Topic: Jetting Advice for Modified 1971 CB750 K1

[HondaMan]

Mark
I think you have a typo....
"More often in the 675A carbs (which are the normal ones for the K1)"
It should be 657A carbs, correct?

Oops, yep! Thanks! (fixed it)
It's my fault for not keeping tight watch on what this particular keyboard types. I got the laptop dirt cheap: if I turn it on from cold it works fine for about 15 minutes, and then the keyboard starts doing things like delaying one keystroke behind another, making all sorts of words and numbers up out of what I typed, and sometimes skipping halfway forward or backward through a paragraph to start typing up/down there instead(!). 'Tis a challenge (and time-consuming) to fix it all in my e-mails and forum posts, which is all I use this one for. After I put Linux on this one it stopped catching all the colds that my old Windows-based one did just from surfing the news.

And sometimes, I don't catch all the typos! 

[HondaMan]

However: if the plastic floats and current checkvalves came from 4into1.com, all these bets are off. Their checkvalves seem to weep constantly unless the fuel tap on the tank is shut off. Their floats are also undersized, not pushing the checkvalves closed very well. I received a "kit" from an owner (some 3 years ago, now) for his roundtop carbs (I was asked to rebuild them with supplied parts from 4into1.com) where one of the float valves was a differently sized (smaller) unit from the other three. It didn't fit the carbs, either.

Interesting! My floats and check valves are from 4into1, where do you suggest I get some better quality ones. I have had some issues with these in the past.

Did your carbs originally have the plastic floats, or the brass ones? There were some late 657A carbs that had plastic floats that I saw in the Fall of 1971, in bikes from the "new factory" at that time. My K2 that I bought new in 1/72, which came from Honda with a K1 engine, all built in 11/71, had brass floats in the 657A carbs (so, not ALL K2 bikes had 657B carbs, which the Internet is so sure about today). The brass-and-solder joints on those floats in 2 of the carbs got eaten away by MTBE during 1996-2001 here in Colorado, and I didn't ride it from 2001 until 2006 because of cancer. The floats were junk after that: then it received plastic floats when I refurbished it in 2006. Somehow all but 1 of the float valves survived that acid and are still in it today.

So, you can use the plastic Keihin floats in any of the roundtop carbs: I run them 1mm deeper than spec in the 657A carbs, but 26mm works OK in the 657B or 7A carbs (or the rarer 086a carbs). The 4into1 floats I have seen are too small and the geometry is odd where they shut off the float valve, making them have fuel-level troubles in the bowls (mostly it is too deep, uncontrollably so). I can look around in my [many] boxes of parts to see if I have 4 floats tomorrow, but I suggest getting float valves from PartsNmore.com or CB750Supply, those are the ones I rebuild these carbs with. If your carbs are the ones that have the tiny fuel filter on top of the float valves, I think Honda still sells them and PartsNmore.com has them from time to time in stock. They were found mostly in the 1975/6 CB750F1/2 bikes, but showed up in some of the late K5 and some of the K6 bikes, too (those with the 086a carbs). Those carbs usually had the idle-mix screws that stick out of the holes: they originally had little plastic "limiter" caps on the screws (emission controls), but those mostly disappeared by themselves by about 1985, just cracking and falling off from engine heat.

[scottly]

If there is any question about floats and valves, the actual fuel level in the float bowls should be verified with the "clear tube" method.

[KickstandDan]

Mine originally came with plastic floats. The carbs have 7A stamped onto them. I still have them but with the 4into1 kit I thought the new stuff would work, but I see how the new stuff is not as precise or as calibrated as the stock stuff. Fortunately I kept everything I have removed from the bike.

[TideWater2]

Is there any documentation, frame #s or manufacture dates, on when each of the three carb jets were used?

Also, any identifiers for determining which factory made the bike?

[newday777]

Is there any documentation, frame #s or manufacture dates, on when each of the three carb jets were used?

Also, any identifiers for determining which factory made the bike?

Not that I've seen 😕

[KickstandDan]

This is what is stamped onto all 4 of my carbs.

[MauiK3]

Yamiya is also a good source of parts, high quality and of course, some high prices.

[HondaMan]

The "7A" designator, as far as I've been able to discover, indicated the new molds that were more precise that made the 657A series carb bodies: they are crisper renditions of the same carb. Those carbs appeared intermittently in the late K1, early K2, the K4, and F1 versions of the 750, based on the notes I've taken as I get to work on them (for decades). Honda had no defining literature on the 7A, but inside they look like more nicely-done 657A carbs. So, be cautious of the 'internet carb ninjas' who swear they know about the 7A carbs...

