I've decided to take a methodical approach to define the problem and draw up a list of tests to track it down. Can you look through and suggest other things to inspect please?
The problem
* The car vibrates wildly when the engine revs are between 2800 and 3350 RPM in all gears and in neutral, at all throttle openings except just off idle.
* Vibrations can be felt through the seat. Pushing against the passenger's door reduces some of the vibrations. Rattles can be heard in the dash, doors and from the rear of the car.
* The car vibrated a little at this engine speed when I got it a couple of months ago, just not very much. Since then, the car has been serviced. The points gapped, the timing and the valve clearances set. The tunnel "furniture" have been removed and reset. The door trims removed and reset.
* The problem started after a few minutes of driving at 60 MPH.
* The timing was set to ~20° BTDC when I bought it (set 10° before the wrong mark on the crank case). I have adjusted it by a few degrees either way and road tested it. It runs very sweetly at speeds lower or higher than the problem range. It ran very badly when set 10° before the correct timing mark.
Problem is engine related, not road speed related - happens in all gears and in neutral. It goes away when the clutch is dipped.
Problem is RPM specific - 2800 - 3350
An engine problem must cause vibrations that make the body resonate
It could be that the engine is vibrating more in the bad RPM range
It could be that the engine is fine and that the body has shaken loose enough to rattle with a good engine.
Test table
cause | part | test | result |
something is loose | | | |
| crank | remove the sump and inspect | - |
| cam | full engine dissasembly | - |
| big ends | remove the sump and inspect | - |
| pistons | remove the head and inspect | - |
| flywheel | remove the cover and inspect | - |
| clutch | remove the cover and inspect | - |
| fan / tinwork | inspect | All OK |
| engine / box mounts | inspect | All OK |
| exhaust | inspect | All OK |
something is imbalanced | | | |
| crank | remove the sump and inspect | - |
| flywheel | remove the cover and inspect | - |
| clutch | remove the cover and inspect | - |
| oil filter | remove the cover and inspect | - |
| fan | remove the tinwork and inspect | runs smoothly at all speeds |
fuelling, air or spark | | | |
| fan / air filter / induction | inspect | All OK |
| fuel supply | wouldn't work at high revs but not at medium revs | |
| carburettor | strip and clean | stripped and cleaned - except the inaccessible progression tube |
| plugs | inspect | All OK |
| leads and coil | not likely to be rev specific | works at other speeds |
| distributor | inspect | weights move freely. Contacts open and close right. |
| condenser | check points contact colour | grey not blue |
general checks | | | |
| compression | do a compression test | 100/102 (throttle closed) |
| valve clearances | remove cover and measure | one was slightly off |
| points gap | measure | 0.50mm gauge has some friction. 0.53mm gauge feels very tight |
| spark timing | measure | pully mark is in line with the 10°BTDC line when the strobe is set to 10° |