New one on me...figured I'd toss it up!
Got another one earlier this week so spent the day fooling with it and learning the 'personality'and adjusting to the personality.
I'd say I'm pretty good at getting things set,been doing it on various T's for the last 30+ years... mixture, spark, throttle, etc so at idle to the point you can near count off the 'fires'. So I fiddled with this one all day and can make it purr...but...
It sits there for say 1-2 minutes smooth as silk at idle...on level ground btw...then surges about 10-15% more RPM for about 10 seconds and then settles back all by itself to purr and just keeps in that same repeating pattern at the same times for as long as I'll let it.
I thought something might just need to work its way through but all afternoon it just keeps doing the same cycle over and over, and I'm hands off when it does it!
Been back through everything a few times already, have the coils paper shimmed in good and tight so I know they aren't rattling against the contact with built up vibration.
Has me totally baffled and cost me a half day without success...I can live with it, one more notch and it goes away but idles faster...but....never had one surge before...
Any ideas or others personal experience?
Sounds like a vacuum leak to me. Spray some carb cleaner around the intake manifold and see if it idles faster, if it does you have a vacuum leak.
Are you running the heat stove on your carb? What type of carb? Any slop in the linkage? Have you made any changes to your mixture adj. while it's doing that and any changes? Does it run OK on the road and no surging? What kind of spark plugs? Etc...
Perfectly normal. They do that every time the A/C compressor kicks on.
It sounds to me like the inlet needle in the carburetor is hanging up and the fuel level is dropping momentarily leaning the mixture enough to increase the speed.
That's another good idea!
I resisted the urge to mention that maybe SeaFoam and/or Marvel Mystery Oil might be his friend(s).
LOL, good job!
Old simple carburetors like the Model T's have are not much more involved than a Mason jar with a wick in it. Such carburetors load up from time to time at idle and it is normal. If you inspect the needle valve by removing it and looking at it with a magnifying glass, you may find an annular groove around the needle where it has been forced into the seat.
The needle is made of soft brass and is easily messed up but is also easily fixed. Swizzle it around with a twisting rotating motion on a piece of very fine sand paper such as wet-or-dry number 280 or finer. Keep dressing the needle 'till the groove goes away. and you can get a finer adjustment. I like to buff the needle after I sand it by trailing it on the buffing wheel with light rotating pressure and being careful not to catch it in the rotating wheel. Yes I wear a face mask for this one as that needle can stick in flesh and eyeballs for half inch in a flash.
A really bad needle valve can be chucked up in an electric hand drill or a drill press and dressed to perfection as it rotates. this requires taking the bend out of the needle and then re-bending it after machining. You can find the correct size brass rod in most hardware stores and make a new one more easily than fixing a real bad one.
Some needles have been dressed with a short taper and that is also a no no. You want it to be a longer finer taper than the way a pencil looks. The longer the taper the more turns it takes to adjust the carburetor and the better you will be able to do it. A short taper may only allow a very tiny adjustment span and turning the needle an eighth of a revolution will kill the engine. A long thin taper will allow a half turn or even more for a very fine adjustment
Air leaks are also an issue as mentioned above and those are tested by squirting a fluid at all joints while the engine runs. You can use water, WD40, gasoline, oil from a pump squirt can, brake cleaner, or any liquid. A change in engine speed or exhaust color and smell notates the place where the air leak is. All intake flanges including the actual carburetor should be checked with a straight edge and a light shining from the back. If you find an air leak, un-even surfaces can be corrected with a single cut flat mill file with no-rocking strokes. These are correctly called Bastard files. Not because of their heritage but because of Heraldry. The mark of the bastard was to wear a sash that ran from right to left and not over the heart. All Bastards had to wear that sash. The file is named Bastard because of the direction of the cutting angle.
The carburetor throttle shaft is a known place to find leaks and requires a careful repair. The throttle butterfly and shaft should be removed and the shaft checked with a micrometer to check for perfect roundness and wear. If the shaft is well worn and no longer round it should be replaced with a new one. The correct method is to drill out the hole to a larger size and put a new plug in the hole and drill it to fit the throttle shaft. You either have to fix the hole or the shaft and sometimes you must fix both of them if things are really wallowed out.
I have done quick repairs on carburetors by wrapping fine sewing thread around the throttle shaft right next to the carburetor body and then coating it with crazy glue and then wrapping it some more and tying it off. This creates a temporary stuffing box and if it improves carburetor control it will tell you to fix it right so that the engine will run properly.
I believe that a hunting idle is cause by both bad needle valves and air leaks and that both should be looked after for a nice idle. Needle seats are usually in good shape but you can inspect those with a magnifying glass to check for nicks and imperfections and wallowing out to an out of round shape.
The man's tie on the left below is the mark of the Bastard. The stripe going across the heart is reserved for those with a last name.
The picture below that shows a Bastard file. Bastard is not a bad word it is the name of a mill file that cuts the wrong way !
Very good information Frank. An Old timer told me to always loosen the packing nut on the needle valve before turning it in so you can feel it when it seats, because it's very soft. Then tighten the packing nut after you start to open it.
Interesting info on the ties. I'll probably think of that every time I put one on, which isn't very often.
Frank,both those ties need a volumn control! (G)
I'm confused about the 'bend' in the needle. Could you elaborate?
Hal, I work on lots of old cars and hardly any stock T's so am not familiar with stock T carburetors like the Kingston below in the first picture.
Most of the old one and two cylinder car Schebler carburetors have a 90 degree bend or even two bends in the needle valve where it comes out of the carburetor. those bends are there so you can adjust it with your fingers. If you tried to put it in a Jacobs chuck it would not go. See the yellowish red needle valve in the lower drawing labeled par "C". The spray tube is part "D". You would have to take out the bends in the needle valve to put it in a chuck. The Model T carburetors have that T handle and that has to be removed to put it in a chuck. Thanks for catching that error on my part. Not all carburetors have that bent needle valve, just most of the ones I work on. Anyway carburetor needles will not fit in a chuck unless you get rid of the adjusting mechanisms.
Thanks. I see what you mean.
In the MTFCA book, they show a jig made from a block of wood that holds the needle with the pointy end extending out of a hole at an angle where it can be filed. They use and old mixture adjusting "Fork" cut off and put in a drill to turn it with. They also put a compression spring between the "T" bracket at the top of the needle and the block of wood. I think they kinda just bump it up into the file or sandpaper momentarily while the drill is running and then relax the pressure. I haven't tried it, but it looks like it would do a good job. The two NH's I have rebuilt have had pretty good needles so just some minor polishing with some really fine sandpaper was all they needed.
I am with Kenny E. It is cool and humid in NJ just now.
Feel the upward turn of the intake manifold with your hand. If it is cool to the touch it will soon be very cold which indicates the intake manifold is icing internally. This is caused by the venturi effect of the manifold and the fan blowing cool air over it.
When the engine starts to falter place a rag soaked with very hot water on the bend it should straighten out. Alternatively carefully heat this part of the manifold with a propane torch.
Model T's require a hot air pipe to prevent this phenomenon, believe me I came from Orygun where the Western part of the state is soggy nine months of the year.
Ron the Coilman