Could my leaking fuel pump be causing awful mpg?

stina said:
I've always thought the mixture screws only adjust the mixture on tickover and make little difference at higher revs , that being taken care of by needle profile :?: At least that's what i've read on here . I know all the needles have the same profile over the part of them that covers the tickover range :? Educate me someone :D
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Think of it this way: If you richened the mixture to excess at idle by winding the jet tube down by 1/2" more than it should be (if that were possible) then all the way through the rev range the jet will be 1/2" lower than it should be. The lower the jet is relative to the position of the needle, the richer the mixture.
 
What Warren is talking about doing Stina is attempting to richen the mixture off idle by adjusting the mixture screw. Although this will achieve that outcome to some degree, it will also richen the idle mixture which may already be exactly where it should be.

Ideally if the mixture is too lean off idle and the idle mixture is correct, then the needle should be exchanged for a slightly richer one.

Ron.
 
harveyp6 said:
stina said:
I've always thought the mixture screws only adjust the mixture on tickover and make little difference at higher revs , that being taken care of by needle profile :?: At least that's what i've read on here . I know all the needles have the same profile over the part of them that covers the tickover range :? Educate me someone :D
Click to expand...

Think of it this way: If you richened the mixture to excess at idle by winding the jet tube down by 1/2" more than it should be (if that were possible) then all the way through the rev range the jet will be 1/2" lower than it should be. The lower the jet is relative to the position of the needle, the richer the mixture.
Click to expand...

That's what i would of thought and makes sense , But i'm sure i've read on here it only makes a difference on tickover , maybe i mis read and it makes " more " of a difference to tickover :?

Ron , already typed the above so i'll let it stand .
I got my head round it now , thanks guys :D
 
That's why you mark the screws and adjust just a little, Ron. Adjusting the screw moves the jet up or down relative to the needle, Stina, so it will alter the mixture across the entire throttle range. Exactly how depends on the needle profile. So Ron is right - if anything more than a small adjustment was needed, I'd really want to be investigating different needles.
 
On all SU's the mixture adjustment is by raising and lowering the main jet. Achieved differently on HIF's than on the earlier cars, but the same principle. The piston that lives in the attractive dashpot on the top has a lower limit of travel when it contacts the main body of the carb. The jet adjusts up and down below this level.

At idle the piston - carrying the needle with it, will be sat on the carb body at its lowest possible level. That means that all of the needle is inside the jet, and petrol can only escape past the thickest part of the needle - at its root where it is attached to the piston. So the hole is really very small. Raising and lowering the jet therefore makes a very big difference to the amount of petrol getting out and hence how rich the engine runs at idle.

At full bore, the piston and needle are at the top of their travel. Only a tiny amount of needle is sat inside the jet, and then only the narrowest portion at the tip. So the hole for the petrol to get out of is quite big. Raising and lowering the jet therefore makes relatively little difference to the size of the hole available for the petrol to escape from. Therefore there is very little effect on how rich the engine is running at full bore.

So it is correct to say that the mixture adjustment on an SU primarily affects the running at idle and just above and it's the needle that is in charge when you get up near full throttle and revs. Clearly there is some effect, but it is minimal compared to the effect at idle.

Chris
 
Clearly the needle profile matters then, and where it is sitting in the collar that holds it in the piston.
 
Hi Warren,

In the case of the swinging needles, the needle shoulder needs to be exactly flush with the collar in which it sits, which in turn is exactly flush with the base of the piston. There is no alternative in this regard, for if it isn't positioned this way then a mistake has been made which needs to be corrected.

Ron.
 
Well, mine were all f***ed up, Ron. Neither needle was flush, and they weren't the same. I've heard people talking of raising or lowering the needles, so perhaps it's a cheat's method of altering the mixture profile.

What a difference when the things are set up correctly!
 
I am pleased to hear that "were" was the operative word Warren. Did the gent from the Rover club change your needles or are they the factory BBG originals?

I am going to start a new thread on standard/non standard needles and fuel consumption.

Ron.
 
Warren .
Can you enlighten me more on your Huco fuel pump , where you got it , what model etc ? ( a link would be good , as would a pic of the install if poss ) :D
 
Nice one Warren .
Thanks for the link . If you can a pic would be great . No hurry though , i wont be doing it for a month or so ( long as i get it done before the blistering summer heat :shock: :LOL: )
 
stina said:
( long as i get it done before the blistering summer heat :shock: :LOL: )
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OOh! where are you going then. (not in this country.)

Was hoping to my V8 on the road soon, but circumstances make it look like the end of the year now.
Fitted an electric pump from Simon at BBC? It's beside the coil. Seems to be working fine, but only standing still.

Jim.
 
Here you go, Stina. It's mounted on the chassis rail, and the fuel line is contrived to run through my tin additive Superpower doodad then take much the path it did before. Power comes from a cable run aft, through the bulkhead grommet and into the space behind the speaker, where it is connected to the accessory power, making use of the unused AC connector.
 

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Hi Warren , thanks for that . It looks quite a compact unit . I was looking yesterday and may be able to make up a plate and mount it up by the wiper motor with new pipe work . :D
 
If you mount it where I put mine, you won't need new pipework. I used all existing to plumb mine in.
 
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