Carb Issues - Flooding only under load (1 Viewer)

[RWBeringer4x4]

Hey all -

I put this over in the "clustertruck" thread as well, but I wanted to open this question up to everyone.

For reference: My truck has a 283SBC and a Rochester Quadrajet carburetor. Qjet was rebuilt right after I bought it 3 years ago, and only has about 500 miles through it since then.

I just installed new fuel lines running from the fuel pump up to the carburetor. I wanted to run hard lines instead of the rubber ones that had been there previously. I also tuned up the truck - checked and set timing (20deg BTDC @ idle, 34 @ throttle, vacuum advance plugged), and adjusted idle and fuel mixture (using a vacuum gauge).

Now, it idles great, and has awesome throttle response BUT:

1. If I hold the engine at higher RPM's (~2500) for more than about 5 seconds, it stalls the second I let off the throttle - SOMETIMES. But it still stumbles when you unload it.

and

2. I decided to take it for a spin, and only made it about 100 yards before the carburetor completely and catastrophically flooded. Fuel was sitting in the venturis, coming out the baseplate gasket, and leaking out the accelerator pump. Managed to clear the flood, get it turned around and limped home before the carb flooded again.

It still won't flood at idle, and actually wasn't even stalling when revved sitting still, it's only flooding when I actually put it under load - in gear/driving it.

Any thoughts on what might be causing this?

 

[RWBeringer4x4]

Gonna bump this for some more feedback. A couple guys have responded over in the Clustertruck thread but I'm really interested to hear anyone else's input!

 

[Stumpalama]

#50 carbs are a mystery to me... Sorry.

 

[Pin_Head]

Sticky float?
Junk in the needle valve seat?

 

[Stumpalama]

Maybe all that work on the fuel lines dislodged something...

 

[RWBeringer4x4]

Sticky float?
Junk in the needle valve seat?

This is sort of what I was thinking too - but it doesn't quite make sense to me...If the float was sticking, or if crap happened to make it through 2 fuel filters and into the needle/seat assembly, wouldn't it be flooding all the time? At idle, under throttle, while moving or sitting still, etc?

Why it would only flood when the truck is moving is a mystery to me...

All other symptoms point to a stuck open needle/seat or a sticking float except that if the truck is sitting still, it seems to work fine.

 

[RWBeringer4x4]

Maybe all that work on the fuel lines dislodged something...

Very possible...there was some sediment in my in-line fuel filter that seems to have "vanished" since moving it to its new location...That said, there's a second filter in the carb I would have thought would catch it. But that crap went somewhere!

As I mentioned in my reply to Pin Head, the reason I didn't immediately jump to this conclusion is because it only floods under load...

I did run the carb bowl dry (intentionally) before running a compression test/messing with the cylinders to prevent an unexpected start...I guess the rush of more fuel into the bowl could have picked stuff up off the bottom or dislodged the float or something too...

 

[Pin_Head]

It is intermittent.

 

[RWBeringer4x4]

It is intermittent.

Yes and no. Intermittent that if the truck is in the driveway it will sit an idle fine. It responds to mixture screw adjustments and behaves as it should. However, if I drive it, it can't go 100 yards before it floods, chokes on fuel, and stalls. Then I can hold the gas pedal down, "clear" the flood by cranking the starter, and get another 100 yards before it floods and stalls again.

It also intermittently stalls if I hold the throttle open for an extended length of time then let off the throttle. It stalls as soon as I let off the gas.

 

[Pin_Head]

But you have gas coming out of the bowl vent tubes and this has to be due to a bad float/needle valve system. There is no other possibility.

 

[RWBeringer4x4]

But you have gas coming out of the bowl vent tubes and this has to be due to a bad float/needle valve system. There is no other possibility.

Sounds like I'm yanking the carb then! Was hoping I could limp this one along for a bit but sounds like I'll be investing in another set of gaskets. I'm hoping to replace the old, chokeless, broken, Buick Q-jet with a professionally rebuilt Chevy one soon.

 

[edwjmcgrath]

I don't know about this carb in particular, but on a lot of carbs you can pull the top off if you're careful with the linkage.
Needle/seat/Float can be fixed and reassembled without pulling the whole shebang.
And if you're just looking to get by, if the gasket is fairly new and stays in one piece, I've reassembled carbs without replacing it.

