Triumph Repair/spitfire


what does the bypass screw do on the Stromberg 150 cd carb

my car loads up in traffic its running to rich How do i fix this problem?


Hi Cory,

I first need to clearify the terms. As you face the carburetor there are three items on the right side of the carburetor. First is a long plastic cover which covers a bimetal plate that controls a valve that is temperature controled valve. Normally this is not adjustable unless the long plastic cover is removed and should not be adjusted.

Next in is a large brass screw in the side of the carburetor and that is a trimmer screw used by the factory and is not used afterward so it should be run all the way in tight.

The next item in toward the engine is the bypass valve that is a diophragm with a small screw with a lock nut on it. This was a bypass of the throttle plate and the diophragm usually gets hard and dried out and don't operate unless recently replaced. It was an emmission control device that was not much use even when it was working so we often had to clean the valve and examine the diophragm and if it showed signs of leaking past the valve it often needed to be adjusted so as to force the valve all the way closed. This valve could not cause the carburetor to run rich (puff black smoke out the tail pipe and flat blacken the spark plugs).

If you do have the symptoms of a rich mixture (black smoke and flat black carbon on the plugs) you should first look at the large rubber diaphragm on the piston to see that it has no holes or tares in the diaphragm. Then check the needle position in the piston to see that it is not retracted up into the piston too far. The needle is loose and feels like it is lightly spring loaded and the shoulder of the needle should not be recessed more then a 1/16 inch up into the piston. This is adjusted with a long 3mm Allen wrench down through the top tube of the piston. You also need to confirm that the piston moves freely in the top cover. (there may be some oil in the top tube and that is normal as long as the oil is not thicker then engine oil and not over full.

The next cause of excess fuel is a flooding condition caused by the float not set correctly or a problem with the float itself or the needle and seat that the float uses to control the float level. If you check this you have to remove the carburetor and the float chamber and hold the carb face down so the float is hanging from it's pin. This puts the float in a position so as to close the needle to the seat but not compressing the spring in the needle. Then measure the distance from the very bottom of the float to the carburetor surface that the float chamber seated against. This should be 17mm. If it is less or more then 17mm, you need to bend the tab holding the needle in or out to make the distance of 17mm.

One other item you need to look at and that is the hose form the side of the carburetor that is the float chamber vent. It normally wents to the charcoal canister to vent but due to the age of the car and all the possible wrong routing of hoses over the years, that hose could be stopped up or the canister may be stopped up which can cause the float chamber to build pressure and cause fuel to be forced out the main jet and thus running rich.

Let me know what you find.


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Howard M. Fitzcharles III


Triumph TR-4 up & Spitfire, and Engine theory


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

Associate member SAE EAA member

Import Car magazine

ASE Master Auto with L-1 certification up to 2000

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