Pick your brain

[DoubleDingo]

A couple months back I rebuilt my carburetor with a quality from Cliff Ruggles. Afterwards the truck ran great except for part throttle. At part throttle it had a mild stumble but nothing major. I misplaced my quadrajet books so I have been trying to locate them so I could do the "Tip in" procedure to figure out if part throttle was lean or rich.

Fast forward to today. Took the truck on a trip to get a camper shell. Going there the truck ran excellent and pulled the grades with ease. Coming home I went to go around a semi and the truck acted like it was running out of gas. It was at 1/2 tank. So it stumbled a little and then picked right back up like nothing was wrong. A few miles down the road it happened again. So I decided to keep it cruising about 55 mph and hang in the righthand lane with the truckers. That was going well and then it happened again. It did it three more times on the freeway and that time I was near an exit so I took it. Then it drove just fine getting home.

What do you guys think it is? Float sticking? Control module in the distributor? Coil? Fuel pump? I just drove it to the store and it ran strong with no issues. There was no gas coming out of the carburetor like it was flooding. This has me baffled. I'm going to pull the carburetor anyhow so I can tap the hole over the APT screw so I can fine tune the APT. Anything you can think of to look at on the carburetor while it's out?

 

[DoubleDingo]

Fyi: My data on my phone is about to run out so I won't be able to respond much. But your comments will go to my email, so please send your ideas, I need some other minds to help me figure this out. Supposed to be taking it on a trip in the near future, so I need to get the issues ironed out. Thanks in advance.

 

[bucket]

If you have to let off the throttle for a bit to regain power, I'd suspect the carb is not getting enough fuel or the float level is set too low.

 

[350runner]

X2... I wouldn't adjust the APT screw with out a A/F meter, besides that's unlikely the cause of your stumble.

Sent from the dust in front of you!

 

[chengny]

Maybe you are sucking air into the piping on the suction side of the fuel pump.

Check the section of rubber hose that connects the steel tubing to the nipple on the sender head. Look at mine:

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Those will be replaced before the bed goes back on. And your truck is 5 years older.

If air is getting drawn into the system, it can cause problems with the fuel pump (cavitation and loss of prime) and the carburetor (float operation and screwed up F/A ratios).

Lower tank level and greater engine load will increase the vacuum in the line. If a crack exists, more air gets pulled in as vacuum increases. At some point that air will cause the problems described above.

 

[Quadrajet Power]

Always good to check fuel lines for cracks like mentioned above.
Is you stumble at wide open throttle or just on acceleration? If at WOT, you may need to tighten your air valve tension spring.
What did you set float level at? I would set it at 5/16" on a truck. Make sure power piston operates smooth.

 

[DoubleDingo]

@bucket, my instinct was to feather the throttle, but it basically recovered on its own. Then it ran great, a few miles down the road it did it again. Steady pace, not trying to pass, limping it home so to speak.
@350runner, I don't have an A/F meter, that is on my to get list. Other priorities needing my money.
@chengny, I never thought of those lines. Great idea to change those and the others I haven't replaced yet.
@Quadrajet Power, It didn't happen at WOT, just freeway cruising. The air valve is at a nice setting, it works nice and smooth. The float is set at 12/32" per factory settings of a similar 80's carb I have that also came out of a 3/4 ton truck.

The majority from you all and others I have spoke to say it is fuel delivery related. I will replace rubber lines, fuel filters and check some other stuff while crawling around under the truck and under the hood. But that all got put on the back burner, I messed up my knee last night and can barely walk, let alone climb in and out of engine compartment or crawl under the truck. Actually, crawling is probably doable...lol, just no pressure to knee, oh boy made that mistake already this morning.

 

[350runner]

Hope you heal up soon!

Sent from the dust in front of you!

 

[DoubleDingo]

The bum knee finally healed enough to get to work on this problem. I asked a couple mechanics that do some badass restoration work what they thought of the symptoms as well, and one of them asked if the problem only happened after it warmed up. And it does. When it's cold it runs great, warmed up the stumble crept in. He mentioned the distributor. So today as I helped the girlfriend move to an apartment, the truck was acting up big time. Backing up a little, before moving any if her stuff I got a control module, coil pick up, coil, fuel filters, a new brake booster and a vacuum gauge. I figured if the truck died I stood a better chance of fixing it. It acted up but didn't die which was a good thing.

