Blowback valve does not fit

Another puzzle. After accumulating many different rebuild parts for the carbs on my XK120 and trying to figure out why my carbs don’t look anything like any of the photos after a few seconds with Joe Curto we learned I have HD6 carbs from another vintage. What an awesome resource, thank you Joe! I do not believe there ever was a blowback valve installed as the intake to the starting carb was heavily carbonned. So I ordered the valve and spring, spendy little sucker! I believe I still have the 120 u pipe for the mixture flow as the manifold end adapter has 4 bolts. Dunno if that is any different from the pipe for the 150. The only blowback valve is C-4204 indicated, according to the Moss diagram, to be inserted into the carburetor end of the u-pipe. However the Moss diagram would seem to indicate that would stop flow from the carb to the intake manifold. The Anders diagram seems to show an arrangement that would work, with the valve part of the banjo bolt on the intake manifold side, allowing air fuel mixture to bypass the spring, but defend against backfires. However the blowback valve is about 0.513 inches OD and the banjo bolt is about 0.50 inches. Trim to fit?

I don’t know if the starting carb and anti-blowback valve will be compatible with HD6 carbs.
Here it is with H6 carbs.

The blow back valve is not really needed.
It is also not fitted on later models ( my late XK150 does not have it anymore )
I removed it on my XK140, because of starting issues, never placed it back, as all my starting issues were over.

If the valve is missing or stuck open and you get a backfire, it will blow fuel out the top of the starting carb and all over the engine. It happened to me once.

I just got my XK-140 carbs back from Joe Curto. When Chris called me to say the carbs were ready for shipment he specifically said that it was missing the blowback valve and that I should get one for it. So apparently the ultimate authority thinks it is important on that car. Just FWIW.


You’re right Rob, I have two MK2 bonnets with blistered paint in the same spot.

Notice the splotch on this bonnet of a brand new early steel car 670278 (or possibly 670275) in a Michigan showroom.
The anti-backfire valve was not yet a part of the starting carb design. It was added beginning with engine W.2449