Need Help With A Fuel Issue

[digitech]

Still having issues with my fuel system, '79 Spider running a 34ADF on a single plane intake, mechanical fuel pump. Symptoms are:
Engine starts, runs and idles fine - until I've driven about 15 or so miles.
Then, car cuts out and would sometimes restart and stay running right away, other times wouldn't stay running.
I have replaced the fuel pump with a "good" spare.
I have replaced the coil/ignition module with new parts.
I narrowed the issue down to an idle problem, if I keep the engine revs around 2000 or higher, it will get me home.
I put a new kit into the carb, cleaned/blew it out with compressed air and changed both fuel filters, putting a clear one in the engine compartment to see what was going on.

Today, more of the same. But, I found out the filter is only filling to about a 1/4 full when running at higher revs and much less than that at idle - then it cuts out. The fuel then drains out of the filter before you can count to 3!

Only 3 places for the fuel to go - back to the tank, on the ground or into the crankcase. None on the ground, no fuel smell to the oil so it must be draining back to the tank. How??? And why only when warmed up??? When the car is cold, running on the choke, the fuel filter stays pretty much plumb full!

HELP!

[So Cal Mark]

maybe you're pulling a vacuum on the fuel tank. Did you remove and plug the cannister?

[maytag]

maybe you're pulling a vacuum on the fuel tank. Did you remove and plug the cannister?

I was thinkin the same thing. Next time this occurs, release the gas-cap and try it again.

[majicwrench]

Clear fuel filters are NOT a good diagnostics tool and should NOT be used as such. If car will run revved up it is getting plenty of fuel.
So, car simply dies at idle?? It will stay running if you keep revs up??
Most idle issues on these caused by idle soleniod and/or plugged idle passages.
Keith

[digitech]

Keith, I know you don't like clear fuel filters but they can be used for diagnosing an issue, especially one as clear cut as this. Did you see where I removed the carb, cleaned it out, blew it out with compressed air and put a new gasket kit in it?

It's possible the idle solenoid is malfunctioning - funny it would work most of the time though and has a good strong click every time I test it.

The clear fuel filter allowed me to compare fuel supply from this morning when there were no issues (the fuel filter was full) to later in the day when I had problems ( the filter was struggling to stay an 1/8 to 1/4 full and then drained back immediately when I shut the engine off).

I do have my canister hooked up and a vacuum line from it to the carb; but this has been this way since I got the car put back together and I've been driving it for over 700 miles before this nonsense started. I was thinking fuel tank as well and I did remove the cap but I still had the issue.

I'm open to all ideas and suggestions and will try each one if I haven't already but I believe this is something a little bit strange ......

[majicwrench]

I certainly saw that you had repaired carb. But I have personly overhauled these carbs only to have the idle jet (tiny) plug almost immediatly.
You say it stays running reved up??

I have seen those clear filters many times on running engines with very little fuel showing in them. The fact that it drains back when shut off is interesting, but not sure how it relates to what is going on, what you really need to know is if there is fuel in the float bowl. And whatever is causing fuel to "drain back" from the filter cannot cause fuel to "drain back" from the float bowl anyway.
Keith

[digitech]

More info.....
Today I started the car in order to bleed out the air I introduced into the cooling system when I took off the carb. The car sat in my driveway, idling for 30 min. and running quite well. When I shut the car off the fuel remained in the filter as evidenced in the 15 min. and 45 min. pics.





Got the air out, cooling fan on twice so I lowered the car and took it for a 30 min. drive, covering about 25 miles at all speeds. It ran beautifully and idled well at every stop sign. I returned to my driveway and the car began running a little rough. Raised the hood and this is what I found. When I shut the car off, the remaining fuel in the filter drained out immediately. Even with the engine running, it can barely put fuel into the filter, nothing like earlier, before the drive.



I only have the one check valve installed in the line going to the fuel pump as you can see, none in the return line - but it was like this for the first 700 miles as well and everything was fine then. I will add the other check valve tomorrow but I'm really thinking something is internally wrong with the carb - like a hairline crack that expands with heat allowing the fuel to drain back.

[124JOE]

a leaking float valve?

[digitech]

Nope, put a new float/float valve in - no change. Installed the other check valve into the return line as shown in the maint. manual today - no change.

[124JOE]

i had a similar problem car died wont start then did then died...
it was a ground problem.my bro grabed the neg cable and pulled it right out"after market connection"
also the wires in my dizzy were bared "i taped them and coverd well"

[digitech]

Well, this isn't anything like that - besides, everything is new. I can drive the car on any given day for at least 30 miles and it runs very well. Then, it's like it can't supply enough fuel to keep the engine running. That's the symtom and if you look at the fuel filter, you can see the level get lower and lower while it sits and idles until it finally conks out. After a bit, you will be able to get it restarted and if you keep the revs up when you have to stop, you can keep driving although it may not run quite as well. The next day, it will all be a repeat - start out running and idling great and then downhill from there, if you run it long enough.

I feel like something has gone wrong with the carb as I was trying different jets in the primarymain/air when it last ran right. And before anyone asks, it's all back to original and I have taken the carb apart twice since.

[124JOE]

im thinking your fuel pump is over heating or the filter is cloging
even if there new you can have problems with them

2 pressure gagues with tees, one before the filter and one after the filter will tell you witch one

[digitech]

I have never even heard of a mechanical fuel pump overheating - how is it even possible? The fuel filters are not plugged up - they are both brand new and free and clear.

The problem is the fuel begins to drain back into the tank after a while of running - didn't know it was possible and in all my years of working on cars, I've never seen it until now but that appears to be what is happening. I just don't have any idea what the cause could be other than something wacky with the carb. Picture trying to fill a cup that has multiple holes in it -that's what is happening.

I hate to spend the $$$ but as soon as Ricambi gets some carbs in, I'm buying a new one and hopefully that will be the end of it.

[RRoller123]

Vacuum being pulled on the tank somehow?

[maytag]

it doesn't seem like this is your carb. I can't think of anything that could be going on in your carb which would result in fuel being drawn back into the tank (and let's face it, it's being drawn, not draining. it's not gravity-induced, because your fuel pump would overcome that.) You're getting a vacuum of some sort.
it is possible for your pump to "cavitate" (that may not be the correct term for this sort of pump, but it loses it's prime) and yes, I guess hypothetically it is possible this could be heat induced.

Cheaper than buying a new carb just to "try it out" would be to add a small / cheap electric fuel pump as a "helper-pump" near the fuel tank. run it that way for awhile and see if that solves the problem. My guess is it will, and then you don;t have to spend $400 on a new carb, you can track down the issue from there.