GT6 Rocker box cap removal stops engine !

[Royston W]

Here's a strange one, trying to adjust stronberg twin carbs, and had to remove breather pipes to gain access to thottle linkage. Noticed an immediate change in engine speed slowing and stalling engine. reconnected and ok again. To check undid Rocker Cover cap and again the engine stopped. Never heard of anything like that before. Any ideas ?

[NonMember]

I assume this is a later type with the breather pipes into the carbs rather than the manifold?

The crank case is intended to be mostly sealed but "purged" by this breather arrangement. The hoses connect to the "constant depression chamber" of the carbs, which runs at a fairly steady, small "vacuum". This chamber is a fundamental part of the carb's metering mechanism and must not be disturbed. The crank breather flow is small enough to not worry it, but if you open the cap or disconnect the pipes then you have a huge hole straight into it. Big air flow, loss of regulated vacuum, metering system goes all to pot and the engine stalls. It's expected.

[68vitesse]

Noticed a similar thing on my 2L Mk1 Vitesse, one off the breather hoses had split, a split diaphram in the smiths valve can cause a similar condition.

Regards

Paul

[daverclasper]

I had a recent split diaphram. Engine would hunt up to 2000 rpm at idle, would run ok, though a bit of popping on over run