wow thanks again....so the engine size nor the carburetor (2 barrel vs. 4 barrel) has nothing to do with any of this? (Too much air for too small carb?) "To further test, wait for a warm day (above 50F) or a cold day (below freezing). Ice accumulation is most likely in that temperature range, so test outside that band. If you can run both snorkels now, ice was definitely the problem" I'm a bit confused. I won't get any days over 50 until next spring. So wait till it's below freezing? Carb icing mostly happens between 32-50 degrees? Don't mean to sound pushy with all my questions. I really appreciate the info!

 

Yep, it's a little quirk due to how pressure and temperature drop passing through a venturi. When air passes through a venturi, the air and the metal cools off, maybe 15F colder than ambient.

Below 32F, any moisture in the air is already frozen. Even though the venturi still gets even colder than ambient, the frozen moisture just passes right on by.

With ambient air above 50F or so, even though the venturi gets colder, it still stays above freezing. Any liquid moisture in the air does not freeze.

The troublesome range is from around freezing to 50F or so. As the venturi gets colder, the metal drops below freezing. Any liquid moisture in the air will accumulate on the frozen surface. It's similar to how frost starts to form on a beer mug when you take it out of the freezer. There's one big difference, though. The mug warms above freezing and the frost melts. The venturi remains below freezing, so more and more ice accumulates.

 

You don't know how much I appreciate the shared knowledge. My goal is to get this to be a year-round driver. In North Dakota, I would call our weather "extreme.' Hot, cold, humid, dry in any and all combinations. I will definitely let the forum know what happens with future tests. So neither the engine size nor the carburetor (2 barrel vs. 4 barrel) has nothing to do with any of this? (Too much air for too small carb?) I'm not calling you wrong. If both my snorkels were hooked up to fresh air, your carb icing explanation would make more sense in my small brain. Are you saying this carb icing phenomenon can happen even without air plumbed from outside? As in carb icing can happen even without fresh air plumbing to the single snorkel and the snorkel sucking warm/hot under-hood air? Like maybe I have to unhook air cleaner connections depending on the weather? Thanks so much again!

 

You can't. Well I suppose you could call me anything you want, but it's not theoretically possible.

Don't get sidetracked worrying about air cleaner flow capacity versus engine size. It's not really much of an issue. As long as the element is not clogged, different filter housings won't cause rough running. I seem to remember different styles having a minor effecti on low end torque, but don't quote me on that. Lots of guys run wide open chrome air cleaners. Others keep the stock arrangement, but flip the lid to let in more air. You never hear about such variations causing performance issues. I think a lot of the attraction is feel-goodism, such as how a chrome differential cover increases a vehicle's acceleration.

RE: Underhood temperature? It's not much of a factor, if at all. A cold day at freeway speed? You've got so much cold air swirling around from underneath, and the engine is inhaling air at a fast clip. I bet those air molecules barely pick up any heat, if at all. The air passing into the carb is probably just as cold as booger-freezing North Dakota ambient. It might be a different story at idle, parked on a hot day, but that's not the scenario that is giving you trouble.

Presently you have two sources of air feeding into the air cleaner. One is the stock heated air intake. When the temp-controlled flapper draws heated air from around the exhaust manifold, that is a fairly restrictive path. Your new snorkel is much less restrictive. Even though the stock side is furiously trying to supply heated air, the vast majority will be nearly ambient from the new snorkel. The poor temp-controlled flow is overwhelmed by this new path of less resistance.

Look at it this way. All was good with the stock arrangement. You made a modification and right away the engine ran poorly under certain conditions (a combination of freeway speeds and weather). We have a pretty good idea what caused this change (icing). Either undo the modification permanently or make provisions to undo it temporarily as needed.

Next summer, with carb icing no longer a factor, I'd suggest testing if this second snorkel increases performance like you're hoping. Do some timed acceleration runs and compare mileage, with and without the second snorkel. You may not see much of a difference, if any. Only testing will tell.

 

LOL What now? What's theoretically impossible? Anyhoo....just got done with my test drive. 28 degrees F/82% humidity . So after letting the pickup warm up for 10 minutes, I drove around my small ND town. No issues. (Did not get into 4th gear.) Temp. normal/oil pressure normal. Then I parked for about 20 minutes. After that I took her out on the highway. Got into 4th gear. After just a few minutes, the surging/chugging started. The issue stops when I let off the pedal. When I say let off the gas, I don't mean completely, just not so much throttle. When I do this, the issue goes away immediately. And it stays away until I accelerate again. Then just after just a couple of seconds, the surging/chugging starts again. Then let off the gas ...and smooth. My speedometer doesn't work so I don't know my MPH. Lets' say 65-75 MPH the surging/chugging. At 45-55, no issues. Just checking in to let you know the status of my pickup. Next step, block off 2nd snorkel. Thanks again so much. This forum is wonderful for someone like me who doesn't know all that much but loves to learn from those who clearly know so much better than I.

 

Tonight I did my test drives again. Issue unresolved. Removed 2nd snorkel and blocked hole in air cleaner can. The hesitation still happened. Then, I unhooked the fresh air duct to limit outside ambient air. Same issue. I then jammed the hot air stove open to maximize heat coming from the exhaust manifold. Same issue. I believe I have eliminated the "carb icing" possibility. I'm all ears and thanks for any and all help.

 

Seems that way. I reread post #10, where you described how the problem started suddenly. The truck previously had a mystery air cleaner and the engine ran wonderfully. Very important: Did this include freeway driving to match the current problem scenario? Or was this just around town? If the latter, you don't really know if the same problem existed with that air cleaner.
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Reading between the lines, did you do the dual snorkel mod at the same time as installing the present air cleaner? Or did you run this new one as a single snorkel for a while? I'm just trying to narrow down exactly when the problem started.

