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May 28, 2020, 02:52:14 AM

Fuel pressure usung a standard fuel pump.

This is a discussion for the topic Fuel pressure usung a standard fuel pump. on the board Beach Buggy Engine and Exhaust Help.

Author Topic: Fuel pressure usung a standard fuel pump.  (Read 276 times)

this user is offline Buggybaggy

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on: October 13, 2019, 02:38:15 PM
With a standard fuel pump the more the engine revs the more the pump pumps. But does that mean the fuel pressure to the carb goes up too?

The reason for the question is this.
Paul Smith recently posted on the club FB pagr about how fuel consumption increases dramatically with a Weber 32/36 if the fuel pressure goes over 3psi.
I've done a couple of longish runs this year on my own, so not holding back the horses to keep a convoy intact. I've been revving about 500-700 rpm faster than convoy speeds and my fuel consumption has been eye watering. The fuel guage has been moving faster than the buggy.


this user is offline Manxdavid

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Reply #1 on: October 13, 2019, 04:01:54 PM
Pressure should remain pretty constant, its just the volume that should increase. I got a gauge and T piece off eBay for about £10 when I was setting mine up, money well spent.
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"Ah, Beach Buggies, sure, just a quick cheap way of getting a few more years out of a rusty Beetle. You can throw one together in a weekend." anon.


this user is offline Hugh

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Reply #2 on: October 13, 2019, 04:14:31 PM
Steve I think your consumption is mainly reflecting that big carbs aren’t necessarily efficient especially when fully open and petrol is being drawn in greedily.

Most cars have optimum revs or speeds and for most buggy engines the cruising speed of 50/55mph is probably best for mpg ?

In your daily I’d guess at 75/80mpg your mpg is distinctly down on what it is for 55mpg.

PS. I don’t recall your expert leading cruise speed in the Netherlands being exactly slack !
Hugh


this user is offline snoopy

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Reply #3 on: October 13, 2019, 04:45:16 PM
After the next good run take a plug out and check the colour, the thing to remember is you run on a single choke for economy then open the second choke for power thus fuel consumption goes up.
You may have the second stage jetted to rich.


this user is offline farmer rob

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Reply #4 on: October 13, 2019, 05:48:46 PM
on a standard VW fuel pump, the pump actually pumps the fuel on the return stroke not the push stroke of the rod, so the pressure is regulated by the spring strength in the pump. So if the push-rod is shortened slightly, it doesn't squash the spring as much and therefore the spring has less return strength, so the fuel pressure is lower.

I tried a 32/36 progressive carb years ago, when I first got my buggy. I got it running lovely on the first choke but when you gave it more throttle, it just made more noise and drunk the fuel like mad and didn't really give you any more power, so it got binned. I have since found that the main jet in the second choke has to be a lot larger than the one in the first choke. If you look on aircooled.net site, in there how to do things pages; they have got the jet sizes you need to get them to run properly or you could ask Paul as his is supposed to run well, I don't know how much juice it drinks when he floors it though  ;D ;D

Rob


this user is offline Merscury

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Reply #5 on: October 13, 2019, 07:55:27 PM
1641cc the only jet I've changed is the primary idle which I changed to a 50 (I think the one supplied with the carb was a 60).  On paper the main jets are oversized but it runs so well I don't feel inclined to change them. Like Manxdavid I set the pressure with a temporary gauge inserted in the fuel line, On a long motorway run at a steady 65 to 70mph I get 43 to the gallon, knocking around town stop/start it's more like 32-34mpg and just like Farmer Rob I've no idea how much juice it drinks when I floor it.   For the short period of time when I changed my fuel pump and was running a fuel pressure that was far to high, the motorway mpg dropped down to 35,  fitted a pressure regulator and hey presto it went back up 43mpg
Have fun. Be lucky


this user is offline Manxdavid

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Reply #6 on: October 13, 2019, 09:26:04 PM
I dropped my pressure (34ict Webers, standard mechanical pump) by grinding little bits of the top of the pushrod until it dropped to 2 1/2 psi. Job done!
Photos printed on genuine ILFORD® paper.

"Ah, Beach Buggies, sure, just a quick cheap way of getting a few more years out of a rusty Beetle. You can throw one together in a weekend." anon.