Float level for all 7A carbs with brass floats were 25mm and for plastic ones 26mm, as measured when disassembled: these are actual values I measured, not someone notes, "specs" or claims.

The jetting and needle setting rules for these follow the general settings for the year of the bike. In the K1 bikes with the 657A and 7A carbs made after 8/71 the mainjets were #110 with the needle clips in the 3rd notch (from the blunt top end) of the needles. This required, however, the use of the HM300 exhaust pipes: if the same carb was used in the later bikes with more restrictive pipes (K2 onward) the mainjet was normally #105 with the needle clip in the 4th notch. The 7A carbs came with plastic floats in the later bikes, too, except in the early F0 bikes when they were brass.

There are a few exceptions to this (including my own 750, a mix-and-match bike from rushed production in late 1972). For example, mine came with #110 mainjets and the needle clips in the 4th notch, using the AD125 spark advancer (very K1 early tuning) and an early K1 cam. It fouled sparkplugs in 500 miles, black as pitch, as it came from Honda. It also had brass floats in the carbs until MTBE ate them in the early 2000 era. (Now they are plastic.)

[KickstandDan]

The "7A" designator, as far as I've been able to discover, indicated the new molds that were more precise that made the 657A series carb bodies: they are crisper renditions of the same carb. Those carbs appeared intermittently in the late K1, early K2, the K4, and F1 versions of the 750, based on the notes I've taken as I get to work on them (for decades). Honda had no defining literature on the 7A, but inside they look like more nicely-done 657A carbs. So, be cautious of the 'internet carb ninjas' who swear they know about the 7A carbs...

Float level for all 7A carbs with brass floats were 25mm and for plastic ones 26mm, as measured when disassembled: these are actual values I measured, not someone notes, "specs" or claims.

The jetting and needle setting rules for these follow the general settings for the year of the bike. In the K1 bikes with the 657A and 7A carbs made after 8/71 the mainjets were #110 with the needle clips in the 3rd notch (from the blunt top end) of the needles. This required, however, the use of the HM300 exhaust pipes: if the same carb was used in the later bikes with more restrictive pipes (K2 onward) the mainjet was normally #105 with the needle clip in the 4th notch. The 7A carbs came with plastic floats in the later bikes, too, except in the early F0 bikes when they were brass.

There are a few exceptions to this (including my own 750, a mix-and-match bike from rushed production in late 1972). For example, mine came with #110 mainjets and the needle clips in the 4th notch, using the AD125 spark advancer (very K1 early tuning) and an early K1 cam. It fouled sparkplugs in 500 miles, black as pitch, as it came from Honda. It also had brass floats in the carbs until MTBE ate them in the early 2000 era. (Now they are plastic.)

Wow that is super cool! Thank you for the information. Seeing as my bike has 9/71 on it we could assume that the carbs were possibly stock #110 mainjets and needle in the 3th notch. The mains that were in the carbs were 120 but that could have been my grandpa (who's bike this was originally) changed out?

I will see how 120 works on my freshly rebuilt bike after a few pulls and adjust accordingly.

[HondaMan]

The "7A" designator, as far as I've been able to discover, indicated the new molds that were more precise that made the 657A series carb bodies: they are crisper renditions of the same carb. Those carbs appeared intermittently in the late K1, early K2, the K4, and F1 versions of the 750, based on the notes I've taken as I get to work on them (for decades). Honda had no defining literature on the 7A, but inside they look like more nicely-done 657A carbs. So, be cautious of the 'internet carb ninjas' who swear they know about the 7A carbs...

Float level for all 7A carbs with brass floats were 25mm and for plastic ones 26mm, as measured when disassembled: these are actual values I measured, not someone notes, "specs" or claims.

The jetting and needle setting rules for these follow the general settings for the year of the bike. In the K1 bikes with the 657A and 7A carbs made after 8/71 the mainjets were #110 with the needle clips in the 3rd notch (from the blunt top end) of the needles. This required, however, the use of the HM300 exhaust pipes: if the same carb was used in the later bikes with more restrictive pipes (K2 onward) the mainjet was normally #105 with the needle clip in the 4th notch. The 7A carbs came with plastic floats in the later bikes, too, except in the early F0 bikes when they were brass.

