Starting Carb Solenoid Gasket

Pre-XK
1

I made an interesting discovery this morning on my '38 car.

Yesterday I was investigating the apparent non-operation of the starting carb. The needle was not sucking down as it should, and no characteristic hissing noise.

I have rigged up a manual switch under the dashboard for it. With the solenoid removed, I can switch on and the plunger pulls in as it should, sometimes all the way in, and sometimes about 1/16" short i.e. not quite all the way in, but a gentle touch on the floppy disc caused it to pull in the last 1/16".

With the plunger removed and the solenoid installed, the carb apparently works, needle pulls down and hissing noise.

Measurements time. Inlet port face to solenoid mounting face 1/4". Floppy disc fully in to solenoid face 1/8", but 3/16" when not fully pulled in. Seems like not enough clearance., there ought to be more. Could the manifold vacuum be pulling the floppy disc closed against the pull of the solenoid?

Then I realized there was no gasket in the solenoid mounting seat.
I bought a fiber washer, 1-1/8" OD x 7/8" ID x 1/32" thick, Hillman part #780288 from Ace Hardware, put it in this morning.

Success! The starting carb now works as it should, the car starts immediately on one or two revolutions of the crankshaft, the needle pulls down and the hissing noise is back. Apparently raising the solenoid that 1/32" with the gasket made all the difference.

BTW a tip for you all; I was holding the solenoid in my hand, testing it and switched it off, and the plunger and beehive spring flew out, and I spent 10 minutes hunting for that beehive spring. I put a little bend on the small end so it is loosely captured on the plunger shaft and will stay with it if I pull that stunt again.

1 Like
2

Glad to hear these details and success.

In your under-dash switch wiring, here is a tidbit to consider (and maybe you already did). On my Mark V, I changed to under-dash switch also. I chose to wire it such that the entire wire run in the engine compartment is at ground potential (has no voltage on it) unless the under-dash switch is activated. That prevents accidental spark from that wire run except under the brief warm-up conditions. I’ve seen a few burned bonnets on early Jaguars with the burn centered over the starting carb wiring. The original Otter switch instead requires a hot lead be present in the engine compartment at all times on wires with exposed ends adjacent fuel vapor, a condition that makes me uncomfortable.

3

Thanks, Roger. Exactly what I did.
BTW I concluded that the beehive spring was a little bit too strong; that’s why the plunger was not pulling in all the way every time. I stretched the beehive a bit in the opposite direction, so it is a little bit weaker, and made sure the curl in the small end was just enough to hold it on the 5/16" shaft, but does not catch in the swivel, and now the plunger pulls in all the way every time.

4

iwas told that because unlike the other carbs, the needle only operates at one point, they can develop a wear point at a particular spot which can catch… So perhaps anew starting carb needle when rebuilding them may not hurt.

5

I went to put gaskets in the starting carb solenoids of my XK120 and Mark V.
The Mark V solenoid wouldn’t seat right with the gasket in. So I took it out to see what was the trouble, and discovered this brass cup or lip washer in there. Its been in there for 69 years but I guess I never really noticed it before.
IMG_20190912_111254510
Its purpose is obviously to center the large end of the spring so it doesn’t catch on the solenoid when you put it in.
The gasket ID is smaller than the cup washer and was catching on it, so I put the gasket above the cup washer and it went together fine.
My '38 SS and '51 XK120 don’t have this cup washer.