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Triumph TR6 - Engine not running properly

I would appreciate comments /suggestions on the following engine problem.
INFO: 1971 TR6..compression test done at operating temp; all plugs out,timing ok,valve clearance set to spec; all plugs firing ( timing light),electronic high volt. ignition, new wires, rotor, cap, and exhaust gasket,and carbs rebuilt.

CYLINDER # 1 2 3 4 5 6
COMPRESSION 181 125 175 175 189 186

same test again but 1/2 ounce of oil put in cylinder under compression test
CYLINDER# 1 2 3 4 5 6
COMPRESSION 215 180 210 235
Did not do #s 5+6 as this showed fair comparison.
I am getting a missing sound out of exhaust (spark plug is firing on #2 cylinder)and also a whitish/blue smoke. I do not see any smoke comming out of oil filler cap and dip stik hole when removed with engine running. I put small amount of diesel fuel in #2 cylinder and let sit for 48 hours with no difference...I thought maybe stiking rings. I was told engine was rebuilt but have no proof. Any thoughts or words of wisdom are appreciated.

I suspect you are right in not trusting no.2. I had similar persitent misfire and similar compression test data ( ie nothing alarmingly low). Removal of piston revealed cracked lands around lower ring, but cracking did not reach to top surface. Did not see any smoke in exhaust though. The cause was detonation- the motor is supercharged and I had melted holes in 3 and 5, but being a cheapskate did not replace any others. New piston gave instant cure. Nos 1,4 and 6 are still alive and well.
Peter Cobbold

The rule of thumb has always been that if the compression varies by more than 10% between the cylinders that the engine is getting weak and nees attention. In your case the only weak cylinder is #2 so pull the head off and I bet you will find a valve seat starting to burn. At the same time measure the bores for even wear and make sure they are within tolerance. Since the car is not burning a lot of oil I bet it's the valve seat or sometimes the valve (exhaust).


Steve could also be right. But in my experience with 230,000mile engine, valves rarely burn in normal road driving. Between 1974-1985 I have had three piston crowns crack diametrically at right angles to crank axis, which gave subtle misfire and slight exhaust smoke. The only valve problem I ever experienced was a split inlet valve when I bought the car ( at 36000 miles-presumably a manufacturing fault or wrongly gapped tappet). So get the head off,but if the valves are OK dont refit it til you've lifted out no2 piston.Let us know whose bet wins!
Peter Cobbold

This thread was discussed between 16/11/2000 and 21/11/2000

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