Race Engine Forum UK

Interesting that this has been mentioned, as I occasionally have this problem with mine. I see such one-way valves are readily available on a certain E-auction site, so I may well be investing in one...

A carburettor needs enough volumetric delivery to ensure the float level is high enough to supply the main jet circuit under full power. A full-race atmo 8V unit will do about 8 race miles to the gallon and a 16v less still. You can figure out the galls/hr delivery you need from your car's miles-per-gallon.

I have used and recommended the Facet range of interrupter pumps (12v cylindrical ones) for many years and I think they are excellent reliable things. Unfortunately to get high flow the Silver Top 'Fast Road' or 'competition' spec (6-7psi 30 gall/hr) and Red Top 'competition' (6-8psi 40 gall/hr) versions produce enough pressure to flood the carbs. This is particularly worrying on downdrafts as the fuel fills up the barrel and inlet manifold. The Silver Top 'Road' (4-5.5psi 30 gall/hr) version won't do that - the pressure is a bit lower than the Silver Top 'Competition' unit.

The pressure at the pump, say mounted in the rear of the car, is reduced by the hose lines and the height it has to pump thru but certainly not to the extent that, say, with the regular Silver Top, it would produce insufficient delivery at the front of the car. However on the higher capacity models that pressure-drop alone cannot be relied upon to be sufficient per-se to prevent flooding.

The Malpassi 'Filter King' is a good bit of kit for adjusting the fuel pressure and I recently bought one to calibrate the Silver Top pump for my test rig and also to use for carburettor overhaul checks. It has a nice big filter which is a vital add-on, especially if you get supplied fuel hose which is supposed to be 'fuel grade' ie: nitrile - and isn't - and then bits of rubber particles into your whole fuel circuit. (I eventually turned to Wurth UK for fuel hose, thanks to them for providing auto products that actually 'do what it says on the box', seems to be almost impossible to find decent stuff these days).

The hose lines in my case are very short so almost no loss and I have turned-down the pressure at the carb (with pump running but engine off) to 4.5psi. When the engine is running the pressure will drop fractionally. I have tested numerous GC-overhauled carbs with this and there is no flooding. That pump would certainly supply enough demand for up to 190bhp applications of any atmo kind. I cannot comment on the feature of the Filter King that is supposed to 'save fuel' in that it 'acts' as a 'demand valve' bit like diver breathing apparatus - cannot say I am overly interested in that aspect. But certainly
I have used the Filter King with a Red Top pump and found it to be rather erratic in that it, in one test on my own Gp3 Fiat 124 CSA it went either rich at the top end to get the idle jetting stable or vice-versa. It was probably overloaded for that application because the truth is I did not need Red-Top delivery. My conclusion - and this is important in the context of this discussion - don't run any more pressure than you have to to get the required delivery and 3 - 4.5psi max at the carb with engine running at idle. If you do, be prepared for the high pressure to cause erratic variations in float level at the carbs with consequent jetting problems.

Why Fiat/Lancia used the kind of high pressure pumps used in the Volumex and 130TC makes no sense to me except to say that the design (roller cell?) is pretty robust.

The photos show some of the things I refer to - note in particular the fuel pressure gauge (from Northampton Motorsport 01604 766624) that I use in the workshop. It could be mounted on-engine and certainly should be used for calibration prior to start-up, and also the Racetech (Raceparts UK) dash-mounted fuel gauge. The latter is a must-have on a competition car even though your carburetted engine's fuel pressure is pretty low, because as I found to my horror in the mid 90s in NHRA if the fuel pump misbehaves during racing your engine is going to detonate in an instant. An un-noticed failure there cracked a full-race head.

GC

Very sorry for the massive photos I just don't have the time to write this and edit the pics too.

G

Very informative post Guy thank you for your precious time and info.

After all, what the "doctor" says, shall be done. Facet silver top, Fuel King regulator, and a pressure gauge. I think I'll go for the one in the engine bay, as I'm not running a racecar, just a daily driver with a mildly modified engine. After all maybe the mech pump is doing its job well enough, but just to clear my second thoughts about this issue and never think about it again.

Thank you Guy

Nabih