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MG Midget and Sprite Technical - procedure to identify vacuum leak

Can someone recommend a procedure to find why i don't seem to have the proper negative crankcase pressure on my 1973 1275 midget? I have an oil leak at the front of the engine, it appears to have been slowed by replacing the oil sump seals, but still leaks. I removed the filler cap on the valve cover with the engine running and engine speed did not change.... so i obviously don't have a good vacuum, right?

all the hoses are in the correct places, i.e. the breather from the timing cover is correctly plumbed to the intake manifold and all the other hoses are in place. I have checked that the hose clamps are tight. What other points are potential vacuum leaks and in which order should I check them?

could the air pump/gulp valve/ emissions equipment be part of the problem? I am planning on removing these components this fall, but they are currently installed.

Thanks for all tips/suggestions!
Chris
Chris Edwards

looking back, I'll answer my own question somewhat... here are places I have thought of that I might have a vacuum leak:

1. manifold gasket
2. gaskets between manifold and carbs
3. hose between manifold and gulp valve?
4. connections to air pump?
5. check valve?
6. valve cover cap... should this be vented? I cannot remember if the cap i have is vented or not, it's a "chrome" cap on an alloy valve cover.

anything I've missed?
Chris Edwards

couple months back I pulled the timing cover and the gause in the seperator canistor was completely clooged up and grossed out,,,certianly was restricting air flow and vacume... you might have a look for the same

but yes your list are all good spots to check, on the carb area Ive always used wd40 and sprayed around the loints with the car running and listen for a speed up... if it does, you found an air leak

also make sure any nuts or bolts in the intake manifold are tight, they can get loose and leak

in the end it could just be worn rings ... try harbor feight and get a leak down tester that will help to determine might even hook upa vacume gauge to see whats happening ..googe is your friend on that one


Prop
Prop

Chris. I am not sure what year the closed venting system was used on the Midget. But, I believe that, like the MGB, the Midget went to an elbow on the valve cover, connected to the charcoal canister, by 73. In that case, there should be a rubber hose running from the large, center nipple of the charcoal canister to the elbow on the factory valve cover to provide a source of air input. But, without either the hose to the charcoal canister, or a vented valve cover, you have nor source of air input. If, indeed, you have one of those aftermarket aluminum valve covers, all of them I have seen have had a non-vented cap. If that is true of yours, your breathing system is compromised and will not work as the factory intended. You need some source of filtered air input.

Les
Les Bengtson

In the UK, the test isL undo the vavlve cover cap, and idle speed increases (showing that the cap is restricting air bleeding into carbs, and therefore vacuum is present).

A
Anthony Cutler

Les,
good call, yes the 73 model did have a vent line from the valve cover to the charcoal cannister, and mine is in place. the aftermarket alloy covers do not have this vent, but they do have a round flat spot that I drilled out and installed a breatehr tube (just a standard brass nipple from the plumbing section of home depot with a similar internal diameter to the original breather).

Ant, If i remove the valve cover cap, there is no change in idle speed, so i know there is a leak somewhere... i just have to find it now!

wish me luck!
Chris
Chris Edwards

Chris-
See my reply in thread "and now for the bad news - new project R Harvey" below. The OE breather hole is 3/32" as I recall.

FRM
Fletcher R Millmore

Thanks Fletcher, excellent response!
Chris Edwards

ok, i've had some time to "poke around" under the hood(bonnet) and found some interesting things, but not necessarily an answer! I sprayed the joints from carbs to manifold, manifold to head, and all the joints to the gulp valve, air pump, etc... and there was no effect on engine speed. So i moved on to the valve cover cap, I plugged the hole in the valve cover cap.. no effect. I jiggled the rubber hose from the valve cover breather to the charcoal cannister, no effect. BUT if I plug the opening at the bottom of the cannsiter (which i think is supposed to be for air to sucked in?) the engine will stall. is this normal? I'm running out of ideas!
Chris Edwards

p.s. the breather from the manifold (plumbed to charcoal cannister) is 1/4" so that may be part of the problem. I have had it plumbed that way for.... 10 years or more and never thought to check if that should be a certain size.

re: oil leak, i have just re-checked the oil level and find it is still totally full. after a run last weekend, there was oil on the front crossmember what made me think i still losing oil, but now i wonder if i just hadn't cleaned up the previous leak very well?? I guess I will just keep an eye on it. i have a new timing cover seal and front pulley seal so if i decide it is still a problem, that can be one of my winter projects. I'll be pulling the radiator then anyway.
Chris Edwards

Chris,
Here's another check which may be easier to verify for vacuum as an alternative to listening for revs change: Run the engine, remove the oil filler cap and stretch some kitchen cling film over the cap. If the engine is drawing a vacuum this will depress quite obviously in the middle.

