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S11 3.5V8 high oil consumption - should I fit a pcv valve?


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Hi all, Ive a series 2 with a RR classic 3.5 V8. I've fitted a high torque cam and new edlebrock carb / manifold. i cant say that oil consumption was anything i worried about prior to the conversion but It seems to be using around 1 ltr oil per tank of petrol now and I'm thinking i maybe should add a pcv valve?

The carb came with the pcv port blanked off but I assume its simple enough job to fit one. I'm planning on a rebuild at some point but I'd like to try and establish if the oil consumption is down to the plumbing or good old fashioned wear, although it didn't  seem to burn it so quick before the carb conversion.

Currently I've a pipe from the o/s rocker cover, through a flame trap and into the air filter port and the n/s rocker is the same but from memory into a port on the back of the carb. Could this cause too much vacuum, too little? 

do you think a pcv valve would help?

The engine does blow a puff of smoke on throttle after coasting but it doesnt have valve guide seals so i expect that. It also seems to blow a little on reasonable heavy throttle but certainly never plumes of smoke

thanks in advance

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I'd be checking the air/fuel ratio. A rich mixture will wash oil from the bores and increase oil consumption as well as causing increased bore-wear.

[in practice, matching a non standard carb setup and camshaft _properly_ along with getting the ignition-timing/advance-curve correct will involve a good bit of rolling-road time]
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On the original twin carb setup the air was drawn in via a small pipe at rear of the block below the valley gasket via a small filter looks the same as a fuel filter to a port at the air cleaner. The larger outlet pipes went from both rockers to ports on the vacuum side of both carbs. Thus there is more volume drawn into the carbs than it is possible for the input to supply drawing fumes from the engine into the carbs.

Later engines had a filtered inlet on one rocker cover and an outlet on the other  rocker cover going to the throttle body.

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If you are zoning that set up, and allowing the engine to draw on the crankcase from under the butterflies, you need a restrictor, around 4mm, or it is permanently aucking oil vapour out of the crank case.

The EFI engines used that exact setup, and without the restriction it runs very rough, be surprised if you don't need to return the idle at least, once you have this sorted.

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9 hours ago, Bowie69 said:

If you are zoning that set up, and allowing the engine to draw on the crankcase from under the butterflies, you need a restrictor, around 4mm, or it is permanently aucking oil vapour out of the crank case.

The EFI engines used that exact setup, and without the restriction it runs very rough, be surprised if you don't need to return the idle at least, once you have this sorted.

Ah yes the stubs on the carbs are very thick walled.

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On 9/8/2021 at 8:17 PM, Bowie69 said:

If you are zoning that set up, and allowing the engine to draw on the crankcase from under the butterflies, you need a restrictor, around 4mm, or it is permanently aucking oil vapour out of the crank case.

The EFI engines used that exact setup, and without the restriction it runs very rough, be surprised if you don't need to return the idle at least, once you have this sorted.

thanks all. i dont quite understand what you mean buy drawing [air?] from under the butterflies? is that meaning the n/s pipe i mentioned going to the back of the carb? If so then i what form of restrictor should i use?

Sorry if i'm being thick :)

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Well, if you have the vacuum connection under the butterflies (sometimes called 'manifold vacuum'), at idle, the crankcase will be being put under enormous vacuum, as the engine is sucking hardest in this condition.

The engine, drawing that vacuum on the breather will suck air through the breather pipe, thropugh the crankcase, out the other rocker through the breather you have attached , through the flame trap and into the air filter where it can get all the air it wants.

If the pipe going to the carb is too big it will suck WAAAY to much air through, and pick up oil vapour along the way, giving you blue smoke, especially after idle or as you have said, coasting.

You need a restrictor so the engine isn't able to suck through too much air through the crankcase, a 4mm restrictor is what the EFI engines use, so I would start there. It needs to go just before your breather pipe goes into the carb.

If we knew what model edelbrock you had, we may be able to find a more suitable port.?

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Just to add, it is common for both ends of the pipes to appear to come from a similar location as described above but the air preasures are different. The air in pipe feeds from the atmospheric preasure area before the air filter or at least before the butterfly. This is because the air filter opening is designed to be free from water and dirt and so is a convenient location to source  it.

From my experience, a PCR valve is just a one way valve which opens when the engine is running but will close if there is a backfire or any positive preasure in the manifold, it may not offer any flow reduction.

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OK, so a bit of investigation leads to me say a few things....

- The port on the back of the carb you have connected is designed for the brake servo/booster or vacuum line to the autobox (some use vacuum to decide when to shift). This will be direct to the manifold, and is therefore NOT suitable for your crankcase ventilation without a (~4mm?) restrictor.

- You appear to be missing, or have plugged the PCV port on the front of the carburettor, see image below. This is likely where you need to connect your crankcase vent, it will be pre-butterflies, and draw a lot less vacuum, more to the point, it is designed for it. What did the instructions/seller say?

- Your vacuum advance hose is in the right please, have a cookie 😛 

 

94A2C21F-69C6-49FE-A7C0-9C2337C05A00.jpeg.470c98f6a96c2bab95047d3ae485c97c.jpeg

 

 

 

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