Air leak

[smithy]

I am drawing small amounts of air into my fuel system somewhere. Have covered all pipe work in talc but nothing shows up. I plan to pressurise the system with a footpump and cover the pipes in soapy water, what pressure is it safe to go up to? Is the lift pump a likely culprit?
Beta 28hp and cav filter. Tank with pickup pipe, above engine height.
I have already replaced the glass filter bowl as it had a hairline crack also changed the the fuel valve. Any other ideas?

 

[superheat6k]

As you suggest the lift pump diaphragm, possibly the spill at the main fuel pump, or the seal where the glass on the CAV separator joins the glass. All of these I would expect an external sign.

I would suggest a few psi should be fine.

If you can split the system then split it at a joint and pressurise or vacuum either side of the split, to see where the vacuum disappears. Please note air leaking in doesn't mean fuel will leak out. Also is the pick up tube sealed to the top of the tank, or a single piece of pipe from outside. A leak here would certainly go unnoticed.

 

[Billjratt]

If it's 'always been a bit iffy' there's a possibility somebody has overtightened an olive joint or wrecked a soft copper washer. I had to go over all the joints until I found an olive with a diagonal saw cut...( yes, somebody changed their mind and put it together again!) That was after painting every joint with very gloopy stuff that should have blocked any small leaks - to no avail.
It's great when you find a definite root cause.

 

[duncan99210]

Check that the CAV filter is correctly assembled. Very easy to assemble it just not quite right and that leads to an air leak..... How do I know? I'll let you figure it out!

 

[smithy]

The pick up is all one piece until the joint to the pipe, filter I have disassembled and rebuilt several times and found nothing. I will do the pressure test and let you know how I get on. Thanks for the ideas

 

[jdc]

Did you replace the small (ID ~6mm) O ring around the central bolt through the CAV filter? Few replacement elements come with one, so most mechanics never change it (and may not even know it's there) yet it should be done every filter change. Caused poor running on my generator because of air ingress.