Topic: 1970 750 K0 Carb Questions

[Scootch]

I have a '70 K0 with the original carbs. I think I need to put a rebuild on them so that I know what I have. I was told that the main jets are #120, idle jets are #40 and the air jets are #100. I am not sure if the carbs were ever rebuilt. I ordered a repair kit from Yamiya. I read a lot of discussion that indicated that kits from certain sources are suspect. Yamiya includes #110, #115 and #120 main jets in the kit. They say to start with the #110's. I know that you guys have plenty of experience with the carbs so I want to benefit from your knowledge. What jet would you suggest I use? I think the 120's that are currently in the carbs are too big because the plugs show on the rich side. What position on the needles do you use? Any and all suggestions and input greatly appreciated. TIA

[69cb750]

#120 and #40 are correct for original exhaust.
Original kehin jets are the best.

[Scootch]

It does have original HM300 exhaust. As of yet I am not sure what mfg jets are. I was told to look for stylized K if they are Keihin. Does anyone know what jets you get with the Yamiya kits?

[Johnie]

Check this out:

[Don R]

 Be aware, the little butterfly needle clips can fly into another dimension never to be found. 

[Scootch]

Check this out:

Thank you
What does JN mean?  Also I assume the Float column indicates millimeters or some such thing?

[Johnie]

That's the jet needle clip position you were asking about. Correct, float is mm.

[Scootch]

That's the jet needle clip position you were asking about. Correct, float is mm.

Thank you. And now a question about the jet needle positions. I know I have seen discussion(s) about the numbering but I must have neglected to write it down. What is the consensus regarding how the jet needle numbering goes?

[Johnie]

Needle clip position is counted from the top down.

[Scootch]

Needle clip position is counted from the top down.

Thank you!

[Johnie]

Good luck with it...I remember looking for those answers back in the day too.

[Scootch]

Good luck with it...I remember looking for those answers back in the day too.

Thank you. I am sure I will be back asking many more questions as I get into it... 

[HondaMan]

A tip:
Modern fuels burn much slower than the 1970s gasolines we had when these bikes were designed. So, I'd strongly suggest NOT using premium gas anymore, which will make the black sparkplug problem even worse today: use Regular for in-town riding below 50 MPH and midgrade above that, generally.

When I worked in the Honda shops circa 1970s we changed many of the #120 mainjets in the K0 bikes to #115 for normal use, and even down to #110 if the bike was seldom ridden at speed on the hiway. This helped reduce the plug fouling quite a bit. The K0 was always known for having dark plugs, particularly if ridden in city traffic a lot.

I also recommend, partly due to K0 design issues and partly because our fuels today need extra air to burn well, that the larger holes in the emulsion tubes be drilled out to 0.035" or (if you live at altitudes above 3000 feet) 0.0375" to help prevent the engine from 'loading up' on fuel while riding at low speeds for considerable time. This step is normally done on every set of 750 carbs I rebuild for others here.

[newday777]

A tip:
Modern fuels burn much slower than the 1970s gasolines we had when these bikes were designed. So, I'd strongly suggest NOT using premium gas anymore, which will make the black sparkplug problem even worse today: use Regular for in-town riding below 50 MPH and midgrade above that, generally.

When I worked in the Honda shops circa 1970s we changed many of the #120 mainjets in the K0 bikes to #115 for normal use, and even down to #110 if the bike was seldom ridden at speed on the hiway. This helped reduce the plug fouling quite a bit. The K0 was always known for having dark plugs, particularly if ridden in city traffic a lot.

I also recommend, partly due to K0 design issues and partly because our fuels today need extra air to burn well, that the larger holes in the emulsion tubes be drilled out to 0.035" or (if you live at altitudes above 3000 feet) 0.0375" to help prevent the engine from 'loading up' on fuel while riding at low speeds for considerable time. This step is normally done on every set of 750 carbs I rebuild for others here.

Do you drill out the larger holes on the later carbs too?

[Scootch]

Hondaman what position do you recommend for the needle clips?

[69cb750]

Clips came in center position.
Some main jets were drilled by owners, compare to new kehin jet.

[Scootch]

Clips came in center position.
Some main jets were drilled by owners, compare to new kehin jet.

Thank you.  It will be interesting to see what I find with the jets.
But now I notice something. I have a pic of the carbs and the floats that are supposedly in my carbs. Looks like they are plastic. In looking at the Yamiya site they list floats for the K0 and the K1-K6. The floats they have for the K0 are metal. The floats for the K1-K6 look to be plastic. And the pic that I have supposedly of my floats look to be similar to the floats for the K1-K6 - they even have the two holes in the cross bar. I don't want to jump to conclusions until I actually see what is in my carbs but just going by what I see so far might I conclude that perhaps the wrong floats may be installed in my carbs??

[ekpent]

Both K0 and K1 used brass floats. Plastic in 72 and forward. I have replaced brass with plastic in K1"s but never tried a K0 yet.

[Johnie]

Yes, if you have brass floats might consider replacing them with the plastic. The brass ones get pin holes after 50 years and leak which caused them to sink and overflow.

[HondaMan]

A tip:
Modern fuels burn much slower than the 1970s gasolines we had when these bikes were designed. So, I'd strongly suggest NOT using premium gas anymore, which will make the black sparkplug problem even worse today: use Regular for in-town riding below 50 MPH and midgrade above that, generally.

When I worked in the Honda shops circa 1970s we changed many of the #120 mainjets in the K0 bikes to #115 for normal use, and even down to #110 if the bike was seldom ridden at speed on the hiway. This helped reduce the plug fouling quite a bit. The K0 was always known for having dark plugs, particularly if ridden in city traffic a lot.

I also recommend, partly due to K0 design issues and partly because our fuels today need extra air to burn well, that the larger holes in the emulsion tubes be drilled out to 0.035" or (if you live at altitudes above 3000 feet) 0.0375" to help prevent the engine from 'loading up' on fuel while riding at low speeds for considerable time. This step is normally done on every set of 750 carbs I rebuild for others here.

Do you drill out the larger holes on the later carbs too?

Yep! It helps with the ethanol-laced fuels by aerating a little more.

[HondaMan]

An additional thought: if you have the later HM300 style pipes, the mainjet should not be more than #110. You'll also want to turn the air screws in toward 7/8 turn instead of 1.0 turns out in this case to prevent the engine from 'loading up' on unburned fuel at stoplights. This then blackens the plugs.

[Scootch]

An additional thought: if you have the later HM300 style pipes, the mainjet should not be more than #110. You'll also want to turn the air screws in toward 7/8 turn instead of 1.0 turns out in this case to prevent the engine from 'loading up' on unburned fuel at stoplights. This then blackens the plugs.

Thank you !  This all makes sense and falls in line with everything I have observed and know so far...

[70CB750]

Both K0 and K1 used brass floats. Plastic in 72 and forward. I have replaced brass with plastic in K1"s but never tried a K0 yet.

One of my K0 floats was leaking and I soldered it shut. It worked well.

[HondaMan]

Both K0 and K1 used brass floats. Plastic in 72 and forward. I have replaced brass with plastic in K1"s but never tried a K0 yet.

One of my K0 floats was leaking and I soldered it shut. It worked well.

Me, too!

[70CB750]

Both K0 and K1 used brass floats. Plastic in 72 and forward. I have replaced brass with plastic in K1"s but never tried a K0 yet.

One of my K0 floats was leaking and I soldered it shut. It worked well.

Me, too!

I just remembered that I also checked weight using reloading scale:

[grains]
1. - 165.1
2. - 157.6
3. - 161.1
4. - 162.5

Number one was the soldered one.