update-it was the fuel pump. it got worn out working against the vaccum from the unvented gas tank, so now the tank is vented and a new fuel pump is in place and so far so good




Short story is my car needs work as has died a couple times rolling down the highway causing me to pull over, restart and go on my way. 

long story is bought it a few months ago and am just on third tank of gas.  rebuilt engine and they convert FI to webber carbs and put a new fuel pump on (lower GPS) as prior pump was for FI rig.one day going down highway in 5th gear engine quit running after 15".  i pulled over restarted the engine and went on my way no problems and no real problems since then until today.

it has a loud fuel pump when you turn on the ignition .  fuel pump has been moved to frunk and steel line runs down passenger side of tunnel under carpet.  it is usually very loud but some times makes no noise at all.  i   can't explain the variability.  sometimes i would note a hiccup in the power when going down the highway in 3rd or 4th gear at about 3400 rpms, very brief and subtle perhaps worse going up an incline.  nonhighway driving has been great no problems, no hiccups.t

oday was like the first time just going down the highway no problem and engine died so i pulled to side and this time a little harder to start. got it started died on highway in a few yards, did it again same thing very brief duration of engine running as it should.  i pulled off again and opened frunk and tried to unscrew the gas cap and it would not come off.  monkeyed around with it and tilted it and a large amount of air either went in or vapor came out and then gas cap easily came off, engine restarted and drove it home no sweat.  obviously it is off to my mechanic but what do those of you with 914 experience think about the situation.  fuel pump?  vapor lock? other?
thank you, dll

The first thing that comes to mind is that in changing to Weber’s, no provision was made for venting the tank. Normally the fuel pump would send gas to the engine and excess fuel comes back via the return. The tank is vented through the charcoal canister. You now probably have a low pressure pump going to carbs with no return. Any venting to the tank? Maybe you are generating a low pressure zone in the tank, and the pump cannot move fuel against the pressure gradient.

I wonder if the variability in how hard the pump seesm to be working is a result of partial clogging in the lines. If the pump is occasionally cutting out, I'd check the relays on the relay board.

Sounds like the fuel tank vent is blocked off or plugged. Needs to be vented to flow.

Try running with the gas cap loose to see if that makes a difference

+1 on venting being the issue. Just Googled Porsche 914 fuel tank venting and there are others who faced the same problem with Carb conversions. One thread mentioned adding a fuel tank relief valve but had no details.

Prior to EPA all gas caps were vented but we have a special issue with the cap being in the trunk. That said, instead of a loose gas cap you could try replacing the small hose from the filler neck to the expansion tank (to save the OE hose) with a generic rubber hose that has a very small hole drilled in it. This should help for a while but you'll still have gas odors in the frunk.

Best long term solution is either a vent valve, or I believe one the threads talked about unhooking one of the hoses from the engine bay to the charcoal canister.

Good luck,

Ron

I ran a small plastic hose from the filler neck vent to down under the car below the tank. I have carbs with single fuel line (no return). no gas odors in frunk or inside the car. Can be very small ID and will still vent adequately as rate of fuel flow out of tank is quite small.

Though the gas line vector to solve this trouble is a good one, I was faced with a similar issue on a '75 2.0 I had in Redondo Beach. It would just stop. I would move to the right get to the shoulder and it would start after cycling the key.

I was convinced it was something evil, but turned out to be the ignition switch.

Good luck on the Fuel Front!

thanks for all the comments
maybe it is the gas tank
my gas tank certainly seems to have a negative pressure
the pics below are poor as black on black in the frunk, but it looks like i am missing a lot of the vapor/pressure management pieces
the little blue plug in the attachment on top of the gas tank seems like it should be vented somewhere so i will unplug it and see what it does to neg pressure and performanceClick to view attachment

Remove this plastic tank (8 little screws around gas filler cap), you'll find another small vert fitting down there. connect small hose there & run down just under body

if its not the tank being able to vent then it could be something else i have seen that is similar.

a couple of years ago we had a guy with a 928 driving with us up to bathurst and his car was dying similarly. screwed the gas cap off and the cap hissed like there was vacuum in there. but it did not solve the problem turned out the tank outlet filter was clogged.
everytime you switched off the car or let it rest the gunk would float off the filter and it would run again until it all clogged up. apparently this can happen to old 928s because the plastic tank degenerates and bits of its lining start coming off and clogging the filter.
not sure this can happen with a 914 exactly but could be residue in the tank doing similar. its a thought because it acts just like a tank that is not ventilating.

on that line of thought, what condition is your in line fuel filter in.

i would have thought the vapor vent line that is standard in all 914s is still there somewhere in your car but not connected to a charcoal can, simply venting to atmosphere and venting the tank. you need to see if that line might be there.

what model year is the car. after new year 74 that tank venting line runs through the tunnel to the engine bay.

thanks for the thoughts-the line off the filler cap is there, not sure where it goes. it is 1971 and PO says tank was cleaned up. the expansion tank vent was plugged by PO i unplugged it and put a line on it to outside and i no longer get the vaccum effect but not sure that it has fixed the problem, still has a hiccup in power at 3400 rpm but has not completely died (have not driven enough to test it). the inline filter is ok

I had a problem a little like yours and it just ended up being a bad ground on the fuel pump.