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1970 Chevrolet C20 Custom Camper Pickup
SBC 350, Quadrajet carburetor

I've been having some issues I believe are float related. Wanted to see if my thinking is correct.

Issue #1 - takes a bit of cranking to get it started after it's been sitting. I do have check valve in the fuel filter. This has been going on for a while now. Fires right up when warm. I've also heard this might be the fuel evaporating off. I typically only drive the truck on weekends.

Issue #2 - The other weekend I was driving the truck. Twice, I was pulling out on a road with a 50mph speed limit and a bit of an incline so I was on the throttle about 25-75% for 20 seconds or so when it started stumbling. I let off for a couple seconds and it recovered. I believe it emptied the bowl. That same day I had it out on the highway and it did fine.

After this I changed the fuel filter. I had also changed out my secondary rods. Took it for a drive and it drove great. Good power, good A/F verified with an A/F gauge hooked to a cheap o2.

Issue #3 - This was last weekend. It was about 90* F (32* C) out. Went WOT from ~30mph and it started to stumble almost as soon as the secondaries opened. Later that day, twice, I was pulling away from a light at about 50-75% throttle and when I crossed the street, it started stumbling. Let off and it recovered after a few seconds. Staying less than ~50% throttle, it drove great.

I am thinking it is a fueling issue, not an ignition issue, since when it happens, the engine stumbles for a few seconds, then recovers. I would think if it was ignition related, I would see the issue more regularly and come/go 'right away' rather than having a couple second delay.

Just for reference: Points ignition, 16* base, 18* mech, 12* vac, all in around 2400.

I am thinking the float might be stuck partially open. I am planning on pulling the air horn off and see what is going on in there, and cleaning it up.

Is there anything I should be on the lookout for that would cause these issues?

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  • 1
    Sticky accelerator pump? You'll know when you get the airhorn off. Good luck with that miserable roll pin!
    – SteveRacer
    Jun 16, 2016 at 16:11
  • I've rebuilt many of these back in the day. I agree with @SteveRacer. This sounds like the classic sticky accelerator pump issue. Especially for vehicles that sit for any length of time. If you aren't using fuel treatment, you may want to consider doing so with today's fuels.
    – CharlieRB
    Jun 16, 2016 at 16:35
  • Thanks, I will check that out. Is there a test for the accelerator pump aside from remove the air cleaner and see if it squirts a good stream of fuel? With issue #2, I was pretty steady on the throttle and with #3 it was fine for a second then stumbled. I thought accelerator pump issues would pop up right when I changed the throttle position?
    – rpmerf
    Jun 16, 2016 at 17:06
  • Do you have any way to verify the fuel pressure going into the carb? It seems to me like a fuel starvation issue, like you were saying the bowl is emptying. This may be due to a stuck float not allowing the needle valve to open right, or if fuel pressure was bad it may not be keeping up with demand. I guess an extension of this is not enough fuel flow from the pump itself. If it isn't keeping up, it could be an issue.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jun 16, 2016 at 17:40
  • At this point I'd probably just rebuild the carburetor and resign myself to dumping a bottle of Techron in every other fill-up. Unless you're lucky enough to have a place near you that doesn't sell ethanol-blended gas, in that case start filling up there (although you will still probably need to rebuild the carb).
    – TMN
    Jun 16, 2016 at 18:27

3 Answers 3

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If the float was sticking you would see black smoke when it stumbles and would clear up when its pinned.You're right with the acc. pump, would be a consistent flat-spot, but check for a steady stream anyway.All little things add up, so if points, plugs and ignition leads are not 100%, start there.

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  • Thank You. I will be sure to replace the accelerator pump. I have replaced the plugs and wires about a year and a half ago. I've maybe put 5,000 miles on it since then. I replaced the points and vacuum advance can a couple years ago and set the dwell. I set the timing last summer. The only piece that hasn't been replaced is the coil.
    – rpmerf
    Jun 16, 2016 at 19:14
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    So, it turns out it was an ignition issue. I was getting some bad kickback on hot starts, so I installed an ignition kill switch so I could get the starter spinning, then hit the ignition and not get kickback. I then realized my starter was grinding, so I realigned/shimmed, and now I am not getting kickback. I removed the ignition kill, and it is no longer having issues. I meant to remove that before even posting this question... Also, I looked at my accel pump, and it is only spraying for the first half of the stroke. Seems to be running good though.
    – rpmerf
    Jun 29, 2016 at 16:43
  • Good to hear, usually timing too far advanced makes it "kick back" , but if its o.k. now.
    – boltspinner
    Jul 1, 2016 at 19:24
  • Thats what I thought it was. I was surprised to see realigning the starter fixed it. I was thinking I would have to modify my distributor for more mechanical advance, or purchase another distributor with more mechanical advance.
    – rpmerf
    Jul 1, 2016 at 20:31
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Issue #1

Aren't you supposed to prime the carb by pumping the gas a couple of times before cranking? I think the electrics should be on when doing this. That could at least help.

Issues #2 and #3

They're not called Quadrabogs for nothing. Bogging can be caused by a number of issues. Check that the mobility of the larger flaps and the spring returning them is OK. Also the choke rod and the pump pin mobility.

Could be a number of issues, plenty of info around the web.

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    I had posted in a comment on another answer that I had resolved the issue. It was an ignition issue, not a Quadrajet issue. I do pump it before starting, but the bowl is empty after sitting for a couple days. Takes about 10 seconds of cranking, then pump the gas and it fires right up. I have Lar's document on fixing and tuning Quadrajets.
    – rpmerf
    Nov 6, 2019 at 12:55
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Start the truck pour a half cup of Marvel Mystery Oil right down the carburator a little at a time. Do not let the truck stall. It will smoke a good rate. Once that smoke clears a bit take it for a nice ride. Old school mechanics.

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