V8 Breather pipes - webber 500 conversion MGB GT Factory V8

JC. · Post by **JC.** » Tue Jul 26, 2011 12:28 am

Hi guys,

Not posted up in a while...
I've got yet another MGB GT V8 with a genuine 38k on the clock (this time it's a keeper!) yeah yeah...
Well it IS a keeper this time because I've started throwing cash at it.

Anyway, I've done a webber 500 conversion to it and I just wanted to check that I've got the breather setup OK.

Originally it had twin SU's There was a vent pipe from each rocker cover to the carb and the breather on the back of the block.

I've fitted a large filter to the vent on the back of the block and piped the breathers up like so:

Is this acceptable? I've done about 500 miles on this setup as it is and it seems to run OK save for a bit of difficulty hot starting on occasion?
There are no ports on the rocker covers to fit any additional pipes other than what I have. Do I need to figure a way of running an additional pipe from the other rocker cover to the carb or is it fine as it is?
It's a 3.5 low compression MGB V8 block. Other than the carb it's bone stock.

DaveEFI · Post by **DaveEFI** » Tue Jul 26, 2011 7:38 am

The single breather as fitted to the EFI engines had a rather larger passageway (approx 1/2") into the intake than the carb cars. They had two smaller ones from the rockers and a third from the crankcase. The crankcase one went to the air fliter body.

CastleMGBV8 · Post by **CastleMGBV8** » Tue Jul 26, 2011 8:21 am

JC

The normal way to plumb the system is to run your green tube to the centre port on the front of the Edelbrock although it is acceptable to connect to the filter base and of course there should be a PCV valve in the green tube.

Kevin.

JC. · Post by **JC.** » Tue Jul 26, 2011 9:38 am

Could you explain why will I need to fit a PCV valve when the car didn't have one when I was running the SU's?

Previously there would have been two hoses aprox 7/16" bore running from each rocker cover to the carb.
There is now only one pipe approx 3/4" bore running from the drivers side rocker cover to the carb base.

Will I need to figure out a way of fitting either a breather or another flame trap to the passenger side rocker cover? Currently there is no breather at all attached to the passenger side rocker cover.

CastleMGBV8 · Post by **CastleMGBV8** » Tue Jul 26, 2011 11:53 am

JC

The PCV valve controls the amount of vacuum applied to the crankcase.

You do not require an additional breather, the early P5/6 engines had the (inlet) breather at the rear of the crankcase instead of one on of the rocker covers as per the SD1 and later engines.

The main idea is to reduce crankcase pressure AND scavenge harmful contanimants that will degrade the oil.

Kevin.

Ian Anderson · Post by **Ian Anderson** » Tue Jul 26, 2011 12:54 pm

Please recheck the diagram

The breather is only going into the air filter housing (it will draw fumes into the engine and burn them)

It will have minimal vacuum (if any) and if it has any vacuum then the air filter is not big enough as it is restricting air getting to the carb

Either that or I'm reading it all wrong

Ian

DaveEFI · Post by **DaveEFI** » Tue Jul 26, 2011 1:04 pm

CastleMGBV8 wrote:JC

The PCV valve controls the amount of vacuum applied to the crankcase.

You do not require an additional breather, the early P5/6 engines had the (inlet) breather at the rear of the crankcase instead of one on of the rocker covers as per the SD1 and later engines.

The main idea is to reduce crankcase pressure AND scavenge harmful contanimants that will degrade the oil.

Kevin.

My P6 had a breather on either rocker cover fed to the carbs via a flame trap
And one from the crankcase to the air filter.

JC. · Post by **JC.** » Tue Jul 26, 2011 2:59 pm

Well that's how the MG set up came originally - two pipes each with an inline flame trap routed to each carb.

The screw in discovery wotsit is a flame trap - it's certainly the same as the inline type i.e it's full of wire wool!

It sounds like I'm OK as I am, then?

At least Noone has said "don't start it again otherwise you'll break it" so I'm quite happy with that.

