On my XK120 I have a 7B serial number head that I believe is from a 1960’s S Type.
My question is, that it has from the electric enrichment device (hisser) there are two tubes going into six attached under the inlet manifold.
I see on some other cars with the original style single tubes there is a an anti blow back valve. Why is there not one in my system and what is to stop air being sucked through the six tiny manifold injectors when the hisser is not activated, other than the little brass plate under the light spring from the magnetic plunger. Seems to me to be very crude and liable for air to be sucked through the whole system — or am I missing something obvious in my thoughts. What has prompted this comment is that if I connect a vacuum gauge to the inlet for the screen wash on top of the inlet manifold, I get quite low vacuum reading of about 14 inches. I have checked ignition timing and strobe is showing 10BTDC @700rpm. I have checked TDC Mark several times and sure is correct.
As you have the Stype manifold, due to the 6 feeds, it would not have had a take off for a screen washer, so where is it positioned? Have you checked a reading at the servo take off?
I assume the servo take off is blocked.
Peter B.
Edit
Has the manifold 1 or 2 take offs?
Thank you, the manifold has a takeoff at the far end towards the bulkhead for the brake servo and off the same connector a little tube for connecting to the washer bottle. In fact I have blocked off the tube at that connection to the washer bottle. I have tried the servo connected and not connected. Apart from the driving it makes no difference to the engine running. When I tried the vacuum gauge, I connected to the manifold via the washer bottle connection. The thing is that my engine combination is a bit of a crossbred with that head, but also has two HD8’s. I am not so concerned about the running as I have posted on the technical page about several things like ignition timing and carbs, but really it is my question about no blow back valve in system that to me seems can have suction air leaks as designed ?
Just rereading yours, as I mentioned before it has six takeoffs on the manifold. It goes from hisser into one then splits to two and each of the two has three.
John, the blowback valve was mulled over awhile back. I can only sumise that Jaguar decided that the small diameter of the 6 feed pipes precluded blowback, so no need for
a valve. If the disc valve on the bolt of the soleniod is leaking you should hear a hiss, the only other leak might be at the rubber connections , T piece to each 3 pipe group.
Please post a photo of the connection at rear of the manifold.
Peter B
Here are two photos. It is a bit hard to see, but the take off for the screen wash on the top has a rubber bung and tape over the top. You can see the disconnected screen wash pipe tie wrapped to the tube that goes behind the engine to the servo on the left side of the car.
So the original connection for the servo has been removed and a T piece fitted.
Allthough there should be little difference have you connected to the servo take off
re a reading ? If you hear no hiss at the S / C soleniod I dought any problem there
blowback valve or none.
Peter B
