Fuel return system

DaveHerns

New Member
Can't recall ever seeing much comment on this and if it goes wrong .If the return flow to the tank was too high it could cause fuel starvation , too low and it could cause the carbs to flood . Also , if working , would it allow vapourised fuel to vent off and return to the tank ? What's in the bit where the return feed connects to the O/S SU and does it never go wrong ?
 
Probably because it rarely gives any problems. There's not much scope to mis-route it, and it doesn't actually go back to the tank itself, it T'ees into one of the outlet pipes on the sender, so it's actually lower than the carbs. If the return got blocked it could cause one or both of the carbs to flood, although there's plenty of V8 conversions out there where the return has just been blocked off without problems. If the restriction was removed the lower fuel pressure that would result before that point could cause fuel starvation at higher rpm. It would help with vapourisation, but with the standard set up you run out of battery before you get the temp down enough for it to do any good.
 
It is a very clever system actually. The relationship between the diameters of the main and return lines, the fuel pump supply rate and the carburettor / engine requirements would have been all originally calculated using differential calculus.

Ron.
 
I disconnected my return line when I installed the Weber 500, however a mis-diagnosed problem with my electric fuel pump caused me to reinstate it, the reasoning being that a supply of constantly flowing fuel would be cooler and therefore reduce the chance of vaporization.
From memory the return plastic line has a reducer metal adapter inserted in it to reduce the return flow volume leaving a positive pressure flow for the carb.

Graeme
 
my car has no return feed as I got Holley 4 barrel --no vapour problems either as fitted heat sink(as buick did)--as the rover 3500 is a buick -its a pitty Rochester wouldn't let rover do carbs-and also rover had contract with SU-which on the v8 are not good-ask anyone with a four barrel carb equipped car-much smoother :D :D
 
What an astonishing post Symes! You've obviously never driven properly set up SU's. Certainly in the UK, advertising for 4 barrel carbs relies on people with defective or poorly set up SU's failing to realise this and instead spending money on unnecessary new equipment. There is just about an argument for changing to a 4 barrel carb if the requirement is to increase air flow rates at very high rpm. But you could equally argue that the correct answer in this situation is injection. For low throttle running SU's will invariably produce a result equal or better than a 4 barrel and are considerably easier to set up to the particular engine.

If you have a bare engine with no inlet manifod, then by all means fit what you are used to. But if you already have SU's then either overhaul and set them up properly or go for injection!

Chris
 
chrisyork said:
What an astonishing post Symes! You've obviously never driven properly set up SU's. Certainly in the UK, advertising for 4 barrel carbs relies on people with defective or poorly set up SU's failing to realise this and instead spending money on unnecessary new equipment. There is just about an argument for changing to a 4 barrel carb if the requirement is to increase air flow rates at very high rpm. But you could equally argue that the correct answer in this situation is injection. For low throttle running SU's will invariably produce a result equal or better than a 4 barrel and are considerably easier to set up to the particular engine.

If you have a bare engine with no inlet manifod, then by all means fit what you are used to. But if you already have SU's then either overhaul and set them up properly or go for injection!

Chris
have driven v8,s with su carbs just dont like them--seem to need constant fettling -to be3 honest v8 much better all round with 4barrel-set up once job done unlike su carbs-and my 300 had no inlet when I got it--would do injection but like to keep things simple
 
symes said:
chrisyork said:
What an astonishing post Symes! You've obviously never driven properly set up SU's. Certainly in the UK, advertising for 4 barrel carbs relies on people with defective or poorly set up SU's failing to realise this and instead spending money on unnecessary new equipment. There is just about an argument for changing to a 4 barrel carb if the requirement is to increase air flow rates at very high rpm. But you could equally argue that the correct answer in this situation is injection. For low throttle running SU's will invariably produce a result equal or better than a 4 barrel and are considerably easier to set up to the particular engine.

If you have a bare engine with no inlet manifod, then by all means fit what you are used to. But if you already have SU's then either overhaul and set them up properly or go for injection!

Chris
have driven v8,s with su carbs just dont like them--seem to need constant fettling -to be3 honest v8 much better all round with 4barrel-set up once job done unlike su carbs-and my 300 had no inlet when I got it--would do injection but like to keep things simple

Simple :?: SU's then , :wink:
 
symes wrote,...
have driven v8,s with su carbs just dont like them--seem to need constant fettling

That only happens when either they are worn out and need overhauling or they were not set up properly in the first place :wink:

Ron.
 
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