Fuel shut-off solenoid

I'm replacing my fuel shut-off solenoid on my 34Pict3. I've heard that some people cut off the plunger and effectively run without one. Since they're about $20 I'll just get a new one, but here's my question: Why does the VW motor use one of these where there are lots of other older carbed makes which did just fine without them? At first I thought that it might be due to very high cyl. temps. at & immediately after shutdown, but then I have a 912 Porsche with dual Solex carbs that doesn't seem to need a solenoid. Is it due to fuel left in the intake runner/manifold on the VW? Thanks, Bill.

Reply to
Bill
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Recall that in the mid 1970's the fuel crisis hit (i.e. Arab oil embargo). After that car makers got much more fuel conscious and started taking steps to do whatever they could to raise fuel economy. Also, about the same time tetra ethyl lead (which was used to raise fuel octane) was being phased out. The third hit taken by the auto world was the initiation of pollution controls (which at that time meant "smog" pumps). The combination of low octane fuel, more timing advance (to raise fuel economy) and pollution controls tended to make engines that would run after the ignition was shut off purely due to pre-detonation. I remember a Ford LTD that our family had that would run for a good 10

- 20 seconds after the ignition was shut off (and it absolutely sounded like it was tearing itself up).

The initial response of the automakers was to add a fuel cutoff to the carburettor so that fuel flow was halted when the ignition was switched off. This pretty much fixed the "engine run-on" problem but it did add one more item that could fail in the fuel system.

Cars older that about '75 - '76 seldom came with a fuel cutoff since at the time they were produced, high octane fuel was readily available, timing was generally set for a compromise between best fuel efficiency and best driveability and smog pumps hadn't been invented. In other words they just didn't need them when they were made.

The biggest issue was the low octane fuel of the era. After tetra ethyl lead was phased out it took the oil companies several years to come up with a suitable alternatives. In fact the alternatives were never as good as tetra ethyl lead (strictly in terms of octane boost and valve lubrication) but were cleaner for the environment. However the days of

100 octane pump gas were by and large gone forever.

If you buy at least 89 octane fuel and don't run with too much timing advance (and your engine hasn't been rebuilt with a higher compression ratio) you can probably do without the fuel cutoff.

Reply to
John Crichton

E-85 is 105 octane. I play around with blends in my vehicles from 10 to 85 percent. Timing advance and compression ratios. I've done this for years.

Reply to
Dennis

The fuel shut-off solenoid only works at idle, just for info if someone did not know.

J.

Reply to
Berg

Smog pumps? IIRC 1968 was the beginning. Also, I should restate my question more clearly. Is there something about the inherent design of the opposed 4 aircooled motor that would require a fuel cut-off solenoid when other contemporary cars didn't need one? I've seen these valves on VW's back to the 30PICT and have heard that there are even

6volt versions..
Reply to
Bill

The aircooled VW heads run a lot hotter than watercooled engine heads. That would contribute to self-ignition and run-on. As long as the engine is turning over, the airflow generated will draw fuel from the idle circuit of the carb, any carb. So the best way to eliminate it, is to eliminate the fuel flow through the idle jet circuit.

I suppose under MOST conditions, there would be no run-on, and the idle solenoid is just a failsafe. Many center mounted replacement carbs (Weber progressive, or dual-throat webers or dellortos) do not have any shut-off, and there's still no run-on. So basically, I don't see any reason you couldn't disable the solenoid. You may experience run-on every now and then, or maybe not. If it happens when the car is parked and in gear, it will try to lurch forward. I have witnessed a Volvo 700-series do that, it crawled and bucked and jumped forward a whole car lenght - good thing the parking space in front of it was empty. (it was our rental car)

Jan

Reply to
bugfuel

I,ve never had the slightest trouble with the solenoids. May as well leave them in there if they are not a problem. I suspect my Squareback is carboned up after 37 years and does tend to run on a bit (when I have a spare set of carbies installed that dont have a solenoid), particularly after a longish run. John

Reply to
John

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