Bogging down in third

[Sunny the Model A]

Howdy Yall, got a question for you. last time I drove my 31 pickup, it developed a bad habit of bogging down in third gear from 13-30 mph. it has the 4.11 gears outback, and it shouldn't bog down. timing is good, no brake drag that i know of, first and second are pretty good. now I shift into second at 9 mph and into third at 13-15 mph since it has the lower rear gears. so, what's up with it? doesn't seem to be the carburetor but I will sometime put a marvel on it and see if it make a big difference (it has a zenith currently). also, when descending a hill, it likes vibrating a bit when letting off the throttle. Is this normal for an A or is this worth looking into? thanks for your time

[Rob Doe]

Sounds like it could be a fuel issue. Where is your GAV set? Try opening it a bit. Loosen the gas cap in case you have a vacuum in the tank. Check for fuel flow at the bottom of the carb then at the side where the line goes into the bowl.

You said it doesn't seem to be the carb, what makes you think that?

[Sunny the Model A]

well, i drilled the gas cap so vacuum isn't the problem, and I flow tested the jets and set it all to specs. and I have the gav open when warm about 1/8 turn or just a smidgen open. I tried fiddling with the gav, but it seems to not like over 1/4 open, so it might be running a tad rich. and flow form the sidebowl on my zenith is really good and the tank is clean as a whistle, no rust and no trash in the carb.

[Y-Blockhead]

How is the gas level in your float bowl?

Millions of Model As ran fine on the Zenith carb, I see no reason to switch to a Marvel (altho I have one...).

[Marshall V. Daut]

The vibration while decelerating can be caused by a bad U-joint, loose bolts in the U-joint clamshells or a loose pinion bearing nut. The bolt securing the U-joint to the transmission rear shaft could also possibly be loose or the special notched washer beneath the bolt head was forgotten. These problems miraculously "disappear" when accelerating because of the torque being applied against them. When the torque is released, any or all of these parts manifest themselves in vibrations.
Marshall

[nkaminar]

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Have you checked the compression? Have you played with the ignition advance? Try the other carburetor. Try pushing the car in neutral on a level road. Does it push easily? Is this something that came on gradually or all of a sudden? How is the tire pressure?

Vibrating a "little" while backing off on the throttle could be normal depending on how much a "little" is. Or as Marshall talked about, it could be an indication of something amiss in the drive train.

[Sunny the Model A]

Thanks a million, to all who replied. so as far as i know, gas level is correct in the bowl, since I had an experienced carb man at my club put it together since he is so meticulous. as for wanting to put a marvel on it, that comes from wanting to squeeze my fuel dollar further and keep the zenith as a spare under the seat. I got a free marvel from a friend that need a few things, so it's me wanting to experiment. as for timing, i fiddle with it constantly to keep the engine at top efficiency. I'm not one to run full advance all day. I usually run about halfway to 3/4 on the advance since I don't drive fast. now, as for compression, its good. Got a bone stock engine and it seems to run fire. the vibration only hits on deceleration at speeds over 30, under that it won't. this vibration has been with the car since I first drove it at that speed, which was after I bought it, so it wasn't sudden to my knowledge. it feels like it might be from the u joint forward because it seems to vibrate the front and floor under the seat more than the back end. it can best be described as a frequency vibration that's akin to an engine rev spot. the frequency is faster than tire shake, closer to an engine vibration or something driveshaft related. not so bad mirrors fall off or anything, nor is it tire related since I keep my tire pressure correct and they have been balanced. so, its drive line related in some way. I theorized it could be something like loose hardware, but only way to know is pull stuff and I am not in the position to do that right now as I am using the truck regularly with my old chevy car. I am praying it isn't flywheel or something bad, hopefully it's something easy to fix.

[Sunny the Model A]

oh, and postscript, it rolls EASY. like so light you can push with one hand on level ground easy. brakes just got done, nothing dragging and all grease points topped up nicely. so its not bogging down from that, and it feels nice in first and second, but go to third and it acts like its too tall. other club members a's I have driven act more peppy in third. mine puts its tail in its legs and goes "do I haaaaave to?" lol

[Big hammer]

Clogged exhaust, aka mouse house, muffler falling apart on the inside. Throttle plate not opening fully, aka bent linkage, many years ago my A wouldn’t go past 45 mph going down hill with a tailwind ( throttle plate was opening fully).

[Rob Doe]

Sunny, I've recently been tweaking motor mounts. (Float A Motors/FAM) What mounts do you have and are the rubbers old and hardened? What front mount do you have. There is a leather washer that goes under the front yoke that sticks down through the front x member. Snyder's or Bratton's has exploded views on their website.

[Rob Doe]

As I remember, 15mph is lower than the Model A Instruction Manual calls for shifting into 3rd. 4-11 gears would help this, but try shifting out at 18-21mph???

Your 13-30mph is the low speed range of the carb. The CAP jet circuit in combination with the Compensating jet, Secondary Well, and GAV are supplying the largest percentage of fuel to the Venturi at that speed range. The Secondary Well, the brass insert, supplies the idle speed range with fuel, but it also supplies a brief surge of fuel when transitioning from idle to low speeds. (similar to an accelerator pump on later carbs)

If your issue is a brief stumble when stepping on the gas in 3rd gear from 15mph, our coupe will stumble in cold weather, (below 55* F) if I have the GAV too tight. A small as 1/8 or so turn further open stops this. (Model A Zenith carb)

Carbs vary due to jets and many other variables, but running with 1/8 th or less turn open seems tight, as in lean. Sooty powdery black plugs would be rich You might benefit from resetting the base idle.

[Sunny the Model A]

as for engine mounts, all bone stock. no float a motors. my engine is firing on all plugs and it seems to just not have quite enough grunt at those speeds. if I stamp down on the throttle it will pull fairly well but it really takes off after the 30 mark. if i set the hand throttle at say 30, the engine occasionally makes a slight misfire every now and then, almost indistinguishable unless you listen closely. it's a low popping sound coming from the muffler, sounds like a lean burn or air getting into the muffler. its getting full throttle and all linkages were checked and the idle seems sufficient.

[nkaminar]

In high school, a dirty trick to play on someone was to jam a potato up the exhaust pipe. The car would hardly idle, much less accelerate.

[Sunny the Model A]

well, thankfully it doesn't have that problem of a dam tater in the tailpipe. but I might take the muffler off and hose it out to be safe you know? my used to run really smooth at 30 without that faint muffler pop, but after a while it starting happening, however the sluggishness was present before that.

[katy]

Originally Posted by Sunny the Model A

I had an experienced carb man at my club put it together since he is so meticulous.

Why not ask him to diagnose it ?

[Sunny the Model A]

because he is swamped with em, got 7 carbs in ahead of me and i did an inspection of all the heights, and I gave him flow tested jets, by all accounts it should be fine. but those zeniths are so finicky it isn't funny.

[nkaminar]

Here is a good site about the Zenith carburetor: https://model-a.org/

Here is their symptoms guide and the first topic is problems with driving over 30 or 35 mph. https://model-a.org/symptoms.html

[Sunny the Model A]

the sluggishness in third might be the comp jet from what I read. it might not be getting the correct fuel mixture. but when i get the truck back together as i took the brakes apart, I'll be able to give y'all better descriptions of the ailments. maybe sitting waiting for brake parts will teach my A a lesson hahaha

[1crosscut]

While it is sitting waiting for parts pull the carb off, split if in half and use compressed air to blow through the jets and passages. Easy, quick and simple to do.

[Sunny the Model A]

good idea, i just happened upon a scrap air compressor that'll do the trick, so I'll blow it all out in the meantime while the brakes are going back together.