Talk Morgan Forums Morgan Sports Cars Classic Morgans (pre-CX) SU Carbs - A Cautionary Tale

Grumpy,

MG used to hard pipe the small overflow tubes to P-clips attached to various engine fasteners and routed the outlets to the level of the sump and then left them open to drip to the ground. I'm going to route mine to a catch tank of some kind. Kinda douchey to let raw gasoline drip on the ground. Greenies already hate ICE people, so why give them another reason?

I understand your post a bit better now. Though you make mention of a fuel pressure regulator which is an item I tend to associate with fuel injected engines. Also fuel injection tends to make use of a higher pressure fuel pump which if fitted to a carb fed car could perhaps overcome the needles ability to shut off the fuel at the correct float level, if at all..?

One other thing it seems you may not have mentioned is a fuel filter to inhibit crud from the tank entering the float bowl...?

I have an electric fuel pump and regulator on a carbed engine. It means that you get consistent pressure (set by the regulator) rather than the ebbs and flows you get with a mechanical pump. The regulator can be set to mirror the pressure provided by the mechanical fuel pump so no more likely to overcome the needles.

TBM thanks for the heads up, it is many a year since I had a mechanical pump in bits to replace it`s internals, though I think there may have been an inbuilt means of pressure regulation in the pump, i think that once pressure was at the preset level the diaphragm may have acted on the lever to hold it off the cam, and thus inhibit the pumping action till the pressure dropped.

In the old SU electric pumps pressure regulation was such that at the pre-set max pressure the electrical contacts were not allowed to close until the pressure was at or below the required setting. Those with similar electric pumps will hear the pump ticking rapidly when they first turn on the ign after lay-up until the fuel that has evaporated out of the float bowl is made up and once the correct fuel level has been achieved determined by the float rising to the extent it forces the needle valve to close and the pressure in the line to then rise to the extent that the pump stops ticking, until the engine is fired up and ticking like a heartbeat resumes.... Well at least that is how I remember it...I think.. More than happy to be corrected.

Just had the same issue with HIF SUs on my +8 The float neele valve has a filter above it and this was clogging with bits of rubber/plastic debris. Cleaned the filter and problem sorted. Just need to track down the source of the debris, I assume it due to corrosion because of the E10 fuel.
Have an in line filter but it's near the fuel tank
Luckily my carbs have the overflow tubes that vent below the sump

Luddite, I've read various sources that seem to agree that anything more than 3 psi of fuel pressure can overwhelm the needle valve and cause flooding. My biggest concern isn't the "how" of the flooding, it's how to manage the spill at the "when." I'm going on the assumption that it is just a matter of time before a flooding event occurs.

For what it's worth, your memory of electric fuel pump sounds matches mine.

Many thanks for taking the time to respond GP. GW, I suspect it may be possible that you may be correct re the E10 issue having a degrading effect on fuel lines etc. So far I seem to have been fortunate not to have encountered the issue myself. On older cars there is ever a chance of picking up crud from the tank, and the E10 issue seems to have the potential to complicate things similarly. My +8 has a fuel filter in the engine bay mounted just prior to the Stromberg carbs.

GP, I have messed with various cars with SU carbs over the years and while I have had a few that had needle valve issues it was a rare occurrence. However as cars age and fuel tank corrosion occurs increasing the chances of crud in the fuel, that and the possible issue of bio fuel degradation on fuel system parts, I suspect it is wise indeed to ensure that overflowing fuel is directed away from hot areas in the engine bay.

My recollection of typical SU electric fuel pumps found on BL products in the post war period was such that they operated essentially as a lift pump. For this reason they were usually mounted on the engine bay bulkhead above carb(s) with little more than gravity feed to them boosted only by the diaphragm spring. I have seen 1.5 psi mentioned though.

There must have been variants to the design of these long suck, short blow devices though. I remember re-plumbing the double ended SU pump on a Jag XJ12 at 2am on the hard shoulder of M4 many years ago when one side failed leaving me with a dry tank and 10 gallons of fuel in second that otherwise could not reach the engine. Here the pump was under the boot floor adjacent to tanks, so short suck, long blow

Good point on the possible different types of old tick,tick SU pumps Richard in that it seems likely there was more than one version with not only differing capacities but also in their design as one with more lift capacity and another with more push capacity, or blow and suck if that might be the preferred terminology..(-:

Mini`s also had the pump fitted near the tank similar to Jags. Jags with twin tanks had two pumps and a switch on the dash to switch between pumps and tanks. I well remember having a Mini van on which the fuel pump was in need of overhaul ( Yeah there was a time when repair was an option) the van would loose power and I could reach round and give the floor above the tank a bang with whatever came to hand, which was enough to get the pump back into action. With the level of intermittence of failure increasing in time I was eventually forced to buy a new set of spring contacts as the old ones would eventually burn as the result of back EMF generated in the coil, if and when the capacitor/suppressor gave up the ghost.

From memory my Father's Jaguar MK10 could also pump petrol between the two tanks, as on one trip on our holidays down to Brean in Somerset, in the days when the M5 wasn't all connected up and in sections, we left a trail of petrol thru a town centre, as the petrol was being pumped into an already full tank from the part empty tank when my father switched the tanks over Whilst in a queue of traffic the points duty Policeman tapped the window to say you have an problem of trailing fuel. The local Jaguar dealer was still open and they fixed it. I don't remember his 420G having the same ability to pump between tanks, or maybe it just worked without issue or the MK10 created its own unique problem.