Triumph Repair: Spitfire engine revs rising when warm, crankcase pressure, vent pipe

Question
QUESTION: I have a Spitfire 1500, 1977, when the engines cold it runs perfectly at around 950-1000 rpm, however, after 10 or so minutes the engine revs rise to about 2200 on idle. When I remove the rocker cover breather pipe to the twin SU's the revs drop back down to normal, any ideas as this is doing my head in. The car has had new spark plugs, points changed and set and then the engine has been timed with a strobe. Many thanks for your help, Nigel

ANSWER: Hi Nigel,
Since you have twin SUs this is not a USA car, correct? You will have to tell me where the valve cover vent pipe is connected on the SUs. If it is connected to the intake manifold do this, With the engine warmed up and idling at 2200 RPM or so, remove the hose as you did to make the idle come down. Then with the hose off close off the pipe to the manifold to see what it does and then close off the vent pipe on the valve cover to see if either changes the engine speed. Also note if there is any pressure from the valve cover.

When an engine vents the crankcase via a valve cover into the intake manifold they use a PCV valve to take in and burn the vapors in the crank case. If the crankcase pressure is too high this pressure and vapors can cause an engine speed to raise and you may even see blue smoke from the tail pipe on acceleration.

If this vent pipe goes to the air filter pull it off of the air filter and rev the engine several times to see if any oil or smoke is coming from the valve cover.

let me know,
Howard

---------- FOLLOW-UP ----------

QUESTION: Hi Howard,
This is a UK model. The valve cover vent pipe connects to the sides of the SU's after the fuel mixture. I removed the vent pipe and run a pipe direct to each of the SU's and it ran perfectly. I have a small amount of pressure from the valve cover but nothing excessive, just like a breath pressure. It seems like the oil vapours are being burnt off and this is adding a fuel to the petrol mix.

Cheers
Nigel

Answer
That can happen, and it is dangerous to the engine because the oil has a very low octane rating and will cause detonation which can cause a lot of piston damage.
You should do a "Dry & Wet" compression test. Run a normal compression test (throttle open & at least 4 or 5 revolutions)
Then put about a teaspoon of engine oil in the same plug hole and run the same test. The first "Dry" test should net you from 135 to 170 PSI and the second "Wet" test should be higher but not more than 10% to 15% higher. A "Wet" test of over 20% higher is a good indication of a ring problem which results in excessive "blow-by" of the rings (high crankcase pressure).

Most cars that run crank case vapors into the engine to burn have an orifice if they vent into the air filter or into the carburetors (before the throttle plate) or in the case where they vent into the intake manifold or into a carburetor on the engine side of the throttle plate need a PCV valve of some kind. I am not familiar with the UK version of the SU so you will have to tell me if the engine reves high at idle with no pipe at all on the carbs.

Let me know,
Howard