Truck struggles under acceleration

[haruman]

Works fine at idle, or a steady speed...

If I do a real slow acceleration I don't notice anything. If I do a normal acceleration she she sputters and jerks but eventually gets there. If I really give it some gas it struggles more and occasionally backfires.

Just changed the fuel filter and same deal.

Any ideas?

[geezer101]

Check - vac advance on the distributor, then a visual check on the counterweights and springs inside of it (they could be gummed up/broken spring etc). Next is secondary vac servo/solenoid on the carb. If it's split it will create an air leak into the carb + disable the secondary throttle butterfly (no mid range to top end power). Other possible problems - fuel cut solenoid on the carb (not likely but still possible) disconnect the wire running to it and run power straight from the battery+ to check function (it should have an audible click when the circuit is connected/broken). Ignition coil - they break down after 10 years and this will cause power loss, poor fuel economy and misfires under hard throttle. Blocked jets - try purging the carb. Engine running, cover the throats of the carb with one hand while opening the throttle. This will force the carb to pull excessive amounts of fuel through the jets and sometimes this is enough to unblock them.

Tune - check base timing with a timing light and maybe try to dynamically tune it (no timing light required). Loosen off the distributor retaining nut, start the engine and hold the throttle open @ about 2,000 rpm. Gently swing the distributor between advance/retard until you find a sweet spot and the engine rpm increases by itself and lock the retaining nut up and test drive it, listening for engine rattle or feel for power loss or something amiss. I slightly under gap my spark plugs to allow for a more aggressive engine tune and use premium fuel instead of regular. Physically look at the spark plug electrodes for a sign that there is an issue (fouled plug, plug too clean - anything that indicates one or more of the plugs aren't doing their thing) Look at the engine running in darkness to see if you can spot any arcing from spark plug leads or the coil, visually inspect the inside of the distributor cap and rotor for tracking or burnt terminals.

Last thing - vacuum leaks. They are tricky to hunt down. Check the manifold gaskets and spacers, vacuum lines, brake booster line and check valve. This should keep you busy for a whole weekend :lmao:Good hunting, hope you find the glitch.

[xboxrox]

Hi haruman -- With an already repaired & good ignition system, new fuel filter; Vernon, a local neighborhood DIY mechanic, advanced my trucks timing & installed a new fuel pump... I put in a can of Seafoam into the gas tank & now run 92 octane gas -- most of your problems are no longer happening to my truck...
:checkeredflag:
Driving with Aloha :jumpinganger:

[FMS88]

Which carburetor do you have? The OE Mikuni or a Weber? Also, when idling and in neutral, if you quickly open the throttle, does it rev okay or sputter and stumble?

[haruman]

I have the Weber, and mechanical fuel pump. Weber's been installed for about 4 years.

I'll go check the idling in neutral in a few minutes.

[geezer101]

OK. Using a Weber changes diagnosis - my bad for not remembering you've done the swap. Are you using the mechanical pump with a regulator?

[haruman]

OK. Using a Weber changes diagnosis - my bad for not remembering you've done the swap. Are you using the mechanical pump with a regulator?

No regulator.

[geezer101]

Check fuel flow. The mechanical pump might be on it's way out and can't supply the Weber with enough flow to keep up demand. I'm surprised it's held up this long without issues. Time to think about a HVLP electric pump :shrug:

[haruman]

Which carburetor do you have? The OE Mikuni or a Weber? Also, when idling and in neutral, if you quickly open the throttle, does it rev okay or sputter and stumble?

So I tried it at idle in neutral and it still sputters and stumbles.

[haruman]

Check fuel flow. The mechanical pump might be on it's way out and can't supply the Weber with enough flow to keep up demand. I'm surprised it's held up this long without issues. Time to think about a HVLP electric pump :shrug:

So what's a good way to check the fuel flow?

[FMS88]

So I tried it at idle in neutral and it still sputters and stumbles.

I'm thinking that the accelerator pump or nozzle has failed. With the engine off and the air cleaner removed, if you look down into the carb while opening the throttle, do you see a stream of gas from the accelerator nozzle? If not, the nozzle is clogged or the pump has failed.

[haruman]

I'm thinking that the accelerator pump or nozzle has failed. With the engine off and the air cleaner removed, if you look down into the carb while opening the throttle, do you see a stream of gas from the accelerator nozzle? If not, the nozzle is clogged or the pump has failed.

I see a dribble...

[FMS88]

I see a dribble...
I think that's the issue. A dribble isn't sufficient so it's starving for fuel when you try to accelerate. I'm not familiar with the Weber's internals, but another member or an internet search should tell you how to access and fix the accelerator pump circuit.

[haruman]

I think that's the issue. A dribble isn't sufficient so it's starving for fuel when you try to accelerate. I'm not familiar with the Weber's internals, but another member or an internet search should tell you how to access and fix the accelerator pump circuit.


Just ordered the part, looks pretty easy to swap. 4 screws and it's easily accessible, shouldn't take but like 10 minutes...

