Help with fuelling issue

Cyanide · 3 Jun 2016 at 09:05

I'm hoping someone can point me in the right direction for fixing my wife's bike as I'm pulling my hair out trying to sort it.

It's suffering from fuel starvation when at full throttle for more than 15-20 seconds. You get a splutter as it starts to die but lifting off for a few seconds and/or running at ~80% throttle fixes it.

It wouldn't be an issue but it's only a 125cc 4 stroke (Aprilia Mojito 125) and she goes down NSL roads so full throttle is needed sometimes!

I've done the obvious things such as changing the filters, spark plug & vacuum line to the fuel pump. I've also replaced the fuel pump itself as it looked a bit grotty and I wanted to eliminate it as the culprit.

All of the vent pipes are clear, the fuel cap isn't vented but I've ran it with the cap loosened and it still cuts out.

Other than stripping and rebuilding the carb, is there anything else it could be?

MowyTone · 3 Jun 2016 at 09:26

Is the wiring for the fuel pump ok? I had an issue with the fuel pump relay that stopped the pump from firing.

edit: Are there any other vacuum hoses? If it is a vacuum fuel petcock and there is a leak in a vacuum hose it could cause issues

Cyanide · 3 Jun 2016 at 09:28

MowyTone said:
Is the wiring for the fuel pump ok? I had an issue with the fuel pump relay that stopped the pump from firing.

It's a vacuum pump, it's just got pipes for fuel in & out and a vacuum tube.

Dogbreath · 3 Jun 2016 at 13:18

Seems like a common problem. Have you stripped the carb and made sure the needle valve is clean and that the float is moving freely?

The problem with vacuum powered fuel pumps is at full throttle there is virtually no vacuum!

Cyanide · 3 Jun 2016 at 13:24

Dogbreath said:
Seems like a common problem. Have you stripped the carb and made sure the needle valve is clean and that the float is moving freely?

The problem with vacuum powered fuel pumps is at full throttle there is virtually no vacuum!

Haven't touched the carb yet, I was trying to eliminate all the cheaper/easier options before getting to it. I still have nightmares from setting up the carb on my Bandit

Looks like that'll be this weekend's job!

Dogbreath · 3 Jun 2016 at 13:37

The other thing to check is that the vacuum line to the pump is not blocked (e.g. kinked) or holed, and that fuel lines are not kinked.

Cyanide · 3 Jun 2016 at 13:42

Dogbreath said:
The other thing to check is that the vacuum line to the pump is not blocked (e.g. kinked) or holed, and that fuel lines are not kinked.

Already replaced everything with reinforced pipes to make sure they weren't kinking/collapsing under vacuum.

Dogbreath · 3 Jun 2016 at 16:25

Nothing partially blocking the outlet on the tank? Pull the fuel hose off and see if it runs out (into a suitable container) at a decent rate. If not could also be a fuel tank vent (possible built into the filler cap).

Apart from that sounds like you've covered everything but the carb.

A thought; does filling the tank to the top help to relieve the problem or does it make no difference?

Cyanide · 3 Jun 2016 at 20:26

Dogbreath said:
Nothing partially blocking the outlet on the tank? Pull the fuel hose off and see if it runs out (into a suitable container) at a decent rate. If not could also be a fuel tank vent (possible built into the filler cap).

Apart from that sounds like you've covered everything but the carb.

A thought; does filling the tank to the top help to relieve the problem or does it make no difference?

Fuel seems to be flowing ok, I'll double check it tomorrow though.

Fuel cap isn't vented but even running it with the cap loose doesn't help so it's not a venting issue.

It was running alright with a full tank which is what made me think it's the pump but it cut out the other day when it was full too

wazza300 · 3 Jun 2016 at 20:35

Faulty ignition coil? Esp at high revs

Dogbreath · 3 Jun 2016 at 20:49

wazza300 said:
Faulty ignition coil? Esp at high revs

I did wonder about an ignition problem, but it doesn't fit with it recovering when you back off for a few seconds; that's a classic fuel starvation symptom.

Cyanide I did have another thought, have you ever replaced/cleaned the air filter? Years ago a guy at work had a scooter where the pipe leading from the carb to the airbox was collapsing at full/throttle high RPM because the air filter was clogged up.

