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MG MGB Technical - Emission and PCV issues
|I have a single SU HIF4 carb fitted to my '80 engine and the emission controls have been disconnected. The vent tube from the top of the valve cover has a hose going down the firewall to atmosphere with the center nipple on the cannister that should go to the valve cover (this car has two cannisters) not connected. The check valve is hooked up directly sans air pump to the gulp valve and the rest of the hookups seem correct including the EGR valve. I would like to simplify the system for correct PCV action and have thought of installing a mushroom PCV valve although I have heard of problems with it. |
I would also like to plug off the gulp valve and EGR valve and unrelated vacuum ports. If this is possible and the EGR valve is disconnected from its vacuum would that cause other problems? Possibly removing and blanking off the EGR valve might be a solution to removing the EGR valve? I would like to remove the air rail and wonder what the threads are for plugging the 4 ports.
This car does not have a vapor tank in the trunk, I wonder what that does to everything. Emission controls are not required where I live for a car that old and I drive it not often. The car seems to run ok as is but with some hesitancy upon acceleration perhaps due to untuned carb at this point. I am thinking of investing in a color tune. Any help is appreciated,thanks
|SU carbs used on MGBs from 1968 had a PCV port - a brass tube about 3/16" bore sticking up between the dashpot and the mounting flange. If yours has one of those simply plumb that to the pipe which should be on the front tappet chest - that will provide the suction on the crankcase. If your carb does not have that port then you should fit a PCV valve to the inlet manifold (see below) and plumb that to the front tappet chest cover.|
There needs to be a filtered and restricted fresh air inlet which was originally the oil filler cap, but on cars with the charcoal canister the restriction was in the port on the back of the rocker cover, and the filtering in the charcoal canister. All you need to do is put a small filter on the rocker cover port.
The tank vented via a separator in the boot/trunk, and the charcoal canister. If the boot does not have the tank then either the port to that from the tank was sealed and a vented filler cap fitted, or the tank is venting from an open port somewhere, which usually gives rise to fuel smells in an around the car.
The EGR and Gulp valves can be removed from the inlet manifold and the ports plugged, which will leave space for a PCV valve if needed.
That should leave just the charcoal canisters and the anti-runon valve. Without all the emissions plumbing being in place and functioning correctly the anti-runon valve will do nothing, so can be removed, and its pipe to the inlet manifold, another port to be plugged. You may as well then remove the charcoal canisters, and plumb the carb overflow down past the exhaust to atmosphere.
Others will be able to advise about the screws to seal the head ports.
I only used a Colortune once many years ago and couldn't get on with it, since then I've always tuned my carbs with the lifting pin as per the book and had good results.
|I believe that you have an SU HIF44 carburetor fitted to your '80B, rather than a HIF4. It should be a direct bolt on replacement for the original Zenith Stromberg emission carburetor. Paul's advice is spot on and using the '65 to '67 mushroom PCV valve is a good idea. Deleting the EGR valve is a definite plus. RAY|
|Let me see if I can answer some of your questions.|
First, no vapor seperator in the trunk. How is the gas tank vented then? There should be the vapor tank on the right side of the trunk. A line runs from the fuel tank to the can and a second line runs from the can to a hard line in the trunk. This hard line runs out of the trunk, along the right chassis rail and up into the engine compartment where it is connected, via a rubber hose, to the charcoal canister. Without this system in place one of two things are happening--either you are venting the tank to the open air, inside the trunk, or you do not have a functional vent and the system is venting as it can.
In the former case, venting into the trunk, you are building up fuel/air fumes in the trunk near the electric fuel pump. The possibility of explosion is quite real.
In the second case, venting as the system can without any formal controls, can lead to fuel starvation at highway speeds. One quick check is to take the car out for a fast run, pull over to the side of the road or into a parking lot, and remove the non-vented gas cap. If you hear a rush of air into the fuel tank you are not venting properly. Either of these things would be a quite serious problem.
The line from the rocker arm cover needs to be connected to the charcoal canister's larger nipple. That is the source of filtered air into the engine's positive crankcase ventilation (PCV) system. It would seem that, currently, you are allowing unfiltered air into the rocker arms, the oil in that area, then down into the crankcase for the oil to be pumped to various moving parts--including the rod and main bearings. This needs to be connected correctly to prevent further engine damage.
Air rails, from memory, are 7/16" UNF and the holes can be plugged either with allen head set screws or hex head machine bolts. The set screws make a more attractive installation.
The line from the front tappet cover is the vacuum source for the PCV system. It needs to be hooked up to a vacuum source (intake manifold or carb) through some form of valve system. The older style of "mushroom" valve has worked fine on my 68 GT for many years. Once, I had to replace the rubber diaphragm in it, a relatively easy job. On some carb conversions one simply runs the line to the air filter through an elbow fitting in the metal part of the air filter. Certain MGs used a similar system as did many US cars over the years.
This thread was discussed between 24/04/2012 and 30/04/2012
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