Why does choke make such a difference in driving?

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Model T Ford Forum: Forum 2015: Why does choke make such a difference in driving?
Top of pagePrevious messageNext messageBottom of page Link to this message  By Chris Bartol on Monday, September 28, 2015 - 07:53 pm:

Ok, this motor is not a rebuilt one and is definitely tired (smokes out the valve cover, none out the pipe). I've been farting around with it before we put a good rebuilt one in it that we have ready and had a question.

This TT actually moves along pretty well at speed until it "tops out" and then starts to run very rough, sputter out the carb and buck. Enriching and leaning out the carb does not seem to help, but today, for kicks, I pulled the choke out a bit and it seemed to straighten out. Once I pushed it in, the buckin' bronco was back.

Any thoughts as to what I'm missing here?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Les Schubert on Monday, September 28, 2015 - 08:01 pm:

Chris
I encountered a similar situation on a tour on Saturday. I have not definitively figured it out but figure;
1. Fuel flow from the tank
2. Ethanol blended gasoline may be contributing
Best of luck and I will post back if I come to a conclusion


Top of pagePrevious messageNext messageBottom of page Link to this message  By Derek Kiefer - Mantorville, MN on Monday, September 28, 2015 - 08:04 pm:

I agree with Les that it is likely a fuel flow problem.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Donnie Brown North Central Arkansas on Monday, September 28, 2015 - 08:14 pm:

Chris, there are two internal passages (NH carb). One of them is the primary jet that you adjust, and the other is basically for idle. When you close off the choke it forces the carb to also pull more fuel thru the idle passage. If the primary(main) passage is slightly restricted because of rust swelling to the internal walls of the passage, it may be restricting the amount of fuel that the main needle valve can supply, so when choking the engine you are adding some more fuel thru the other (idle) passage ... Just thinking out loud here, someone else may chime in and say Im all wet, but ???? maybe ..


Top of pagePrevious messageNext messageBottom of page Link to this message  By G.R.Cheshire on Monday, September 28, 2015 - 08:19 pm:

With a vacuum leak closing the choke (primer) would richen the mixture and make the motor run smooth again. I would check the intake system carefully.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Vic Patterson. Grande Cache, Canada. on Monday, September 28, 2015 - 09:59 pm:

Do you have the hot air pipe installed? I have had similar problems with the carb icing up when the pipe wasn’t installed, even on a warm but humid day.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Wayne Sheldon, Grass Valley, CA on Tuesday, September 29, 2015 - 04:52 am:

Sounds to me like you are outrunning your fuel flow. After a couple minutes burning it faster than it flows, the level in the carburetor bowl drops low, and the engine starts running way to lean. Begins back-firing, bucking, losing power, etc. When you pull the choke, it restricts the air going in, matching (roughly) the restricted fuel flow. For a minute or two, it will run better, but still losing power (restricted fuel flow equals less power). If you slow down a bit, then the flow of fuel may catch up, you can speed up, but only for a short time.
Do check the carburetor passage ways, but also fuel line flow. I use a bucket to catch it and open the carburetor drain to see how fast it runs out. Check the entire path of fuel flow, fuel filters, kinks in lines, bad routing, etc.
Those are my first thoughts.
Good luck!


Top of pagePrevious messageNext messageBottom of page Link to this message  By Frank van Ekeren (Australia) on Tuesday, September 29, 2015 - 07:04 am:

I think Chris got it right with it's a tired engine, the valve guides are just worn out! restrict the air by closing the choke to compensate for what vacuum is pulling by the valve guides.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Les Schubert on Tuesday, September 29, 2015 - 01:11 pm:

I fixed my car this morning. The problem was fuel flow. The sediment bowl was plugged up with fine rusty particulate
Consider that I have driven this car for 40 years and many thousands of miles with never this type of problem. The car sat for a couple of months with ethanol blended gas in the tank. The carb had been run dry, but acted up a bit on starting it and then things cleared up. I had added some "Sea foam" fuel treatment (I have had good luck with this stuff) and a couple gallons of fresh gas. The car ran good for about 40-50 miles and then started losing power and would only run with some choke on (Holley sway back NH). As I was the tour organizer I just loaded it on the trouble trailer as the tour only had about 7 miles to the end at my place
Today I checked and virtually no fuel flow. Removed the screen from the sediment bowl and a bunch of "crap" came out and then the fuel flow was good. I plan to be much more careful where I buy my gas for my T!!!