[Salteen]

Yeah I had to do this with a weber 38 38 installed on a ford pickup (1978 f150) and it is literally take the top half off and change it throw it back on check your floats and ride on.

Now if your timing is pig rich it will blow accelerator pumps and power valves like no other, I literally got a pop from the engine when the accelerator pump went out.

[FMS88]

Sounds great. If the Weber is only four years old, chances are the nozzle is clogged. Compressed air may clear it, but often only temporarily because any foreign material still inside can replug it since the end opening is typically quite small. When you take it apart, look for crud on the old part and in places where it can collect inside the carb. Clean out any you find and consider installing a small filter between the fuel pump and the carb as extra insurance. When its back together and any lost fuel replaced, opening the throttle quickly should send a strong, steady stream from the nozzle.

[haruman]

Are there two nozzles? One on each side? I can see a stream from one but not the other...

[haruman]

So to date:

New accelerator pump
New fuel filter
Can of HEET in the gas (in case there was water)
New gas cap (old one seal was crumbling)

Still doing it, I can see a stream on one side but not the other, leading me to think that jet is clogged?

[Salteen]

yes there are 2 jets. so yes you do need to clean it. this is probably your whole problem if its on the primary butterfly

[geezer101]

Try purging it. It might clear it out (if you haven't already attempted to do so...)

[FMS88]

Here's where you can find diagrams of the various Webers that fit:
https://www.carburetion.com/Weber/weberdiagrams.asp

For the DGEV/DGAV the accelerator pump jet/nozzle is #96. It looks the same although numbered differently for the other models. It's on the primary barrel and appears to have two nozzles. (I don't have a Weber so I have to rely on the diagram.) If one side is plugged, you may be able to clear it with compressed air, but you might have to remove the top of the carb to get to it the screw that holds the jet/nozzles.

[haruman]

Try purging it. It might clear it out (if you haven't already attempted to do so...)

How to purge?

[Salteen]

compressed air and some carb cleaner is my best guess, idk lol.

[geezer101]

compressed air and some carb cleaner is my best guess, idk lol.

Blocked jets - try purging the carb. Engine running, cover the throats of the carb with one hand while opening the throttle. This will force the carb to pull excessive amounts of fuel through the jets and sometimes this is enough to unblock them.

Suggested it on an earlier post on this thread. :thumbup:

[haruman]

Blocked jets - try purging the carb. Engine running, cover the throats of the carb with one hand while opening the throttle. This will force the carb to pull excessive amounts of fuel through the jets and sometimes this is enough to unblock them.

Suggested it on an earlier post on this thread. :thumbup:

Oops I missed that, will give that a shot! Thanks!

[haruman]

Blocked jets - try purging the carb. Engine running, cover the throats of the carb with one hand while opening the throttle. This will force the carb to pull excessive amounts of fuel through the jets and sometimes this is enough to unblock them.

Suggested it on an earlier post on this thread. :thumbup:

Well that didn't work. I guess I can take the top part of the carb apart and clear the nozzle that way...

[haruman]

Hmm. Looking at this youtube video, is it possible that the other side shouldn't have a hole?

https://youtu.be/jlEXR2hXUFg?t=163

[geezer101]

It shouldn't as the accelerator pump jet should only feed the primary throat. It should have a nice steady stream of fuel coming out of it when its operating properly. You can see they're relatively easy to pull apart and you don't have to remove the entire carb from the manifold to work on it so if the accelerator jet is blocked, you can take it out and push some compressed air through it.

[haruman]

Ok. I think my accelerator jet is fine then. Once I changed the gas cap & accelerator pump I get a good stream from the jet on the primary side.

This leaves the fuel pump I think...

[FMS88]

Here's a thread from a Jeep forum where a member has a Weber with the same no-load and with-load stumbling you're experiencing.

https://www.jeepforum.com/forum/f12/weber-32-36-dies-when-secondary-opens-867114/

Some of the replies mention possible causes, things to investigate and tests to try. I'd ignore suggestions to change jet sizes, especially if it ran fine before the hesitation problem appeared.

[geezer101]

At this juncture I'd think about an electric fuel pump install which is always advised when doing a Weber swap. Depending on what pump you get, you can do this relatively cheaply and they're easy enough to rig up. Guys like the Carter P4070 fuel pump but I'm not a fan. They're expensive and noisy - plus I've had one fail on me (not only did it fail, but it caused issues downstream of the fuel pump when it coughed up a heap of fine brass shavings)

[haruman]

So basically I need an electric pump, and a relay to kick the pump on when the ignition is on. Correct?

[geezer101]

Yep. Use the hot wire that originally went to the carb as the trigger for the relay, and have the main power feed fused straight off the battery to the relay and out to the pump.

