Have I simply popped a valve?

[Mike Nevitt]

I’m sure this will have been covered at some point, so excuse the repetition. The engine in my 1976 car has something of unknown provenance but it shows 76,000 miles on the clock and has run well since acquiring it in March this year, without missing a beat. Sadly, on Sunday it suddenly quit, two minutes after starting, whilst idling. It did so with a nasty clatter, reminiscent of a broken timing chain. There was no loss of fluids. I’ve taken the rocker covers & distributor cap off and the plugs out. When I turn it over it looks fine and sounds perfect. Bemused, I decided to check tappet clearances. All but one are well within tolerance. The inlet valve for cylinder five should be between 0.008” and 0.010” but it is actually 0.021”. This may of course not explain everything but I really would welcome thoughts, guidance and advice. Thanks.

[Motorsport Micky]

Snake neck camera through the spark plug hole ? OR Whilst you have the plug out knock out the ceramic and points area and weld a schrader valve to take an airline or footpump into the steel body and screw in with both valves closed and pump it up, if a nasty on the valve head the air will escape down the inlet. Remove the carbs and listen for escaping air.

Micky

[Mike Nevitt]

Thanks MM. really helpful.

[flying farmer]

If you have a bent valve then the valve clearance will be too large on that valve and Mickeys tip will show the problem, though I have achieved the same results by wedging the end of a length of heater hose over the plug hole and blowing down it. Lung power is sufficient I find for initial diagnosis.

The question is, if a valve has bent, how has it come into contact with the piston?

I have found it on engines that have stood for years and the valve has stuck open, but not on a running engine.

I would look carefully at the cam timing marks and make sure everything is lined up correctly, its possible the chain has jumped a tooth on a worn crank sprocket, or the tensioner has run out of travel and the chain has become loose enough to slip a tooth.

Neil

[jbuckl]

…+ 1 for previous advice…. or , just check compressions