Looking for OEM exhaust

[Dandy Man]

Hi, im new to the life of the cb750’s and as this is my first post ill give my little story. Im currently in school to be a small engine and equipment technician so i know a thing or two about fixing things, i have never had experience working on or riding motorcyces until this past year when i saw a STEAL of a deal on facebook marketplace. That being said i found a 1980 honda cb750c in non-running condition for super cheap, but all it really needed was a carb overhaul and some tlc. Fast foreward a few months and i have a relatively well running bike to learn how to ride on and all it needs is a few little things here and there to be a clean and complete bike.

But enough rambling its time to get to business. Im looking for an OEM or similar to stock exhaust system. My current exhaust is a four into 2 and it looks like it was custom made by farmer joe. It has awful welds joining the pipes into the mufflers and what looks like jb weld covering the possible exhaust leaks on the joints. It also has a megaphone muffler with what looks like removable baffles however i dont know if i can remove the baffles as they are ceased up. From the outside everything looks clean because there is a heat shield covering the joints but from the bottom it looks aweful. so id like to just replace the exhausts with the original 4 to 4 pipes, which would have the benefit of allowing me to feel the differences in pressure between cylinders 1 and 2 because im unsure whether i have an occasional backfire or misfire.

So… does anyone know where or how i can find an oem or similar exhaust in good condition near the twin cities MN? Or would i be better off finding a professional to custom make an exhaust.(sounds expensive)

Side objective: can i run a bigger front/smaller rear sprocket to get a bit more top end out of the bike? If so by how much can i change from the stock sizes?at 70mph i would be at ~6k rpm and id like to be at 3-4k for cruising, any thoughts?

 

[Old_Crow]

Not sure if your exhaust is stock or not, but all the pictures of a '80 CB 750 Custom that I find on the 'net show the 4 into 2 exhaust. I believe that was one of the differences of the Custom vs. the standard K model. Mine also has a 4 into 2, but it's aftermarket and not stock on the LTD.
My '79 LTD runs just about the same speed at the same rpm, the engine sounds busy, but not like its straining. I'm a bit on the heavy side for the bike, so I don't want to lose the low-end torque that would happen with the gear change. These engines were made to rev, and they'll do it happily.

 

[Dandy Man]

Not sure if your exhaust is stock or not, but all the pictures of a '80 CB 750 Custom that I find on the 'net show the 4 into 2 exhaust. I believe that was one of the differences of the Custom vs. the standard K model. Mine also has a 4 into 2, but it's aftermarket and not stock on the LTD.
My '79 LTD runs just about the same speed at the same rpm, the engine sounds busy, but not like its straining. I'm a bit on the heavy side for the bike, so I don't want to lose the low-end torque that would happen with the gear change. These engines were made to rev, and they'll do it happily.

Perhaps i was wrong about the stock exhaust being 4-4 but the point still stands that my current exhaust is not in the best condition, is there a way to remove the baffles without detroying them?
The two pipes merging into one then that one being restricted by a baffle with only a 3/4 into hole in the end seems like a waste and is possibly causing some of the problems with how the bike is running

Also i have put about 30 miles on some new spark plugs and the ends of the threads are black like id assume they should be but the electrode and ceramic are clean as a flute with absolutely no discoloration
This indicated its running lean right? It has really bad hanging revs at like 3k rpm after letting off the throttle, when i have some speed it is especially bad as if theres just air being crammed into the engine or something…..

Im still trying to figure it out but when im stopped i can get the idle to settle, whereas if im still moving (clutch pulled and/or in neutral) the rpm WILL NOT settle and will stay at 3k until i eventually stop

 

[brettp]

Sounds like air leak causing lean condition.

 

[PWBII]

Hi, im new to the life of the cb750’s and as this is my first post ill give my little story. Im currently in school to be a small engine and equipment technician so i know a thing or two about fixing things, i have never had experience working on or riding motorcyces until this past year when i saw a STEAL of a deal on facebook marketplace. That being said i found a 1980 honda cb750c in non-running condition for super cheap, but all it really needed was a carb overhaul and some tlc. Fast foreward a few months and i have a relatively well running bike to learn how to ride on and all it needs is a few little things here and there to be a clean and complete bike.

But enough rambling its time to get to business. Im looking for an OEM or similar to stock exhaust system. My current exhaust is a four into 2 and it looks like it was custom made by farmer joe. It has awful welds joining the pipes into the mufflers and what looks like jb weld covering the possible exhaust leaks on the joints. It also has a megaphone muffler with what looks like removable baffles however i dont know if i can remove the baffles as they are ceased up. From the outside everything looks clean because there is a heat shield covering the joints but from the bottom it looks aweful. so id like to just replace the exhausts with the original 4 to 4 pipes, which would have the benefit of allowing me to feel the differences in pressure between cylinders 1 and 2 because im unsure whether i have an occasional backfire or misfire.

