Runs smooth, then sputters, then smooth, then sputters

[DoubleDingo]

What is on the truck: Vortec heads. National carburetor quadrajet for an electric choke 3/4 ton truck. All the factory tubes and pieces. No EGR, just smog pump and plumbing for that.

The new carb had been doing a great job since October 2022, and about 4 tanks of gas had been run through it without issue. It was running like a top. Sitting a red light, idling smooth. Light turns green, I am at the front of the line, I hit the throttle and start heading up the bridge over the rail yard, and it starts sputtering. A noticeable loss of power, and running rough. It was like it was running on 4 cylinders. I never got into the secondaries, and rarely do.

I parked it and finally got a chance to check it out on Saturday.

What I have done: Checked vacuum hoses with a vacuum pump, checked choke pull-offs, snugged up the carb mounting bolts, they were a tad loose but nothing major, etc. Everything seemed to check out. As an experiment, I added a couple of vacuum tees, so I could run the distributor and the choke pull-off from the base plate, as well as the line that goes to the air cleaner housing.

With that change, I hooked up the vacuum gauge to the line that goes to the air cleaner housing and fired up the engine. It pulled a steady 16 inches. This truck requires checking the timing with the air cleaner housing installed, but with every port connected accordingly, it was like the air cleaner was installed so I checked the timing. Timing is at 10 BTDC. Right where I set it when I stabbed the engine years ago. Timing is good. I adjusted the air mixture screws and got the vacuum to 17 inches.

There are audible fuel squirts with the engine off, no play in the throttle shaft, no vacuum leaks I can locate.

With the engine good and warm, and I noticed the vacuum gauge was moving while idling, not bouncing, just slowly moving back and forth from 17 to 15 and the idle fluctuated according to the change in vacuum. I shut it down to change the fuel filters. Fired it up, and the vaccum was still fluctuating between 17 and 15. And then all of a sudden it smoothed out again.

Since it smoothed out I took it for a test drive. It started off smooth and then it started sputtering again. I noticed if I feathered the throttle or got up to speed it would usually clear up and run smooth.

Any ideas?

 

[fast 99]

things to check

Fuel filter, pressure
secondary air valve opening too fast
PB booster leaking vacuum
if HEI centrifugal advance spring broken
possible ignition issue, weak coil

 

[DoubleDingo]

Fuel filters replaced. What is pressure secondary valve? I'll check the booster lines and distributor

 

[fast 99]

Secondary air valve is that big butterfly above secondary throttle plate. Should close easily and have spring tension when closed. There is a spring wind up specification included in most carb kit directions. Usually is 3/4- 1 turn. There is a small Allen screw on top housing right rear that needs to be loosened and a straight screw head on the spring shaft. Unless the air valve hangs up or doesn't close, most times adjustment is not required.

 

[DoubleDingo]

Would that secondary have any affect on the idle or low rpm partial throttle? I thought those only came into play with full throttle?

 

[DoubleDingo]

Saturday night while driving up a grade, it did sputter and backfire, and then picked up again and ran like a top.

 

[DoubleDingo]

It still fires up with a tap of the key.

 

[DoubleDingo]

Drove around after work. Let's eliminate secondary discussion, I am not driving WOT. I will eventually check that, but the secondaries worked fine before this started happening, and the one time I had to gun it to get out of the way of a truck with this new carb, it laid rubber.


The issue is literally it runs great, then stumbles/sputters/whatever metaphor or verb one needs to enter there, and then it will run great, stumble/sputter whatever, repeat.


I am suspecting vacuum leak because I had some a few years ago that I fixed. Some of what it is doing now is similar to what it did back then, it just never cleared up like it does now.

 

[Radiohead]

My mind keeps going back to a fuel starvation issue.

 

[DoubleDingo]

My mind keeps going back to a fuel starvation issue.

If it is, it's intermittent. Once it clears up it runs strong like normal. And going down the road at higher speeds it doesn't seem to happen.

I'm wondering if a vacuum leak can cause the vacuum advance to fluctuate, thus altering the ignition timing? Seems like it could.

 

[Radiohead]

Is/are the fuel tank(s) venting properly?
Perhaps the fuel draw at low volumes isn't enough to overcome a return restriction but at higher flow the vacuum gets equalized.

Just spitballing ideas... easy enough to test by loosening the gascap and driving around.

 

[DoubleDingo]

I tried loosening the cap. Didn't help. I was wanting to adjust the choke before leaving this morning, was running late, then I went to leave and some dude is acting weird and doing weird stuff near the house. My wife was leaving late today, so I called in to let my supervisor know I was going to be late to make sure this dude acting weird didn't do something to my wife. I had already loosened the cap and started the truck while I waited to see if the dude was going to clear out. He did not, so I shut it down, went inside to let my wife know what was going on, and that she would not be going to her car alone.

I took that time to as an opportunity to adjust the choke. When I fired it up again it did run smoother. I thought I had fixed it. Nope.

It's still sputtering, but it does seem to be less than it was before. I will run it some more with the cap loosened to see if that helps, since it was a short run this morning.

I did notice another issue, though, the oil pressure gauge is now pegged to the right, it does not move. OY!

 

[DoubleDingo]

My mind keeps going back to a fuel starvation issue.

Just talked to Pop about this. He said it sounds like the power circuit has a clog. We discussed that there is probably water in the fuel because of all the rain we got during the winter months. He said to get a bottle of HEET, and do the Mexican (to some)/Italian (to others) tune-up. So, I will grab a bottle of HEET and fill up the tank again. I have used a half tank since Saturday trying to get it to clear and to diagnose the problem.

 

[Radiohead]

Run more fuel out with the heet before you splash on more. No point in having a tank of half crap. Get as much of that out before fresh is my motto.

I'm going through this with the mini truck on the farm, I poured in 5 gal of what turned out to be contaminated fuel to top off the tank. Right away within 2 miles I knew I was fornicated. Put a bunch of heet in, and at half a tank topped it off. Now I have a half fornicated tank of gas. Damned if I'm pouring out $50 over this, but it will take weeks to go through it. If it was a daily, this would be over with by now.

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Gotta get the square going....

 

[DoubleDingo]

At this point I have two bottles of motor-purr in the tank, and 11 gallons of fresh premium was added Saturday, the second bottle of motor-purr was added Tuesday at 3/4 tank. One bottle treat 21 gallons. I like your idea of having a stronger mix of HEET before adding fresh. Going to drive the truck the rest of the week to try and clear things out.