Can you reverse the mods back to the last time the truck ran well? Reinstall the previous air cleaner, for example. I'm still not convinced the new air cleaner itself is the problem. Your latest testing seems to rule out carb icing, especially the part where you rigged the flapper valve to run full hot. My TheoryDuJour, subject to wind direction and my medication levels, is something else was disturbed during all this work. I'm thinking something simple like a vacuum line knocked loose or reconnected improperly.

​​​​​​​I suppose it's possible a new (unrelated) fault coincidentally popped up at the same time as the other work. Maybe the fuel filter partially clogged or the fuel pump is starting to go. I'd still suggest reviewing (and reversing) all recent work first before digging elsewhere.

 

Thanks again. Yes I ran the mystery air cleaner at highway speeds. I was having no issues. I put a period correct air cleaner on (the filters are the same size.) No issues, many miles at highway speed. The only other thing I did was "snug up" valve cover bolts. In the air, spark, fuel equation, I am now thinking fuel. I have 2 fuel filters. One screws directly into the carb. The other is an inline pre-pump filter. First, I plan to remove the pre-pump filter and test. If that's clogged, removing it for testing should pinpoint the issue. I am pretty sure the carb fuel filter was new this summer when my uncle rebuilt the carb. I do have a new one if need be. My vacuum lines are minimal. The EGR is blocked off and not hooked up to any lines. I have a PCV line to carb. There is vacuum from the distributor to the carb. Manual brakes so no brake booster. I do not have the vacuum lines figured out on my air cleaner can. There are sensors and valves involved that need to run on vacuum but I don't have a vacuum tree on my manifold. I agree, it must be something simple....that's why I can't figure it out. This summer when I redid the rear brakes, broken parts and pieces fell out when I took off the drum. Quite an obvious problem. Thanks again for all your help!

 

Were things the same way when the engine was running well? You're going to need the thermal flapper control system hooked up properly, or are you talking about some other features on the air cleaner?

For the lines in the system that controls the thermal flapper thingy, please see this other discussion, post #3:

https://www.ford-trucks.com/forums/1...l#post19607983

In a nutshell, if the lines are hooked up backwards on the thermal control valve, you inadvertently induce a vacuum leak when the intake air is warm.

Did you touch the vacuum line to the distributor? The rough running under load at high RPM reminds me of similar symptoms I experienced with my '84 351W. The line to the distributor had a leak. I replaced that and then also found the vacuum advance actuator was leaking internally. The engine ran fine at lower speeds and low power demands. I fixed both problems and performance returned to normal. On my truck, it has a direct vacuum line between the carb and distributor. Some models have a temperature switch mounted on the air cleaner, part of a system to disable vacuum advance under certain conditions. If one or more of these lines is not connected correctly, you may have a vacuum leak.

 

I have 9D612 air cleaner vacuum motor (on top of snorkel) and 9E607 air cleaner temp. sensor (metal, through top of lid, 2 ports). I do not have 9E862 vacuum delay valve (plastic, 2 ports, goes through air can wall.) Nor do I have the bi-colored in-line valve from the vacuum motor to the metal lid sensor (9E607). My air cleaner has never been hooked up to vacuum as I have no "tree." Does the air cleaner assembly system make its own vacuum when the engine is running and the air cleaner is plumbed to ambient air intake and exhaust manifold heat? I have a functioning PCV breather fitting that goes to a small filter inside the can and I have a fitting that hooks up to the charcoal canister. That's also hooked up with a flexible according tube. I didn't touch the vacuum line to the distributor. Right now, I only have 2 air can hook up options. Hook the hose up from the air cleaner motor to the right or left outlet on the lid valve. I will try it both ways with the other side capped and see what happens.

 

UPDATE: Switching air tube hook-up on the metal top of the lid valve did nothing. So I decided to check fuel filters. The bolt-on carb filter was clear. However, the in-line pre-pump filter was full of brown (rusty?) sludgy no-good-ness. I let all this gunk drain out of the filter. I could not blow air through it. Tomorrow I will install new in-line filter and test drive. I don't know if my issue will be resolved, but that filter needed to be changed nonetheless.

 

No, not at all. The preheat flapper door (out on the snorkel) is driven by an actuator powered by vacuum tapped off the intake manifold. If you have no intake manifold connection to supply this muscle force, then the actuator has been in the spring-loaded position (cold air?) all this time. Don't get too sidetracked by this at the moment. You had previously mentioned physically blocking the flapper door in the hot position with no improvement, so I don't think this is part of the problem at all.

Can you please post a picture of the vacuum lines present, whether connected or not? Also, we'd like to see the vacuum diagram, usually found on the radiator support or engine valve cover.

 

Fingers crossed! It would be very odd that an unrelated fault occurred immediately after you changed air cleaners, but stranger things have happened.

Keep us posted.

 

UPDATE: Problem solved. Replaced in-line pre-pump filter with clear, WIX type. No chugging/surging/hesitating at highway speeds. Thanks so much for all your help guys. Here are photos requested.






I don't have vacuum routing sticker anywhere that I can find.

 

If that is the truck you are working on, a dent side, it is a little older than ours so don't know where or if they had a sticker.
Ours would be on the radiator support, about where the air filter is in your picture. Anything on the passenger side firewall?

You might have better luck in the area of your year truck when it comes things like "how is this run" or in your case the vacuum lines as guys could go look at their trucks.
We can only help so much as a general how things get run.
Good luck
Dave ----