Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - CRankcase ventilation with Weber DGV

I have been reading in the arcive about the pros and cons of the Weber DGV conversion. I find the car car to be easy to start and with lots of torque ond no flat spots. Only thing I cant see a solution for is that the engine is idling at 12-1300 rpm when warm, although the idle speed screw is not even touching the adjuster plate on the throttle shaft.

Another issue is that the engine is gooing up with emulsified oil. I have no consumption of cooling liquid and no oil or air bubbles in it, so cyl head gasket seems to sealing OK.

No wonder I have gooh maybe with a solidly packed up with grime filter in the oil filler cap, and the charcoal canister stub on the cyl head cover connected to the stub on the carb K&N airfilter together with the venting tube from the pushrod/tappet cover. A guaranteed way to avoid any air circulation I presume

I have now washed out the oil filler cap filter, mounted a small K&N crankcase vent filter on the stub for the charcoal canister and removed the restrictor in the inlet stub in the carb air filter box as suggested by John Twist in one of his Youtube videos. Hopefully this will clear up the gooh. Fresh oil and filter has been arranged with. Long tets drive planned this weekend.

Has anyone on this list any experience with the John Twist fix for lack of crankcase ventilation with a Weber DGV?

Picture: Setup before I modified it as described last night.



Jan Emil Kristoffersen 79 MGB has much the same set up as your picture works well. As far the idle issue, check for a second idle screw which is used to set the high idle on the automatic choke. The base idle screw and the high idle screw are close together. One may be in the way of the other. My base idle screw is adjusted by using a 5/16 wrench, not a screwdriver. Idle speed is good at 1000 RPM.



PS. My dad was born in Fredrikshald (?). Any where near you??
gary hansen

Is the throttle hanging on the cable? I don't know the Weber but AFAIK on any carb you should be able to reduce the idle to a stall with the idle screw(s). If you can't something must be keeping the butterfly open, or mixture is finding a way round it when fully closed somehow. A vacuum leak is often mentioned, but while that will result in a higher idle speed for a given idle screw setting, an engine can't run on air alone i.e. fuel at well must be getting into the inlet manifold somehow.

Very early MGBs had a very crude ventilation system which consisted of a hose from the rocker cover to the front air cleaner, and another hose on the front tappet chest cover hanging down beside the engine. That was very hit and miss as to which way any ventilation went, if any. A common complaint was oil fouling the air filter, and for that to happen the engine must have been sucking in unfiltered air through the tappet chest hose!

The first stage of positive ventilation was a PCV valve on the inlet manifold, which can be problematic, soon replaced by a feature of constant depression carbs like the SU and Stromberg that provided a continuous source of low-level vacuum without any components to go wrong.

With a Weber or other non-constant depression carb you will need a PCV valve on the inlet manifold, piped to the front tappet chest cover to apply a controlled suction to the crankcase. Then you need a filtered and restricted fresh air inlet elsewhere. Non-emissions MGBs used an oil filler cap that did both, emissions MGBs had a port on the rocker cover that was restricted, and that was piped to the charcoal canister which did the filtering. With that port but no canister put a small filter on the port.

You only need one of either a vented and filtered oil filler cap, or the port on the back of the rocker cover, not both. If you have the port then it is better to put a filter on that and use the original solid oil filler cap, rather than have someone inadvertantly replace your vented cap as being 'incorrect' with a solid cap.

Any port on the front of an original air cleaner should be on the dirty side of the filter, so it doesn't need to be closed off.
Paul Hunt

Thank you for so much useful information, I will get back with a test report at the end of the coming weekend!

Gary; Fredrikshald is a town which was at one point in time renamed to Halden. It is 70 miles from where I live, a beatiful town with a historically very important fortress on a hill above it. You should go there one day!

Google Halden Norway and you will find lots of pics and info.


Jan Emil Kristoffersen

I was considering a Weber at one time. So, I asked Brit-Tek a well respected distributor for Weber kits what do for crankcase ventilation. They recommended going to a draft tube. I wasn't really happy with this solution but didn't pursue any alternatives.

I decided to stick with SU HS4's and the Smith's diaphragm type PCV connected to the intake. The valve cover has the vented plastic cap. (All original to my car).

With the SU it is pretty simple to compensate for the PCV air flow via normal tuning and the correct needle.

The PCV maintains a reasonably steady crankcase vacuum. So the flow will be what is allowed by the valve cover vent hole plus combustion gases that get into the crankcase. So, I think you could tune the idle on the Weber to compensate. Hopefully you wouldn't have to rejet.

