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Shimming fuel pump?

On my way home from work today I stopped to get some antacids.... An old feller walks by and checks the '71 Super out, gives it a thumbs up. He said he used to work on them all the time, loves them. then he said I should try putting 3 gaskets under the fuel pump.Said it'll get me another 8 mpg. Something about the pump putting out 26 psi when the carb only needs about 9 psi. Adding the extra gaskets decreases the stroke of the fuel pump. Anybody else heard of that? I can see where adding the gaskets would decrease the stroke and fuel pressure, but how would that increase the fuel mileage? Does the stock configuration of the fuel pump add 8mpg worth of friction?

Just curious

Von Erik

Reply to
Von Erik
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And if you decrease the stroke of the pump, you also decrease wide open throttle fuel volume, possibly you will have an overall lean mixture, spark knock. I mean, that idea is just nuts, proper carb jetting and ignition timing, and good overall engine condition is the only way to performance and economy, short of fuel injection. That's what I say, anyway. Douglas

Reply to
Douglas

yes some put out way too much fuel pressure. Mine pushes 18 so I needed a regulator because weber carbs can only handle 3 to 4 lbs. Putting in extra gaskets works but as the other poster has said it also decreases the amount of fuel the pump can move

Reply to
Tim

Dual Webers can be easily fed at 3-4 pounds without starving at speed, so how can the stock carb starve at the same pressure?

Reply to
jjs

This is an old "trick",from back in the Mobil fuel run days.It sometimes works,not only on VW's.Before CNC machining,the base height dimension would vary slightly day to day,machinist to machinist.Hence gaskets and shims varied the effective stroke of the push rod.This in turn had an effect on line pressure.Low pressure=low performance,hi MPG.The difference between pumps can also be substantial.Ask a Formula Vee racer about blueprinting a "stock 1200".Steve

Reply to
Ilambert

Hmmm.... Thanks guys. Just curious...

Sounds like an electric fuel pump might be the best way to go...

Von Erik

Reply to
Von Erik

Keep the pressure between 3 and 4 pounds. You don't need a regulator with the right pump.

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has 'em.

Reply to
jjs

one problem is many aftermarket pump spacers are too short. This raises the fuel pressure (I've seen over 13psi from the stock pump). Adding gaskets between the 2 until you are 3-3.5psi is what you need to do, but you have to get a pressure gauge to know where you are at, and when to stop. The chrome "regulators" with the dial and arrow on them should be thrown into the nearest trash can.

John Aircooled.Net Inc.

Reply to
John Connolly

Check your manual. You'll find that VW says to add shim gaskets until the pump stroke is between certain limits. This has almost nothing to do with fuel pressure because the pressure varies very little over the range available by this means. The 26 psi is certainly BS.

What you're really trying to do is to put the operating point of the pump diaphram in the place which puts the least strain on it. This gives you the max lifetime of the pump.

I'd be really surprised if you could make as much as 1/2 psi difference in the pump output pressure with any number of gaskets that you could fit there with the stock studs. With just 2-3 extra gaskets, this is unlikely to make as much as 0.1 psi difference.

Keep in mind that adding shims does NOT decrease the stroke; it just moves the end points of the stroke up or down by the thickness of the shims.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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Reply to
Jim Adney

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