Top of pagePrevious messageNext messageBottom of page Link to this message  By Charlie B actually in Toms River N.J. on Tuesday, September 29, 2015 - 01:24 pm:

I'm not sure about fuel flow because I think it wouldn't get better by pulling out the choke if the fuel wasn't there (in the bowl)in the first place. I'm more with the carb/clogged passages guys. But if you pull/re-do the carb flow will be easy enough to check at that point. It's one or the other.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Stan Howe Helena, Montana on Tuesday, September 29, 2015 - 02:26 pm:

There is no vacuum. It is a myth. There is only the constant pressure of the atmospheric air trying to fill a vessel of less pressure. In a carburetor there are only a few places air can enter to fill the vessel of less pressure, in this case, a cylinder with a descending piston creating a larger space for air to flow into, ie: creating a larger vessel. The outside air pressure will try to find a way to the area of lesser pressure. If you pull the choke, the air flow to the lower pressure area is reduced and the outside air pressure will force any non-restricted area to balanced pressure by forcing fuel into the bowl, mixture into the cylinder, etc. What you are doing is causing the flow of air to the lesser pressure area to flow over an area in the carburetor where fuel is present as well as filling the bowl with fuel. That's the simple explanation. The longer one takes several textbooks and some formulas.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Frank van Ekeren (Australia) on Tuesday, September 29, 2015 - 09:14 pm:


Top of pagePrevious messageNext messageBottom of page Link to this message  By Stan Howe Helena, Montana on Tuesday, September 29, 2015 - 10:29 pm:

What people commonly call vacuum is the pressure differential between atmospheric pressure and a vessel of lesser pressure. Nothing will enter the vessel of lower pressure unless there is greater pressure forcing it in.

Any Physics book will tell you the same thing. Once you understand that concept you are on the way to understanding how carburetion works.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Frank van Ekeren (Australia) on Tuesday, September 29, 2015 - 10:59 pm:

That's right Stan, commonly called vacuum so every average Joe knows what we are talking about, label it "in Hg" and you've lost every body!


Top of pagePrevious messageNext messageBottom of page Link to this message  By Michael Garrison - Rice Minnesota on Tuesday, September 29, 2015 - 11:00 pm:

Sounds like we've got a couple similar yet different definitions what is a vacuum. In physics a vacuum seems to be a lack of atmosphere. However, in a mechanical situation (which to my uneducated mind seems to relate to physics) a vacuum is a description of the "pulling" of the atmosphere from one pressure vessel to another. In a way, this whole vacuum thing and not understanding it kind of sucks.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Stan Howe Helena, Montana on Tuesday, September 29, 2015 - 11:16 pm:

Well, I don't have time to take anybody back to basic high school physics. I understand how carburetors work pretty well tho.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Frank van Ekeren (Australia) on Wednesday, September 30, 2015 - 01:05 am:

Stan
As you can see I've got to much time to play when on office duty at work, I can see your concept on vacuum when it comes to a carbie and high school physics, different animal when talking manifold vacuum. For an engine to pull air or oil from a worn part like valves and butterfly shaft etc, it's the manifold vacuum doing that not carbie/venturi vacuum that's doing it. So hense the widely used term, engine vacuum.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Stan Howe Helena, Montana on Wednesday, September 30, 2015 - 02:36 am:

I already know all that Frank. You fail to see that the original question asked why the carburetor runs better with the choke pulled. If you feel better about your superior explanation, go for it. I will stand by what I said, what I have studied for the last 50 or so years and what I taught:

Vacuum is a convenient term for pressure differential between two vessels. Nothing is "sucking" anything in to a vessel of lower pressure. It is forced in to the vessel (or chamber if you wish) by the atmospheric pressure outside the vessel or in cases of super or turbo charging by mechanical pressure.

Go take a college course in physics and post back here when you understand the concept.

It is no wonder so many people no longer post on this forum. I could name 50 guys with a world of knowledge and hundreds of years of experience who will no longer read or participate in the forum. I'm pretty much one of them. I apologize for trying to offer an explanation to a question asked and promise in the future I will refrain from trying to make anybody any smarter than they want to be.

