rumpfy
01-11-2012, 01:30 PM
Did a rebuild of the two vacuum motors on a carby TP Magna.
The old girl was running badly at large throttle opening.
A full check of everything showed;
Float valve needle was showing a shiny spot on one of the ribs, due to rubbing.
Crud behind the Float valve filter.
Damaged diaphragm in the Bowl Vent Valve diaphragm due to crud under the diaphragm.
Secondary diaphragm and Air Preheat diaphragm vacuum motors were leaking air.
All other thermostats and valves seemed to be OK.
I have had experience with vacuum systems particularly self playing pianos. These instruments operate on vacuum between 1 and 13 kPa.
The motor on the air snorkel was rebuilt with a rubber/ cotton diaphragm material but the result, while functional and OK, was not going to be good enough for the secondary diaphragm.
For this, I used a polyurethane coated nylon woven material. This material stood up to immersion in petrol for 24 hours.
I have details of the diaphragm for anyone who needs it. I tried to post pictures/instructions, but the forum rules prevented it.
The BVV diaphragm was repaired with a dob of Sikaflex 227. This material is a air/moisture cross linking polyurethane compound.
The original secondary diaphragm material seems to be of a poor choice of material. It hardened severely and this resulted in cracking of the material.
On reassembly of the vacuum motors, the Air preheat motor functioned well even though on the bench test, the motor leaked; taking about 3 seconds to lose the vacuum. When the engine is at normal temperature, this motor is relaxed and not required functionally.
The Secondary motor took about 12 seconds to lose vacuum. For their specific uses, the leakage performance of both motors is considered to be OK.
After reassembly of the complete carby system, the car performed very well at full throttle openings.
I hear stories of the cost of new secondary diaphragms at prices between 90 and 190 dollars. My rebuild took a week of development and about $15 for some rubber/cork gasket material.
The old girl was running badly at large throttle opening.
A full check of everything showed;
Float valve needle was showing a shiny spot on one of the ribs, due to rubbing.
Crud behind the Float valve filter.
Damaged diaphragm in the Bowl Vent Valve diaphragm due to crud under the diaphragm.
Secondary diaphragm and Air Preheat diaphragm vacuum motors were leaking air.
All other thermostats and valves seemed to be OK.
I have had experience with vacuum systems particularly self playing pianos. These instruments operate on vacuum between 1 and 13 kPa.
The motor on the air snorkel was rebuilt with a rubber/ cotton diaphragm material but the result, while functional and OK, was not going to be good enough for the secondary diaphragm.
For this, I used a polyurethane coated nylon woven material. This material stood up to immersion in petrol for 24 hours.
I have details of the diaphragm for anyone who needs it. I tried to post pictures/instructions, but the forum rules prevented it.
The BVV diaphragm was repaired with a dob of Sikaflex 227. This material is a air/moisture cross linking polyurethane compound.
The original secondary diaphragm material seems to be of a poor choice of material. It hardened severely and this resulted in cracking of the material.
On reassembly of the vacuum motors, the Air preheat motor functioned well even though on the bench test, the motor leaked; taking about 3 seconds to lose the vacuum. When the engine is at normal temperature, this motor is relaxed and not required functionally.
The Secondary motor took about 12 seconds to lose vacuum. For their specific uses, the leakage performance of both motors is considered to be OK.
After reassembly of the complete carby system, the car performed very well at full throttle openings.
I hear stories of the cost of new secondary diaphragms at prices between 90 and 190 dollars. My rebuild took a week of development and about $15 for some rubber/cork gasket material.