The Stovebolt Site Forums The Shop Area Engine Shop lurching at near-idle speed

Hello:

This is for a ’51 ½-ton 216. I need some help in identifying the cause of what I would describe as “lurching at very slow speed”. Let me first say I do know how to drive a manual transmission, so it’s not a case of having to learn that.

When the truck is warmed up, it idles just fine, very smoothly. When I shift into first gear or reverse and let out the clutch, the truck might move smoothly for a few feet at very low speed, but after a few seconds it begins lurching, and I have to give it gas to smooth it out. And not just a little gas, but maybe up to 7-10 mph (guessing). My expectation (perhaps incorrect), is that once the clutch is disengaged, on a level, smooth surface, the truck should be receiving enough fuel to roll, without depressing the gas pedal. If I’m backing out of a garage, it should be possible to ease out, but this truck wants more gas and therefore faster speed.

A possible clue: Years ago, for a reason I can’t remember, I disconnected the vacuum hose from the windshield wiper motor. This hose is now open to the air, and the truck won’t run if I reconnect it to the wiper motor. This may not be related, but the symptoms are very similar, in that covering the vacuum hose also cause lurching (because the engine is starving for air, I understand!).

Maybe I have the wrong expectation about this truck being able to roll in gear at idle speed, but it just seems like some adjustment is out of whack. Ideas or tests?

If your wiper hose is connected to the intake manifold and the truck will not run with it plugged, then you most likely have an issue with your carburation. In addition to that, you may have some weak engine/trans mounts.

Maybe start by plugging the vacuum line and then re-setting the idle speed? See where that gets you?

That wiper line being wide open represents a gigantic vacuum leak. If your engine seems to idle better with it wide open like that, you have a serious issue with your carb idle mixture setting and maybe the ignition timing as well.
It is no surprise to me that you have a driveability issue once you move away from idle. That huge vacuum leak will cause the engine to run way too lean.
Carl

Norcal Dave, thanks. I'll try plugging the vacuum line back to the wiper motor and make adjustments from there, but my recollection is the truck dies almost immediately if I do block the hose (don't remember if it's when I place the hose on the wiper motor or when I place my finger on the end of it, completely blocking it). I'll experiment.

52Carl, thanks. To clarify, it seems to drive fine at "running speeds" (> 10 mph). The rough running (lurching) is at or slightly above idle speed while in gear. But give it some gas and it runs (relatively) OK. Does that change your view of things?

DJ, off idle, at "running speeds" your vacuum leak will not make near as much of a difference as it does at an idle. 52Carl is correct, you need to plug that line and get it running properly at an idle with that plugged. Once that is done, and timing checked/set, then , as Dave said, look for soft motor mounts. Soft mounts may allow the engine to rock, moving the throttle linkage, (same as you pressing on the gas) then it will lurch and move the throttle linkage the other way (like letting off the gas) and keep going back and forth.

The wiper hoses I've had are pretty small. It can not possibly affect the motor when plugged. It's plugged when wiper is off. The motor gets it's air from a huge opening called the carburetor. Plugging the vacuum port at manifold would increase venturi flow. You are not getting enough gas/idle circuit. Carb problems as said already. Motors surge/stumble because of vacuum leaks not plugged wiper hoses. Are you plugging it right at the manifold? Is your fuel pump also a wiper pump?
Is your choke stuck closed? Do you have a small "Hot Rod" air cleaner, Is your air cleaner plugged?

If all you are asking is why it won't go forward in idle with a standard transmission, set the idle up to specs. Check the clutch. Be in first gear. Is this a 4 speed with granny low (compound low)? or a 3 speed or what?

Believe it or not, it does take practice if this truck is new to you or has new drivetrain components.

Thanks all. First let me answer the questions:

1. This is a Type B downdraft carburetor (not asked, but relevant).
2. I have not plugged the vacuum inlet at the manifold, only at the end of the hose that plugs into the wiper motor.
3. I don't believe my fuel pump is a wiper pump; there's a separate motor with a switch (if I've understood your question).
4. Choke is not stuck closed, because I must manually open / close it from under the hood (another topic for another thread), and I always remember to open it after starting.
5. Air cleaner is the large, original oil bath with horse hair filter element (or whatever that material is).
6. Air cleaner could be plugged, I suppose. Haven't inspected it lately.
7. This is a 3-speed, on the tree.
8. There are no new drivetrain components; this thing is 98% original. I've possessed it for 10 years, it's been in the family for 50 years. My mom's husband did some work on it 15 years ago, but it was mostly trim, maybe some front suspension.
• 
  What I plan to do:
  1. Plug the vacuum inlet at the manifold and adjust the idle mixture and idle speed to 450-500 rpm. (I don't have a tach, so not sure how to know the rpm).
  2. If that doesn't help, I'll adjust the timing. I've never done that before, but I know the procedure has been written about abundantly on this site.
  3. If that doesn't help, I'll remove, disassemble, inspect and clean the carb, with emphasis on the idle system.
  4. In any case, and maybe as a matter of safety, read the service manual section on motor mounts and inspect them.
  • 
    Thanks all. I'll keep you posted. Any more info and suggestions are appreciated.

