Snapping at idle

[signature65]

So I have been mia from pelican for awhile. Bought a 997 and the 73 kinda went to the wayside. Well I'm back to the 73. Just got it back from paint and finishing the details on it.

Well I have a snapping sound at idle. Listened with a screwdriver to my ear to the chainboxes, throttle bodies, underside, case and MFI pump. The snapping is loudest from the distributer. I took a video. maybe someone has has similar experience? Thanks in advance guys.

http://youtu.be/BwukZNhfFBM

Darren

[scarceller]

Could it be arcing sound? Look into the engine bay late at night for arcing.

Open the distributor just to see if you spot something?

[emac]

Darren- I think your dizzy cap in off center, the sound you hear is the rotor hitting inside the cap.
Ernie 81sc

[signature65]

The sound is mostly intermittent, the video just caught a run of it. So I don't think it's the cap. It could be a worn dist shaft tho.

[signature65]

Anyone else have any expertise to chime in? I don't think it's arching. I have had my hands all over the wires and stuff while it's snapping and nada. I,m perplexed

[DRACO A5OG]

Almost sounds like an exhaust leak. Place a rag in the tail pipe, see if it becomes more pronounced. That is an odd sound for sure.

[motomiata]

This sounds exactly as my 72TMFI engine. The sound in my engine appeared after the third start up after 4 years sitting during the restoration of the rest of the car.

It's clearly a high pitch metal-against-metal sound, intermittent, non rytmic, and only at idle. I didn't had the nerves to keep the engine running for further investigation, but has started dismantling in order to check the timing chains and the valve train components. The only work on the engine during the 4 years has been upgrading the chain tensioners to Carrera type, setting the valve lash and retiming the cam shafts.

I will post any progress.

[wayner]

Any chance it is actually backfiring through the intake, caused my old fuel gumming up the works and making it run lean?

My ear isn't good enough to tell.

[motomiata]

In my case I am very certain it's a purely mechanical issue. The sound does not upset the idle as a misfire or backfire would.

[motomiata]

A late update on this: I have isolated the noise to be on the right hand bank and it is clearly felt on the rods connection the butterflies, as a blow with a small hammer.

My first impression was that it could be a dropped washer or nut bouncing on an inlet valve and randomly hitting the butterfly. I thorough investigation with a bore-scope did not reveal any foreign objects or sign thereof (scratches or the like).

Could it be a misfire blowing back the inlet hitting the butterfly wit a snappy sound?

Any suggestions how to proceed for eliminating a possible misfire?

Thanks

[scarceller]

Using the bore scope do you see anything on the intake valves? Like one being real dirty? indicating a cracked, bent or poorly seated valve?

[motomiata]

I was mainly looking for objects and didn't study the valves closer. I'll have a closer look next time around the car.