close, but at higher engine revs, the valve is so sluggish that it never opens and closes properly.

- Bill,
If only #3 was low, you have a mechanical engine problem. Check valve lift, springs, rocker arm etc. If the are all about the same (Yes, running compressions will be lower than static), the problem is outside the engine.

Note that running compression will be affected by a gross error- such as plug wires not in correct firing order, one carburetor not at all working, etc.

-Good luck
Joaquin

Thanks Joaquin! I learned something new today. A little quick googling turned up this:
http://www.misterfixit.com/dyncompr.htm

It talks about how to perform a dynamic compression test and how to interpret the results -- basically a more detailed description of Joaquin's summary. This page claims that a dynamic reading of about 50% of the static test is good (about 50-75 psi)....

Cheers!
Rob Edwards

This thread was discussed between 28/11/2003 and 01/12/2003

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