help with jerking while driving

[Emeraude]

I just got my 25 roadster going after buying it from the previous owner who had little knowledge of it,s history. When I started driving it the engine starts easily idles fine and accerates fine and no problems shifting from low to high. The problems is any speed below 20 mph it starts jerking and gets worse the slower it goes. ANYTHING BELOW 15 mph is almost impossible. If I accerate above 15mph it smooths out and the ride is smooth.
In getting it going i put on a new texas t distributor, rebuild carb from snyders new spark plugs and wires and nerw coil. while driving at low speeds and the jerking starts I have experimented with many different combinations of spark and mixture settings and nothing seems to help. Any suggestions on how to correct this?
The only thing I can think of is the drive train gearing causing this as what would happen if going to slow in 3rd gear in a 3 speed transmission?
would appreciate any suggestions

[speedytinc]

Check the rear end ratio. What you describe is usually mitigated by retarding the spark to a happier position. If you have 3:1 gears this exacerbates the problem you are having. 3 revolutions of the crank = 1 revolution @ the wheel. Pull the spark plugs & test on flat ground. Stock gearing is 3.66.

I have 3-1's in my T. If I dont shift into Ruckstel (under-drive) I can get a shuttering & bucking taking off from a light.
You didnt mention having a rux. If you do, it could be as simple as being in direct with hi speed gears.

Its not unusual to find hi speed gears in lighter roadsters & speedsters without a rux.

[Les Schubert]

Couple of thoughts;
1.If a stock rear axle could it be a babbit thrust washer that has died.
2. I assume it’s not front end shudder caused by clearance/ front end alignment issues

[Mark Nunn]

Is there any type of mechanical noise when it is jerking? If yes, it could be a very worn U-joint that would feel smoother under acceleration or higher load.

[Emeraude]

from the above reply’s I would lean to Babbitt washers or u joint. Is there any way to check these without dismantling the rear end or pulling the drive shaft? Or is it more likely the rear end gear ratio and will have to live with it?

[Jerry VanOoteghem]

Sounds like a misfire under load. Not all new spark plugs are good spark plugs. A friend was telling new about misfiring with brand new Autolites.

[speedytinc]

Check the rear end ratio to rule it out. If it is 3:1, you can modify your driving. Use more low pedal or run faster around the corners.
Or tear it down.

The Babbitt thrust washers can be checked by pulling the rear wheels. check for movement in the internals. If the left is gone, the axle can move back & forth 3/16" You would have to use an endoscope on the right side for a visual confirmation. It may be quite obvious looking @ the axle grease. If it's grey, it's got decentigrated Babbitt in it. You can also fish with a wire for chunks.

The U-joint can be viewed enough thru the large grease cup hole. A better view by removing the DS flange & looking thru the larger hole in the DS ball.

[TXGOAT2]

Have you tried greasing the ujoint? Pump about half a tube of good grease into it.

[Jerry VanOoteghem]

Have you tried greasing the ujoint? Pump about half a tube of good grease into it.

Is that going to remove the wear that some feel is present in the u-joint?

[TXGOAT2]

I don't feel any wear in the ujoint at all. If it's dry, greasing it may improve its function and help pinpoint the source of the problems the car owner is having. It's fairly easy and inexpensive, and if it does no good, it will do no harm.

[Art M]

I had a similar problem. I tightened the nut on the carburetor that locks the mixture needle shaft. This reduced the problem.
I smeared ultra black around the nut/carb region. The problem went away. I am wondering if someone knows the correct procedure to solve my problem. I am sure there is a better way out of this problem.

Art Mirtes

[TXGOAT2]

A great many things could cause a car to behave like that. A problem anywhere in the ignition system could cause it, including a problem with the switch, the timer wiring, or the coil box insulation or coils not held tightly enough to maintain good contact or problems with one or more coils or the timer itself.

Loose motor mounts could cause it, as could a loosely mounted lower steering bracket, a loose throttle plate or a loose throttle lever on the carburetor, or extreme wear in the throttle linkage or loose control shaft levers. A choke or choke linkage issue might cause it. As pointed out, mechanical issues in the driveline could cause it. One or more weak valve springs could cause it.

A person with T experience driving or riding in the car could probably quickly narrow down the range of possibilities.

[BobUkPipedream]

Am I the only person who saw the title and thought ‘Dude… don’t do that whilst driving’

[speedytinc]

Am I the only person who saw the title and thought ‘Dude… don’t do that whilst driving’

Scratched my head, then I got it.

[RVA23T]

Am I the only person who saw the title and thought ‘Dude… don’t do that whilst driving’

I did as well but then I realized I need help to not be a jerk behind the wheel whenever and whatever I am driving!!!

[DHort]

Robert, I got both meanings.