So… does anyone know where or how i can find an oem or similar exhaust in good condition near the twin cities MN? Or would i be better off finding a professional to custom make an exhaust.(sounds expensive)

Side objective: can i run a bigger front/smaller rear sprocket to get a bit more top end out of the bike? If so by how much can i change from the stock sizes?at 70mph i would be at ~6k rpm and id like to be at 3-4k for cruising, any thoughts?

Larger on front sprocket will get you more top end & less RPMs, also less torque. Same with smaller rear sprocket, check chain clearance & availability for larger front sprocket. Sources for these vintage machine parts are drying up. I do this on all my SOHC models when building them myself. Stock exhaust is going to be hard to find & expensive when you do. I think Mac still makes after-market exhausts for this bike. Way cheaper than OEM. Making exhaust from scratch will also be big $s. Keep in mind this bike was jetted to be on the lean side in the first place, so, get a good exhaust on it with some back pressure.

 

[Old_Crow]

Not really familiar on the baffles. Both of my other 750s had 4into 4 pipes and the 4 into 2 on my current bike is aftermarket. I've never looked at taking the baffles out because I think it's already too loud.

 

[Dandy Man]

Sounds like air leak causing lean condition.

It had a vacuum leak after the first time i took off the carbs but i think ive got that sorted out now, while the bike is running i can spray carb clean all over everything and the rpm doesnt rise.
Although i checked the valves in november and they were very tight, like bare minimum clearance. it was on the lowest end of the manuals specs but i didnt have the tool or replacement shims so i was unable to do anything about it, i plan on doing that soon.
Could tight valves cause the sort of air leak that would enduce a hanging idle?

 

[Dandy Man]

Not really familiar on the baffles. Both of my other 750s had 4into 4 pipes and the 4 into 2 on my current bike is aftermarket. I've never looked at taking the baffles out because I think it's already too loud.

Fully agree on the bike being loud enough as it is, just wanted to see if i were able to take them out could i get the bike to run better or do i need the restriction for whatever sciency reasons

 

[brettp]

It had a vacuum leak after the first time i took off the carbs but i think ive got that sorted out now, while the bike is running i can spray carb clean all over everything and the rpm doesnt rise.
Although i checked the valves in november and they were very tight, like bare minimum clearance. it was on the lowest end of the manuals specs but i didnt have the tool or replacement shims so i was unable to do anything about it, i plan on doing that soon.
Could tight valves cause the sort of air leak that would enduce a hanging idle?

The manual is incorrect. Honda admitted those clearances were too tight. .005" is the correct spec.

 

[Dandy Man]

Larger on front sprocket will get you more top end & less RPMs, also less torque. Same with smaller rear sprocket, check chain clearance & availability for larger front sprocket. Sources for these vintage machine parts are drying up. I do this on all my SOHC models when building them myself. Stock exhaust is going to be hard to find & expensive when you do. I think Mac still makes after-market exhausts for this bike. Way cheaper than OEM. Making exhaust from scratch will also be big $s. Keep in mind this bike was jetted to be on the lean side in the first place, so, get a good exhaust on it with some back pressure.

Im well aware i would lose a little torque but i really dont need it, im simply flying through gears and just want to have a quieter and more comfortable ride, which i would achieve by having longer gearing.
At ~35 mph im chilling at 3k rpmwhich is nice except thats 5th gear, if i could cruise 35mph@3krpm in 4th or even 3rd gear then i would be closer to 4k rpm at 60-70 mph in 5th gear rather than 6k. current/stock gearing is 18 teeth on the drive sprocket and 43 teeth on the driven. Would it be too much to size up to 20 on the drive and 36 teeth on the driven? My redneck calculations put me at 4k rpm in 5th gear going 70mph. Probably not very accurate but it cant be too far off. Any thoughts? Am i wrong about anything?

Also now that ive gotten some more seat time ive noticed that the clutch is a little slow to fully engage, like when i let go of the lever it takes a second to fully engage if i get on the gas a little early, is that something worth looking into or should i just keep trying to get better with my timing and control. It seems to stay engaged if i whack the throttle well after i let go of the lever, its just what i assume is a weak return spring or gummed up shaft or something of that nature. (unless its normal and im just paranoid)

 

[Old_Crow]

Fully agree on the bike being loud enough as it is, just wanted to see if i were able to take them out could i get the bike to run better or do i need the restriction for whatever sciency reasons

All my Hondas (up until my current one) have had stock exhaust, so my experience with modifying exhaust is on old Harleys with drag pipes. Over the years I found that just a smidge of obstruction in the end of the pipe helped with the low end and didn't hurt the top end. Google "snuff or not". It was a baffle you could put in the end of the pipe with a knob that you could turn to either align the baffle with the exhaust flow or restrict the exhaust flow. I started out by welding washers to the head of a bolt so it looked sort of like a lollipop on a stick, drilling a hole in the end of the pipe, and placing the washer in the exhaust flow. Started with the washers perpendicular to the flow and then turning it until it was parallel. On my old Sporty it ran best with maximum restriction. On the 93" stroked Shovelhead I ended up with just a 1/4" bolt protruding into the exhaust flow. It didn't look like much, but it sure improved low-end torque. Ahhh, science.