Throttle bypass valve

is it possible to eliminated the throttle bypass valve on the Stromberg 175 CD2 . My 1969 has a runaway idle that’s hard to tame. The carbs have been cleaned and overhauled but idle problem came back.

I don’t use them on my S-III -and I found one of them was apparently either stuck open or otherwise maladjusted and was causing that carb to run lean even with the needle jet fully up.

If you’re not going to use them, I’d suggest taking the valve off the carb and separating the halves, then adjusting the nut on the spring so that more tension is put on the spring, ensuring that the valve stays closed. While you’re in there, I would also clean the brass valve disc and the mating surface on the valve body and put a dab of grease to work as a (reverseable) sealer.

Mike B

I would imagine that you can just replace the gasket between the valve and the carb body with a piece of solid gasket material, essentially blanking it off.

Some seem to have success with blanking-off or otherwise neutering the valve, but not me. I was tired of coming to a stop-light and having to “blip” the throttle in an attempt to bring down the wildly high RPM’s.

I made the bypass work by replacing the diaphragm (mine was bad) and setting it to open at 22 inHg, using a hand vacuum pump with gauge. In your case it looks like you have 2 of them, as yours is the later setup, one for each carb, but basically the same carb as mine- the 175CD2, so same vacuum setting should apply, just twice.

Good luck, whichever route you take.

69 2+2

Could a loose vacuum connection make rpm go up in idle? Since you rebuilt your carbs already maybe check vacuum hoses?

Hey, I have a similar runaway idle on my 1971 Series III V12 with Strombergs and automatic. Runaway to 1800+ rpm occurs when hot and in neutral or park, like sitting in the driveway. When in drive, the auto tranny prevents much runaway. Carbs have been rebuilt and cleaned, vacuum retard/advance replaced, and damper oil added. I though the low damper oil was the problem, but no. Is the “throttle bypass valve” the same as the “no return valve” (i.e., check valve) on the return line to the gas tank? I have ordered a new one and will see if that works. There’s another recent post regarding adjusting that valve by adding copper washers. I suspect a vacuum leak and am tracking that down.

I have been bothered by high idle with hot engine on my S2 for a couple of years. I went through many checks and adjustments, including bypass valve rebuild. Finally i completely blocked off the throttle bypass valves on the Strombergs and the problem immediately disappeared.

Cleaning them would help for a few miles, but not for long. John Twist (of MG carb notoriety) recommends just leaving them blocked.

I also picked up a nice little exhaust burble on throttle down.

Bob, could you please provide a description of what you did to which parts?

I removed the throttle bypass valve (TBV), which is on the right side of the carb. I replaced the gasket between the valve and the carburetor with a gasket i made, that had no holes in it. Original gasket was like item 1 in the bottom pic below. My replacement gasket covered the hourglass shaped hole in the middle and the small hole near the top. I had only the three screw holes left. Reinstalled the valve. Did both carbs.



I did this when I rebuilt my Strombergs as a preventative measure (and to gain the burble on deceleration 'cause why not). Basically disabling the few emission related “features” on these carbs makes them quite capable and serviceable.

I could look it up but… can someone explain the purpose of this valve and what, if anything, you give up by blocking it off?


Geo- the valve opens under deceleration to allow more air into the air/fuel mixture to lean out the mixture. This reduces the amount of unburned fuel getting into the exhaust during deceleration.

Got it - thanks! Lorem ipsum