Engine Issues

[Teletom]

Ok to get quickly too it I have a '77 fiat 1800 that has an odd issue. This issue has persisted through a bottom up engine rebuild as well.

When driving once I hit about 75MPH or 3600-4000 RPM it gets jerking issue where it feels like it is running out of gas or misfiring. Now the there is no visble change in the RPM's and this issue gets worse the less fuel I have in the tank. Now the issue never goes away just "More gas in tank" = "Higher top speed" is the best way of putting it.

When this issue originally occurred I though it might be the fuel pump or something in the tank. Tank was completely clean and bought a new electric pump. Since I originally had the issue I rebuild the engine from the ground up, changed the dizzy to electronic point and cleaned up most of the emissions stuff so there is very little left to check.

I can only think this is from the carb (which is a weber I believe) or still a fuel sending issue. Could excess fuel do this or only too little, I have considered that it might be a vacuum leak but I can not recreate the situation while in the driveway. I dont believe it is the tranny either as it does this in all gears once the fuel level get low enough. It is a weird situation and hopefully someone has dealt with it before.

I will answer any questions you guys have, help!

[Arved]

If its not the fuel, I would look at timing. Check your mechanical advance to see if that is stuck. A good way to do this is hooking up a timing light and see if you can see it advance with increased engine speed. A symptom of late timing might be the engine running hot.

Arved

[klweimer]

Obvious culprit is your fuel filter, less obvious might be an issue with the fuel vapor venting system. Does it do the same thing when the gas cap is left loose or removed?
Kirk

[fiatfactory]

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[PhillySpider]

I had the same issue, came down to a bad check valve on the vapor line...

[Teletom]

Ok new update on this, trying to run the gas as low as possible and keep it running . Currently at half tank and it will not reliably get above 40MPH. I replace the fuel filter in the front and removed the old plastic check valve in the front as well to see if either of those would help. Now I am thinking it may be the fuel pump and considered too much fuel coming through as a possibility but then thought that would not make sense. For your information I have the FACET pump from autoricambri (the small square one) which they state is rated for 1.5 - 4.0 PSI @ 25 GPH is this too low? or okay. I will get a more expensive pump asap with better PSI if that is the issue.

Anyone have any more ideas? Also i can not replicate the issue while idling, which originally made me think maybe a transmission issue . However whenever I fill the tank to full the issue changes to allow me to go faster so I have ruled that out as an issue. Also when the engine was rebuilt the tranny was checked and found to have no issue or extensive wear.

Thank you in advance.

[Teletom]

One other thing, is this possibly a vapor lock issue? it has a fuel return line from the carb that should prevent this right. Sorry if I am incorrect in my assumption

[seabeelt]

What does the fuel sending unit look like? Perhaps the sock at the end of the pick-up tube is varnished/blocked and you only get fuel when the tank is high enough to get over the top part of it?????. Just a thought

[baltobernie]

For your information I have the FACET pump from autoricambri (the small square one) which they state is rated for 1.5 - 4.0 PSI @ 25 GPH is this too low? or okay. I will get a more expensive pump asap with better PSI if that is the issue.

No, the Facet pump installed correctly is right for your car. Webers cannot tolerate more than 2 psi.