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Crankcase vent / idle question

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Crankcase vent / idle question
  This topic is about my 1977 MG MGB
Old Salt Gold Member David Yager
Bushnell, FL, USA   USA
1977 MG MGB
I have a 1977 MGB that has had the air pump and gulp valve removed and a pair of HS4s installed ( no poppet valve in throttle plates). Also a circa 1964 mushroom type PVC valve has been installed on the intake manifold pulling vacuum on the forward lifter side cover. The original charcoal canister has been left in and hooked up. The oil fill cap is non-vented. The anti run on valve is still in place and hooked up. Vacuum advance is hooked to the manifold port through a solenoid valve triggered by the 3rd / 4th gear transmission switch, there is a port on the aft carb that is blocked off. Also the HS4s do not have the ports drilled between the barrels and the throttle plates.
I have been chasing an idle problem. Tuned both carbs (several times) and set idle at 850 RPM while engine is at operating temp ( fans are running and temp gauge needle in middle of span). Take the car for a 10 minute test run and idle jumps to about 1200/1500 rpm. After the car has sat and cooled down it starts up as soon as I turn the key and idles at 850 again until the next test drive. I removed the distributer last week and checked and lubed up the vacuum advance and the mechanical advance. Removed both carbs and install new throttle shafts, bushings had minimal wear and new shafts fit well and no slop. The idle will increase about 200 RPM when I remove the filler cap and return to 850 when I replace it. Hoping I can get some suggestions and comment about this CC vent set up and possibly some direction where I might go to fix this problem.

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pinkyponk Avatar
pinkyponk Gold Member Adrian Page
Berwick, NS, Canada   CAN
Maybe you're getting a vacuum leak from something that only happens when it's hot?


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barry s Avatar
barry s Silver Member Barry Stoll
Alexandria, VA, USA   USA
1972 MG MGB GT
1974 MG MGB
1976 Triumph TR6
1980 MG MGB
What Adrian says seems plausible since the removal of the oil filler cap suggests that your fuel mixture is somewhat rich. You might try misting water around the manifold carb mating surfaces when the engine is hot and elevated rpms are occurring. You could also try propane but I'm not sure if elevated rpms might lessen sensitivity to the additional 'fuel'. Water should cause a stumble (I think).

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78 B PaceCar Avatar
78 B PaceCar George Funderburke
Kennesaw, GA, USA   USA
David, If you have the mushroom type PCV I would expect to see a breathable oil filler cap. The older B's had a more open crankcase ventilation system. The valve cover on these earlier engines did not have the metal pipe with the small opening connected to a carbon canister. They didn't even have a carbon canister. On later engines, a seal oil cap was used with the metal pipe valve cover. The small opening on the metal pipe limited the air flow to the carburetor preventing the engine from going to lean. This arrangement created a constant vacuum in the crankcase. If you remove the cap while the engine is running at idle, the engine RPM would increase because of the leaner mixture. On the mushroom type PVC arrangement, you could remove the breathable cap and not see any change in RPM's.

It appears you need to go to the full mushroom PCV system by adding the breathable oil cap or remove the mushroom PCV, plug the hole on the manifold and route a crankcase vent hose to both carburetors. If you remove the mushroom PCV, you will need to leave sealed oil cap and make sure the metal pipe on the valve cover has the small opening. If you have the early HS4 carbs they will not have the vent pipe. Therefore you will not be able to use the later arrangement, Which ever direction you go, you will need to readjust the mixture on your carbs.

I am not an expert on this, but these are my thoughts.

lewk Avatar
lewk Silver Member Keith Lewis
Cambridge, ON, Canada   CAN
X2 what George said, good advice.

ozieagle Avatar
ozieagle Gold Member Herb Adler
Geelong Victoria, Australia   AUS
1958 Wolseley 1500 "Wooly"
1966 MG MGB "Bl**dy B"
I would check the diaphragm, in the PCV valve.

I don't know how the charcoal canister works with PCV, but you must have a vent into the engine, somewhere. A vented oil filler cap was one way this was provided.

Check here for full details of all MGB venting systems.


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RAY 67 TOURER Avatar
RAY 67 TOURER Ray Marloff
Fort Bragg, CA, USA   USA
1967 MG MGB "My Girl"
Make certain that the linkage, between the carburetors, isn't binding and has proper clearance per the Workshop manual. RAY

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Old Salt Gold Member David Yager
Bushnell, FL, USA   USA
1977 MG MGB
Thanks for the suggestions and advise. Helps a lot to give me some directions to go in.

Donthuis Avatar
Donthuis Don van Riet
Rijswijk, ZH, Netherlands   NLD
If none of the earlier tips work, modify your vac advance from the later manifold system used to ported. It requires modifying the rear HS4 to add a suction pipe.
See older threads on this, HS4 has to be drilled frrom the top, much easier than with HIF's via the underside. Picture shows such a pipe installed (mistakenly put on top of a HIF though!)

BTW, in principle such a change would also require another type of dizzy, but in my experience the lower vac advance of a manifold oriented dizzy does no harm, while the opposite a ported one with say 20 degrees crank vac advance on a manifold oriented system would get you in even worse trouble with rising idle rpms confused smiley

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vacuem vervroeging op eerste carborateur 021.JPG    32.4 KB
vacuem vervroeging op eerste carborateur 021.JPG

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