This assumes that, like UK cars, air is drawn in through the vented filler cap. If yours has an alternative air input you might have to temporarily plug this (finger) to get the same effect. Of course if the film is sucked in it also indicates that your other source of air feed in is already blocked, which sounds like a possibility from what you say.

Guy
Guy Weller

Chris -
Since you have a homemade cover/breather, you need to fit a restriction in the breather from cover to center charcoal canister port (and be certain the cap IS sealed). The restriction hole is 5/64", just looked at one. The standard hose was almost 1/2" to fit over the elbow on the stock rocker cover that the restrictor was put into.

The tank vent line goes to one of the side canister ports, and the carb vent line (2xcarbs>tee>canister)to the other.
There is no line to the manifold.

The bottom port on the canister is for atmospheric air. If you block it, you are applying crankcase vacuum to the carb vents, and the engine stops. That's close to what happens if you fit an anti run-on valve, as on MGB & Spridget 1500.

IF you have been running with a "big" vent, then you must have the carbs set way rich in order for it to idle. Trouble is that means that while the idle mixture comes out about right, the mixture gets progressively richer as throttle opening increases. You might be pleasantly surprised at your fuel mileage after you get this sorted!

FRM
Fletcher R Millmore

Thanks Fletcher, it sounds like the first thing i need to do is reduce the size of the vent tube from the valve cover. I have jsut taken a few pics of the engine bay so we can all see that we are on the right page:

here you can see the valve cover cap (with small vent hole that doesn't seem to be suckng air, since plugging it doesn't make a difference) and the hose to the charcoal cannister.




Chris Edwards

here's a shot with the hose removed, you can see the outlet is 1/4" which you tell me should be 3/32 or 5/64?
Chris Edwards

oops, pic


Chris Edwards

here's the carbs and manifold (what a mess of hoses!) the middle one from the y on the carbs is the one to the timing cover, gulp valve to air rail still in place... for now!


Chris Edwards

last one, here's the charcoal cannister. One hose from the evap line from the tank, one hose to the carbs, one from the valve cover. at the bottom of the cannister you can see the open port which the engine is sucking air through. (if i plug it, the engine stalls).

even though i plan to remove the air pump and air manifold, gulp valve etc soon, i would rather know that this is all plumbed correctly before i start messing about with it. eliminate "x factors", etc.




Chris Edwards

Chris -
"valve cover cap (with small vent hole that doesn't seem to be suckng air, since plugging it doesn't make a difference)" - that's because the much bigger hole in the rocker cover vent/canister overwhelms it. Plug this cap hole and be sure the cap has a decent gasket to seal on the cover.

"you can see the outlet is 1/4" which you tell me should be 3/32 or 5/64" - Yes, 5/64" Just make a plug and drill it.

The vent to the canister allows a small amount of filtered air in, but not enough to equalise pressure, hence crankcase vacuum. Keeps the engine from sucking dust through the works. And sucks gas vapour into the engine to burn, but not enough to overwhelm it if there is a lot of gas vapour in the canister.

FRM
Fletcher R Millmore

Wow, the complexities of the USA version carbon filter cars! Interesting, but I think I prefer the simpler UK versions!

So it sounds as though at some stage the little restrictor plug has dropped out of the end of the hose - and that is all!

Chris that's a nice clean engine BTW, just too many hoses!

Guy
Guy Weller

Guy -
Not really. The restrictor was an integral part of the OE rocker cover elbow. Like many things people change, it can bite you!

FRM
Fletcher R Millmore

Ah, I see, - so missing because of the change to the aftermarket alloy cover.

Guy
Guy Weller

This thread was discussed between 16/08/2010 and 20/08/2010

MG Midget and Sprite Technical index

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