TR5/TR250 Bypass valve

[mrw02536]

I’m rebuilding a set of stromberg cd-2 175 carbs. The vacuum inlet on the bypass valves have been soldered closed. Should I reopen them or use them shut off? I will be installing the carbs on a tr250.

 

[poolboy]

Are you talking about the brass nipple like the TBV on the left in this picture or where it opens inside the valve ?

 

[mrw02536]

prev owner soldered the brass tube closed in the left hand photo.

 

[mrw02536]

Are you talking about the brass nipple like the TBV on the left in this picture or where it opens inside the valve ?View attachment 79921

The brass pipe that attaches to the manifold vacuum has been soldered closed, in the left hand photo.

 

[mrw02536]

Are you talking about the brass nipple like the TBV on the left in this picture or where it opens inside the valve ?View attachment 79921

I also have another question about the gaskets that came with the diaphragm kit. I replaced the diaphragm and used one of the gaskets between the carb body and bypass valve, what are the extras for?

 

[mrw02536]

Are you talking about the brass nipple like the TBV on the left in this picture or where it opens inside the valve ?View attachment 79921

Here’s a photo

 

[mrw02536]

As I’ve said, as a project I purchased a set of cd-2 175 to rebuild. My 250 is in storage in Florida until I get there on December, when I’ll install the rebuilt ones. I dissembled the carbs completely and just waiting for parts to reassemble. I needed to replace the jet and adjustment screw as well as the throttle shaft and seals which were quite worn. The diaphragm in the bypass valve was brittle. Naturally all new gaskets and o-rings. Everything was cleaned in chemdip. The carbs are a slightly later version of what on my car, having adjustable needles and a plug in brass drain. Every else looks the same. This is my first and I wonder if there are any last minute things I should be aware of. I don’t no what to expect when I install them.

 

[poolboy]

If you want to keep the TBV functional you may find a variety of gaskets in the kit. the one you'd use with that version of the TBV is the one in the middle.
Personally I prefer to disable it's function entirely....an be done with it.

 

[mrw02536]

If you want to keep the TBV functional you may find a variety of gaskets in the kit. the one you'd use with that version of the TBV is the one in the middle.
Personally I prefer to disable it's function entirely....an be done with it.
View attachment 79947

Now I’m confused. From the photo, vacuum from the photo gets to the carb through the small hole that fed by the brass tube that you suggest i cover with the gasket. What do I have wrong? It looks like the one on the right is the correct fit.

 

[poolboy]

The type of TBV that was used on the TR250, the 69TR6 and the 74 TR6 is like the one you have..The sensing port for the vacuum that makes the TBV function is a hole down at the bottom of that nipple.
On the rest of the years the sensing port is internal and must remain open if you want the TBV to function,
BUT. the small hole gets covered in the 250, 69 and 74 versions
This 69 TR6 illustration is exactly the same as the TR250

Roadster Factory: Auto-ignition Vacuum Details--carb Models, 1969 :TR6 Assembly Manual Volume 1 (page 51)

I'd disable the thing entirely if I were you...make a substantial gasket,one capable of standing up to high manifold vacuum..in excess of 25 in-Hg during engine braking..a gasket that covers all but the 3 threaded screw holes that attach the TBV to the body of the carb.

 

[mrw02536]

If you want to keep the TBV functional you may find a variety of gaskets in the kit. the one you'd use with that version of the TBV is the one in the middle.
Personally I prefer to disable it's function entirely....an be done with it.
View attachment 79947

 

[mrw02536]

How does the newer bypass work without a connection to the manifold? How does it sense vacuum?

 

[poolboy]

Thru this hole on the manifold side of the throttle disc

 

[mrw02536]

The type of TBV that was used on the TR250, the 69TR6 and the 74 TR6 is like the one you have..The sensing port for the vacuum that makes the TBV function is a hole down at the bottom of that nipple.
On the rest of the years the sensing port is internal and must remain open if you want the TBV to function,
BUT. the small hole gets covered in the 250, 69 and 74 versions
This 69 TR6 illustration is exactly the same as the TR250

Roadster Factory: Auto-ignition Vacuum Details--carb Models, 1969 :TR6 Assembly Manual Volume 1 (page 51)

I'd disable the thing entirely if I were you...make a substantial gasket,one capable of standing up to high manifold vacuum..in excess of 25 in-Hg during engine braking..a gasket that covers all but the 3 threaded screw holes that attach the TBV to the body of the carb.

This is how I currently have it blocked.

 

[mrw02536]

This is how I currently have it blocked.

Could you explain your reasoning on why I should disable it with the rebuilt carbs?

 

[poolboy]

If you can tell me why you think it's needed ?

 

[mrw02536]

If you can tell me why you think it's needed ?

Bypass Valve: The bypass valve provides a channel for air to flow around the throttle disk. A very high depression can exist in the intake manifold if the engine is operating at a high RPM with the throttle closed such as might occur when decelerating. Poor fuel combustion occurs in this situation giving increased exhaust emissions. This is frequently evidence by exhaust popping during deceleration. The bypass valve senses the high depression and allows some air to go around the throttle plate to reduce the depression to the desired level.

 

[poolboy]

Right...the reason I disable it is because it's emission related only under those rare conditions that it opens.
Disabled, I don't have to bother with the procedure for getting it properly adjusted and the idle will never be affected by a bad adjustment or by the inevitable deterioration of the TBV's diaphragm
BUT if you have a 250 (or a 69) it's important to not only have the TBV properly adjusted but also the Vacuum Control Valve that's mounted and activated by the front carb which must be installed and also properly adjusted.
If you have all that and feel the need for the TBV to function when you use 'engine brake' then do not deactivate it.

 

[mrw02536]

If you can tell me why you think it's needed ?

 

[mrw02536]

This is what i read:
Bypass Valve: The bypass valve provides a channel for air to flow around the throttle disk. A very high depression can exist in the intake manifold if the engine is operating at a high RPM with the throttle closed such as might occur when decelerating. Poor fuel combustion occurs in this situation giving increased exhaust emissions. This is frequently evidence by exhaust popping during deceleration. The bypass valve senses the high depression and allows some air to go around the throttle plate to reduce the depression to the desired level.

what is the downside if everything is working properly?