 

[unklwedy]

If power valve is sticky and not shutting off after full throttle exhileration that wood cause over rich mixture. But it don't spain fuel spilling out of main nozzles. You mite want to monitor fuel pressure. Is the fuel pump mechanical or electric?

 

[RWBeringer4x4]

I don't know about this carb in particular, but on a lot of carbs you can pull the top off if you're careful with the linkage.
Needle/seat/Float can be fixed and reassembled without pulling the whole shebang.
And if you're just looking to get by, if the gasket is fairly new and stays in one piece, I've reassembled carbs without replacing it.

I probably COULD work on it in the truck, but a quadrajet consists of about 11 tiny screws that you need to remove in order to take the air horn off - too much of a risk of one going down the manifold. Plus, it's about 4 bolts and a couple hoses to get the carb off the engine completely so it's not a big deal. Disassembling is more of a pain than anything (plus new gaskets, etc.).

 

[fzr400guy]

I would definitely just pull the carb and throw a reduild kit on it. Whatever you do don't replace it with an aftermarket carb (Edelbrock/Holley). I do have to admit that I am currently collecting parts for a TBI conversion. I'm done with carbs!!

 

[RWBeringer4x4]

I would definitely just pull the carb and throw a reduild kit on it. Whatever you do don't replace it with an aftermarket carb (Edelbrock/Holley). I do have to admit that I am currently collecting parts for a TBI conversion. I'm done with carbs!!

Yeah, Quadrajets, especially, are annoying, but they're awesome off-camber and decent on fuel so I'm planning on keeping it. Someday if the 283 bites it, I'll probably bite the bullet and go to a 350 TBI setup.

Right now, I'm strongly considering pulling the trigger on a Sean Murphy Inductions (SMI) rebuilt quadrajet. They fix all the known issues (leaking welch plugs and throttle shafts) and set one up custom for your engine. I spoke with a guy there today - sounds like building a Quadrajet specifically for a 283 (which wants about 1/2 the carb that a Qjet provides) has been done a number of times. I'm currently running a Qjet that I believe came from a Buick Wildcat, which is very possibly 800CFM. Qjets meter air "on-demand" so size doesn't matter, really, but it's probably also jetted really large for such a small V8.

Sounds like the lead time from SMI is about 6 weeks. As opposed to wasting money on (another) rebuild kit, I might wind up money ahead just ponying up for the SMI unit and knowing I'll get something that works, and is properly set up for my engine. Even after I rebuilt my current Qjet 3 years ago, performance has always been marginal, and fuel economy has been pretty low.

 

[Pin_Head]

The Rochester 2GC is the definitive carb for the 283 and is (literally) half the carb of the Q-jet. No doubt the Q jet will work too if set up properly for the 283.

 

[RWBeringer4x4]

The Rochester 2GC is the definitive carb for the 283 and is (literally) half the carb of the Q-jet. No doubt the Q jet will work too if set up properly for the 283.

Yep - it sounds like that's basically what SMI does - limits travel on the secondaries to force it to run off the primaries alone (which, as you said, is all a 2GC is).

 

[BroderickDover]

I would definitely just pull the carb and throw a reduild kit on it. Whatever you do don't replace it with an aftermarket carb (Edelbrock/Holley). I do have to admit that I am currently collecting parts for a TBI conversion. I'm done with carbs!!

i second that motion

no edle

carbs suck

 

[RWBeringer4x4]

Ok, to everyone helping so far - I took some video tonight. I got some REALLY strange readings off my pressure gauge.

The fuel pump is brand new, and should be a 5.5-6.5PSI pump...doesn't appear to be all that correct - but I'm questioning my (also new) pressure/vacuum combo gauge too...

I hooked it into the fuel line where one of my filters was (there is still the one in the carburetor) using a brass T-fitting. You can see it in the video.

The needle is jumping all over the place, but in the first attempt - seemed to smooth out under higher RPM's - it showed a reading of about 6psi.

TURN SPEAKERS DOWN - the first video is pretty blown out - I was up close and personal...



I shut the truck down because my neighbor stopped by. While we were talking, the engine flooded (as usual) so I had to open up the throttle to restart it. This time (and each subsequent time) the gauge was all over the place - completely unreadable...



Last video - this is the quadrajet's primaries. It seems to be dumping a lot of fuel, but I'll let you guys be the judge - I'm honestly not sure what's "normal" and what's excessive fuel...

https://youtu.be/5ca80rJhhKk

Any thoughts?

When shut down, the fuel pressure stays at about 7PSI