AAAHHHHHH!!!! Fuel problems

[jbarron]

Truck seems to be starving for gas. Started a couple weeks ago. When I get into the peddle to pass or get up to speed it will start stubbling and I have to let off to let it recover. It was happening only when the secondaries kick in, but now it will do it with moderate sustained throttle. I thought it might be something in the carb so I took it apart today and all was well. I changed the neddle and seat just for grins and checked the float level, still no help. I changed the fuel pump still no help. I had changed the fuel filters about 3K miles ago and they all look good. I'm running external filters for the dual tanks and all the fuel hoses have been changed. No filter in the carb. It does the same thing with either tank.

This is on a stock 350 with a Q-Jet.

[leddzepp]

Did you check your ignition system? Seems as though you have everything covered concerning fuel delivery.

[jbarron]

I haven't check the ignition. Doesn't feel like an ignition problem the way it surges when it starts stubling. I updated to an HEI system about 5K miles ago. I try a new module just in case.

[DB Smith]

Originally Posted by jbarron

When I get into the peddle to pass or get up to speed it will start stubbling and I have to let off to let it recover..

Could be spark advance. Do you have a vacuum advance hose from the distributor to the carb?

[jbarron]

Yes I am running vacuum advance. It will rev all day while parked and not do this.

[leddzepp]

If you are only having problems as you increase RPM's and not at an idle, I would say you have an ignition problem. If it "surges" I would also suspect an ignition problem. Just my opinion

[jbarron]

It will rev fine in park with no problems. It revs fine under load, but then falls off like it is starving. I'll check out the ignition system and advance just in case.

[DB Smith]

So, it's idling and revving normally with no load, right?

It sounds like it's doing it under load only...like hills, sudden acceleration, and passing?

A little less advance on the timing might help performance under a load. How many degrees BTDC are you running?

[Bus Ted Knuckle]

changing the needle and seat won't help the secondaries, is it a quardrajet? If so, sent the wind-up spring tension. The secondary flapper is probably opening up too early.

[jbarron]

The carb is responding right. The secondaries open up fine and stay open until it starts to stubble, then they shut down. I can make this happen without opening up the secondaries, and it had been running great. When it started doing this it was only doing it after holding the secondaries open for quite a while, then it started getting worse over a couple weeks. Now it will do it with moderate throttle, no secondaries. At highway speeds if I try to speed up just a little(60 to 70) it will start stubbling and surging like it is running out of gas. I can back off and cruise and then slowly build speed.

[dwcsr]

whats your fuel pressure. Sounds like it running out of gas.

[WorkinLonghorn]

When I run into a problem like this that totally baffles me I've learned to not take ANYTHING for granted. If it was me I'd disconnect the fuel line at the carb and watch/measure the flow and also check the pressure. Check the accel.pump action while manually opening the throttle linkage and make sure it sprays.
Then I'd check the spark. Pull a plug and watch the quality of the spark.
Then put a timing light on it and watch the advance.

[Boog]

I think it'd be a good idea to get the fuel pressure checked first.
And even though your fuel filters look good it is possible the pickup screens in the tanks are clogging up thereby restricting the fuel supply upon acceleration with a load on it.

[jbarron]

I put a fuel pressue guage on it. I've done this before and it always bothers me, since the guage will drop down to almost no pressure. This is right at the carb and after the fuel filter. I'm not sure if the pump is suppose to read full pressure even at idle when it doesn't need the pressure. Either that or the guage is bad. The low pressure is one reason I replaced the fuel pump, but I had checked this in the past and it has always been this way. I get some wide spreads in pressure. If you rev it it really doesn't do much. The pump is pumping, since you can see it at the filter(clear plastic type).

I will pull the fuel line off and check flow rate.

I was also thanking about pulling the fuel line off at the pump and blowing the lines out back to the tank.

[smoked68]

I had the same problem, it was the fuel pump, so I changed it and still had the same problem. After a lot of whining I replaced "THE NEW" pump with another and problem fixed. Bad pump. Good luck...

[jbarron]

I hate to hear that. That pump is a pain to get off. If anybody needs to change the pump remember to use the trick of putting in a longer bolt in the bolt hole that lines up with the pump rod. It holds the pump rod in place so you don't have to mess with it.

[Boog]

Go with an AC pump and you may not have to do this again. I've never had any complaints with them.

[dwcsr]

If you have a 3 line pump plug the small return. It cleared up my problems.

[jbarron]

Fixed it. I had one fuel hose from the tank switch to the hard line that I did not replace when I was putting on new lines. It looked good at the time and still looks good, but the inside is goo. This new gas is eating rubber lines fast. I'm going to replumb all my lines after seeing this.