Help with an unusual noise

[gereed75]

Put this on the engine rebuild board but no response. Maybe someone here can respond

I am running a 2.4 built on fully worked mag cases, 9.5 JE's, Mod solex cams, webers with rebuilt 930 tensioners with guards and Henry's bushed tensioner arms. Single plug running the PartsKlassic spark unit and optical points.

Pulling through 5500 rpm I get a kinda of raspy metallic jangling sound. The noise builds as rpm goes up and then fades to nothing by 6000 rpm. From there It pulls to 7200 redline with no noises other than that sweet aircooled six wail.

The motors pulls without hesitation through the noisy RPM. It sounds very much like a valve train harmonic. kinda like one of those party noise makers that clicks a spring with a toothed wheel. It is very consistent. The motor makes no other unusual sounds at any RPM. The valves were correctly adjusted fairly recently. Oil pressure is normal.

This is in a 914-6, so engine noises are much more apparent than in a 911. Being in a 914 also makes dropping the chain covers not so easy.

I built this motor and the noise has been there since day one, has not gotten worse or better. Guessing maybe 60 - 80 hrs on it.

Anybody else ever experience something similar?? Any Ideas Thanks

[Reiver]

If it sounds like ball bearings being shaken in a glass jar you are getting detonation.....either not enough high grade/ron fuel or way to lean in that RPM range for your compression....that can do real damage if not addressed.

[Bob Kontak]

Originally Posted by Reiver

If it sounds like ball bearings being shaken in a glass jar you are getting detonation.....either not enough high grade/ron fuel or way to lean in that RPM range for your compression....that can do real damage if not addressed.

The raspy "click" sound is not something I have encountered and I have never experienced detonation so arm-chairing this one.

I would think Reiver's thoughts should be treated as an important variable to eliminate or confirm quickly.

Back your timing off five degrees and listen for a change.

[gereed75]

Thanks for the thoughts. That description of the noise is not far off. Running a pretty conservative timing already at about 28 degrees. Retarding it a bit more would be good quick check and I’ll try it. The thought of detonation is scary.

the noise occurs very consistently like clock work only at that exact rpm and does not seem to correspond to load conditions. Nor does it appear at any other rpm as you might expect if it were detonation.

The distributor is a completely mechanical T distributor Can’t imagine a mode where it somehow advances consistently for only a certain 500 rpm range. Nor can I imagine a mode where the carbs go lean. I guess it would take only one cylinder to go lean to detonate and cause the noise.

It is Because of the consistency that I suspect some sort of mechanical harmonic.

[pkabush]

AFR gauge is cheap insurance. Takes out the guess work.