There are a few exceptions to this (including my own 750, a mix-and-match bike from rushed production in late 1972). For example, mine came with #110 mainjets and the needle clips in the 4th notch, using the AD125 spark advancer (very K1 early tuning) and an early K1 cam. It fouled sparkplugs in 500 miles, black as pitch, as it came from Honda. It also had brass floats in the carbs until MTBE ate them in the early 2000 era. (Now they are plastic.)

Wow that is super cool! Thank you for the information. Seeing as my bike has 9/71 on it we could assume that the carbs were possibly stock #110 mainjets and needle in the 3th notch. The mains that were in the carbs were 120 but that could have been my grandpa (who's bike this was originally) changed out?

I will see how 120 works on my freshly rebuilt bike after a few pulls and adjust accordingly.

Yeah, back "in the day" when some riders noticed that the K1 was "slower" than the K0, and Honda had reduced the mainjets because they also had changed the cam and the pipes (insides were different inside, particularly at the upswept bend), a LOT of riders ordered #120 mainjets and plugged them in, thinking they found the "secret" of the lost power. But, between the K0 and K1 the final-drive gearing (sprockets) had changed, the airbox had changed, the intake passages were slightly longer, and the cam was tamed a few degrees (as was the spark advancer, twice by the end of the K1) all to reduce sparkplug fouling and after-run on hot days (the mixture was so rich the bike wouldn't slow down immediately upon releasing the throttle on hot days).

There were lots of changes made between 1969 and 1971!

[KickstandDan]

Wow, that is super cool. Thank you for the information. I have also really enjoyed reading your cb750 book so thank you for that as well!

[TideWater2]

Thanks Hondaman, very interesting. I'm working on my 3rd K1 and still struggling getting the first one tuned as good as the second one. They are like women, they are the same, but still different. 

[seanbarney41]

Tidewater, that is what I have found.  You can only get pretty close using documented carb settings with these bikes.  Although you may end up way off.  You may end up spot on.  The one thing you can count on is every cb750 is a littlle diferent.

[MauiK3]

And yet many are very obsessed with the early K0's.
For all their "faults" they take their place as important markers in motorcycle history

[HondaMan]

Tidewater, that is what I have found.  You can only get pretty close using documented carb settings with these bikes.  Although you may end up way off.  You may end up spot on.  The one thing you can count on is every cb750 is a littlle diferent.

Boy, is THAT ever the truth now!

[KickstandDan]

Jetting Advice: When to go from 120 to 122+ Main?

I’ve been running a few hundred miles with stock jetting: 120 main, 40 pilot, needle in the middle, and stock airbox (with my engine upgrades and 4 into 1 exhaust).

I’ve noticed that if I turn the air mixture screw too far in, it runs rich and bogs at low RPM. If I turn it too far out, it gets lean and I hear some popping or misfiring in the higher RPM range and it seems to lose power. Right now, I have enough adjustability to clean that up, and the bike feels strong through the rev range.

My question is: how do I know if it’s time to move up from a 120 main? If I have enough tuning range in the air mixture screw to dial it in, does that mean the 120 main is still correct? Or should I still consider bumping up based on the high-RPM lean symptoms I saw before adjusting?

Appreciate any input.

[newday777]

"My question is: how do I know if it’s time to move up from a 120 main? "
By doing plug chops to see the color of the spark plugs how they are burning. You have been given starting sizes multiple times, for stock motors.
With changes in any motor build from stock, you will have to do plug chops to find if you have the correct set up. No one will be able to tell you a set parameter, your altitude plays into jetting factors. I'm guessing you don't know how to do this?

[KickstandDan]

I haven't ever done a plug chop. I know it consists of high rpm followed by killing ignition and checking plugs to see how it is running. I live in closer to the city in Philadelphia and I don't have as many areas to rev to higher rpm and kill it without doing this on the side of a busy highway. What do you suggest is the best way to plug chop with a CB750?

[denward17]

While I'm no expert, if you see richness, leanness by adjusting an idle mixture screw, I'm not so sure it's the main jets fault.

What RPM range are you feeling/seeing these conditions?  Where is the throttle?  1/8, 1/4 or ?

Could it be a needle position?

