Hi Folks,

I got a new Bonez straight pipe/presilencer to replace the cat assembly on my 83. The flange faces are uneven enough that I am getting leaks, enough that it's backfiring where it didn't before.

I am using new, standard 2" gaskets that came with the pipe, but they aren't thick enough to seal the uneven flange. I've done all of the obvious wire brushing, no remains of old gaskets.

I was thinking maybe there's a thicker exhaust gasket out there, but I couldn't find one. The other ideas are some sort of squeeze tube stuff that can withstand the heat, or maybe something like that on the standard gasket and squeeze it all together.

Anybody had this problem and solve it?

Thanks!

Tom

 

A flat surface and some sandpaper would probably get the job done, if you've got some time.

 

I did lap it a bit with a metal cutting file but the "proud" surface area is the majority. I was hoping to find a gasket that would be malleable enough to fill the relatively small area rather than slog through hand filing the large area down to meet it. It's only like a ~.020" gap when everything is clamped down tight, but it's enough to leak.

 

I've seen Permatex High-Heat RTV used to help seal exhaust surfaces. You might get away with that.

Run a small bead on the faces of both sides of the gaskets maybe a quarter-inch wide. Tighten it up until it just starts to squeeze down. Let it sit an hour and tighten it up all the way.

Not really a long-term solution, but might work.

Probably should flatten the flange surfaces, though.

 

Yeah you're right about that. I worked at it and flattened them enough to make a much better seal and I also reassembled it making sure to align everything as perfectly as possible. So far so good, no leaks and the back firing is mostly gone.

Thanks

 

Hmm, I have the test pipe and assumed the backfiring was due to the missing cat! I’ll have to go back under the car and take a look.

 

For me, the back firing started as soon as I put in the pipe/exhaust leak, there was none with the cat in place. I've only got ~36K on the car and no engine mods other than direct fire (12A). And I said mostly gone for the backfires because I can still get it to pop when the engine is cold, and it's choked on startup. Once warmed up though, no backfires. So I think the most important thing in my case was a combination of filing the surface of the Bonez flanges and aligning everything...there's enough room in the bolt holes that you could be pretty far off and not realize it. Oh and I'm a complete amateur :P.

Also, on the backfiring, it would bang big when I would rev to 6 or 7K and get right off the throttle on a shift change. But if I moderated it a bit, held onto the revs a second longer before I depressed the clutch, it would be okay.

That smell: Ari at Bonez was very clear about this when I chose the pipe over the high flow cat: it's really smelly. Still, I had no idea. At a red light with the windows open, you want to close them. I think I might be too rich at idle so I'm going to tweak that. Once warmed up, idle is solid but at 500 rpm. I'm going to adjust the speed and mix and see if that smell gets better.

Edit: For leak testing I did what GSLSEforMe suggested in another thread: plug the rear with rags and rev it.

Performance wise, this thing is great! Not a huge bump in power, but it's there and more consistent, smoother feeling. Makes the car still more fun.

 

I've tweaked the idle to about 750rpm per the tach and leaned the mixture a bit. Of course the "smell" is subjective, but it's still pretty strong, not unburned gas odor, but strong. Is that just the way it is in your guys' experience?

And I suspect any exhaust without a cat would be that way, including the full RB? Do you RB exhaust guys notice a strong exhaust smell?

 

In my limited experience it just depends on the tune. Any gassy smell is unburnt fuel. So either your too rich, or your too rich at idle. But maybe too rich at idle is just right at other rpm ranges. Its just a carb after all.... an EFI system can be programmed to go richer or leaner depending on other variables.
Sorry, I dont have an answer for you. But my pro-tuned modern Subie smells way, waayyy stinkier than my carbed RX7 exhaust. '85 12a with full RB exhaust, Holley 390cfm carb.

 

The car runs great, and the Nikki is stock, no rebuilds AFAIK. I would think if anything a dirty carb would make a leaner mix over time as crud built up and blocked passages. There are probably a 1000 ways carbs can get weird.

I suppose a vacuum leak in the emissions stuff could cause it run rich? I don't really understand any those emissions parts so I stay the hell away from them. :P