Transmission diagnosis

[Roland Paetsch]

I bought it in LA 6 years ago. Done about 10k miles. No change in noise. Came with 135 jets in the Solexes, overjetted for 1582 cc and S cam, very annoying noise at deceleration from above 3500 rpm. Now jetted with 127.5, correct jet but still noisy though reduced noise level. Have tried 122.5 jets (too small) and the noise was nearly gone. I am not clever enough to deduce the significance (if any) of this behaviour.

[Martin Benade]

Very mysterious!

[Roland Paetsch]

Maybe the engine just has more breaking power with the larger jets. Then the noise is proportional to the "re-loading" of the clutch/transmission.

Searching the net, the problem could be caused by too little preload of the pinion bearing. Hence a larger load produces more noise.

In the other current thread "deceleration noise" here on 356Talk the same problem is dealt with, many valuable suggestions.

[Jacques Lefriant]

Hi
if there is a lack of preload of the carrier bearings deceleration will suck the gears into a bad contact. This is a fairly common mistake by transmission builders that assume they can use the same shim stack and just get the backlash right by making corresponding changes. the gaskets that are supplied now are slightly thicker and need a corresponding correction in the shims. The Cadrobbi conversion uses a much more robust bearing set and stronger side covers.
j

[Roland Paetsch]

Jacques, would a cracked differential housing upset the bearing preload?

[Jacques Lefriant]

Hi
it is the other way no preload will let the diff flex more and allow the crack to grow more.
j

[Thomas Frisardi]

Hi
if there is a lack of preload of the carrier bearings deceleration will suck the gears into a bad contact. This is a fairly common mistake by transmission builders that assume they can use the same shim stack and just get the backlash right by making corresponding changes. the gaskets that are supplied now are slightly thicker and need a corresponding correction in the shims. The Cadrobbi conversion uses a much more robust bearing set and stronger side covers.
j

But doesn't bad contact usually sound like a whine or a howl? I know that sound of a loose pinion, which can move back and forth relative to the ring gear. It's a howling sound. Not sure how a ring gear moving side to side sounds, but I would have guessed it's a howling sound or whine, not the grinding sounds described here.

[Roland Paetsch]

Yes, it is not a whine or a howl. It sounds more like a load of bolts and screws are poured from a metal bucket

I am away now but I'll have my mechanic to look at it end of coming week.

[Steve Proctor]

This type of sound was present on a Roadster that a friend of mine owned. It was horrific. He had the trans rebuilt and then had it gone back through again right away because the sound did not go away. Nothing was found to be remiss enough to cause the noise. The flywheel was determined to be way too light and the sound either went away or was GREATLY decreased when a stock flywheel was installed. The offending flywheel had been race lightened in the usual way with LOTS of material removed from the perimeter behind the ring gear, but then had large holes drilled everywhere and smaller holes drilled in the spaces between the large holes. Can't imagine what this flywheel must have weighed, it was Swiss cheese.

For the record, I am in favor of light flywheels and with a street lighten as performed by Competition Engineering, I do not have this problem. It must just exist with over the top lightening. This must be a well-known phenomenon with all the molested cars running around out there?

[Thomas Frisardi]

It sounds to me as though the heavier flywheel acted to dampen the vibration that was causing the rattling sound, not that the flywheel itself was causing the rattling.

[Steve Proctor]

Yeah, other than being loose, can't imagine why a flywheel, itself would rattle. I have seen that John Wilhoit engines are running harmonic balancers and though I can't confirm, would think the flywheels are lightened for responsiveness and that the harmonic balancers are employed to eliminate the torsional vibration without the mass a greater distance from the crankshaft centerline that a stock flywheel provides. In the case I cited, would seem that the noise caused by torsional vibration through drivetrain that worked transmission gears, ring and pinion, etc., against their respective backlashes??