Topic: CB750 K1 Resurrection

[ZTatZAU]

Hello everyone!

I'm new to the site and this is my first post.  I didn't want to hijack twright3's recent thread on removing his stuck K1 carbs, (CB750 K1 stuck carb removal (sohc4.net)) but some of the replies in that thread were quite helpful and informative and I have a few questions of my own.

As for me, I've been riding all manner of two-wheelers for 60+ years.  I was building and riding  Whizzers and Cushmans while still in grammar school and riding and wrenching on Harleys and British bikes by high school.  I bought my CB750 (mfg. 12/1970) VIN: 10721XX brand new in 1971 and best I can figure, it's an early K1, as opposed to a "new factory K1" mentioned below... but my VIN tag just says CB750.  Did Honda ever put CB750K, CB750K1, K2, etc., on later model VIN tags? 

I rode the 750 to both coasts a time or two from the mid-west in the 70s & 80s and the 750 was everything I wanted in a motorcycle... until I got bitten by a Goldwing bug... first a GL1100 and later in 1994, a 1989 GL1500 which became my regular ride...  near or far.  So the 750 was parked in 1994, and it sat unloved and unattended to, in my shop, until 2012 when I put some fresh gas in the tank, drained the carb bowls, changed the oil and spark plugs and started it right up.  It actually ran fairly well on the few rides I took and a lot spiffier than the GW.  I knew the front brake needed some TLC and a set of fresh tires wouldn't hurt, but I parked it again, thinking (mistakenly) that I could resurrect the old 750 Four with no sweat, anytime I wanted.  Huh!

Fast forward to the fall of 2021 when for a variety of reasons, this 75-year-old biker decided it was time for something a little sportier and a bit lighter than my Goldwing, for everyday riding.  I'm not sure why, but I decided to get another of my 70's vintage bikes on the road again... a '73 XLH1000 Ironhead Sportster... the antithesis of everything I liked about my Hondas   I have no regrets on the choice, as there is something magical about the feel and the sound of a stock mufflered Ironhead.  The Sportster is starting, and running, and stopping, and charging, and looking just great again.   And I must say that the 2022 riding season on the XLH was one of the most enjoyable I've had in years.

But I'm thinking some here may understand how much my 750 still wants to be ridden and won't think I'm crazy when I hear it whispering, "Ride me!", whenever I walk by.  So, after sitting for another 10 years, I've decided my next project will be to get my CB750 back on the road... so here I am.

Carbs - As removed


So far, I've pulled, disassembled, and cleaned the carbs in an ultrasonic cleaner.  Dawn detergent in distilled water only.  It did a pretty good job except for heavy varnish on the flat rack plate and on the bottom of the inlet end of a couple of carbs.  (i.e. the ones that were leaking.)  I've downloaded a Honda SM and the K1-8 supplement as well as a fairly comprehensive parts catalog, but they don't mention a Keihin model number.  The top of each carb's mounting flange is marked "7A" on the left side, and all are marked "IL" on the right side... except carb #2 which is marked "IK" on the right side of the flange.  If anyone can explain these markings, please do.

I found a really good thread here on rebuilding later model PD42(?) carbs with a lot of pictures, but nothing similar for these carbs..

One reply in twright3's thread...

... The plastic floats in a K1 indicate the bike is a New Factory K1. In the future, you might find that to be helpful info, supposing the floats were not changed by someone before you. 

Re modern gasolines (notes for when you get it running again): the K1 carbs usually (I would say 'always', but not all bikes are factory original today) need to have their emulsifier tube's larger holes opened up a little more, like to 0.039", with a size drill. They were too small by circa 1985 (MTBE fuels) and are STILL too small with modern ethanol-laced fuels, so they make the plugs foul quickly. The OEM emulsifier holes are only 0.033" in most of the K1 657A series carbs.

... has me thinking my carbs are likely the "K1 657A series carbs" that HondaMan mentioned but my carbs have metal floats.  So, does that indicate my bike is an "Old Factory K1" as opposed to HondaMan's "New Factory K1"?  Any explanations here on the terms/history of New Factory vs Old (?) are appreciated.

