V8 Fuel Starvation ?

Having a seriously weird fuel (?) problem on my '83 110 V8.

She starts & runs fine on LPG, but - when started on petrol started with difficulty, ran roughly for about 30 seconds then cut out - repeatedly. Pumping the accelerator prolonged this to about 45 seconds. That was yesterday - today she won't start at all on petrol.

Possibly relevant facts/related symptoms ?

- LPG is a simple single point induction system, injecting into the air intakes of the (Stromberg) carbs. Petrol system is standard as to pump & filter.
- Petrol in tanks is only 2 weeks old.
- New fuel filter fitted about 2 weeks ago, but had been running fine.
- Fuel pump makes the right fast then slower ticking noises on startup.
- Had been experiencing petrol vapourisation problems when running on the hot side in heavy traffic.
- Had just fitted braided fuel hose from filter - fuel solenoid (part of LPG system) - carbs, to try to alleviate vaporisation problem.
- Serious backfire when starting from (very) cold on LPG had blown front silencer box 4 weeks ago, so there is a new box (a pipe run to Y pipe) fitted. This sounds a bit rough on start up on LPG since the start of the petrol problem - with an intermittent burble/rattle from the area of the silencer box until the engine is warmed up.

What I have done so far:

- Reinstated the original fuel pipe; checked all fuel connections for petrol/air tightness.
- Checked the top half of the carbs. Cleaned pistons & needles & checked diaphragms (OK).
- Run the system in the emergency, petrol-only, override mode - no joy.
- Disconnected the LPG solenoid to see if it was the LPG ECU playing up - no joy.

Thoughts and/or advice urgently needed. Not a crisis - can still run on LPG - but we're meant to be going on holiday, towing the boat to an LPG-starved area (West Pembrokeshire) from next Monday !

Sorry for the rather long post - but if I had any hair left, I would be tearing it out !

Thanks.

Nick

Checked the timing?

The carbs don't like running without petrol in them.
Luckily for you there are few passages and jets in those Strombergs.

I suggest you replace the diaphragms, AND carry spares. Every time the engine backfires into the inlet the diaphragms get blasted. They can't stand it.

Ideally when running on gas you would jam the carb pistons fully up. This would help the mixture problem, but when the thing does backfire (common on gas) the pressure of the bang would tend to try to blast the diaphragms DOWN, but to a much LESS extent because with the pistons jammed up the blast can pass through.

Then you need to use petrol regularly to wash out the aluminium oxides that form the white fluffy powder that clogs it all up. I think you could try methanol to clean this stuff out. I think aluminium oxide dissolves in methanol (which is not the same thing as methylated spirits).

It seems to me that the problem most likely lies in the fuel delivery and level system in the carbs - the float chambers and the passages to and from them. About the only way to sort that is to take the carb off, open it up and clean it.
Critically,

you MUST have a clean passage IN for fuel. Dry gas running may clog this.

the float MUST be intact. Running on gas means the float is bumping and vibrating in AIR, without the benefit of fuel to keep it cushioned, and to stop it moving much. The float can get a hammering, even get holed so it sinks.

The needle valve inlet jet (a prime suspect) must be clear, and must stop flow when the float rises. Blow into the fuel inlet; then lift the float and blow again.

The passage from float chamber to main jet must be clear. It isn't easy to check this on the car

The jet itself must be clear.

Once you've sorted all that it should be OK, but it will happen again soon. Carbs do NOT like running dry.

CharlesY

Soooo leading on from that Charles, is LPG a bit pony then? I wuz thinking about converting my V8 at some point, but if it's going to give me endless gyp with me SUs I can't be arsed!

Thanks all - good advice.

Timing should be OK - had a 6,000 mile service & MOT 2 weeks ago.

I run on petrol at least once a week to try to avert some of the problems with prolonged dry running that Charles mentioned.

Looks like the floats & needle valves need checking. Ho hum - carbs off ! Had planned to switch to SUs, anyway, so no may be the time.

Nick

Problem identified (but not yet solved - waiting for parts); blocked fuel tap on the petrol solenoid of the LPG system.