Fuel pressure, carburetor

[Reinhoud]

Gents,

 

My bike has some weird issues lately. After say 15km the bike start running like crap, when I keep going it just slowly dies. Turn it of for about 30 seconds, and off it goes again like nothing happened. Symptoms of an empty tank.

Checked the fuel supply, that's all good. Later I thought, because of the way it dies, it might be the ignition, changed over the CDI unit, that's not it.

 

So, today I thought I'll check the fuel pressure. Something went wrong with tapping the NPT thread apparently, so the gauge is leaking a little bit, that means the reading isn't very accurate..

Looks like the pressure is about 0.5psi. It used to be 1 psi. With 2 psi the needles in the floatbowls started leaking.

There's enough pressure in the up-pipe that the bike doesn't run when the pitot tubes aren't connected... Even on idle. Not sure if this is important.

 

My question; what happens when the fuel pressure is low? Could it cause the problems I have? Can't get another gauge today unfortunally..

Do the Malpassi regulators need re-adjusting occaisonally?

Other question, how long should it hold pressure after the ignition is turned off? Not that that really matters I think, but it shows there's a leak somewhere if the pressure drops

 

Thanks

Edited February 8 by Reinhoud

[clivegto]

I'm running 1.5psi, could be a kink in a fuel pipe. 

[Paulm]

Mines running 2psi,as Clive says check for kinks in any pipes and make sure the fuel is definitely returning to the tank,in my experience of Malpassi fuel regs,you don't need to check the fuel pressure unless you're having issues.

[badger]

Fuel pump blocked? I've got an in tank pump, the little sock filter pre pump had got a load of crap in between the 2 membranes. Gave these symptoms but I know most people don't use an in tank pump so could be the pump or a blocked filter if you have one? 

 

[Reinhoud]

Thanks guys.

 

First time it happened I thought it was the fuel filter. Deleted that after I noticed the fuel tap has got a filter, same problems. Fuel pump seems to be working alright, I pumped 10 liters into jerry cans without problems..

 

[TLRS]

Anything changed around the time this problem started to occur?

[TLRS]

Any chance boost makes it’s way into the fuel delivery? Probably won’t empty the float bowls but maybe pushes into the fuel line and changes the flow to the bowls.

[Reinhoud]

On 2/16/2025 at 12:14 AM, TLRS said:

Anything changed around the time this problem started to occur?

No, don't think so...

 

First it did it occaisonally, sometimes it did it, sometimes I could ride it for 100km and nothing happened.

But lately it happens quicker, first around 50km, then around 40km, lately around 25km..

[Reinhoud]

On 2/16/2025 at 12:22 AM, TLRS said:

Any chance boost makes it’s way into the fuel delivery? Probably won’t empty the float bowls but maybe pushes into the fuel line and changes the flow to the bowls.

It needs pressure in the floatbowls. When I disconnect the hoses from breather floatbowl to pitot tube, it dies straight away..

It gets progressivly worse. I can feel it coming up, first a littlle bit, and when I keep going it's getting worse. Then leave it for a while, and she goes again like nothing happened...

Looks like a clogged filter. Deleted the fuel filter, it has one in the tap, same problem.

When I check everything, everything looks fine.

I can pump 10 liters into jerrycans with the fuel pump, and nothing weird happens..

 

[TLRS]

1 hour ago, Reinhoud said:

It needs pressure in the floatbowls. When I disconnect the hoses from breather floatbowl to pitot tube, it dies straight away..

It gets progressivly worse. I can feel it coming up, first a littlle bit, and when I keep going it's getting worse. Then leave it for a while, and she goes again like nothing happened...

Looks like a clogged filter. Deleted the fuel filter, it has one in the tap, same problem.

When I check everything, everything looks fine.

I can pump 10 liters into jerrycans with the fuel pump, and nothing weird happens..

 

Makes sense to have pressure in the floatbowls on a boosted engine. Is your fuel tank pressurized though.. could float valves leak boost?

[peter1127]

check for kinked or ruptured fuel/vacuum/boost hoses. Sometimes hard to find. Or ruptured membrane in the malpassi.

Could also be clogged fuel tank vent, that gives this symptoms too.