Do you have a gas filter? Is the filter on your potato clean?

[Emeraude]

To answer some of the questions the u joint is fully greased clean tank and lines. It has a distributor and that has been checked. As far as I can tell the motor mounts are ok. I am going to try and check if I have a worn u joint.
I appreciate the help

[Norman Kling]

It would be helpful to let us know how the car idles, and if the jerking comes when you are accelerating or when driving at a constant speed. Low compression on one or more cylinders could cause it. The compression should be even on all cylinders. Somewhere around 50 psi. It can change with modifications to the engine such as high dome pistons or type cylinder head. Also variable by altitude, however, it should be nearly the same on all cylinders. A weak spark could cause misfiring while the throttle is pulled down but would smooth out when the engine is at a constant speed. I have a 3-1 in one of my T's. It came that way when I bought it. On any start out on a grade I must use low Ruckstell or it will be too hard a strain on it. I must also rev more before shifting to Ford high than my other T's which have stock gears. Since you have a distributor be sure your points are good and the dwell is correct. Also have a good condenser in it. Be sure the plugs are clean and the gap is correct.
Norm

[TXGOAT2]

Sometimes it's what you know it ain't.

If the engine is misfiring under load and causing the car to jerk along at lower speeds in high gear, it points to an ignition problem. It could be as simple as ignition cables shorting to one another or cross-firing due to being bundled together.

On the other hand, a mechanical issue serious enough to cause the car to jerk and buck might be expected to present multiple symptoms, such as jerking, knocking / grinding noises, and to show at least some symptoms at most any speed or load or in any gear, whether pulling, coasting, or or when applying the transmission brake.

[speedytinc]

As I mentioned earlier, extra retard helps your described situation.
Did you add enough retard with the distributor install?
Did you confirm your axle ratio yet?

[Emeraude]

To answer the questions I have done both compression and leak down test and all the cylinders were at 55 lbs. the car starts and idles well. It averages well from a stop but after shifting tonight and trying to drive in the 15 mph range the jerking starts and continues unless i increase the speed to near 20 mph it stops. I haven,t determined the rear axle ratio. How do you do it?
Appreciate all the help

[Norman Kling]

It can be done by leaving it in high gear with the brake off. Easier with spark plugs removed. Mark with chalk the spot where the rear wheels are touching the floor. Then turn the crank and see how many revolutions the engine makes for one revolution of the wheel. If the engine turns 3 times you have 3:1 ratio. Standard would be 3.66:1.
So standard would be approximately 3 2/3 turns of the crank to one turn of the wheels. If 3:1 ratio you would need to go faster in low before shifting to high.
Norm

[Art M]

It's easier to lift only one wheel off the ground, with hand brake fully forward, turn the engine one revolution The rear wheel will turn .5 revolution with 3 to 1 axel. He wheel will turn .67 or so with a 3.68

[Scott_Conger]

Art, are you sure about that?

[TXGOAT2]

The difference in 3.63 and 3.00 is about 20%. It seems to me that a stock T or near-stock engine ought to run smoothly at 15 to 20 MPH in high gear on level ground at light throttle and moderate spark advance whether it has a 3.00 or 3.63 gear set, assuming the tires are of normal diameter.

[Art M]

Scott, shame on me. My method is correct, my memory for the numbers were off.
For a 3.6363 to 1 axle, and the engine turns 1 revolution
The rear wheel turns about .55 revolution

For a 3 to 1 axle, the rear wheel turns 2/3 of a revolution

Example calculation. 2/3 = .667 turns. or 8 o'clock.

2/3.636 = approximately. 55 turns

I was on a tour with Tim Wrenn when I posted. Later on the tour while eating ice cream, I though about the wrong numbers.

Art Mirtes

[Jerry VanOoteghem]

So, if I understand it correctly, it would normal for a Model T to jerk along after shifting to high if you have a 3:1 rear end? I've never run a 3:1, but I don't think so...

[Jerry VanOoteghem]

Scott, shame on me. My method is correct, my memory for the numbers were off.
For a 3.6363 to 1 axle, and the engine turns 1 revolution
The rear wheel turns about .55 revolution

For a 3 to 1 axle, the rear wheel turns 2/3 of a revolution

Example calculation. 2/3 = .667 turns. or 8 o'clock.

2/3.636 = approximately. 55 turns

I was on a tour with Tim Wrenn when I posted. Later on the tour while eating ice cream, I though about the wrong numbers.

Art Mirtes

This is correct, when you realize that you're doubling the speed of the ONE lifted wheel, by virtue of the ratio within the differential itself.

[Scott_Conger]

Art!

no one was injured during the calculation, and all is forgiven!



hope the tour was nice - we're having spectacular weather here