Just so I understand correctly:
1. The motor "won't run at all" with hose plugged?
2. The truck won't crawl in first gear whether hose is plugged or not?
3. It won't crawl like you think it should after warm up?
4. When was the last time it would crawl like you think it should?
5. You have never said how it runs down the highway after warm up. Stopping at stop signs and acceleration and so on.

I think I would try the hose plugged with air cleaner removed, to see if there is a change in anything. I suspect it will run OK then. This will also let you visually see that choke is opening all the way. It seems like it runs good after warm up like all trucks do after warm up. I don't see any reason to mess with timing. You don't seem to have a very serious problem or headache here. So take it slow and wait for more opinions. I am still worried about the choke, especially with your explanation of how you operate it. If you can't see the butterfly, you do not know if it is open all the way.

If you can hear every cylinder fire, idle too slow. If it idles like a sewing machine, good. Until you get a tach.

Bartamos, thanks for your interest. Answers below.

1. The motor "won't run at all" with hose plugged? CORRECT
2. The truck won't crawl in first gear whether hose is plugged or not? WON'T CRAWL WITH HOSE NOT PLUGGED; DIES WITH HOSE PLUGGED
3. It won't crawl like you think it should after warm up? CORRECT
4. When was the last time it would crawl like you think it should? DON'T REMEMBER / WASN'T AWARE
5. You have never said how it runs down the highway after warm up. Stopping at stop signs and acceleration and so on. I DON'T DRIVE IT MUCH; IT'S NOT STREET LEGAL AT THE MOMENT. THE OTHER DAY I PULLED SOME SHRUBS FROM MY YARD. IT WAS WARMED UP VERY NICELY. I DROVE IT AROUND THE CUL DE SAC AT 10 MPH AND IT DROVE FINE. PULLING IT INTO THE BARN AFTERWARD, IT WOULDN'T CRAWL ON LEVEL CONCRETE. I HAD TO GIVE IT GAS AND WAS MOVING FASTER THAN I WOULD LIKE TO WHILE TRYING TO PARK. REALLY HAD TO WORK THE CLUTCH TO LIMIT MY SPEED

As you suggested, I removed the air filter, ensured the choke was wide open, started it, let it warm for a minute so it would idle, then placed my finger over the vacuum port at the intake manifold. It died immediately. (I typically don't remove the air filter to verify the choke is open, because I know how the cable linkage is oriented when the choke is open or closed. Long story as to why I don't use the choke cable at the moment).

It's raining here so I can't do much testing with the engine running, because I don't want my truck to get wet (nor breathe carbon monoxide). Do you think I need to adjust the idle, or do I have a vacuum leak?

I better back off because you have a vacuum leak at the open hose and I never heard of a motor dying when a vacuum leak was plugged. I have a mental block on this right now. Will think on it. The leak is after the venturi. Throttle plate closed, Air cleaner, which does inhibit air low somewhat, had no effect when removed. It still died when plugged. The hose must be 1" in diameter.

I think you have me on the right track, Bartamos. I adjusted the idle mixture screw (I think it had been backed too far out), by screwing it in, but not overly, then backing it out about 1.75 turns. I also adjusted the idle speed screw for a faster idle. Then I put my finger over the vacuum port, and it kept running, not smoothly, but running. When it stops raining and I can roll the truck outside, I'll pursue this.

BTW, I'd say the vacuum air hose is 1/4" inside diameter.

Thanks for the help!

You are the one who did it, not me. 1/4 ID is bigger than I thought. Good job. Glad to offer encouragement. The guys before me had the ticket already. (Dave, Carl, Cuda). Carb just needs tweaked and maybe a rebuild. Now get some electric wipers for all that rain I wish we had.

Since the idle mixture screw does not actually control mixture, but controls volume, you had too much volume to be atomized. It needed more air flow with the throttle closed. So it got it from the open hose. You slowed down the volume of air/fuel into manifold and it does not need the "extra" air. Just what air the throttle plate lets past and/or idle bypass circuit provides. I think I would call it that it was running "rich". I guess it was flooding out. Not sure about all this. I made it all up.

Yes, thanks to Dave, Carl, and Cuda, who also suggested I plug the vacuum port and adjust my carb idle mixture. I'll do a little more adjusting then drive it, and I'll let you know how it goes.

