Topic: HM341 Exhaust on A CB750 K1

[Bryce919er]

Hello, I just got a CB750 K1 I want to fix up.  I am new to these but have wanted one for awhile.  I am just learning have heard the K0-K1's had HM300 exhaust & K2 and later had HM341.  I am looking to buy an exhaust & want to buy the correct one--here is my problem.

MY K1 has HM341 on it.  I have no reason to believe they were added later unless you all convince me that is the only way.  It is a very late K1--built 9/71, registered as a 72 (I have heard at least in my state up until 1973 the year it sold is the year it was registered as), but VIN says its a K1 (VIN CB750-11175XX).   Since it is so late is it "correct" for it to have HM341's?  I have all the other K1 identifiers...

Also.

Did any K1's come with the handlebar mounted idiot lights?  I do not have the lights but I have one K1 speedo & one K2 & later speedo (no tach) for some reason. 

How can I spot a K1 airbox?  Is there a link to changes in the bikes that aren't obvious?  For example I can see the painted fork ears, tail light size, seat, differences, etc.  clearly.  But poking around here I learned the K1 has a different airbox than the later models & at some point the petcock switched sides of the tank---any more stuff like that so I don't buy the wrong parts?

Thanks

[Johnie]

Welcome to the board...you have found the best forum for restoration of your K1.  You have the correct air box top.  Looks like the bottom half is missing.  Easily found and I bet someone on the board here has one in the for sale / wanted area.  K1 comes with HM300 which are available for over $1,000.  No question about that.  341's are a lot cheaper which is why someone probably changed them out to save a buck.  K1 does not have lights on the bars.  They are located in the speedo and tach. Looks like a nice find.  Have fun with it.  Click on the pic to enlarge it...

[736cc]

I had a very early serial # K2 that had K1 parts (r. fender and tailight) and also had a very late K1 that was registered as a 1972. Its possible transition parts occur. If the HM341's ain't rotted, just use 'em. Check what main jets are in your carbs; K1 had 120's, K2 had smaller mains (110's?)

[ekpent]

Welcome Bryce. What a difference a bath makes-that WAS one dirty K1. Looks like a nice start,how many miles are on it? Have you had it started yet? If not start right with cleaning out the gas tank and carbs,good tune-up-tappets-plugs -timing-oil change and that baby should come to life. Keep us posted with some more pics----Good Luck---Eric

[Johnie]

The 69, 70, 71 all have that smaller tail light.  You look period correct there.  I see the petcock is open...oh, oh.  Was there gas in it.  I agree, do not be in a hurry to start it up.  If that bike had basic maintenance it will run.  These are tough engines here and very reliable.  Most have a least a slight head gasket leak.  Many guys do not bother to change it until it gets bad...and that may be never.  Change that oil, filter, clean and seal that tank, clean the carbs, change the points, condensors and plugs, adjust the valves...bet it starts right up.  Nice pics...

[ekpent]

I am with Johnie on those steps as I mentioned also,as for the major tune-up ,fluid changes etc.. You do look to have some oil leakage around the valve,head,tach seal or muffler area.May need new head gasket. When I get these bikes I like to do a very thourough cleaning degreasing and after the tank clean-tune-up phase try and get them started and running before tearing anything on the engine apart to get a better assessment of leaks,srange noises etc.. Don't worry if your clutch is not working right now,the plates stick after sitting and need some fresh oil worked in on start-up to break loose. Also when you do carbs you should find brass floats if original,check those very carefully for leaks,they can be a problem. Both Johnie and I have swapped ours out with more modern plastic units.Lots more but dig in and ask questions.

[Johnie]

Welcome Bryce. What a difference a bath makes-that WAS one dirty K1. Looks like a nice start,how many miles are on it? Have you had it started yet? If not start right with cleaning out the gas tank and carbs,good tune-up-tappets-plugs -timing-oil change and that baby should come to life. Keep us posted with some more pics----Good Luck---Eric

Dang ekpent...that paint on your K1 looks nice in that pic.  Can't wait to get mine done this winter.  Lookin' good there!!!