I see $25 carb gasket kits on partzilla with float bowl gaskets, washers and o-rings and the Honda parts listings and diagrams (E-26 Carburetor Keihin) I've downloaded also list kits and don't show individual part numbers to search for.  Is there a better way to buy individual gaskets and o-rings for these Keihin 657A (?) carbs?

I've also discovered a very small pinhole in one of my metal floats in a "pre-ultrasonic dunk test".  That particular bowl was the nastiest of the four carbs and was probably leaking and evaporating gas in there for a long time.  I've removed all the water from my dunk test, but I can still hear something rattling around inside the float.  It sounds like a single very small "glob" of something and I'm thinking perhaps coagulated particulates of something left behind from evaporating gas.  Anyway, my questions are...

1)  Has anyone had any luck soldering up a pinhole leak in a Keihin metal float?  I know plastic floats are available but I'd kinda like to stick with my metal floats.  Suggestions welcome!

2)  Assuming I am successful in sealing the leaking float with solder, is there enough excess buoyancy in the pair of floats to mitigate the extra weight of whatever remains inside one of the floats on the float bar?

3)  I think I read somewhere, but can anyone explain the practice, or the theory behind, setting one float in the same bowl higher (or lower) than the other float?  Or did I dream that up?

I also read in twright3's stuck carb thread that you all like pictures...

...  Post us up a pic of the bike someday - we all love pics !

...so I dug up this photo of my 1971 CB750 in about 1973.

The Rally Winners


My first and current wife of 47 years and I, had just won a local motorcycle rally!  Some may notice, it only took me two years (in all my youthful wisdom), to ditch the original clunky looking, upswept, wimpy sounding, stock 750 mufflers, for the "so much cooler" and "so much louder", 4 into 2, unbaffled collector setup.  (I sure wish I could remember what happened to the original mufflers!)

ZT

[Mark K]

Welcome to the forum from southeast Alabama ZT. Wish I had the knowledge to answer just one of your questions. I have found this forum to be extremely helpful in my attempt to rebuild a '73 CB750. You'll receive a lot of great expert advice, suggestions and support from the members. Good luck with your reconditioning of your bike!

Mark K

[ZTatZAU]

Thanks for the nice wishes, Mark!

I really enjoyed getting the old Ironhead going again with a lot of help and advice on one of the Sportster forums.  I'm looking forward to the same with the 750.

ZT

[newday777]

Welcome aboard ZT.
Congrats on 47 years with your wife and keeping your bikes all these years. (I never wanted to jump into the Harley thing after having a couple friends that had them in the 70s and 80s, the first was a classmate who bought a new 75 sportster, which every time we went riding it broke down and he had to jump on the back of my Honda to go get my pickup to haul his sportster back to get it fixed.)
I bought new a CB450 in 74, a new CB750 in 75 and in 76, rode the 76 until 83 and sold it for family needs. 
In 2009 I bought my 1st wing, an 83 Aspencade,  got a couple 99 1500s and now have an 08 1800 that I bought in 2013 and have put 150,000 miles on it.
In 2018 I bought a friend's 75 CB750 from his estate and have been having a blast riding it again.
As to will your leaking float be repairable? I don't know the answer but try it. It might work. If not, I have some brass floats in a box from the early carbs that came with a bunch of parts I got that I could go through and test.

[Oly]

Welcome to the forum.  I'm also in the process of putting a K1 back on the road.  I found under FAQ the writeup by Hondaman very informative, along with his "My CB750 Book".  He use to be a Honda mechanic, racer, etc. in the day and continues to offer great input and perspective on issues, as do all other forum members.

[grcamna2]

Welcome to the forum here ZT;it was good reading your motorcycle story to gain perspective on your journey 

[Shtonecb500]

Welcome, I'm jelly of the guys who were able to buy these things new and still have them today, good on you. I've only really ridden these Hondas so hearing everyone else's experiences on other makes and models is great.

[ekpent]

I have fixed a pinhole leak in a brass Suzuki float before with a product called Seal All. I think solder would be fine or look for another brass one in good condition. I just replace with the plastic and move on. Probably got some old brass around. Try and clean all the old jets and re-use especially if it ran well before. Like I mentioned in that other thread I have done many 750's with that style of carb and never replaced a stock float bowl gasket and I just leave them in place. Only one I ever replaced was an aftermarket rubber one that liked to swell up and go nasty.
  Good luck on the project and congrats for keeping it so long.

