Uneven tickover

[clarkie46]

I have just finished reconditioning and fitting an engine to my stag. Rebored,new pistons and rings,crankshaft reground, new main and big end bearings and reconditioned cylinder heads.
The engineer who reconditioned the heads has not done a great job as they are a bit tappety. After doing 300 miles on running in oil i have change to 20w50 and i noticed that when running on tickover if i removed the oil filler cap to listen better to the valves the engine lost revs and almost stalled and if i put my hand over the hole the engine picked up to normal tickover.
Could this be a problem with the sump breather system or does anyone have an idea wht the problem may be.

[sujitroy]

This is considered normal. It means that you have good vacuum and no leaks.
Sujit

[Phil S]

this is because you are allowing air into the closed breather system and weakening the mixture
Phil

[wilf]

Yup, normal. In fact it is the standard test for airtightness of the crankcase evacuation system. Worry only if the revs don't drop when you take the cap off.

[clarkie46]

Thanks guys, that puts my mind at rest.All i have to do now is set the valve clearances myself. Should have done that in the first place but i thought i would save time by getting the engineer who did the overhaul to do it.