JC. · Post by **JC.** » Tue Jul 26, 2011 3:02 pm

Ian Anderson wrote:Please recheck the diagram

The breather is only going into the air filter housing (it will draw fumes into the engine and burn them)

It will have minimal vacuum (if any) and if it has any vacuum then the air filter is not big enough as it is restricting air getting to the carb

Either that or I'm reading it all wrong

Ian

You've got it. I'm feeding the breather pipe from the o/s bank through a LR disco style screw in flame trap to a hose tail mounted on the underside of the air filter housing.
The air filter is 14" accross and the element is about 2" deep. Not the best, but I can't fit anything thicker under the bonnet. The wing nut is already leaving an impression in the bonnet noise supression material!

DEVONMAN · Post by **DEVONMAN** » Tue Jul 26, 2011 3:36 pm

JC. wrote:Could you explain why will I need to fit a PCV valve when the car didn't have one when I was running the SU's?

.

When you were running the su's the breathers were connected to the carbs at a point where near constant depression occured. This gave an Active breather system. This connection point is located between the dashpot and the butterfly. At this point of connection there is a near constant vacuum (not much though) which sucks the fumes out of the crank case.
The PCV valve does a similar job when it is connected via the inlet manifold and it controls the vacuum applied to the crank case via your breather tube. The PCV valve need to be connected to the inlet manifold to work correctly and putting it in the pipe (Green) to the air cleaner will not work properly.
What you have at the moment is a Passive set up where you are just collecting the fumes that are blown out of the breathers and burning them in the engine via the air cleaner and carb. For best results the fumes need to be sucked out. However there will be some suction inside the air cleaner. It works but not considered to be the best way and may lead to oil leaks due to crankcase pressure.
Much depends on the condition of your engine and the amount of piston blow by.
I suggest that you at least fit a second breather to the other rocker cover to give some flow throughout the engine and to prevent the buildup of mayonaisse in the rocker cover.

Cheers Denis

sidecar · Post by **sidecar** » Tue Jul 26, 2011 3:55 pm

My 2 pence worth....

There will be very little vacuum at the base of the air filter, or at least there should be very little vacuum unless your filter is blocked.

I'd run another breather with a PCV valve from the other rocker cover and connect this breather up the the front of the carb on the big connector. There is a lot of vacuum at this connector. The air flow through the engine will be in from the breather connected to the air filter, this will be nice clean air. It will then be pulled through the engine out of the other breather into the front of the carb.

The carb is actually jetted to 'expect' this 'air leak'. It will still too rich for a 3.5 lump. (I've posted my jet and rod combo's up else where)

Also the carb does not work too well with the Lucas vac advance system, I and others have posted up about this too!

sidecar · Post by **sidecar** » Tue Jul 26, 2011 3:58 pm

double post

badger · Post by **badger** » Tue Jul 26, 2011 9:09 pm

Having done quite a bit with these carbs and engines, here are my suggestions.

1. DO NOT connect a breather pipe to the large connection on the carb body, or the oil consumption will shoot up, potentially inducing pinking because of the oil entering the combustion chambers.

2. DO connect a second breather pipe from the other rocker cover, for the reasons already given.

3. If possible, fit the rover inline flame trap to the new rocker cover pipe, no need for a PCV valve of any type.

DEVONMAN · Post by **DEVONMAN** » Wed Jul 27, 2011 7:55 am

badger wrote:Having done quite a bit with these carbs and engines, here are my suggestions.

1. DO NOT connect a breather pipe to the large connection on the carb body, or the oil consumption will shoot up, potentially inducing pinking because of the oil entering the combustion chambers.

2. DO connect a second breather pipe from the other rocker cover, for the reasons already given.

3. If possible, fit the rover inline flame trap to the new rocker cover pipe, no need for a PCV valve of any type.

I think it's a bit drastic to rule out the use of a PCV valve altogether. If the valve is located where it can pick up oil splash then yes there will be increased oil consumption and problems with oil getting directly into the combustion process. So the PCV valve take off point should be protected from direct oil splash and in the case of the Rover the baffle inside the rocker cover on later engines would do the job.

However, in this case a simple semi-passive breather system will probably do the job ok.

cheers Denis

Robrover · Post by **Robrover** » Wed Jul 27, 2011 8:33 am

I run a pipe from one of the P6B rocker covers to the front of the carb with an inline PCV valve fitted. On the other rocker cover there's a small K&N mushroom type filter. The rear carb pipe goes to the brake booster.

You may want to leave the vacuum advance pipe off - the Eddy and the Lucas dissy don't seem to get on too well. I have a Scorcher dissy with no vac advance.