[Salteen]

thats what i am doing, pretty simple and straightforward

[Giovanni89]

I would hook a timing light up before you go to town on the carb. An ignition stuck advanced will cause sputtering and backfiring under load.
The weber accelerator pump nozzle looks the same for both the 32/36 and 38/38. They use the same casting. The 32/36 only has a nozzle hole drilled on the primary barrel side however. It is normal for a 32/36 to only have one stream of gas. The 38/38 has both nozzles drilled. The speed at which you open the throttle determines the flow of gas. Open the throttle slow and it will dribble. Snap it open and you should get a stream. Failed accelerator pumps only cause momentary hesitation on throttle opening however. They bridge the gap between the idle circuit feeding the engine and the main circuit running the engine when you open the throttle.
If it's fuel and not ignition, rough idle can be caused by fuel starvation or over fueling. Fuel starvation at idle, and there probably wouldn't be enough fuel to feed the engine running down the road. Rough idle and running caused by over fueling is usually smoothed out when you open the throttle.
Acting fuel starved and over fueled makes me think the ignition system may be to blame. Can't hurt to check it out.

[Giovanni89]

Somehow I missed the second page when I replied to this. One thought, a mechanical pump could be underperforming at low engine speed and overwhelming the carb at high engine speeds. So while it is possible that it is ignition like I thought, I would definitely install an electric pump as recommended. My truck came with a stock mechanical pump and a Weber installed. It ran much smoother when I went electric after the mechanical pump failed. But not before I replaced the mechanical pump and it failed again a few months later.

[haruman]

I think I'll do the pump swap first, Giovanni, and see where that gets me. Hopefully all the way. :)

Geezer is this the pump you recommend?

https://www.ebay.com/itm/Top-Electric-Gasoline-Fuel-Pump-5-16Inch-42-GPH-2-3-5-PSI-12V-1-2A-2-Wire-Design/292173987502

[geezer101]

That's the one. I've bought that pump for my Weber install but I'm not ready to swap everything on my engine. This should (hopefully) solve your fuelling woes.

[haruman]

I have some more questions... What is this thing here that the vacuum line is attached to:

24397

And what's this black thing in the red circle, and what's this vacuum line in the blue circle for?

24398

[geezer101]

The thing in image #1 is a vacuum triggered switch. It has come up before on the site and performs some kind of 'ignition on' circuit connection. A member had one fail recently and their truck wouldn't run. The thing in the red circle in image #2 is a vacuum barb cap that I think is part of the EGR system. Trace the vacuum hose in the blue circle to wherever it's going. It might be going to a control circuit or scavenging vapour/gas from the valve cover.

[Giovanni89]

you'll have to follow the blue circle to find out where the hose goes. red circle, the cap is ported vacuum port. typically the vacuum advance on the distributor is hooked to ported vacuum. The lower hose is manifold vacuum port. some folks hook the vacuum advance to manifold vacuum. They are the same except when the throttle is closed. manifold vacuum when the throttle is closed provides a full vacuum signal. ported vacuum kills the vacuum signal when the throttle is closed. Personally, i find ported vacuum to be better. It provides better engine braking, and acts the same from part to wide open throttle. If your vacuum advance is hooked to the manifold vacuum, i'd recommend swapping the hose and vacuum cap.
As geezer said, that vacuum switch prevented someone else's truck from running. You could unplug the connector, attach a jumper, and see if the truck will run.

[haruman]

Any recommendations for fuel pump relay kit?

[camoit]

OK. The red circled port must go to the vac advance on the distributor.
The other vacuum switch you show on the firewall is the AC idle kick up. But since you have a Weber it does not do anything anymore.
As for the relay you would just use any old relay. You just build the wires.

The reason you have no power is because the vac advance is not working correctly.
You should have a manual what came with the carb. If no there is one in the manual section I think.

[haruman]

OK. The red circled port must go to the vac advance on the distributor.
The other vacuum switch you show on the firewall is the AC idle kick up. But since you have a Weber it does not do anything anymore.
As for the relay you would just use any old relay. You just build the wires.

The reason you have no power is because the vac advance is not working correctly.
You should have a manual what came with the carb. If no there is one in the manual section I think.

Why would the carb run fine for years then? I will try that and see what happens... just confused as to why its been fine and all of a sudden this symptom has come up.

[geezer101]

It has to be a mechanical fault or the pump has quit it's day job (or a combo of both). There is a specific clue that's eluding you at the moment. Once you spot it you'll get it running like new. If you have to pull the carb apart to inspect it, well that's just how it goes. Hopefully you'll fix it soon and get your power back :)

[haruman]

Installed an electric pump with relay and that fixed all my issues. Starts way faster & easier now too. I have a block off plate for the mechanical pump, I may take it off and open it up to see what went wrong.

My dad seems to think that the ethanol in our fuel could have deteriorated the innards in the pump. On that note I've started adding this to my fuel:

https://amzn.to/2SGxWUq

We'll see if that helps.

[geezer101]

Yeah ethanol is sketchy if you haven't modified fuel delivery to handle the stuff and it will affect tuning without rejetting the carb to run on it. I've gotten into the habit of adding synthetic 2 stroke oil to my DD with premium fuel (95+) It runs like a top - I'm only using 100mL for every 10 L of fuel but not on every tank (maybe every second/third refuel...) The fuel stabiliser is a good idea if you drive your car only once every few weeks.