Shorrock supercharger question

[DaveAhrendt]

Well, after a fair bit of delay the Shorrock is back in the MG-TC and has been run a few times in the garage. I'm still working out a choke linkage that doesn't need Hercules to pull the knob.

But, I have another issue to ask about. After running in the garage for 15 minutes, when I shut down, a minute or so later some fuel will drool from the carb throat. I'm not running with an air cleaner installed so it's easy to see.

Someone told me that it's normal for some fuel vapor to condense and drip after shutdown but I'd like to confirm. Has anyone experienced this? Normal?

[John Brownhill]

Hi Dave,
This is typical of the TC installation with the caburettor below the supercharger every time you stop there will be a small quantity of fuel plus oil in the carburettor, all you can do is to ensure the carburettor is not set too rich on idling. The fuel and oil will be drawn in next time you start; look inside the carburettor you will see what looks like quite a lot before the carburettor bridge .
You may notice when the car has been standing for a while say a couple of weeks it is difficult to start because of the oily mixture in the carburettor; I have had this!
My solution is to fit a small drain tank on the engine side of the carburettor before the bridge by drilling into the manifold at the bottom then draining every so often.
John

[DaveAhrendt]

Thank you for the confirmation. Feel a lot better about things now. If it wasn't normal I had no idea about how to fix it.
I hadn't considered oil but I've not run it long enough. Oddly, oil was my biggest issue. When the blower was rebuilt in New Zealand some years ago they put the oil seal between the pressure section and front drive in backwards. When I tried to run it I was getting oil running out the carb and very poor running.

Thanks again.

[John Brownhill]

Hi Dave,
For info
Choke cable lubrication- I worked for a company who sold many thousands of cables; we did experiments on lubrication and efficiency.
What we found was-best lubricants were not oil or grease- but thick wax or thick graphite or a combination of the two after degreasing the cable first.
Piano wire inner was better than stranded.(we could not use piano wire because of fixing the end nipples on)
Best of all was a liner inside the cable acetal being better than nylon , ptfe being too soft
John

[DaveAhrendt]

I've been running the TC in the garage for a bit and ran into the following situation.

Engine running, up to temperature, idle very stable at around 850, sounds smooth. Rapidly blip the throttle to 3K or so and let off. Engine rpm drops way below idle and generally stalls. Maybe 1 out of 5 times it will catch itself and stumble back to idle.

I was thinking vacuum leak but I'm not sure. Got any ideas?

Thanks.

[John Brownhill]

Hi Dave,
I generally have the tickover a bit faster perhaps 1000
Check the damper oil in the carb aim for around 20 grade you will notice the carb runs quite cold so thick damper oil may give the problem of the piston not returning quickly enough.
Of interest I did a run of 50 miles the other day about 60 mph, I checked my small catch tank under the carb next day and it had got a small quantity of oily fuel mix in it.
I guess we have to live with this due to the carb position.
John

[DaveAhrendt]

Hi John,

Thanks for the damper oil tip. I'm sure there's 20-50 in there now. I'll dump it out tomorrow and try with straight 20.
When you said wax as the better lube for the bowden cable were you referring to something like candle wax or something more industrial?

[DaveAhrendt]

Hi John,

Bingo! Changing the damper oil to 20 weight appears to have solved my issue. Now when I drop the throttle the rpm drops just a bit below the idle point and rapidly recovers. I assume this is just a bit of overshoot. Of course, with the TC tach I know the exact RPM 2 seconds ago.
I had generally thought of the damper oil purpose was to slow down the piston rise. Didn't think about slowing the drop.
Thanks again.

[John Brownhill]

Hi Dave,
Yes candle wax or any solid wax the idea is that it forms a solid surface for the inner to slide on rather than pushing through the lubricant.
John