Leaky Valves

[BillS]

I have a current problem that I would appreciate help with. I have been looking for a slight exhaust leak, it sounds almost like the head gasket at times but is variable. A compression test shows 120 lbs for each cylinder so that looks excellent. I was tuning the carbs and the engine slowed, faltered and then resumed its beat, I then noticed exhaust fumes coming up from number 3 cylinder exhaust valve, through the springs. I therefore repeated the exercise and the same thing happened together with the blowing exhaust noise I had been looking for. So the compession test shows all ok but the symptoms indicate otherwise. So before I take the head off to have a look (I would rather be using the car!) any thoughts? Thanks, Bill

[daniel p]

try another compresion tester to make sure the one you are using isnt at fault

[marvmul]

When the exhaust valve starts to open,that is way before BDC during the ignition stroke, there is till a lot of pressure in the cylinder, so I suppose it is normal to see smoke escaping by the valve guide when you didn't get ignition on that cylinder?

[BillS]

I took the head off tonight - and found the problem. The head had been rebuilt by a specialist for the previous owner about 700 miles ago. What I found I have not seen before, the valves are rattling around in the guides, not just worn but either the valves are way way to thin or the guides are way too large. The rest of the head looks fine, it is just a surprising lack of pride in their work. But at least I can now get on and fix it. Bill

[Rhodri]

The Moss catalogue shows different sizes of valve guides to overcome the possible problem you have here, of the wrong size stem to guide bore- page 90

[marvmul]

Bills, are the valve guides of the bronze type? These guides need to be reamed to size after fitting and maybe the specialist was very wrong when reaming the guides. The steel guides don't need reaming.

The standard exhaust valves have wider stems than the inlet valves : MOSS is selling exhaust valve guides with thicker walls and then exhaust valves with the same size of stems like the inlet valves can be fitted. Wider stems are better for cooling of the valves, but the exhaust valves with smaller stems are used in all uprated heads without problems.