Engine dies when I let throttle off

[JoseyWales]

Hi all. When I am on the gas and let it off sharply, my engine dies. Any suggestions?

Some of the conditions...
- It happens when the engine is hot and cold.
- The fuel level in the float bowl of my Motorcraft 2bbl carb is 7/8" from the top with the engine idling.
- I have eliminated vaporlock issues.
- Engine idles at 600 RPM in gear with 14 in Hg vacuum.
- I've also had the problem with the engine idle set much higher.
- I'm running an MSD ready-to-run distributor with no control box... vacuum AND centrifugal advance.
- Timing is around 10 degrees advanced.

Other engine features...
- Long tube headers
- Weiand low-rise dual-plane intake with 1/2" thick carb adapter
- Mechanical fuel pump
- C4 tranny
- Crane powermax 2020 roller cam

 

[acarlotto]

That's kind of funny, I just bought a Bronco and mine is doing the same thing. The previous owner thinks it has to do with the carb. Once it cranks up after it stalls you can smell real strong gas vapors. Let me know what you come up with.

 

[DirtDonk]

Just a thought here. With the manifold, spacer and headers and cam, you might be running lean under certain circumstances (or most of the time) with that stock carb and it's just losing it's explodability in the cylinder when you shut the throttle blades. Has it been re-jetted?
The spacer is an adapter I presume? If not, are you using it for another reason? A phenolic spacer is good for heat, but with that setup you probably don't need any additional plenum volume and that additional volume might be effecting your setup in a negative way. IMO.
Try richening up the jets a size or two for starters.

You might even be able to test my theory by blocking off your PCV valve temporarily. Just for a few minutes (after it's warmed up) run it plugged and see if things change.

Seems like you have other issues too. 14 inches of vacuum at idle seem awful low. What are the cam specs? Sounds like a lo-po efficiency cam from the name, but a hi-po performance cam from the vacuum signal. Should have at least 19 (mine ran at 21 for most of it's stock life) and this could also be an indicator of either a lean mixture or retarded timing. I know you said you have it at about 10°, but try more. If it pings with more, again, that might be a sign of leanness.

Paul

 

[DirtDonk]

Are you getting a vacuum signal at the distributor's advance tube when idling? If so, then your throttle blades are open farther than they should be. That wouldn't necessarily cause your stalling in and of itself, but the symptom might indicate other issuse and certainly cause your low vacuum reading at idle.

Paul

 

[broncnaz]

I agree with the possibility that its to lean running a 2 barrel carb on a 4V manifold can create issues. I also think you should be pulling more vacuum at idle.
Might have a vacuum leak. do you have any other driveability issues? A vacuum leak will cause a lean condition and low vacuum readings.

You cam should be relatively mild I believe its designed for non mass air EFI so the vacuum signal should be high like stock.

 

[JoseyWales]

With tranny in neutral, idle is 800 RPM with 18 in Hg vacuum. No change when I plugged the pcv port.

Cam has .530 lift and 262/270 duration. Cam spec sheet pdf is attached below. The Crane recommended valve springs are installed.

I designed the carb adapter because I don't like the tall aluminum ones they sell at Autozone. It is milled from a 1/2" steel plate.

Vacuum gauge tube is attached to the little tree splitter at the back of the manifold as is the distributor advance tube.

Timing is advanced to the point just below where the engine starts pinging. I'll have to borrow a timing light to check it because mine tore up.

I don't know how to check the jet size. I suspect it is indeed lean though. A local mechanic suggested that too. There is a very slight pop in the idle that concerns me.

Thanks for the replies... Please provide more if you can. ;D

Forgot to add... no driveability problems. Engine is very strong and smooth on the road.

 

[DirtDonk]

With tranny in neutral, idle is 800 RPM with 18 in Hg vacuum. No change when I plugged the pcv port.

Did you try driving it that way too, just to see if it still dies?
Looking at the pic (like I should have earlier), I see that you're using a non-stock PCV valve. What's the application? I use a valve with a 90° elbow too, but the wrong valve can add to your tuning woes by leaning out the mixture even more. Usually though, you'd notice a slight stumble off idle too. And since you didn't mention one, maybe your valve is well matched to your engine after all.
Still, worth asking about.

I designed the carb adapter because I don't like the tall aluminum ones they sell at Autozone. It is milled from a 1/2" steel plate.

Cool. Did you verify that there are no vacuum leaks though? Like was mentioned.
Nice looking spacer. Probably no leaking, but again, it's always worth double-checking.

Vacuum gauge tube is attached to the little tree splitter at the back of the manifold as is the distributor advance tube.

Well, there's an issue right there. Although some engines like direct manifold vacuum to the distributor, most don't. Including "most" Broncos. Some here are running like yours with good results, but more often than not, the engine will run much more happily with a timed vacuum signal from the carb to the distributor. Find the ported-spark fitting on the carb (usually the higher of the two) and hook it there instead. Re-time it and see what you get.

I don't know how to check the jet size. I suspect it is indeed lean though. A local mechanic suggested that too. There is a very slight pop in the idle that concerns me.

Popping through the carb?
Although a lean mixture can give you all sorts of popping out either end, another thing can crop up too. Did you break in your cam using the recommended procedure when you first fired the engine up with the new setup?

