351w stumbles

[Madgyver]

I can't narrow this issue down. I'm thinking fuel pump. Or carb (qjet).
It stumbles going uphill in gear under load as I throttle. Goes away when I let off the accelerator but then I'm slowing down.
Any ideas?

 

[bronconut73]

Accelerator pump diaphragm.?

 

[DirtDonk]

Classic clogged filter and/or weak pump.
As is always the case in life, this is not guaranteed. But it's exactly how those two things start showing up.
No matter how new the filter, or how new the pump. Either can happen at some point.

The accelerator pump is a good idea too I think. But it would not stay bogging during a climb. The pump is only for the transition phase of opening the throttle blades. Once the throttle is held at a steady stage, the pump's effect disappears.

Paul

 

[DirtDonk]

Hmm. So Mad, is it only when you're actively trying to accelerate? Or the whole time you're under the load?
If it's only during throttle-up, then bronconut's idea of the pump is more likely the culprit.
That, or a vacuum leak leaning the mix way out.

Paul

 

[Madgyver]

Last time it happened in 06, it was that I couldn't go over 40mph, it was the mechanical stock fuel pump. I put in a Carter electric and it was great up until last year. It was also missing sparks and found that I had a bad coil terminal. Fixed it, now to fix the stumble climbing an incline at speed.
Im thinking bad fuel pump. I have another rebuilt qjet that i could swap in to see if problem persists.

 

[Madgyver]

Hmm. So Mad, is it only when you're actively trying to accelerate? Or the whole time you're under the load?
If it's only during throttle-up, then bronconut's idea of the pump is more likely the culprit.
That, or a vacuum leak leaning the mix way out.

Paul

it goes away when I let off the throttle but then I slow down. I ease into the pedal slowly to keep up. Or downshift.
Only happens on inclines at speed.

 

[okie4570]

Small internal screen on the qjet clean?

 

[B RON CO]

Hi, also could be float level not set correctly. Good luck

 

[gnpenning]

As mentioned filter or pump. I would go with the filter first. One load of bad gas will do it. What filter are you running? ?

The stock carb one or something else?
If stock is it the long one or short one?
Cheap easy fix, plus you can cut the old one open and check how dirty it was.

Got a smok'n deal on my plow truck because of a bad $4 filter. Guy even said the last time it was acting up It was the filter. He was to lazy/ cheap to check. Cost him a few grand. Long story, but I didn't feel bad in the end.

Volume and pressure test are quick and easy while you're doing the filter.

 

[wildbill]

;D;D;D Mite look at the plug wires had this happen on a 400 ford any hill or under power it would stumble ran ok other wise good luck.%)%)%) Bill ;D%)

 

[75junebug]

Mad, are your secondary choke plates adjusted right(whatever that means)....My Qjet would kick in momentarily then bog and stutter until I let off....I changed both filters and changed over to a carter rotary fuel pump on mine before I started jacking with the tension spring on the 2ndary choke plate...Finally found a happy medium between too loose and too tight.

 

[Boss Hugg]

Sorry bud. Gotta say it.

Don't go uphill! ;D

I know nothing of a Qjet so I really can't help with that.

The carter pump...it was great until you started the rebuild project last year? or when did it become "in-great?"

It shouldn't be hard to change the carb just to try it, should it?

 

[Rustytruck]

First 2 things with a Quadrajet to worry about is a bad carb base gasket and the stupid bronze rock fuel filters if your running those.

If replacing the base gasket look carefully at the gasket and the carb. There are different ones and if you get the wrong one you will have a vacuum leak under the carb.

After that I would count the turns on the mixture screws and remove them and spray carb cleaner through the screw holes and put the screws back in. Also check for vacuum leaks on the intake and the carb connections. Good steady vacuum is important to the Quadrajet other wise it it screws with the fuel transitioning between idle and mid-range fuel control circuits in the carb.

 

[Justafordguy]

Hey Mad, I'd check the vacuum advance in the distributor also. Mine had the same kind of hesitation when the diaphragm busted.

 

[pcf_mark]

If it is stumbling it must be drunk..sorry someone had to say it.

Verify the vacuum advance is working. Disconnect the hose from the carb and put manifold vacuum to it or just suck on the end. If the idle speeds up it is working.

I can't see it being a fuel pump, filter or float level if downshifting makes it better - each of those require more fuel not less and the incline is still the same.

I think it is part throttle lean. As a test once you clean the idle screws as Rusty Truck suggested then install them as a you found them x number of turns out. Then back them out a half turn each and take a test drive to see if it fixes it. This should make ut run richer at part throttle.

 

[DirtDonk]

...I ease into the pedal slowly to keep up. Or downshift.

I can't see it being a fuel pump, filter or float level if downshifting makes it better - each of those require more fuel not less and the incline is still the same.

Good catch Mark. Yeah, what he said Mad. If you can get rid of it by dowshifting, it's not likely the pump or filter. Unless...
However, downshifting under just the right circumstances does sometimes use less fuel, not more. But more often than not, you're going to "suck air" just as much either way.

Q-jets have LOTS of things that can go wrong with them. They're amazing devices really, and work really well most of the time. But if a metering rod breaks, or gets stuck, or something else inside gets gummed up, you could have strange issues.

So to clarify, just where and when does downshifting help the problem go away? Got video?;D

Good luck.

Paul

 

[Madgyver]

Good catch Mark. Yeah, what he said Mad. If you can get rid of it by dowshifting, it's not likely the pump or filter. Unless...
However, downshifting under just the right circumstances does sometimes use less fuel, not more. But more often than not, you're going to "suck air" just as much either way.

Q-jets have LOTS of things that can go wrong with them. They're amazing devices really, and work really well most of the time. But if a metering rod breaks, or gets stuck, or something else inside gets gummed up, you could have strange issues.

So to clarify, just where and when does downshifting help the problem go away? Got video?;D

Good luck.

Paul

I'll try get a video.
It did it again yesterday driving on a level road and happens on acceleration at about above 50% throttle in 3rd gear. I let off throttle and it goes away but it slows down. I've been making sure no one is driving behind me when I'm messing with the throttle.
Driving around town (stop and go) traffic there is no issues, only on the upper end of the throttle it does it.
I drilled a hole in my non-vented gas cap and also drove around without the gas cap and problem persists.

 

[Madgyver]

Could it be a spark issue?
Like in the dizzy, vacuum advance?

 

[Madgyver]

https://m.facebook.com/permalink.ph...&notif_id=1505617646415817&notif_ref=m_beeper

 

[pcf_mark]

You are much further into the throttle than I had expected this more than cruising along.

I am not a Q Jet expert hopefully someone can chime in. It seems very sudden more electrical than carb. Does your tach jump around or drop to zero?