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Grace
Hagerty Employee

Piston Slap: Proper ventilation of the Midget's crankcase?

Tony writes:

 

I have a 1976 MG Midget in which the original SU carburetors and manifold have been replaced with a progressive two-barrel Weber DGEV and its associated manifold. With this arrangement the PCV valve and plumbing to the intake manifolds no longer exists, so the valve cover vent port is now plumbed to an enclosed catch can that vents via a hose to atmosphere below the engine (like back in the ’50s). Will the lack of manifold vacuum to draw off of the valve cover cause over-pressurization of the rear seal and such? I’ve noted considerable oil consumption and oil leaks (as in every classic British car).

 

Read the full article on Hagerty.com: https://www.hagerty.com/media/maintenance-and-tech/piston-slap-proper-ventilation-of-the-midgets-cra...

6 REPLIES 6
patgarity
Intermediate Driver

That’s not an SU that’s a Zenith. The motor in your Midget is a 1500 Triumph Spitfire. British Leyland opted to squeeze that motor in the Midget rather than trying to make the A series motors meet emission standards. The authors suggestions won’t do you much good as there are no rapper inspection covers with breathers. The open venting of your motor should work fine unless you have a state emissions test. The deposits on your plugs are probably oil or carbon from a mixture that’s too rich. I have no experience with your combination but most carb conversions are pretty lean out of the box. My advice: just run it.

patgarity
Intermediate Driver

PS Fuel hose doesn’t work well for emission hose. Fuel line is used for pressure not vacuum. Get emission hose for your breather set up. When the motor starts “pumping oil” from too much blow by.....sell the car.

Sajeev
Community Manager

Good point, thank you for bringing that up!  Apparently there is a dual use fuel/emissions hose that would likely be better to buy since you can use it in both locations. 

https://ph.parker.com/us/en/fuel-line-vapor-emission-hose-series-395

patgarity
Intermediate Driver

OK I’ve been sucked in. I dug out my factory manual for the 76 Midget. The factory ran a hose from the breather in the valve cover (oil separator inside) to a tee fitting which ran to the charcoal canister for the evap system and a restricted fitting on the Zenith/Stromberg carb. The fitting tapped into the carb in front of the butterfly. We call that ported vacuum. Do not under any circumstance tap into manifold vacuum! You’ll be sucking oil into the carb. So run the vent hose from your catch can down the side of the motor so it extends below the motor. The airstream under the car will produce a slight vacuum to vent your crankcase like old cars used to do. The Bernoulli effect. 

TonyT
Instructor

I would recommend a leakdown check. That will tell you the condition of the rings and valve seats. Obviously, if you see more than a ten percent loss past the rings or valves, some work is in order. What a leakdown won't tell you is the condition of the valve guides and stem seals. To check the guides, remove  the intake manifold and visually check the bottom of the valve guide in the port for oil. Wet is not good. Do the same on the exhaust; proceed accordingly.  Venting the crankcase is a must! Remember, every time a piston goes down, it's moving air beneath it, and compression losses past the rings tend to accumulate in the lower part of the engine. Enough of that accumulation combined with those superior (!) British crankshaft seals and other gaskets results in oil going where physics tell it go. You mentioned that you have upgraded the aspiration to side-draft carburetors, so you might consider equipping the engine with a small vacuum pump to evacuate the crankcase. The discharge can be plumbed to a vented catch can which can then be emptied either back into the engine if you wish or into your used-oil storage container in your garage. I recommend the latter. If you decide that a vacuum pump will work for you, you will want to seal up the factory side cover vent to gain the most out of your new system. Another benefit of the vacuum pump is a slight bump in horsepower due to reduced pumping losses, but it's doubtful that you would feel it.

Sajeev
Community Manager

Leakdown test is a great idea!  Well, it's great until one gets the bad news they are trying to avoid. 🙂