Forgot to add... no driveability problems. Engine is very strong and smooth on the road.

At least that means that your setup is somewhat compatible with the full manifold vacuum to the distributor, but try the ported vacuum anyway.
Might just help with your main issue anyway.

Paul

 

[Socal Tom]

The distributor vacuum advance is hooked to the wrong source. The dizzy should be hooked to a vacuum source above the throttle plate so that it gets no vacuum at idle.

Timing should be set with the vacuum line disconnected from the dizzy ( and plugged)

I believe what is happening is that when you are "on it" the vacuum level drops and the timing retatrds back to the base timing. Then when you let off the gas, the vacuum level goes up really fast and then the timing increases rapidly and this creates a situation that causes it to die.
What should happen is that when you let off the gas, the throttle plate closes, and the timing doesn't change from the base it was at under hard throttle.
This will be even worse if you set the base timing with the vacuum line hooked up.
Tom

 

[broncnaz]

Its a roller cam no specail break in was needed so I wouldnt worry about it being related to that.


I would also bump up the idle speed a little when in gear.

 

[JoseyWales]

Ok the jets have "52F" stamped on them. The carb casting says 1.08.

I plugged all the vacuum ports on the intake manifold. Hooked the distributor to the ported vacuum fitting on the passenger side of the carb. The stalling condition still exists.

I'm not sure how to verify no vacuum leaks around the carb adapter, but I tightened all the nuts a little more. No effect on the gauge reading.

I don't know what application the PCV valve is for... I previously had stuffed foam in the connected hose to restrict the vacuum to the point where it reads the same as when it's plugged.

The popping i mentioned is in the exhaust. It's more like a slight erratic 'put', only occasionally occurring and more often in neutral.

I think the vacuum gauge is a few inches off in reading. It was damaged previously, but I didn't realize until now that the reading is off. So I'm going to buy and install a new one.

 

[DirtDonk]

I'm not sure how to verify no vacuum leaks around the carb adapter, but I tightened all the nuts a little more. No effect on the gauge reading.

When the engine is running, spray some carb cleaner around the area. I like carb cleaner instead of gasoline because it's less flammable and plenty easy to use.
If, when you're spraying around all potential sources (manifold seams, carburetor, vacuum lines, brake booster) you get a change in the idle (either up or down) then you've found a leak.
Just don't spray the cleaner haphazardly so you can tell fairly precisely where the leak is and don't get excess amounts on your paint and engine.

I don't know what application the PCV valve is for... I previously had stuffed foam in the connected hose to restrict the vacuum to the point where it reads the same as when it's plugged.

I used a 90 degree valve with a 3/8" barb for convenience too. But I tried about ten different models and some of them (mostly mid-seventies Ford apps) made it run so lean that it stumbled off idle pretty badly. Finally found one (Corvette oddly enough) that ran like the stock Bronco one did.
Not saying this IS your problem, but it's one possible source of too much air entering the system under certain circumstances.

The popping i mentioned is in the exhaust. It's more like a slight erratic 'put', only occasionally occurring and more often in neutral.

Classic lean condition causing occasional misfires in the combustion process. Sometimes the mixture is so lean it will either not fire completely or at all. It's actually called "lean-misfire".
So, still leaning towards an overly lean condition.


Paul

 

[JoseyWales]

I learned this morning what I'm pretty sure my problem is... I'm missing an "anti-stall dashpot" that cushions the closing of the throttle. A PO must have removed it. Also I was missing a linkage and hose that connects to a vent valve at the top of the carb body. That is also supposed to relieve the stalling problem somehow.

What year Corvette PCV valve?

 

[DirtDonk]

Over the years Ford used several different dashpot and solenoid configurations on the carbs. Some were just for setting your idle (like on my '71) and shut the carb plates down even farther when you turned the key off. That was called an "anti-dieseling solenoid".
I've seen the little cushion type pot that you describe, but wasn't really sure what they were for. And yeah, most get removed for some reason or another. Damn PO's...

The only hose and linkage setup on top of a '75 carb that I can think of is for the evap canister setup. Just vents to the float bowl though, so I'm not sure how that would effect your stalling.

The PCV I use is, I'm pretty sure, from an early to mid-seventies Corvette with a 350. Say like a '73 to '75 or so. There was such a drastic difference between the 10 or so different ones I tried that I ended up keeping the super-sucker Ford unit ('77 Granada w/302 I think?) as a "high-altitude compensating" replacement. I'll see if I have any part numbers for you. But I don't hold out much hope after all these years they're probably wiped off and the boxes deteriorated.
It leaned out the mixture so consistently that I would replace the normal "sea-level" one with it each time I got into the mountains.
Worked pretty good actually. Kind of a controlled vacuum leak that saved me from having to either re-jet the carb, or suffer with too-rich a mixture, each time I'd go wheeling in the Sierras.
Anybody remember traveling with your parents when you were a kid and the cars would barely make it over the high mountain passes? Blowing black smoke and overheating all the way??? Ah, the good ol' days...

Paul

 

[JoseyWales]

The dashpot was considered a smog device I think. Probly why it was removed. Its name (anti-stall) makes sense though. I think it would work.

My PCV setup isn't workin well. It seems like the hotter the motor gets, the more vacuum is required to suck the gasses out.