Got home after helping her move and got busy. The distributor was a tooth off so I got that straightened out after pulling it out. I didn't replace the coil pickup but did replace the control module and coil.

After stabbing the dizzy I went to time it. The mark was way off so I eye balled it and put everything on so I could test drive it. The timing mark and the engines mannerisms were not jiving, so I went by ear and feel. Then went to test drive it. It would barely start. Weird. Grabbed the timing light and went to set it again. With the air cleaner on and only the vacuum advance plugged off, I set the timing to 12* and went for a spin. Not sure what happened there but learned something tonight, I need to have it all put back together with just the vacuum advance plugged off to time it.

Now there is no more stumble when driving. So it may have been a vacuum leak, bad control module, or coil. Tomorrow I'll change the fuel filters to see if they were clogged too.

 

[DoubleDingo]

The vacuum gauge read right in the middle of the green so I didn't touch the carburetor adjustment except for the idle speed. And I adjusted the modulator on the transmission to shift later. That needs so more tweeking. But the truck is much more alive now.

 

[350runner]

Good deal bud! Good to hear things are going well.

Sent from the dust in front of you!

 

[DoubleDingo]

This is all I did yesterday, I kept falling asleep when I posted those others.

Replaced: Coil, Control Module, Brake Booster Grommet.
Adjusted: Timing, Idle, Position of Rotor.
Checked: Vacuum.
Tightened: Carburetor Bolts.

The carburetor bolts needed to be tightened, the front ones were a little loose, the brake booster grommet was dried up. I need to replace fuel filters and test drive it again. So with all I did I don't know which thing fixed it, but there is a chance it was multiple symptoms. Either way I'll carry the old coil and module. When I hooked up the vacuum gauge I unplugged the transmission hose from the tee and plugged in the gauge there. It was steady at 18hg which is why I left a/f mixture alone. What puzzled me though, is why the timing would not adjust properly with the air cleaner off and the vacuum lines plugged. But with the air cleaner on and just vacuum advance plugged, the timing adjustment was like it's supposed to be.

 

[DoubleDingo]

This is all I did yesterday, I kept falling asleep when I posted those others.

Replaced: Coil, Control Module, Brake Booster Grommet.
Adjusted: Timing, Idle, Position of Rotor.
Checked: Vacuum.
Tightened: Carburetor Bolts.

The carburetor bolts needed to be tightened, the front ones were a little loose, the brake booster grommet was dried up. I need to replace fuel filters and test drive it again. So with all I did I don't know which thing fixed it, but there is a chance it was multiple symptoms. Either way I'll carry the old coil and module. When I hooked up the vacuum gauge I unplugged the transmission hose from the tee and plugged in the gauge there. It was steady at 18hg which is why I left a/f mixture alone. What puzzled me though, is why the timing would not adjust properly with the air cleaner off and the vacuum lines plugged. But with the air cleaner on and just vacuum advance plugged, the timing adjustment was like it's supposed to be.

After doing the work mentioned above back in March, the truck ran much better and the stumble disappeared. For a while that is...then about a month ago it started doing a mild stumble again when sitting at a stop light. Thinking it may be flooding a little, because sometimes it would blow black smoke when starting it hot and would not be a tap of the key start like normal. Yesterday I pulled off the carburetor and popped it open. Fuel was getting into places it shouldn't. All the screws for the air horn weren't as tight as they need to be, and the base plate screws were loose too. So I suspect there was a vacuum leak of sorts. Speaking of vacuum leaks, I noticed the rubber fitting that slides onto the side of the air cleaner has a split in it, so that is leaking. Decided to just plug the hose directly to fitting on the air cleaner to remedy that leak. While I had the air horn off I snugged up the needle seat, adjusted the float down a hair, and readjusted the APT screw one half turn down. Removed all teflon tape that I could find on the inlet fittings, and added a little bit of thread sealer paste instead. Also replaced the carburetor inline filter while everything was apart. Installed the carburetor back on the truck, fired it up, got the mixture screws set, and the truck has a more crisp sound out the tailpipe. The throttle response seems much snappier and idling in gear is better. May have to redo the sealer paste on the inlet fitting because that thing will not seal well. I may have got it snugged up enough, but that thing needs to be checked again with the truck running before heading out on the open road.

Had to go to dinner next door for Father's Day so I didn't get to go for a test drive.

By the way, Happy Father's Day a day late to all you fathers out there.