I do not have time to argue with anybody on this forum or any other. I put in almost 16 hours in the shop today turning out carburetors for applications ranging from a 1928 Meteor to a 1902 Knox, a Schebler that has already been to two other shops and still didn't run right, a Stromberg for a 1921 Lincoln race car at the Indianapolis Motor Speedway museum and machining parts for my reproduction of the only NOS choke setup for the U & J. I answered a dozen phone calls, most for free information; answered several emails and foolishly pissed away time trying to answer a question on this forum. I came in from the shop at 12:30 AM and will be back out there by 8 if not earlier. I'm ready for bed. Carry on, carry on, carry on. I'm pushing 75 and need my beauty sleep.

You win. You are right. I believe every word you say. I am an idiot. My college education and years in this business were a waste of time, money and energy. There, are you happy? Good, because I am going to bed.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Stan Howe Helena, Montana on Wednesday, September 30, 2015 - 02:42 am:

I forgot. I also spent two and an half hours tonight recording my radio show for next week. I spent about 3 hours last night getting material together so I could record the show tonight.

21 years of VOLUNTEER show production to keep traditional music on the air for our thousands and thousands of fans and supporters around the world.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Frank van Ekeren (Australia) on Wednesday, September 30, 2015 - 03:28 am:

Stan, I see you are tired and need that sleep, but getting a bee in you bonnet over the term "vacuum" is a ?! why?, what you stated was,

"There is no vacuum, it is a myth"

So tomorrow when you are all rested you can invent a new terminology for the operation of the fuel system for more than a 100 years on the internal combustion engine to replace "vacuum!"


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jerome Hoffman, Hays Kansas on Wednesday, September 30, 2015 - 08:40 am:

Are you running a hot air pipe?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor - Kenilworth, NJ on Wednesday, September 30, 2015 - 10:29 am:

Stan, Frank, just wanted you both to know others (me at very least) found the discussion very educational and sincerely hope you do not withdraw from forum participation over a differences in terminology.

If I understand correctly, applying choke restricts the airflow through the carburetor and therefore alters normal venture action of pushing fuel up the nozzle into the air stream but the descending piston on the intake stroke still produces lower pressure (aka "vacuum") in the intake manifold which causes atmospheric pressure to push down on fuel in the bowl up the nozzle into the restricted (choked) air flow; enriching the fuel/air mixture.

The fact that enriching the mixture manually does not have the same effect of smoothing out engine performance leads me to believe other variables may be responsible for the operation Chris observed. Pulling on the choke slightly may be acting to limit engine RPM in addition to enriching the fuel/air mixture. Ignition timing becomes more critical at higher engine RPM. Variation in ignition timing at higher RPM can cause significant degradation in engine performance causing it to run rough. Does backing off the throttle slightly have the same effect to smooth it out?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Michael Garrison - Rice Minnesota on Wednesday, September 30, 2015 - 11:31 am:

Wow, I said this subject "sucks" as a bit of sarcasm. It seems when I mounted a whistle on my intake manifold the air was being "sucked" in and the little whistle made a noise. I don't have several years of college, never took a physics class. However years ago the timing advance in the distributor of my cars was actuated by the suction that was created by something I heard people call vacuum. Some called it vacuum advance. The same thing drove my windshield wipers, which I know had a tendency to slow down when I increased the rpms of the engine. People referred to that sucking effect as "vacuum". This conversation, though interesting, grew tiring when it became somewhat confrontational. This thread really
does "SUCK"!

Over the years I've heard people refer to "things being done in a vacuum". I've been told that outer space is a large "vacuum"". Submarines are crushed under great pressure because of the "vacuum" that's created by the out side pressure of the water becoming much greater than the inside pressure in the cabin of the sub. People have told me that the difference in pressure created a "vacuum" inside the sub and it collapsed into itself.

These are the some of the examples and definitions of the word "vacuum" I've heard throughout my years and I've always considered them as fact. I'll continue to use the word "vacuum" in conversation and because I don't plan to go to college, I spent my 40+ years in the factory and retired with a lousy pension that still seems to put food on the table and allows me to buy parts for my Model T.


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