Topic: 1978 CB750K Exhaust Pipe Question?

[bob fliegel]

I am fortunate enough to have an extra set of exhaust pipes. The pipes on my bike have rusted thru and it is time to change them. Before I put on my "new" exhaust pipes I was trying to find a way to protect them so they will not rust away like my old ones.

It seems like the pipes rot out from the inside from moisture. What I was thinking was painting the inside of the pipes with "POR-15" to try to slow down this process. Naturally, I would try to keep the outside polished and waxed, but it is the inside where the real rusting takes place. Anyone have any other better ideas on protecting their exhaust pipes. I know I will never be able to find another set!

One other question, does my CB750K need any "back pressure to float my valves". The material inside these exhaust pipes was removed, by the previous owner, long ago. They are just straight pipes now and I am concerned that they will have no "back pressure". I am only using the bike maybe 1,200 miles a year for "Sunday Driving" and cruising, I definately, do not push the bike. I would hate to damage my valves while taking such good care of my bike in all other ways.

[eurban]

You say that your "new" pipes have had their internals removed by the previous owner?  This is a bit confusing as the stock reverse megagphone pipes for the 77/78 do not have removeable baffling as the endcaps are welded on.  Did PO cut off the endcaps and remove the internal baffling?  Or did he cut the mufflers completely off? Or are you running an ealier set of pipes with the baffles removed?  . . . .Anyways, running pipes with no baffling shouldn't cause any damage (except to you and your neighbor's ears) as long as the carburetion is properly tuned.  If, for instance,  the baffless pipes create an overly lean condition in certain running conditions then you certainly could do some engine damage. . . . .As to coating the inside of the pipes, If they have been used, I think you will find it pretty difficult to prep them properly so that a paint would stick.  Also if you "paint" the header portion, the paint would have to handle high temps.  They do sell coatings for the header portion of the pipes that are designed to shield the chrome from extreme heat and prevent blueing but again this is designed for fresh pipes and is probably not all that effective.  Overall I think that the best way to keep the pipes from rusting out is to use the bike frequently, and in such a way that it fully heats up and burns off the condensation inside the pipes.  I do believe that the stock pipes for the 77/78s actually have a weep hole built into the pipe on the underside near the joint between muffler and the header pipe.  Keeping this clear might help. You could also fog the inside of the pipes with WD40 etc in the winter/during down times.  . . . .BTW original pipes can still be had for the early and mid year 750s and I just noticed that David Silver Spares is now offering a reproduction set of pipes for the 77s and presumably the 78s.  Hope this helps.

[Loudpipe]

No backpressure is OK as long as you keep an eye on the mixture.  Run it a bit rich, and you should be alright.  This is a good article written in layman's terms by UUC, a large manufacturer of performance BMW parts.  They can explain it much better than I can.  It's directed at fuel injected cars, but the concept can be applied to pretty much any engine.

http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm

[lrutt]

On all new pipes I install on bikes I completely coat the insides with the highest temp paint I can get. The exception is of course for glass pack style mufflers. But I do the head pipes and baffled mufflers. When I put new replacement pipes on my 76 I poured a generous amount of paint in each pipe, taped the ends, and thoroughly rotated the pipes around so as to spread the paint. I then hung them up so all excess would run out. I've done this to my HD, my old Triumph, my 750, and a few others.

None have ever blued or rusted yet. But these were all brand new pipes. Don't know if it would work with old ones.

Of course the first few 100 miles or so are pretty smelly but it does protect the pipes.

[Hop on a Honda]

IF you have the loot $$$ Try to get the pipes rechromed first with copper inside and out .
Then Nickle and chrome etc. The insides should be polished too IF possible with a long rotary mop.
Smoother the better.That is to reduce the parasite drag caused by all the carbon build up.
Worked for me on a  set of 4>1s that were 30 odd years old. They came out 100% and looked great afterwords.
Dave Australia.

[jalfonso]

I heard really good things about jet hot coatings.  High temp and it lasts for ever.  www.jet-hot.com   Might be worth a look.

[Geeto67]

I heard really good things about jet hot coatings.  High temp and it lasts for ever.  www.jet-hot.com   Might be worth a look.

doesn't last forever - I have tons of car headers I bought new with jet hot coatings and they have all rusted over over time. I even sent them a  pair of stock cast iron manifolds and they have rusted over on the outside too. (BTW the headers I had coated inside the inside coating held up ok).

The best thing to do to keep your pipes from rusting is to actually ride your bike - a lot. Most of the inside pipe rot comes from condensation that is formed while the motor is warming up, once the motor reaches peak operating temp the condensation slowly evaporates. Starting yout bike and letting run for a few seconds while in winter storage is probably the worst thing for your pipes because the engine has a chance to form the condensation and not evaporate it. I have seen load of bike over the years that were pristine except for the pipes, because the owner was starting the bike to "keep everything" working but not letting it run long enough to keep the condensation from damaging the pipes.

[jalfonso]

Thanks for the heads up Geeto67.  We don't get too much rust out here in Phoenix, way to much heat but not much rust.