#3 header visibly vibrating - Fixed

[seestheday]

I just noticed something weird on my 1981 CB750K.

This weekend I took off my headers to address an issue of the #3 one not seating correctly.  I could feel exhaust puffs when I put my hand near it, so I know that it wasn't sealed correctly.

So I took them off, and used a fair amount of high temp gasket sealer following all of the instructions before putting it back together (waited 24 hours before running etc).  The bike ran amazingly well afterwards (I also fixed a jetting issue, ensured that the cam chain tension was adjusted and made sure that the carbs were still in sync).

With the new jets, the exhaust note is lower, and I like it a lot more.  The puffs also went away completely.

I took it out for a highway spin last night and noticed a little more engine noise was starting again.  

When I started it up this morning I noticed that the #3 was visibly vibrating and rattling.  If I push on the exhaust with my foot causing the headers to flex just a bit the rattle goes away completely and it sounds like the engine is running perfectly.  The exhaust puffs are also back, as I imagine the gasket sealer has been perforated.  The aluminum flanges with the heat sink fins built in are definitely tightened all the way down and the copper crush washers are installed (along with lots of high temp gasket sealer).

I haven't noticed the header vibrate before, but it might be because the exhaust note is now lower, which would change the resonant frequency, coupled with the fact that I can now get a steady idle at 1100 rpm's when before the rejetting I had to keep it up around 1500 or it would stall too easily.

The headers are no name/uinknown 4-1's and the exhaust is a no name/unknown slip on.  Note that I said they're 4-1 headers and not 4-2-1 headers.

Has anyone run into this sort of thing before?  Is there an easy fix?  Do I need to figure out how to bend the header pipe so it will fit better?  

[rrgunslinger]

Did you remove the muffler before trying to tighten the exhaust?  If not I would.  They should be tightened with no other pressure on them. 

On Tue, Jun 5, 2012 at 1:38 PM, seestheday [via Honda CB750'S] <[hidden email]> wrote:

I just noticed something weird on my 1981 CB750K.

This weekend I took off my headers to address an issue of the #3 one not seating correctly.  I could feel exhaust puffs when I put my hand near it, so I know that it wasn't sealed correctly.

So I took them off, and used a fair amount of high temp gasket sealer following all of the instructions before putting it back together (waited 24 hours before running etc).  The bike ran amazingly well afterwards (I also fixed a jetting issue, ensured that the cam chain tension was adjusted and made sure that the carbs were still in sync).

With the new jets, the exhaust note is lower, and I like it a lot more.  The puffs also went away completely.

I took it out for a highway spin last night and noticed a little more engine noise was starting again.  

When I started it up this morning I noticed that the #3 was visibly vibrating and rattling.  If I push on the exhaust with my foot causing the headers to flex just a bit the rattle goes away completely and it sounds like the engine is running perfectly.  The exhaust puffs are also back, as I imagine the gasket sealer has been perforated.  The aluminum flanges with the heat sink fins built in are definitely tightened all the way down and the copper crush washers are installed (along with lots of high temp gasket sealer).

I haven't noticed the header vibrate before, but it might be because the exhaust note is now lower, which would change the resonant frequency, coupled with the fact that I can now get a steady idle at 1100 rpm's when before the rejetting I had to keep it up around 1500 or it would stall too easily.

The headers are no name/uinknown 4-1's and the exhaust is a no name/unknown slip on.  Note that I said they're 4-1 headers and not 4-2-1 headers.

Has anyone run into this sort of thing before?  Is there an easy fix?  Do I need to figure out how to bend the header pipe so it will fit better?  

1981 CB750K

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[sgtslag]

The book specifies that the copper crush washers be replaced every time you drop the pipes...  No gasket silicone should be used, or even necessary.  Considering the high temperatures, and the very high pressures, I would not trust any silicone gasket material to withstand the conditions present -- there is a reason they chose a copper gasket material for this application.  I'd try new crush washers, but I would also carefully examine the headers, and the block, for irregularities.

I have a sealed set ('79 K) I purchased a few years ago, but did not use.  If you want to pay the postage, they are yours (I can get you a quote for US Air Mail -- the least expensive option; otherwise, it will be Global Priority Mail -- no surface mail option possible, I've checked previously).  Let me know.  Cheers!

[shinyribs]

We run into a lot of strange harmonics at work some times. Vibrations can do some crazy things! Not sure how to cure your issue,though. Thats a toughie. Are your pipes chrome or painted? If painted I would suggest welding a tab in a non visible area to the next closest pipe. just to help stabilize it or maybe even help distribute to vibration. I wouldnt think it would come from the tuning you did,but maybe when it was leaking/loose the vibrations could escape,whereas with it tight, now they are being transmitted.

[clyde the ape]

I would check the studs.  Once everything is tight nothing should move.  Sounds like one of the studs may be walking out creating the gap.  Check to make sure the nuts are still tight, could be a bad lock washer or lock nut loosening up.  

[seestheday]

To answer some of the questions:

Muffler was attached to the headers while I tightened them.  I tried to get it off when I painted them flat black but they're rusted on pretty good.  The muffler was not attached to the bike, but it was propped up so there wasn't much force pulling down on the headers.

Regarding crush washers, this is the 2nd new set I've put in and I always seem to have the issue on the #3 header.  That said, I'm willing to give it another shot as there is a chance the 1st set didn't seat correctly and the 2nd set could have had too much gasket material (they're also super cheap - even from the honda dealership).  I'll just put a dab of gasket material on the new crush washers to ensure that they stick in the right spot and seat correctly.

I really hope that it isn't the studs that walked out.  If it is, how could that be fixed?  Aren't they pressed into the head?  I'll measure them vs. the others when I pull the headers to see if that is the issue.

I also thought that there might be an issue with the bendable c-clamp that goes between the finned flange and head.  The c-clamps hold the flanges to the headers.

When I have them off next I think I'm going to use the opportunity to put some wrap on them.  I've been inspired by Shiney's build thread and that has been something I've been thinking of for a while.  Knowing that they'll quiet the exhaust down is also a bonus.

[seestheday]

Finally figured out the issue and boy do I feel silly.

The bendable c-clamp between the header and the flange was too tight around the header, causing it to drop into the cavity of the flange and not press against the copper crush washer.  Once I bent it out so it was over the lip the flange it sealed perfectly.

No gasket sealer required.  Bike is now much quieter without the rattle from the header vibrating.

Now I can hear a very faint valve slap that I'll need to take care of.