Hi Team,
I think I have solved the SU problem, touch wood, with the purchase of a Warneford manifold that was designed for early Holens. It relocates the rear carby forward, and the front one to the rear, by a bit over 1 1/16". My next quest is to sort out the fuelling side of the house.
I have a rebuilt red motor fuel pump that delivers 3-4 1/2 psi.
My understanding is that SU's are designed to operate between 1.5 and 3 psi.
The "fuel rail" I have incorporates a fuel return line (see photo below):
(https://i.postimg.cc/BjqVnSwb/Triple-SU-dry-fit-1.jpg) (https://postimg.cc/BjqVnSwb)
If I understand the fuel rail system correctly, fuel pressure is maintained at an appropriate level by fitting a fuel regulator on the return line to bleed off any excess fuel/pressure. This is opposed to a fuel block system, that has no return, which requires a regulator on the supply side.
The benefits of a fuel rail system, if I understand it correctly, is that it allows a reserve of fuel to be stored in the rail for any surge requirements and the pump is able to feed more fuel to recover as supply is not restricted.
Is my understanding of the two fuel system options correct? If not can you please point me in the right direction.
This brings me to the fuel return line. If I decide to keep the system I have, is there a preferred routing option? My current thought is to route it around the back of the head to avoid the extractors and back to a 90 degree barb threaded into the lowest point of the fuel filler tube using a flexible fuel line as it is at low pressure.
Finally, is there a recommended pressure regulator for this setup?
Thanks in advance for sharing your advice and wisdom.
Cheers n Beers
I have simply run a hard line from the mechanical pump in the stock location. I had a Speco regulator sitting above the thermostat housing, which started leaking profusely, when the diaphragm split after two months. On learned advice I eliminated the regulator as “not requiredâ€, and not an issue since, apart from teething issues sorted by cleaning and adjusting needle and seats. (https://uploads.tapatalk-cdn.com/20250912/daed4082e35146e867fdc009dfba5444.jpg)
(https://uploads.tapatalk-cdn.com/20250912/05f3bd091cf606fb05d26883072c45c7.jpg)
(https://uploads.tapatalk-cdn.com/20250912/6308bb06c04ed38964f62ad3b340ee6c.jpg)
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Quote from: Errol62 on September 12, 2025, 11:04:45 AM
I have simply run a hard line from the mechanical pump in the stock location. I had a Speco regulator sitting above the thermostat housing, which started leaking profusely, when the diaphragm split after two months. On learned advice I eliminated the regulator as “not requiredâ€, and not an issue since, apart from teething issues sorted by cleaning and adjusting needle and seats. (https://uploads.tapatalk-cdn.com/20250912/daed4082e35146e867fdc009dfba5444.jpg)
(https://uploads.tapatalk-cdn.com/20250912/05f3bd091cf606fb05d26883072c45c7.jpg)
(https://uploads.tapatalk-cdn.com/20250912/6308bb06c04ed38964f62ad3b340ee6c.jpg)
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Thanks for the advice. I expect that I can simply block off rthe retrun line and hard plumb it in then. That is definitely the KISS principle.
The beauty of that approach is I can always add it later; however, just in case I may plub in a block off a return feed to the tank. Better to have it and not need it than the other way around.
Cheers
Good thing about a return feed it keeps the fuel and pump cool. If to use quality parts, not Speco, it won’t fail but yes, I am a big fan of the KISS principle.
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And the SUs really are a beautifully simple carb.
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Clay is right. Run them without a regulator for starters. A lot of people find no issue, but depends how strong both your fuel pump and the floats are. If the needles pass, you will know about it (either gently by a very rich mixture, or a bit more flamboyantly by the fuel bowls overflowing). If this happens, find a Malpassi fuel pressure regulator. They will run as a deadhead regulator (no return line).
Keep the spare return line for when the grey motor performance slippery slope kicks in, and you need either a nitrous feed line or a mechanical injection pump return :)
Cheers,
Harv
Quote from: Errol62 on September 12, 2025, 07:29:44 PM
Good thing about a return feed it keeps the fuel and pump cool. If to use quality parts, not Speco, it won’t fail but yes, I am a big fan of the KISS principle.
And the SUs really are a beautifully simple carb.
Quote from: Harv on September 13, 2025, 07:56:00 AM
Clay is right. Run them without a regulator for starters. A lot of people find no issue, but depends how strong both your fuel pump and the floats are. If the needles pass, you will know about it (either gently by a very rich mixture, or a bit more flamboyantly by the fuel bowls overflowing). If this happens, find a Malpassi fuel pressure regulator. They will run as a deadhead regulator (no return line).
Keep the spare return line for when the grey motor performance slippery slope kicks in, and you need either a nitrous feed line or a mechanical injection pump return :)
Cheers,
Harv
Thanks for the guidance Clay and Harv. I will try to get some paint onto the motor next week while I wait for the Warnerford manifold to turn up. Then I can transfer it to the test stand, add all the bits and do some experimentation with the fuel system. I will start with the return blocked off temporarily and work from there.