[ekpent]

Thanks Johnie. Wish I could bite the bullet and get some new HM300's. Just hate to spend more on the pipes than I got into the bike and these other deals keep poppin up. Working on a pretty descent green K1and a snotty 73 right now,both buried so far back in the guys barn(since 98) we can not get it out right now.I do very much respect your restorations and commitment to your projects keep up the good work-----Eric

[HondaMan]

The HM341 pipes require 105 mainjets in the carbs to stop fouling of the sparkplugs, but the stock jet was 110 if HM341 was fitted to a K1. Mine is a K1 with the K2 dashboard and instruments, and taillight and seat, one of the very first K2 bikes. It has 110 mains and HM341 pipes also, but could not keep a plug for more than 1000 miles until I changed to match the later K2 at 105 mains.

The larger taillight bracket was a mark of the K2, yours is certainly K1.

The sides-of-the-head wispy oil leaks were K0/K1 traits after about 15,000 miles, so it sounds like your speedo is correct. These usually go away after the first rebuild, if the cylinders are bored a step size and the head and deck ground flat at a machine shop. After reassembly, run about 5000 miles, then reach in through the fins (even now, as a test) with a flat 10mm wrench and tighten the two little 6mm bolts that you can see between the fins: you will find them to be relatively loose. They should be tightened to 90-100 inch pounds, but the head gasket collapsed with expansion and contraction of the head and cylinders until the metals cured (about 10k miles). After that, they are more stable. There are 2 more in the center of the head, but you can only get to those when the engine is apart. A step bore (.25mm or so) will give you back about 5-6 HP that was lost due to uneven reshaping of the cylinders during the cure phase. 

[Sgt.Pinback]

Did late K1 came with 341 when new?

[Phoenix]

Welcome aboard.  You came to the right place.  These guys know what's going on.  I have a 1971 like yours.  Mine was built 8/71 so it might have been built as a hybrid.  I'm pretty positive it was not touched b/4 I bought it with 10,000 miles on it.  It had 341 pipes and 120 jets.  The plugs would have carbon on them whenever I changed them.  It pulled like an ox w/ no problems.  I put 16,000 miles on it and it ran like a champ.  I talked to h/m and discussed this.  I had melted a piston and was really scared to lean out the system.  I'm still in the process of getting it together.  I'll start with the 120's because I know they work.  I'll then, after success, try the smaller jets because we know if Mark say it, take it to the bank.  btw, I took the tips out of my 341's.

[HondaMan]

Did late K1 came with 341 when new?

Nope. I have one of the very first K2 bikes to reach the USA (11/71 build), which is purely K1 except for the rear fender/taillight, chrome fork ears, dashboard, and...HM341 pipes. They didn't even appear yet on the Honda parts fiche by that September (1972). My carbs were still jetted K1, but with the needles dropped a notch, so it appeared that Honda's Planning group had not expected the HM341 to show up yet. It is also an Old Factory bike

In September, 1972, a guy on a Suzuki waterbike rear-ended me, flipping the bike out from under me, crushing the 3-4 mufflers. When I contacted Honda about new pipes, giving them the "HM341" designation on the pipes and with the serial number on my frame, they said, "Huh?". It took a month to get a new pair for the right side! Our parts fiche (in my shop then) still showed the HM300 for the K2, as well. (!). We got new fiche that December, and they were correct.

I had the HM300 on my K1 (3/71 build), and loved them until I started long-distance touring. Then they got a little annoying at 80+ MPH. When I got the K2 with the Vetter, I knew this was going to be worse: I was dead wrong, and VERY glad - ever since! They are great touring pipes, IMHO. 

[Kevin D]

I have an 11/70 K1. It came with the 300's and they rusted out in about 3 years. I got a used set of 341's with the end caps pulled, and they have been on the bike for 43 years. It was a plug fouler unless you were riding 70-80 mph or better. A couple years ago I got a jet kit from Sirius:
http://www.siriusconinc.com/pro-detail.php?pid=&product_id=179
and put the 115's in and no more plug fouling, also went with the Denso plugs per Hondaman recommendation and liking them too.

[Sgt.Pinback]

Thanks for input. Nice to hear some old stories from these days.