If you use the valve cover port and not a ventilated cap do some checking to determine orifice size. You don't want too much air coming into the engine and hence into the intake.

Robert McCoy

Jan. It wold be interesting to know what year your vehicle is. Normally, a vehicle having the rear elbow on the valve cover was designed to be used with a charcoal cannister which was located in front of the firewall on the passenger side of the engine compartment. That same cannister was also connected to the fuel tank and provided the vent for the fuel system.

When the system is set up without the charcoal cannister, the system you propose works well. Rocker arm elbow goes to either the cannister or has a small air filter attached--it is the source of air intake. The front tappet chest cover is connected to an elbow on the air filter for the Weber carb--it provides a very low level of negative pressure in the engine's internal area, keeping if from building up any significant internal pressure which could force oil outwards past the various seals. Such a system has provided me with years of relatively trouble free driving with quite moderate oil consumption. Not as good as a new set of SU carbs, but less expensive for those on a budget. But, I would save up for a good set of SU carbs and the correct intake manifold as a long term project.

Also, I would want to check out how the fuel tank is vented after seeing your rocker arm cover.

Les Bengtson


I believe Jan's car is a 1964. Refer to his earlier link ("New List Member") in the MGB Technical.

Larry C.
Larry C '74 B/GT

My car is a 1964 MkI but with later 5 main bearing engine fitted.

I have now removed the restrictor in the tube that goes to the air filter, and removed the tube from the cylinder head cover, plus installed a brand new vented and filtered oil cap here. The restrictor in the tube to the air filter was also a disc type non return valve, and was not easy to blow open, so probably more of a stopper tha a restrictor?

Did 400 miles this weekend and the engine is now much drier on the outside, so I probably had a bit of postive pressure before. The emulsified oil in the cyl head cover is almost gone. Coolant level stable. I think I will drive on for now and just monitor the situation. Next step will be a PCV valve if gooing does not go completely away.

Thank you for the hints and tips,


Jan Emil Kristoffersen

Jan... You have to drive these old cars. Since we moved from Adk Mts., of New York to the west coast of Florida, I drive the MGB much more than before. Now the car runs better, oil consumption is way down, and there is more power than before.

Keep on driving


gary hansen

As Paul said, you need a PCV. And what Les said is true as well - you need to have a slight crankcase negative pressure.

Long ago I replaced the SU's on my '73 with a Mikuni PHH-44 and had to figure out how to make crankcase ventilation work correctly. Like you I have the crook neck pipe off the back of the rocker cover to go to the charcoal canister. As a point of observation, the end on this pipe should be rounded with a small hole. This orifice is necessary for proper functioning.

I first tried just about any PCV I could find in the line between the inlet manifold and the frint side engine cover without success. They small ones usually have a ball check in them and must be mounted vertically.

The key is the orifice in the end of the rocker cover pipe. Everything must be sized for this flow or it will not work correctly. In my case what did work was to use the original style PCV that came on the earlier cars - Moss P/N 360-630.

Use a K&N rocker cover filter on the crooked pipe of the rocker cover. And IMPORTANT!!! you must use a SEALED oil cap on the rocker cover. The purpose is to get a fresh air flow coming into the top of the engine through the crooked pipe and passing through the oil trap and then through the PCV to be burned through the engine. The orifice and low flow is necessary to keep the A/F ratio from going lean.

Find a suitable spot on, say, the forward branch of your inlet manifold to install a hose fitting. Mount the PCV on a short length of fuel resistant hose from the bottom of the PCV (the bottom of the funnel) down to the manifold. Connect the side branch of the PCV to the oil trap on the front side cover.

It is important to do it straight into the manifold and let the PCV itself control the vacuum on the crancase. Don't put it into the air filter upstream of the carb because the negative pressure there is not sufficient to generate the suction needed.

I did this about 25 years ago and it has worked a charm. No oil mist inside the engine bay and the negative crankcase pressure actually helps the oil stay in the engine (a little, lol). Best of all, no oil fouling on the plugs.

You may need to replace the diaphragm in the PCV from time to time but they are cheap.

Hope this helps!
Richard Smith 1

Thank you Richard. I have one of these rocker covers wth restricted bent pipe. If my current setup continues to produce emulsified oil Iwill find a PCV and do it your way,

Jan Emil Kristoffersen

This thread was discussed between 05/08/2015 and 01/09/2015

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.