[MauiK3]

Welcome and Aloha
Looking forward to the thread.
I suggest (if you haven't already) getting Honduran's book, it will save you much headache and money.

Congrats on your marriage, we just got to 44 years, you are in the lead!!

[BenelliSEI]

Great write up and love that you still have your bike. Following closely! John D.

[grcamna2]

I have fixed a pinhole leak in a brass Suzuki float before with a product called Seal All. I think solder would be fine or look for another brass one in good condition. I just replace with the plastic and move on. Probably got some old brass around. Try and clean all the old jets and re-use especially if it ran well before. Like I mentioned in that other thread I have done many 750's with that style of carb and never replaced a stock float bowl gasket and I just leave them in place. Only one I ever replaced was an aftermarket rubber one that liked to swell up and go nasty.
  Good luck on the project and congrats for keeping it so long.

Eric,do you have a link for Seal All ?

[rotortiller]

1)  Has anyone had any luck soldering up a pinhole leak in a Keihin metal float?  I know plastic floats are available but I'd kinda like to stick with my metal floats.  Suggestions welcome!

Yes with brass floats.

2)  Assuming I am successful in sealing the leaking float with solder, is there enough excess buoyancy in the pair of floats to mitigate the extra weight of whatever remains inside one of the floats on the float bar?

Minor effect but check float level with a clear tube method if you can. I have soldered floats in the past with careful drying, cleaning and flux then checking for bubbling immersed in liquid.

3)  I think I read somewhere, but can anyone explain the practice, or the theory behind, setting one float in the same bowl higher (or lower) than the other float?  Or did I dream that up?

Might add a bit of side loading on the pivot axle but the engine has vibration to mitigate that.

[ekpent]

I have fixed a pinhole leak in a brass Suzuki float before with a product called Seal All. I think solder would be fine or look for another brass one in good condition. I just replace with the plastic and move on. Probably got some old brass around. Try and clean all the old jets and re-use especially if it ran well before. Like I mentioned in that other thread I have done many 750's with that style of carb and never replaced a stock float bowl gasket and I just leave them in place. Only one I ever replaced was an aftermarket rubber one that liked to swell up and go nasty.
  Good luck on the project and congrats for keeping it so long.

Eric,do you have a link for Seal All ?

https://www.autozone.com/sealants-glues-adhesives-and-tape/sealant/p/seal-all-clear-multi-purpose-adhesive-sealant-2-oz/417319_0_0
  Works good on boots and tons of other stuff. No shop should be without it !!    I used it once to patch a main jet emulsifier crack in the carb casting and also on another as a sealer when doing the .45 ACP cartridge repair trick and it held up well in the gas.

[ZTatZAU]

Welcome aboard ZT.... Congrats on 47 years... and keeping your bikes all these years...

Thanks for the welcome and kudos newday777!  But I sure haven't kept all my rides all these years.  I usually had to sell my current ride to acquire the next one.  And there's quite a few of them that I wish I still had.  The 71 CB750 was the first vehicle I ever bought brand new... which may be the reason I still have it.

(I never wanted to jump into the Harley thing after... a classmate who bought a new 75 sportster, which every time we went riding it broke down...)

By 1975, AMF was in full swing at HD and reliability suffered.  I've heard some say that the 73 XLH was the best Sportster year because AMF hadn't had time to screw things up.  With that said, I think most bikes will eventually turn their riders into mechanics.

I bought new a CB450 in 74, a new CB750 in 75 and in 76, rode the 76 until 83 and sold it for family needs. 
In 2009 I bought my 1st wing, an 83 Aspencade,  got a couple 99 1500s and now have an 08 1800 that I bought in 2013 and have put 150,000 miles on it.  In 2018 I bought a friend's 75 CB750 from his estate and have been having a blast riding it again.

Good on you newday!  That's an impressive string of Hondas and exactly what the HM marketing team had in mind.

As to will your leaking float be repairable? I don't know the answer but try it. It might work. If not, I have some brass floats in a box from the early carbs that came with a bunch of parts I got that I could go through and test.