Cyanide · 3 Jun 2016 at 21:29

Stripped the carb down, gave everything a good clean (didn't realise carb cleaner would melt a bath panel - oops!) And tweaked the float height - the thing's tiny though so there was t much room for adjustment!

It's a tiny bit better but still spluttered and started to die at speed, though it took longer to go and recovered faster than it did before and on an almost empty tank.

Air filter was done not long ago and it all seems fine.

I'm going to double check the fuel lines tomorrow, it seemed to be flowing ok when I checked them yesterday but the fact it gets worse as the tank empties suggests a pressure issue and with the pump replaced that only leaves some sort of blockage as a possibility. For the sake of £2 I might replace the fuel lines anyway just to be sure !

4T5 · 3 Jun 2016 at 21:36

Have you checked the carb manifold properly for cracks/leaks ?

Correct me if I'm wrong but is the carb manifold black & made out of hard plastic ? Check it for cracks & damage against both ends that seal it, Check it's not been over tightened & distorted etc & check the "tube" section for cracks

Also when you say you cleaned the carb & everything did you blow out the jets ?

Cyanide · 3 Jun 2016 at 21:49

4T5 said:
Have you checked the carb manifold properly for cracks/leaks ?

Correct me if I'm wrong but is the carb manifold black & made out of hard plastic ? Check it for cracks & damage against both ends that seal it, Check it's not been over tightened & distorted etc & check the "tube" section for cracks

Also when you say you cleaned the carb & everything did you blow out the jets ?

It's a rubber manifold, it did look to have a small hairline crack in it - I don't think it was right the way through but I sealed it with HT silicone until I can get a replacement just to be sure.

Doused everything in carb cleaner, checked the jets were clear, pulled the slide out to check it etc.

Given that it's improving slightly with everything I do it could well be a combination of lots of little bits wrong with it. It's a 9 year old 125, I could well be expecting too much of it!

4T5 · 3 Jun 2016 at 21:54

I'd suspect the manifold myself as it just sounds as if its leaking back. I don't know whether leaking back is the right term for it but I had issues with a manifold like that before. Trouble with those small manifolds is they look worn but don't actually look split when they are in fact leaking air.

tl45 · 3 Jun 2016 at 22:31

Have you checked the fuel filter within the tank, if it has one? could be inside the pump which can make it a trickier job

Dogbreath · 3 Jun 2016 at 22:44

A manifold leak would be worse at small throttle openings when there is a vacuum and the amount of air going through the carb is very small.

Cyanide said:
Air filter was done not long ago and it all seems fine.

Didn't happen to be about the same time the problem started?

4T5 · 3 Jun 2016 at 22:48

Dogbreath said:
A manifold leak would be worse at small throttle openings when there is a vacuum and the amount of air going through the carb is very small.

Yer you're right there he'd have dodgy tick over & idling problems & a flat spot at take off.

Cyanide · 3 Jun 2016 at 23:04

tl45 said:
Have you checked the fuel filter within the tank, if it has one? could be inside the pump which can make it a trickier job

Doesn't have a filter in the tank afaik, the only filter is after the pump and it's still fine.

Dogbreath said:
A manifold leak would be worse at small throttle openings when there is a vacuum and the amount of air going through the carb is very small.

Didn't happen to be about the same time the problem started?

Sadly not, it was last changed September - October time and the issues started March-ish. Check the filter every month and it still seems fine. I've got a spare so I could change it again just to be sure.

Cyanide · 27 Jun 2016 at 21:05

Update on this - still not fixed, in fact it's getting worse! It's dying at 30mph up hills now

When it's dying now it's not sorting itself when the throttle's backed off. The fuel filter looks completely dry and it takes a huge amount of cranking to fill it back up.

With the bike idling the fuel does flow but the flow rate doesn't look to increase with more throttle and it's not exactly a vigorous flow to start with.

I've now stripped the engine down and checked valve clearances so the current list of things checked or replaced stands at -

air filter,
fuel filter,
vacuum pipes,
inlet manifold,
fuel pump,
spark plug,
carb stripped and cleaned.

There's obviously something wrong with the fuel or vacuum systems but I can't find it and it's going to end up costing me a fortune to keep replacing bits in an attempt to fix it so I've decided to replace the vacuum operated system with a 12v low pressure fuel pump. If that doesn't fix it I'm going to torch the thing