This is interesting! The SSIs didn't have an O2 bung, so I had to have one fitted. I wonder if that could be the issue?

Maybe the cutout fell inside the SSI...

JJ - Wish it was that simple, but definitely experiencing power loss too.

I Should have watch the video…

Following Willcall theory, post a pic of the SSI with the O2 bung to compare with a factory one.

Be very careful running high RPM until problem fixed. Thinking AFM is too lean because of more free flow exhaust. Noise may be pinging that will eventually break rings and pistons. Richen fuel mixture and test. Your '80 does not have O2 sensors. Suggest AFM meter to test. Dyno shop will have one.

I can't tell from the video. But, is there a chance that one of the gaskets for the muffler is bad? I have a car that really likes to blow exhaust gaskets (yes, I know the flange is not flat). But, when it happens it makes a bad noise. To me it sounds more like a tapping valve, but also causes vibrations in the sheet metal adjacent to the port. I don't necessarily think that is what is happening here. But it is a noise and it came after the exhaust was changed.

Jay - In your experience, when it blows an exhaust gasket is it possible to 'only' hear the noise under load? If I sit in the garage and rev the engine by hand, I don't hear any bad noises, even all the way up to redline. But if I drive up a hill and rev over 5K, I will hear it every time.


turborat - Hope you don't mind, I've just copied your PM into here for the benefit of the group:

Can't think of a connection of power loss and exhaust replacement beyond the AFM. The CIS is a passive injection system. A more modern OBD I or OBD II system would make adjustments from O2 sensor input. The CIS acts more like a carburetor where one would have to rejet, the CIS will need adjustment......unless something was disturbed during the install. Check all vacuum routing, fuel pump and fuel filter. Also look at ignition. Cap, rotor, wires and box. Is ignition OE OR change to something else? Might also pull the injectors one by one and spray them onto a clear glass jar. Looking for a wide spray pattern. They are fairly cheap to replace and cannot really be cleaned. An AFM under load at higher RPM will tell for sure. Paul
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There is actually a 'basic' O2/lambda system on the '80 model. However my understanding is that at WOT it doesn't do anything - it only works on improving fuel economy below (I think) 35% throttle or so.

JJ- Which is why I'm leaning towards ruling out the placement of the O2 sensor as a problem, since it (shouldn't) be a factor at WOT, which is when I hear the noise (above 5K)

Ok. I thought '81 was first year of Lambda. I have had four 911 SC and all were/are '78 or '79. On OBD I engines like the 3.2L, one can disconnect the O2 sensor and AFM will richen up considerably. You could try same thing.

The OBD started with the 3.6

Detonation.
Change in flow via the SSIs, richen up the A/F mixture via the 3MM adjustment screw. That level of detonation, or any, over time will cause damage. I did mine seat of the pants, worked fantastic.

subscr! +1 on A/F mixture...

If it is detonation, you may also need to pull out some timing. I'd try richeining it up first, but if that cause start and idle issues, I'd look closely at timing. It will ultimetely come down to a balance between the two (and quality/octane of gas as well).

Josh has it right. After talking to Jerry W. about the subject, timing and fuel are variables that can tuned for. If I remember Jerry suggested 32 deg total advance as a starting point and adjust from there. It's all about the tuning.

To close this out....

It took me over a year, but finally got around to chasing this problem down and resolving once and for all.

I eventually bit the bullet and bought a new Dansk sport muffler. I swapped it out and...problem gone! Noise gone, power loss gone, back to running perfectly.

It took me a year to get over the 'I don't want to drop another grand and still have the problem' fear...sure glad I eventually did.

The Monty id's now gathering dust in my garage til I figure out what to do with it.

Thanks to all for your many and varied ideas.

Glad you found the source of the problem. No more being passed by Hondas. :)

Certainly the muffler change was less than the cost of rebuilding the engine or other random parts replacement strategy. However, you could have also substituted a factory muffler to confirm if the Monty was the source of the issue (that's called hindsight diagnosis).

It sounds like (pun) there's something happened to the internals of the Monty muffler. They may be interested in a post-purchase repair/adjustment for PR/reputation purposes. They're in your neighborhood, yes? If not, might be worth the effort to sardine-can it and see if there's a potential repair, otherwise it's of no use to the next owner or alternate use as a pricey paper weight.

Sherwood

Thanks for posting the resolution!!! Lots of threads where the fix/solution/cause is not given.