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65 Riv; slight stumbling issue

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My Riv wants to stumble a bit on moderate acceleration whenever the fuel vapor return line is hooked up to the fuel filter. I can unhook it, plug it off, and use a standard two port filter and it runs fine, but I feel that that line was put there for a reason, so I would like to use it. Is it possible that I may need to replace the rubber part of the hose due to age? The stumble isn't awful, but it really shouldn't be there, so I am trying to pinpoint the cause.

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          First thing you need to do is  remove the fuel filter and blow backwards through it and check for rust particles in the filter.

You may have a rusted tank screwing up fuel flow with trash in the system. If you find  rust, it's time to install a new repro gas tank

and a new sock on the fuel sending unit and blow out all the lines and replace the fuel filter and fuel pump. If you find zero signs of rust,

replace the fuel pump and fuel filter and the sock in the tank.

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No rust; blew thru the filter today. Nice clean gas. New fuel pump and sending unit. Tank is clean; had it down a couple times adjusting the arm on the sender to make the gauge read better. Blew some air thru the return line towards the tank, and got gas coming out the retun port on the filter, so it's not a clogged line. Car just stumbles thru first when accelerating FAIRLY hard, but not wide open. Clears up into Second, and is fine from there. Put the 2 port, non-a/c filter back on tonight, and it cleared up and ran like a different car. Plugged the return hose when I did it, and it seems fine. Guess it's ok that way, but I kust figured they used that line for vapor lock reasons, and it should be there. Sorta got me stumped, but at least the test provided somewhat of a solution.

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The return is ONLY on A/C quipped vehicles.  I take it yours has A/C???  Take a fuel pump pressure test without & with the return line & report back.   Also remove the main fuel line hose & take a volume test with & without.

 

Tom T.

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Yep it HAD factory ac and has been converted to Vintage Air by a previous owner. Fuel pump test sounds like a good idea. Car has a new AC Delco pump, but it's probably Chinese made like a lot of other crap, so you never know how good they are. I thought it may even be a carb float issue, but it runs so much better with the 2 port filter that I kinda ruled that out. I'll try to check the pump next when I get a chance.

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Not to ask an insulting question, but are you sure you have the filter hooked up correctly (return line to the tank, output to the carb)?

 

If you have a bad return line, you could be sending more gas down the return line than is intended.  It's not a bad idea to replace all rubber fuel lines with ethanol-resistant lines (e.g. Gates Barricade) as a matter of course anyway, and you may find that it fixes your problem.  If not, test the metal return lines for leaks.

 

You might also have an internal problem in the filter itself, but if the problem disappears with the AC filter installed and the return line plugged, that would seem to indicate a potential source of the problem.

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Something else I just thought of.  Is the outlet for the vapor line in the end of the fuel filter have a smaller hole as a restriction????   It is not supposed to be the opening of the 1/4" hose.  IF it's a china filter it MAY NOT have the restriction in it with the smaller inside diameter.

That's the only thing I can think of. Flowing to much fuel back to the tank & when you plug it now has the pressure/volume the engine needs.

Just another thought.

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3 hours ago, telriv said:

Something else I just thought of.  Is the outlet for the vapor line in the end of the fuel filter have a smaller hole as a restriction????   It is not supposed to be the opening of the 1/4" hose.  IF it's a china filter it MAY NOT have the restriction in it with the smaller inside diameter.

That's the only thing I can think of. Flowing to much fuel back to the tank & when you plug it now has the pressure/volume the engine needs.

Just another thought.

I had actually considered that the 3 port may be defective as well. It came from O'Reill's so it is probably of cheap manufacture. I need to look down in it and see if it has the restriction hole since I did save it. Great suggestion telriv; I'll check it out. Any ideas on where to get a high qualuty  orrect filter that is made properly?

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I would put the car on an oscilloscope and watch which cylinders go lean under the stumble condition. An alternative would be an inductive spark plug voltage tester on each wire (that's what I have now). Once you see which or all of the lean cylinders you can start looking deeper into their performance. Something as simple as a cracked vacuum hose on an intake runner mounted vacuum tap has given me similar problems.

Plugging and unplugging the return line may have some affect on a deeper problem.

 

You might find you have one or more lean cylinders all the time and acceleration on the launch just pushes them beyond their limit.

 

For starters, if you have a vacuum tap on a runner you can cap it to see if there is a difference.

 

I find normal spark voltage is about 3500 VDC. A lean one will be around 2000 VDC or less.

Edited by 60FlatTop (see edit history)
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Posted (edited)

Have the same problem since I had my carb off for rebuild. Installed it with a new AC fuel filter.

My first thought was I did something wrong on my rebuild.

inspired from this post I blocked the return line and the stumble disappeared. I can turn this effect on and off by adjusting this clamp on the return line. 

When blocking the return line completely, my fuel pump becomes noisy.

 

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Edited by OldGerman (see edit history)
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I installed an OLD orange Fram fuel filter that I bought of an NOS parts place. I know, I know, I ain't hip on Fram either, but it was an experiment. The stumbling seems to be greatly reduced, if not gone. Gonna keep driving it and see if it could have truly been that Purolator filter that was on it.

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Did you check the fuel pressure and flow?  It may be a crippled fuel pump.

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2 hours ago, KongaMan said:

Did you check the fuel pressure and flow?  It may be a crippled fuel pump.

Not yet. The fuel pump is a new AC Delco, and the symptoms exhibited themselves before I put this pump on.

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