Diagnose fuel problem 2.25 petrol

Two symptoms, probably connected, another probably not.

Warm day, car starts perfectly and runs smoothly for five miles. Park, shop for ten mins, restart, drive a mile, stop and shop for five mins. Restart and drive 30 yards, falters, catch it with choke for a few seconds but it dies. Ties to fire with choke for a couple of turns, then nothing. Has to be fuel, check tank, plenty. Clear bowl behind pump is full, give the lever on the pump a few strokes, nothing. Buy screwdrivers, disconnect fuel pipe, squirts when pumped. Top off carb, aha, empty bowl. Float and needle out, jet seat looks clear and clean. Plan springs to mind, got to be worth a try so fill bowl from spare can, reassemble and it starts and drives perfectly. Drive home.

Second symptom, sometimes get a surging/kangarooing in top half of rev range, choke helps nurse it with tiny throttle opening up to speed. Sometimes a minute after a cold start, other time hot, so no obvious trigger. Usually passes after five or ten minutes.

Third symptom, a 'leak' type whistle on lift off from time to time.

Could it have been a blocked jet, is the fuel pump dying? Engine was hot when it happened, could the carb have boiled dry in five minutes? Or is the pump just not delivering enough? Can't see why it would be intermittent Are there any other filters than the glass bowl that could be blocked? Lightweight SIII. Fuel cap doesn't fit well enough to be causing low pressure.

I'd start at the fuel tank filler cap breather, make sure it works and can easily vent fuel vapour and not clogged, which can create a vacuum that effectively stops fuel flow and can give surges of fuel.

Then check the fuel pipes for leaks. Clean the pipes, dry them (All of them!!) run for a while and see if there are any leaks. It's not likely to be the leaking petrol that's the problem as such, but when petrol leaks it lets air in, sometimes a lot, sometimes a little!

Whatever Fuel filters are on the system, that they're sealed and are receiving fuel and letting it through.

Fuel pump, that it's working with enough pressure/volume for the carb to fill ... and that the carb and it's jets are totally clean! Can't remember, but if there's a rubber carb to manifold bit, or if it's a gasket, but check for leaks there too ..

An airline helps blowing through all the fuel lines, drying them off, and cleaning around the carb etc before removing anything!

Pray .. don't matter who to ...

Right, it was stopped for about an hour, same problem, empty bowl. Filled it and drove a couple of miles, no probs. It sat for an hour and drove back home perfectly.

No leaks on the pressure side of the system, tank cap is loose, and no vac detected lifting it off anyway. I'll pressurise the tank and look for leaks on the low pressure side.

Pretty sure the leak noise is at the carb/manifold join, so I'll get an overhaul kit for the carb and make it lovely again.

Really suspecting the fuel pump - I assume that it's a diaphragm type?

Sorry, can't remember ...

Yeah fuel pump is a diaphragm type. Don’t cost much to replace.

Overhaul kits for pump and carb should be here today/tomorrow! We shall see.

If the carb is a Weber, there's a gauze filter in the fuel inlet, between the inlet pipe and the float valve, you can get to it via a large screw cap under the fuel inlet area of the carb. If that were blocked, that could explain the dry float chamber symptoms.

Tall Ratbag said:

If the carb is a Weber, there's a gauze filter in the fuel inlet, between the inlet pipe and the float valve, you can get to it via a large screw cap under the fuel inlet area of the carb. If that were blocked, that could explain the dry float chamber symptoms.

Click to expand...

Thanks, it is a Weber, so I'll check it when I do the refurb - pump kit arrived, but the carb one hasn't - and let you all know what I found and where.

Tall Ratbag said:

If the carb is a Weber, there's a gauze filter in the fuel inlet, between the inlet pipe and the float valve, you can get to it via a large screw cap under the fuel inlet area of the carb. If that were blocked, that could explain the dry float chamber symptoms.

Click to expand...

@Tall Ratbag gets a brownie point!




Which has fixed the cutting out problem, and made the kangarooing a lot better. I fitted the carb overhaul kit and that was worth doing anyway, all a bit marginal seals, diaphragms etc.. and nice to have a shiny new jet and needles.

I thought that I'd found the 'overrun whistle' leak because the carb/manifold gasket had what looked like a dry patch, but no, no difference. Maybe the manifold gaskets need replacing.

Fuel pump next, which is also going to full of clag. Not easy to get at the back nut though.

Nice one. Good to hear a winning result ..

Arse. It's done it again. Nothing in the filter this time, float bowl empty again. Pump then tank and lines cleaning I reckon. At least I'm getting a tan.

Ahhhhh ... damn ....

Thanks for keeping us updated.

Cheered up by noticing that I can service the 'wet' parts of the pump without taking the lower body off. No visible oil leak at the junction with the crankcase, so why disturb it?

Yes, I know, the lever might be falling off, but let's think positively.

There "should" also be a gauze on the end of the fuel pick up pipe in the tank, and another above the seal in the fuel sediment bowl by the pump - for so much grot to end up at the carb there's a problem upstream - or the fuel pipe itself is rotting from the inside.

I shall be looking into them.

Blow the pipe back into the tank, I agree that there's too much grot so perhaps the pipe is rusting inside. Sediment bowl and strainer should stop it so perhsp there's a lot of fine material.

rob1miles said:

Blow the pipe back into the tank, I agree that there's too much grot so perhaps the pipe is rusting inside. Sediment bowl and strainer should stop it so perhsp there's a lot of fine material.

Click to expand...

It's never simple. The tank pickup has no filter, but there's none of that clag in the tank, a tiny bit in the glass bowl, none in the pump. Pump overhaul kit is nbg, wrong end on the plunger, so new pump coming tomorrow. New pipes bought and fitted, just to eliminate any questions of air leaks or being the source of crud. Blowing through produced nothing.

Could be just a small bit of crud in one of the jet's.An old trick when this happened was to keep motor running without the air filter pipe on then slap the palm of your hand over the carb intake for a moment .The extra vacuum sucks the bit through.

New pump on and it started without having to prime anything. Looking positive for a spin to the pub.

Old pipes look OK on close inspection. I think that the crud in the carb may be years old.