I give it a shot and see what happens but really appreciate your offer should I be looking for a replacement float.

Do you, or anyone else, know anything about the idea or rationale for setting one float higher than the other in the same bowl?
ZT

[newday777]

You probably won't have to worry about setting it to a different height if it is sealed up.
A cement boat floats as does a plastic boat, just takes more to get up to hull speed.

[ZTatZAU]

Wow!  I appreciate all your replies!

Welcome to the forum.  I'm also in the process of putting a K1 back on the road.  I found under FAQ the writeup by Hondaman very informative, along with his "My CB750 Book".  He use to be a Honda mechanic, racer, etc. in the day and continues to offer great input and perspective on issues, as do all other forum members.

Thanks Oly!  I've seen a number of Hondaman's informative posts and I'll be sure to check out the FAQ section.  Good luck with your K1 project.

Welcome to the forum here ZT;it was good reading your motorcycle story to gain perspective on your journey 

Thanks grcamna2!  I appreciate your nice comment!  It's been a great journey too!

Welcome, I'm jelly of the guys who were able to buy these things new and still have them today, good on you. I've only really ridden these Hondas so hearing everyone else's experiences on other makes and models is great.

Thanks for your nice welcome Shtonecb500!

I have fixed a pinhole leak in a brass Suzuki float before with a product called Seal All. I think solder would be fine or look for another brass one in good condition. I just replace with the plastic and move on. Probably got some old brass around. Try and clean all the old jets and re-use especially if it ran well before. Like I mentioned in that other thread I have done many 750's with that style of carb and never replaced a stock float bowl gasket and I just leave them in place. Only one I ever replaced was an aftermarket rubber one that liked to swell up and go nasty.
  Good luck on the project and congrats for keeping it so long.

Thanks ekpent!  I remember reading your comment about the bowl gaskets and did leave them installed when the carbs went into the ultrasonic cleaner with no ill effects.

Thanks for the tip and your subsequent link to the All Seal product.  I'm going to first try dragging/wiping some solder over the pinhole first.  I'll be sure to clean the area well and use plenty of flux.  I don't want to add any unnecessary weight as there is already a bit of "foreign debris" inside the float.  If I knew I could find a replacement metal float, just in case, I might even try desoldering/disassembling the float to remove whatever is inside there... and then seal it all up again... just to see if I could.

The carbs and jets all cleaned up nicely in the UC with a mild Dawn Detergent solution.  I plan to re--use as much as I can but would welcome any suggestions on sources for the o-rings without buying a complete gasket or rebuild kit???



I haven't yet, but I'm curious to measure the small holes in my emulsion tubes as Hondaman suggested in the "Stuck Carbs" thread.  In hindsight, whatever didn't seem quite right when I last got it running 10 years ago could very well be due to the newer blends of unleaded alcohol laced fuel.

Welcome and Aloha
Looking forward to the thread.  I suggest (if you haven't already) getting Honduran's book, it will save you much headache and money.
Congrats on your marriage, we just got to 44 years, you are in the lead!!

Aloha to you too MauiK3! 
I've noticed that Hondaman's book is highly recommended here!  Congrats on your 44 years!

Great write up and love that you still have your bike. Following closely! John D.

Thanks for your nice comments John!

1)  Has anyone had any luck soldering up a pinhole leak in a Keihin metal float?  I know plastic floats are available but I'd kinda like to stick with my metal floats.  Suggestions welcome!

Yes with brass floats.

2)  Assuming I am successful in sealing the leaking float with solder, is there enough excess buoyancy in the pair of floats to mitigate the extra weight of whatever remains inside one of the floats on the float bar?

Minor effect but check float level with a clear tube method if you can. I have soldered floats in the past with careful drying, cleaning and flux then checking for bubbling immersed in liquid.

3)  I think I read somewhere, but can anyone explain the practice, or the theory behind, setting one float in the same bowl higher (or lower) than the other float?  Or did I dream that up?

Might add a bit of side loading on the pivot axle but the engine has vibration to mitigate that.

