MTFCA Forum

I've gotten my 1926 coupe to idle smoothly and start up without a hitch. It has new fuel, new plugs, rebuilt carb, etc. but any time I get the RPMs above probably 1500-1600 it sounds and feels like it's about to shake apart! This happens when driving and when it is just sitting in the driveway. I heard it could be the fan (new, 4 blade on there) so I disconnected it with no difference. What should I check next??

Its a balance issue. Every part needs to be balanced.
Pistons, wrist pins, rods, crank, flywheel, triple gears & each drum plus the output & flywheel must run true.
I have seen a mix of lite & heavy rods.

What was done to the engine recently? Or did it always shake?
I have experienced this, in my case it was caused by a tight piston wrist pin.

Worn main bearings are another source of vibration.

Have you done a plug short out test @ different speeds? You will find if you have a dead or weak cylinder. Even compression #'s?
Any knocks/noises? as in loose rods or mains. A loose flywheel can make her run rough.
Run spec is 1800 rpm =45 mph.
@ higher rpms, say over 2k you can get a harmonic imbalance point. Been there.
Retarding the spark & slowly advancing can get you thru that imbalance.
Try varying the spark. If you are running on battery, advance adjustment is infinite. If you are running on magneto, there are 3 or 4 nodes. Best to be centered in the node.

Check for loose motor mounts, loose generator, front spring clamp nuts loose, loose or cracked front cross member, etc.

You mentioned that you’ve got the car to idle smoothly but was that after you worked on the engine, after it was rebuilt, added new parts or ?
Did it just start the rough running recently ?

I had a similar experience with my T.
I could get it to idle but had no power and a lot of vibration after going to high gear.
Also had extreme shuddering or bucking going from low to high gear.
Did a short test of cylinder plugs and found #4 cylinder no change and #3 cylinder questionable.
Next did a compression test on #4 cylinder and found no pressure. Did the same test to #3 and got a result of
several psi.
Pulled the head and found the head gasket blown between #3 and #4 cylinder.
Replaced the head gasket and fired her up and found a NEW car.
This might help.

Johnr

Even the spark timing of the coils can vary if they are not tuned to spark at she same amperage for each coil. if you have some other coils which have been tuned up or if not, substitute coils from a T which runs smoothly and see if it makes a change. if it smooths out, the problem is your coils.
Norm

Rob,

Did it ever run smoother, or has it always done this? In other words, is this a new thing?

So the question is was the car running and shaking before you changed the plugs and etc or was it doing it before the work was done.

Plug wires on wrong? (firing order) Gaps set improperly?

I had identical symptoms. The vibration was horrible. My problem was that one of my connecting rods was the old heavy version. It weighed 42 grams more than the others. I replaced all four rods and balanced the rod-piston assemblies. That made a huge improvement.