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MG MGB Technical - Float Valves ... not all equal...

Many people have posted here of rough running at idle and many good bits of troubleshooting advice have been given. However, here is something I just discovered which may explain the unexplainable for some folks who share this problem.

In replacing the float valves recently, I noticed that the particular valve design/units I had in the carbs really did NOT shut off solidly when the full stroke of the float arm pressed against the needle assembly.

The units in question were the type which have a nylon body with a hard metal sealing tip and spring loaded plunger that rests against the float "arm". The problem was that the float arm would indeed raise the needle and compress the plunger, but the nylon body of the needle assembly was a bit too short, allowing the float arm to stop against the shoulder of the brass seat, stopping just a tiny bit before full closure was attained. Therefore, some pressurized fuel would leak past the needle and seat. Not exactly as the factory intended.

The replacement units I bought locally had what appeared to be the exact same brass seats, but the needle assembly was made of extruded aluminum with a Viton (rubber)sealing point and a spring loaded steel ball at the float end. AND, its body was at least a full 1/16" longer than the previous nylon bodied needles.

After installing these new float valves and adjusting, the occasional rough idle seems to have disappeared altogether in my case. This scenario may not be true for all folks, but it is worth adding to the list of possibilities when you rebuild your SUs or think you are having float valve problems. FWIW.
Bob Muenchausen

With such variations it would also be worth checking the float heights to ensure that the fuel levels were correct before reassembly. Not that there is a great deal of scope to adjust with the all plastic floats.

Rog
Roger Parker

True, Roger.

Also, I should add that these nylon needles were misleading for float level setting in that the floats rest on the spring loaded pin with the needle against the seat *while the assembly is upside down.* However, the spring loading is not great enough to overcome the fuel pressure in and of itself. Once the float/valve assemblies are put into service, the float can rise due to excess fuel, come in contact with shoulder of the float valve seat and the fuel pump pressure can be sufficient to blow the valve off of its seat since the force of the float is against the shoulder and not the needle itself.
Bob Muenchausen

This thread was discussed on 21/10/2001

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