Just to put a bow on the initial complaint, and then follow-up with one of the suggestions (for which maybe I should start a new thread?).....

I followed the suggestions to adjust the idle mixture & speed so that the truck would run when the vacuum port on the fuel manifold was plugged off. This greatly improved my original problem of the truck lurching or chugging at slow speeds, such as when backing down a long driveway. I could either endure the lurch and have my neck snapped (and scare the buhjeezus out of my grandson who was riding with me), or I could feather the clutch to make it smooth, but I'm pretty sure that's hard on a clutch. Anyway, problem solved, and thank you guys.

However, the truck then ran and idled rougher in general. I decided to change my spark plugs because they were fouled, I presume from my idle mixture having previously been set too rich or giving too much fuel. While I had the plugs out, I checked my compression (irrelevant, another topic, not good), and I gave the plug electrodes a gentle cleaning with diesel fuel and a soft wire brush. I put them back in, and the truck ran great! Amazing what a clean or new set of plugs will do.

Now for the new, but related topic. Some of you suggested I check my timing. Never done that before, so I read the shop manual (in addition to being aware of all the discussions here). My "octane selector" (page 6-19 of the shop manual), is currently set at 8 deg retarded, whereas the manual specifies "5 deg before top center" (p 12-26), and "5 deg BUDC" (p 12-41).

Q1: Does 8 deg retarded seem way wrong?
Q2: How does 5 deg BTDC relate to the "octane selector". Is that 5 deg advanced? If so, my truck is 13 deg away from that (+5 vs -8) on a 20 deg scale. Yikes!

Can somebody advise on these two questions? Should I start another thread?

Thanks!

Q1.......Set octane to spec (0).
Q2......That is the the actual timing at the "ball" or "mark" on the bell housing, using a timing light. 5 deg BTC Before Top Center. BUDC (Before Upper Dead Center).
Now-a-days it's called BTDC Before Top Dead Center. TDC= When no.1 piston reaches the top of the compression stroke.
Bottom line is: you probably have a UIC, I think that means Upper Center, and the "I" is the hash mark for Upper Center. You don't use that. You set the pointer, that you can see in the hole above starter on passenger side, to point at the BB ball which has a 5 next to it. That's what the pointer wants to be pointing at when the light flashes. (Da Ball). That is 5 Deg BTDC.
You do this with a timing light reading no.1 cylinder firing. Don't take the distributor out.

Re read the manual under timing at 6-19.

"Timing" refers to coordinating the spark, the resulting explosion and the resulting force to be applied to the top of the piston just as it starts down. So it has been decided that on a 216 and all the other motors I know about, you fire just before (in "Advance") it gets to the top and the lag time is such that it blasts and builds pressure after TDC. (beginning of power stroke). If it fires too early, it tries to push the motor backwards, loosing power. If it fires too late, you loose compression, loosing power...and, of course, either way may cause bad noises. This is why you get it as good as you can and then take it for a nice little ride................and floor it.

Then of course your valves have to be "timed" to open and close at the correct time to get gas in, compress and get fumes out.

Bartamos, couple quick follow-ups.

1) I understand to set the octane selector to 0, but it seems that it never comes into play in the timing adjustment process. Maybe I should just go do it, but I need to buy a timing gun first. The octane selector seems like a red herring.

2) I think you're telling me that when the engine is properly timed using the ball bearing, it will, by definition, be at 5 deg BTDC. I don't need to figure out how to achieve 5 deg BTDC, like reading some scale, other than moving the distributor per the timing instructions. (Hopefully that question is clear).

Thanks!

1) From 52Carl in 2016:

"The octane selector was originally for fine tuning the timing on the fly between gas fill-ups from different suppliers which had differing octane ratings. You could advance or retard the timing with the octane selector without a timing light or vacuum gauge and never lose your place as to where the needle is on the ball (zero on the octane selector). This is no longer needed as octane standards are more strict in modern fuels from one gas station to another.
I personally, would rather have a manual lever to adjust the timing while driving, similar to what came on Model Ts. But I am weird, I have been told.
Carl"

1a) Set it to zero and leave it there.

2) Yes, that is correct. If you have a ball, that is what you set the pointer on. Just check it first, it's probably OK. If not, rotate dizzy till it is, but remember where it was in case it runs better where it is at....or where at it is, or at what place it was at. At that point, it is what it is.

Got it. Thanks!

Pleasure. Keep the follow ups coming. Very interesting. I am learning too.

Message to Carl: Have you hooked up a spare choke cable to your octane selector yet?

If I were to do anything like that, I would rather have a lever on the steering mast like they did in the old days.