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MG TD TF 1500 - Missing a cylinder

Rough idle, not accelerating as it should... Age-old diagnostic..... pull plug wires one at a time. #1,2,4 the idle drops. #3, no change.

Swapped plugs around, new wires, plenty of spark all around, watch the rockers go up and down, no funny noises. All's right with the world.

Except, when I pull the wire on #3 nothing gets worse, it just holds a steady rough idle.

Any ideas for something else to look at?

.......
MAndrus

Next step is to do a compression test. How much use on the engine?
LD Palmer

Compression is even across all 4. Not particularly high at 105 to 110, but consistent.

.....
MAndrus

You may have a bad plug that is shorted out under compression. Swap out #3 with a new or known good plug, and see if that makes any difference. Plugs can show normal spark when out of the engine and may be faulty when placed under pressure.
...CR
C.R. Tyrell

Make absolutely sure you have good spark out of that wire and always try a new plug. If good compression and spark, check for failed intake gasket- try spraying carb cleaner- but that should affect 4 also? Did you check compression after the roughness developed?
George Butz

Put a timing light on the number 3 lead. Chances are you aren't getting any fire. Pull the dizzy cap and have a good look at it. Check the lead into the cap. Very easy for corrosion to block the current. If you see nothing there, then leave the cap off and rotate the engine and see if the points open at the 3 fire position. Just some thoughts.
MG LaVerne

Bad distributer cap.
-David
D. Sander

Could be leaky manifold gasket. A little air can disrupt idle but let it run at higher rpms.

I'd say to hook a vacuum gauge on, but most don't have a spot to hook up the tubing.

Like George suggests, you might spray a little propane or starter fluid around the intake/head port and see if it responds, or just install a new gasket.

I've had a lot of problem with intake gaskets with the extra weight of supercharger bouncing around.
JRN JIM

I'm with Dave on this one,,#3 could be arcing inside the cap,, wipe / clean the inside of the cap,, check for corosion on the inside of #3 terminal ,,

You have already swapped plugs, so it isn't a plug...but you did say that you had plenty of spark with new wires,,,,,,


SPW
Steve Wincze

Compression is even across all 4. Not particularly high at 105 to 110, but consistent.
MAndrus

This thread was discussed between 15/03/2015 and 16/03/2015

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