Thanks for your thoughtful reply rotortiller.  I appreciate you sharing your experience and expertise.  I like the clear tube idea.  Is there a spec somewhere on setting the proper "fluid height" in the bowl, rather than setting a "float height"  If not, I guess one could set the correct float height in one carb to determine the resulting fluid height is in a clear tube... and proceed accordingly.  Eh?

You probably won't have to worry about setting it to a different height if it is sealed up.
A cement boat floats as does a plastic boat, just takes more to get up to hull speed.

I'll buy that newday777!  Or as they say, "A rising tide floats all boats!"
ZT

[MauiK3]

You can do a clear tube float test to satisfy yourself it’s floating properly

[BenelliSEI]

Make sure your replace the “0” rings that seal the two cross tubes that transfer fuel 1-2 and 3-4. For sure, the old ones will leak. If you have trouble finding them let me know...... somewhere I have a bag of 30-40 of them!

[ekpent]

Yes 8 new fuel pipe and 4 new drain plug o-rings are a very good idea while all apart. I keep extra spares on hand of those as well as all the 550 o-rings.

[rocket johnny]

welcome from western colorado !     would a smear of j b weld seal it ?    in 73 i bought my first 750 k2
 i installed a hooker 4 into 1  and tossed the 341's into the dumpster .   back then no one wanted the stock pipes.     i'd give anything to have that set back .        i've enjoyed your story and photos ,  keep it up

[ZTatZAU]

You can do a clear tube float test to satisfy yourself it’s floating properly

Thanks Maui! I understand the concept.  Do you use a barbed hose adapter fitting in the fuel bowl drain?
Do you happen to know the thread size of the drain plug?  I haven't yet transitioned to a proper "metric mindset"!

Make sure your replace the “0” rings that seal the two cross tubes that transfer fuel 1-2 and 3-4. For sure, the old ones will leak. If you have trouble finding them let me know...... somewhere I have a bag of 30-40 of them!

Yes 8 new fuel pipe and 4 new drain plug o-rings are a very good idea while all apart. I keep extra spares on hand of those as well as all the 550 o-rings.

Yes!  Thanks guys!  I mentioned above that I wanted to replace the O-rings and asked for suggestions on sourcing and (metric) specs if I wind up going generically, off the hardware shelf; so to speak.  You may have missed it.  The trouble is, the parts manual I have does not list individual part numbers or size/specs on either of the O-rings.  The O-rings are only identified as part of a "Gasket Set" p/n 16010-300-014 or a "Chamber Set, Float" p/n16015-300-004.

I'm thinking there must be a better way or a better place to purchase the individual O-rings I need without shelling out $25 or more per carb.  If I knew what I was looking for, I suppose ebay would be the best bet.  Any info, suggestions, suppliers, or recommendations are much appreciated.

I also probably should replace the rubber insulators which are rock hard... unless someone can suggest a good way to "rejuvenate" the rubber.  Ditto for the rubber inlet air horns and I definitely need four new rubber dust caps.  Should four of each of these three items really cost me $300 or more?  Or am I looking in the wrong places?

welcome from western colorado !     would a smear of j b weld seal it ?    in 73 i bought my first 750 k2
 i installed a hooker 4 into 1  and tossed the 341's into the dumpster .   back then no one wanted the stock pipes.     i'd give anything to have that set back .        i've enjoyed your story and photos ,  keep it up

I don't know if it will come to JBW or not, but I appreciate the suggestion and your nice welcome.

I haven't gone any farther since my last post as I've been distracted with other things.  Among them, three days of snow now, with 3 or 4 more inches expected tomorrow and my snow plow was still a lawn mower until this morning.

ZT

[HondaMan]

Here's a single kit for $7+:
#48-0791
at PartsNmore, here:
https://www.partsnmore.com/parts/honda/cb750k/?filters[category]=carburetor&filters[fitting]=custom

[ZTatZAU]

Here's a single kit for $7+:
#48-0791
at PartsNmore, here:
https://www.partsnmore.com/parts/honda/cb750k/?filters[category]=carburetor&filters[fitting]=custom

Thanks Hondaman!

Is there anything special about these Keihin carb O-rings?  Can one easily spec them from Grainger, McMaster-Carr, Ace Hdw, etc., ?  Might you be able to provide the proper sizes for the fuel tube and drain plug O-rings?