Edited Monday at 09:53 AM by peter1127

[Gixer1460]

1 hour ago, Reinhoud said:

When I disconnect the hoses from breather floatbowl to pitot tube, it dies straight away..

It gets progressivly worse. I can feel it coming up, first a littlle bit, and when I keep going it's getting worse. Then leave it for a while, and she goes again like nothing happened...

First line - that shouldn't happen! Even if the pitot was open, the engine should idle - badly, but it should idle as there is no significant boost at idle.

Second line - sounds electrical with those symptoms - something heating up to a failure point - cools down and it works again.

[clivegto]

As Peter said check tank vent. 

[Reinhoud]

On 2/17/2025 at 8:49 PM, TLRS said:

Makes sense to have pressure in the floatbowls on a boosted engine. Is your fuel tank pressurized though.. could float valves leak boost?

That's what I'm starting to think..

[Reinhoud]

On 2/17/2025 at 8:51 PM, peter1127 said:

check for kinked or ruptured fuel/vacuum/boost hoses. Sometimes hard to find. Or ruptured membrane in the malpassi.

Could also be clogged fuel tank vent, that gives this symptoms too.

I'll have a look..

[Reinhoud]

On 2/17/2025 at 8:51 PM, Gixer1460 said:

First line - that shouldn't happen! Even if the pitot was open, the engine should idle - badly, but it should idle as there is no significant boost at idle.

Second line - sounds electrical with those symptoms - something heating up to a failure point - cools down and it works again.

My turbo blows a bit on idle already..

With previous turbo's it did idle without the breather hoses connected

[Reinhoud]

I ordered a recon kit for the carburettors..

[Reinhoud]

Blocked tank vent, should that give already problems when there's a liter of fuel is used?

 

Where does it vent through?  Will a bath in an Ultrasonic cleaner do any good? Or do I get water in places I don't want it? (lock barrel)

[clivegto]

Go for a ride withe the fuel cap slightly open to check tank vent. 

[Joseph]

If you notice that you can ride the exact same distance between departure and engine cut out, it would tend to point towards fuel starvation from tank vent or kinked hose.

A partially blocked fuel line due to being kinked in my experience cut the bike out in around 1-1.5km

Tank vent partially obstructed may take longer ? However on a turbo bike the draw of fuel sucked by the pump is massive compared to a gravity system so it may not be related (you'd think it would bottom out much sooner) but those things are the easiest thing to check and fix 

[peter1127]

8 hours ago, Reinhoud said:

Blocked tank vent, should that give already problems when there's a liter of fuel is used?

 

Where does it vent through?  Will a bath in an Ultrasonic cleaner do any good? Or do I get water in places I don't want it? (lock barrel)

GSXR has a tank vent which is sufficient. If only vent in the cap this might not be.  Imo best to check all hose routings/connections for kinks/leaks anyway. We all know the challenge to get it properly and keep it properly. Drive with open cap might prove messy, I would just remove the valve bit in the cap so its for sure unrestricted and try that. 

With these problems its a process of elimination. Just check all the stuff you can think of and at some point there are not many things left it can be. Since it worked before its not a system design issue. Since it works the first 15km and works again after a short while again that eliminates quite a lot.  in order of likelyhood I would say:

1. tank vent

2. hoses kink/leak

3. Fuel filter almost blocked and/or crap in carb bowls

4. Enrichment system problem: plungers are pushed out by boost. But that remains after a short while unless you manually switch off enrichment at that moment

5. battery undervoltage by fuel pump drain/charge system failure. Battery recovers a bit, you can drive a short while again. At some point you cant start anymore so you would have noticed that :-)

6. coils fail getting hot. Unlikely they recover that fast if at all

7. weird electronic gremlins with connector loose or bare cable touching something. Solves itself because you brake and come to a standstill. Unlikely that would repeat itself exactly the same way.

8. Voodoo crap

 

Best to look at the simple/likely issues first. Hope you dont get to 8.

 

Edited 10 hours ago by peter1127

[Captain Chaos]

7 hours ago, peter1127 said:

8. Voodoo crap

If you do get to this, we need pics @Reinhoud