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MG Midget and Sprite Technical - Engine rpm vs. Oil Filler Cap

Afternoon all, do you have any ideas about the following two problems I noticed yesterday, please? I can't think of any reasons for this to happen.

Since I balanced the twin HS2s last week (the rear one was not flowing any air at tickover), the Sprite performance was much better, even though the tickover was a bit too high. Yesterday, I found my (accurate) tachometer, and found the tickover to be 1190 rpm (no choke).

Whilst I was "tinkering", for whatever reason, I removed the oil filler cap- the rpm dropped from 1190 to 930 and was very unstable. What would cause a drop of 260 RPM? I am aware that the engine crankcase is scavenged through the intake, but why would the RPM drop if I remove the oil filler cap? I've not noticed any oil consumption problems of any sort. I've just read a website (http://pdljmpr.tripod.com/techin.htm) that says that the RPMs should rise if the oil filler cap is removed, as the crankcase should be under vacuum.

Secondly- the unstable tickover at 930 rpm- although this may be a red herring if there is a problem with crankcase ventilation- it's an Ivor Searle engine. I'm wondering if it has a 'hot' cam, as it was unstable at 930rpm? I have asked them if they hold details of the specification of this engine, but as it was fitted in 1993, I am not confident they will be able to produce any details.

Any thoughts appreciated- even if it's to point out that I'm missing the obvious!

Thanks,
Andy.
Andy Collins

Quick reply from Ivor Searle- no specific records, but the type number indicates a standard unleaded engine.
Andy Collins

As you say the breather cap is part of the scavenge system. Removing the cap creates a leaner mixture. Mine does the same. Don't worry.
Bob Beaumont

Removing filler cap should make it easier to draw in a greater volume of air and this is sucked into the carb. In theory this should weaken the mixture which initially would normally increase the revs. I suspect though that the oil fumes from the crank actually cause a short term richening so the revs drop. I think if it was left running like that they would increase a bit and then stabilise.

If, with the cap on, you cannot get the tickover revs low enough, then its likely drawing in a bit of air through worn throttle shafts.
GuyW

Andy you might like to check that the air inlet port under the edge of your oil filler cap is clear. It's small and not easy to see, but it's vital that it's clear for the PCV valve to work correctly.

HTH.
Greybeard

Id also add to the above comments to check the oil sump pan bolts are tight and that no air is getting sucked inaround the oil pan connection

and a very popular air to leak into the engine for a higher reving engine is the both the oil dip stick and the plastic oil dip stick holder

For the dip stick and the dip holder use O rings and rescratch the oil levels on the dip stick

In my opinion the dip stick and its holder are the worst areas for sucking in extra air ...exception carb butter fly shafts
prop

my thinking is that it's probably normal but you might have your idle mixture set up a bit lean and then when you remove your oil cap it goes extra lean causing the revs to drop off
try this
Have it idling and undo/remove the cap very slowly
Just as it starts to let a bit of air in the revs should pick up a little, then as you remove the cap further it should then drop a bit
Having an unstable idle can be a sign of the mixture being out either by adjustment or by sucking air somewhere
willy
William Revit

This thread was discussed between 03/05/2016 and 04/05/2016

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