And might you have any suggestions on rejuvenating the other rubber parts I mentioned above.  I.E., Insulators & Air Horns?  Or do they need to be replaced when they get hard?

BTW!  I've just been reading your FAQ - "Thoughts" articles.  Lots of good info there!
Thanks for sharing!

After reading your bio, I kept thinking we might have crossed paths in the "western burbs" in the early 70s.
ZT

[HondaMan]

Here's a single kit for $7+:
#48-0791
at PartsNmore, here:
https://www.partsnmore.com/parts/honda/cb750k/?filters[category]=carburetor&filters[fitting]=custom

Thanks Hondaman!

Is there anything special about these Keihin carb O-rings?  Can one easily spec them from Grainger, McMaster-Carr, Ace Hdw, etc., ?  Might you be able to provide the proper sizes for the fuel tube and drain plug O-rings?

And might you have any suggestions on rejuvenating the other rubber parts I mentioned above.  I.E., Insulators & Air Horns?  Or do they need to be replaced when they get hard?

BTW!  I've just been reading your FAQ - "Thoughts" articles.  Lots of good info there!
Thanks for sharing!

After reading your bio, I kept thinking we might have crossed paths in the "western burbs" in the early 70s.
ZT

Well, by sizes, they are:
Fuel Tees 1.9 x 7.9mm ( I use 2x8 because of ethanol's propensity for expanding and then shrinking these)
Bowl drains: 1.6 x 4.8. Also the 1.6 x 4.7mm size works, easier to find, too.
The bowl gaskets are available per each at PartsNmore, too. They work great!

If you lived in the Peoria-Pekin area circa 1969-1971, I worked then at Brown's Sport Center, which was Peoria's Honda shop then. I was in NASA's training then, at Bradley University.
In 1971 I went to Macomb and WIU there, as the Apollo program was suddenly truncated, wrecking my space-oriented dream. I went into LEA then, cramming 3.5 years of school into 2 and opened my own bike shop there to pay for it (and my 1st wife, who came from the midst of the bike scene). It was named Western Wheels (after I told Gus Treader, the guy who also owned TNT Enterprises in Missouri and owned the sales end of my shop's store, that his "Wheels of Western Illinois" original name was just dumb...).

This was all at the peak of the SOHC4 experience, and I could make them go faster than anyone else in the Peoria-Macomb-Blandinsville triangle, which, if you followed NASCAR then you'd recognize as the 3 dealership's towns who sold the 100 each NASCAR special vehicles that had to be sold to let a "stock" car into the racing circuits back then. This was where the cars like Charger Daytona and Plymouth Super Bird, among others, roamed the streets on Saturday nights. The 750 was there, showing them how it's ACTUALLY done between streetlights, as those monster cars were made for 200+ MPH top ends, not 2-block races. The Kawi Mach III 500 showed up, too (when their electronic ignitions weren't turning to Silly Putty and heating the seat) and the later 750 version, which could run heads-up with the Honda. My shop there became famous in the Ill/Mo/Iowa area, and people would truck their 750s to my shop every week, just to have them...tuned up. It was a fun time! I paid off the college bills and had thousand$ in the bank when I left the shop to the 3 guys who worked for me that July. It's still there today.

Saturday nights were never the same after I left there for Colorado...

[ZTatZAU]

I enjoyed your story Hondaman and thanks for the o-ring specs!

I was thinking more of the Elmhurst area in '71 than the PIA/C15 area.  I grew up near Rainbow Beach on the South Side of Chicago and came "out to the country" between Naperville and Aurora in the early 70's.  The area has changed a lot since then.

It may interest you to know that becoming an astronaut was one of my dreams too!  I was in a NAVY program going to IIT for an Aerospace Engineering degree.  My plan was to get my commission, then my wings, then kick some ass in my F4 Phantom, and then be selected as an astronaut.  Most astronauts in those days were Naval Aviators.  I eventually got my MAE degree, but the rest of the plan just didn't work out.  Or maybe it did?  Kinda funny how things seem to work themselves out and we just go along for the ride.  I did go to work for the government, and I've now been retired longer than I worked!  Go figure!  Like I told grcamna2, it's been quite a journey.

ZT