Air in fuel filter

[Atrus]

I've been working some bugs out of my '68 1275. One thing I am noticing is consistently, I can get the car to idle and run great at low RPM's (<2,500 or so) but it just sputters and falls on it's face above that. I cleaned the fuel filter and it helped for a few...

...one thing I am noticing is I am getting a lot of little air bubbles coming up from the feed side of the filter through the filter. The filter is filling with air quite a bit, and I am thinking this is starving the engine.

My question is, where the heck are these bubbles coming from? I have a new tank, sending unit, new line, used electric pump, new carbs. The order goes:

tank > Filter > pump

Any ideas? It doesn't leak gas, so I don't feel I have a leaky connection. Tank is full.

 

[Baz]

Is the new tank self venting?

 

[Atrus]

/ubbthreads/images/graemlins/blush.gif I dunno.

New tank (actually bought 4 years ago off moss, car's just not beed used in 4 years. How can I tell?

 

[Baz]

I'm not sure so quote me on this, but a 68 may not have the desmogger in, which takes a line from the tank and sends fumes through the desmogger.
My thinking is, if you have a non-vented tank without a desmogger, there could be fumes (along with air) going through the filter - pump - carbs.
Again, I'm not totally sure, but it does appear that air is or some gaseous stuff is not being allowed out of the tank.
Should be a vapour separator in the trunk.

 

[Atrus]

Baz, think I should pull the gas cap and see what happens? Would that help it?

 

[Dave Russell]

A tiny air leak between the fuel pickup & the filter. Could be on the outside connections or even inside the tank. Small enough to let air in under pump suction but not enough to let fuel leak out.
D

 

[Baz]

I'm not 100% sure as your car is a year where emissions laws and requirements were changing rapidly as well as the plumbing so I'd personally hesitate until leaks have been eliminated as Dave suggested

 

[Biff]

If you are getting air bubbles in the filter which is before the fuel pump then you have an air leak in the fuel line and/or the fuel tank connection. If you have a non-vented cap on a non-vented tank a vacuum would build up strong enough to stop any fuel from getting to the fuel filter. Try running without the gas cap and see if it helps. But I suspect you have an air leak where the pickup tube goes into the gas tank. If the pickup tube isn't pushed in all the way and held tight before tighting the exterior nut that holds the assembly in, you will/can have an air leak yet not leak any gas. If that is okay then check you gas line connections from the tank to the fuel filter.

 

[Sarastro]

I had a similar problem with my 1960 bugeye. I've been meaning to post something about it, but it's a long story, and I didn't have much time. Your similar problem has pushed me over the edge.

My car had a transparent fuel filter just ahead of the carbs. It was full of fuel when cool, but when the car warmed up, it formed a huge bubble, filling most of the filter--I was amazed that fuel was getting through it at all. When I stopped at traffic lights, though, the thing would start to sputter and almost die--speeding up the engine kept it running, and finally it smoothed out--at least until the next light.

Close inspection showed bubbles coming up into the filter from the fuel pump. A fair number of them. No sign of any leaks that might cause air to be sucked into the line--one solid copper fuel line from tank to pump. All of the last few connections were quite tight. So, I suspected it was fuel vapor, not air.

At that time I replaced the mechanical pump with an electric one, mounted near the tank. It had a fuel filter at its input, so I figured the one near the carbs really wasn't necessary. When I removed it, I temporaily installed clear vinyl tubing instead of the usual black rubber fuel line, just so I could see what was going on.

Wow, what a surprise! When the engine was hot, the fuel in the lines looked like champagne--lots of very tiny bubbles forming, coalescing into large bubbles which settled at the high point of the arched fuel lines. The big bubbles did not blow through the lines, and mostly filled them--again, hard to see how fuel was getting through at all. Now, no bubbles were coming up from the fuel tank, though--it was all forming high in the lines.

I noticed that arranging the lines so there was no arch--i.e., the fuel flowed continuously upward, allowed the bubbles to rise to the carbs and blow through the vent in the carb bowl. Once the lines were arranged this way, I tried reinstalling the fuel filter that I had removed, but it formed a vapor bubble again, as before.

So, now, I've reinstalled the rubber lines. The fuel line from the tank goes to a tee, then straight vertically to the carbs. No fuel filter at that point. I keep the lines away from the engine, to keep them as cool as possible, and so far no more problems.

I suspect that today's fuel vaporizes more easily than the leaded stuff common in the 60s. Perhaps more volatile substances are used to increase octane--I've heard that butane is one substance, which is obviously more volatile than tetraethyl lead. Then, the fuel line goes right to the pump, mounted on the engine, which is hot enough to start the fuel boiling. Also, I suspect that the vapor just doesn't move well through a wet paper filter element, so you get a bubble forming in the filter. Some fuel is absorbed by the paper, and blows through with whatever vapor gets forced through by fuel pressure. But, it's not enough sometimes, and the whole situation is really an invitation for trouble.

I plan to put some insulation around the fuel lines, at least in the engine compartment, sometime before next summer. Ugly, yes, but reliability is more important to me right now.

So, what to do in your case? It's a real dilemma--you really need a fuel filter somewhere, I know, but the bubble problem (which, I suspect, is probably vapor, not air) is a serious one. Maybe locate it near the tank, where it is cooler, or use a screen instead of a paper-element filter. I don't know your car well enough, so I can't suggest much else.

 

[Atrus]

Thanks everyone, I'm thinking I may just get new line and do all new line from the tank to the carbs. I have some rubber line I was using from the metal line to the filter, and the filter to the carbs. Also remember not having the best connection at the tank, but I figured if it didn't leak, it was OK. I'll have to check all of these scenerios out, hopefully this coming weekend.

Also, the glass filter I have is a screen filter, no paper element. Ait bubbles collect in the screen, it's almost all air in there, and you can see them pouring out through the screen. Champagne is a good description.

 

[Baz]

[ QUOTE ]
Champagne is a good description.

[/ QUOTE ]

*hic*

Good luck with that, let us know, I'm curious.
What are the metal fuel lines made of? Stainless steel?

 

[Sarastro]

If I were to do it again, I'd use a higher-pressure fuel pump (maybe 10-15 psi?) and a fuel-pressure regulator to get it back to 2.5 psi or so. I'd use a metal-can fuel filter (since the plastic ones might not stand the pressure) and put it between the pump and regulator. The higher pressure would help prevent vaporization, and would allow for greater pressure drop across the filter to blow the bubbles through the filter element. The downside of all this is making sure all the joints are REALLY tight, to prevent leakage or, heaven forbid, something blowing out.

Always possible to think of what one should have done, after the fact, of course. But, it works for now, and so I'll stick with it.

This stuff sure is fun, isn't it? Lots of things to try and to think about...!