[seanbarney41]

Main jet mostly adjusts 3/4 to full throttle.  Pilot screw mostly only adjusts idle tp 1/8 open throttle.  If you are experiencing poor performance when operating at 3/4 to full throttle then the main jet size needs to be adjusted.  If you know the difference, or can determine through experimentation,  between rich/lean, than you will need riding area where the bike can be operated at or near full throttle safely.  Don't worry about plug readings.  Once you get a good performing tune up, you can check at your leisure for problems.

Check for performance problem, what throttle position does the problem occur at?  make one change in a known quantity rich or lean.  Re-test  Did the problem improve or get worse? continue testing until satisfied and no poor combustion signs appear on plugs...

yeah it is extremely tedious and time consuming

maybe think about going to drag strip for test and tune day.

[seanbarney41]

Also guys talk about"through the rev range".  This can be deceiving.  RPM has little bearing on what carb circuit needs adjusting.  It is more a combination of load and throttle position.

[PeWe]

Ride at full speed or very close to it. Reduce the throttle a little.
If it feels like the bike will run better/ faster when reducing- too lean mj

If you can hear or feel a rich d-d-d-d behaviour- too rich mj.


This method works fine on the needle area too.

It is a little thrilling to know before the ride that I have to ride over 160- 180kmh to test full throttle to feel the acceleration and small throttle reductions.

Needle to mj area after 100kmh/+4000 rpm. Small accelerations followed by decelerations by the throttle to feel how it accelerates and behaves.

This is otherwise things that suddenly can happen during rides

CB750 stock carbs are well defined for stock and bigger bore with ported head and hotter cam.
No need to increase pilot jet.

Other carbs where the pilot jet has to be changed:
Bigger pilot will affect needle setting and main jet. Total fuel flow changed.
I have seen 2 sizes bigger pilot need 1 step lower  needle and one size smaller main jet when needle and mj was correct before.
Last year I reduced pilot by 1 step (-2.5).
I had to lift needle 1/2 step by an M3 washer under the needle clip.
Increased mj 1 step too when acceleration  at WOT from 180kmh was bad.
This on my K6 with 970cc and over 100whp.
Runs fine now so I can enjoy cruising around in legal speeds most of the time.

I had also issues with erratic idle, take offs and lower speeds not possible to tune. Like it ran too lean.
Bad fuel.
Changed gas station to other chain this year.
Shell is no more here. Their V-power changed to a bad fuel. Maybe too low octane and more ethanol.

[newday777]

I haven't ever done a plug chop. I know it consists of high rpm followed by killing ignition and checking plugs to see how it is running. I live in closer to the city in Philadelphia and I don't have as many areas to rev to higher rpm and kill it without doing this on the side of a busy highway. What do you suggest is the best way to plug chop with a CB750?

You will have to find some place to do it at properly. Trailer the bike if necessary to a good location.
Or figure out a quiet traffic time to do it.
Or pay for dyno time to do it on a dyno so there is a load on the rear wheel to do a good plug chop.

[newday777]

Ride at full speed or very close to it. Reduce the throttle a little.
If it feels like the bike will run better/ faster when reducing- too lean mj

If you can hear or feel a rich d-d-d-d behaviour- too rich mj.


This method works fine on the needle area too.

It is a little thrilling to know before the ride that I have to ride over 160- 180kmh to test full throttle to feel the acceleration and small throttle reductions.

Needle to mj area after 100kmh/+4000 rpm. Small accelerations followed by decelerations by the throttle to feel how it accelerates and behaves.

This is otherwise things that suddenly can happen during rides

CB750 stock carbs are well defined for stock and bigger bore with ported head and hitter cam.
No need to increase pilot jet.

Other carbs where the pilot jet has to be changed:
Bigger pilot will affect needle setting and main jet. Total fuel flow changed.
I have seen 2 sizes bigger pilot need 1 step lower  needle and one size smaller main jet when needle and mj was correct before.
Last year I reduced pilot by 1 step (-2.5).
I had to lift needle 1/2 step by an M3 washer under the needle clip.
Increased mj 1 step too when acceleration  at WOT from 180kmh was bad.
This on my K6 with 970cc and over 100whp.
Runs fine now so I can enjoy cruising around in legal speeds most of the time.

I had also issues with erratic idle, take offs and lower speeds not possible to tune. Like it ran too lean.
Bad fuel.
Changed gas station to other chain this year.
Shell is no more here. Their V-power changed to a bad fuel. Maybe too low octane and more ethanol.

Was this on roundtop carbs or??