Topic: Jetting Advice for Modified 1971 CB750 K1

[KickstandDan]

Hey all,

I'm wrapping up a rebuild on my 9/71 CB750 K1 and looking for some guidance on carb jetting. I've done quite a bit of work to the motor and want to make sure I'm setting the carbs up right before getting into serious tuning.

Here's a quick breakdown of the setup:
• 10.5:1 compression pistons (stock displacement)
• Head and crank work done by Ken at Cycle X (lightened rocker arms, balanced crank, new valves, springs, Ti retainers and jam nuts)
• Stock airbox with a UNI foam filter
• Delkevic 4-into-1 exhaust mated to a Cone Engineering 18” Quiet Core muffler

The carbs are the stock round-slide units, and I'm just trying to figure out a good starting point for main jets, pilot jets, and needle position based on this setup. My guess is to start with something a little richer then stock.

I'd really appreciate any insight from folks running similar builds. And if @HondaMan sees this, I'd love to hear your thoughts too.

Thanks in advance!
Dan

[scottly]

Try the stock jetting first.

[Finnigan]

+1 stock jetting

[KickstandDan]

Thanks for the quick response. This is a direct quote from HondaMan in another forum talking about carb jetting.

"The K1, depending on which Factory made it, had either 120, 115, or 110 mainjets. It went like this:
The first K1 from the Old Factory had 120 mainjets, middle needle notch, and the K0 cam that opened intakes at 5 degrees BTC.
The K1 Old factory that received the "new" cams with 3 degree BTC intake timing became 115 mainjets, middle needle notch.
Both of the above had the 34-36 degree spark advancers.
The K1 New Factory bikes had the 3 degree cam and a spark advancer with less overall advance: these came with 110 mainjets. These spark advancers are 32-34 degrees."

Because my bike was made in September of 1971 (K1 models were made from August 1970 and November 1971) I assume it was the K1 New Factory and I should go with 110 or 112 main jets?

Here is a link to the other forum: http://forums.sohc4.net/index.php?topic=170103.0

[scottly]

What jets are in the carbs now?

[KickstandDan]

The first time I had cleaned and rebuilt the carbs I naively didn't even check what was on the carbs and installed a 4into1.com carb rebuild kit. This has 40 pilot (which is stock as far as I am aware) and 120 main jets. I still have the jets I took off and today after work I am going to dig them up and see what the carb had on before. With 120 the plugs showed signs of being rich but it still ran "fine."

[scottly]

Aftermarket jets are notorious for being inaccurate. The stock jets should have a stylized "K" on them. Also, on that thread Ekpent said all his K1s had 120 mains.

[Stev-o]

... and installed a 4into1.com carb rebuild kit.

+1...this will cause tuning issues.   Discard them and get genuine Keihin brass parts.

https://www.jetsrus.com/a_jet_kit_street/honda_750_CB750_1969-1971.html

[KickstandDan]

I found this from another forum here: https://www.sohc.co.uk/index.php?topic=8726.0#:~:text=Found%20these%20references%20on%20the,from%20the%20750%20guru%20%27Hondaman

“The K0 and early K1 had #120 jets, which proved too rich, especially if the bikes were run on premium gas and ridden in city traffic. This became #115 by February-March 1971 on the K1 in the 657A carbs (needles still in clip position#3). It then went to 110 on the later K1, and the needles then went to 4th notch from the top in the 657A carbs. The early 657B carbs with the holes in the tips of the idle air screws came with #110 mainjets, clip in position 4, with later "New Factory" K2 bikes usually having #105 mains, regardless of 657A or B series carbs. The 657B carbs with solid-tipped idle air screws had mainjets of #105 size, starting in the K3 sometime. By the K4, the mainjet stayed at #105 size until the 7A carbs mixed in the #110 sometimes: if the needle was the 27201 it had #105 jets, but if 271301, it had #110 mains.
The PD carbs have different numbers, starting in the F1 bikes.”

It looks like stock on my late September 1971 K1 had 110 main with the needle in the fourth position from the top instead of in the center. I will definitely be getting genuine brass but knowing that 110 was stock, should I go with 112 or 115?

[scottly]

From the Honda manual:

[KickstandDan]

It looks like we might be at an impasse here. I have 2 different sources saying they had 3 different types of main jets for the K1. I am currently cleaning the old brass from my carbs and I will let you know what they are when they are all cleaned off.

[scottly]

I have 2 different sources saying they had 3 different types of main jets for the K1.

The second source is actually the same as the first.

[KickstandDan]

I am going to disagree with you on that as well because both links are from different posts on this website however they are just comments so they might not be true. Regardless, I really appreciate your help! 

[KickstandDan]

@scottly I would like to apologize because after cleaning and looking at the old brass, the mains were 120 and the needle was in the 3 from the top (middle). I am going to go with the 120 main as a starting point with needle in the middle position and 40 pilot. I am going to use the oem brass that I just cleaned off and then order some new oem brass from the listed website if I need to move up a size.

Thanks for the help!!!

[seanbarney41]

I have never seen anything but 120's in a K1.  Also have never found the bikes with 105's to run for #$%* until you put 110's at least.  And really, if you are not riding the bike HARD regularly, any main jet 110-130 will make little difference.  When I say HARD, I mean every shift at or near redline and cruising speeds over 75mph requiring rapid acceleration from there.

[scottly]

The manual states that the K2 and K4 had 120 mains, but the K3 had 105 mains??

[newday777]

@scottly I would like to apologize because after cleaning and looking at the old brass, the mains were 120 and the needle was in the 3 from the top (middle). I am going to go with the 120 main as a starting point with needle in the middle position and 40 pilot. I am going to use the oem brass that I just cleaned off and then order some new oem brass from the listed website if I need to move up a size.

Thanks for the help!!!

1st off welcome aboard the forum Dan
As the others have said always use Keihin brass parts, they will have the stylized K so you know they are from Keihin. Aftermarket brass are not machined to the same tolerances and you will be chasing your tail using them.
How much do you know and have experience about tuning carburators? Do you know how to read the spark plug coloring in tuning?

[KickstandDan]

Thanks for the welcome! I’ve actually read through these website threads dozens of times over the past years and finally decided to start asking questions. I really appreciate the advice on Keihin brass.

As for tuning, I’ve rebuilt the K1 from the frame up and have torn down and rebuilt the carbs three times now. I’m pretty comfortable with rejetting, needle adjustments, float height, and general tuning. I’m no expert at reading plugs, but I’ve studied examples of rich, lean, and oil-fouled plugs and use that to guide my decisions.

[newday777]

Have you taken it out for a ride yet? You can't really do a plug chop without riding it.
Do you jets have the Keihin K on them?

[KickstandDan]

My main jets have the K on them and I have ordered pilot jets for jetsrus.

I have ridden it about 10 miles now trying to open up the throttle a little while knowing I am still on a “new” engine. In that first ride I actually got stopped by a guy in his car who said that I am running rich and I will need to tune it because of how it smelt. I will say this was with the 4into1.com brass was installed and I have tried the needle in the fourth position. But that was definitely a mistake. Idle was rough and it would get caught up at 3000 rpm and I would have to adjust the idle screw to bring it down.

[MauiK3]

Think of it as a journey. Start with the jets you think will work and go from there, the bike will tell you what it likes. If you are not really getting into the throttle your main jets aren't in the picture fully open most of the time.

[HondaMan]

The 750K1, as it came from Honda, always had the needles in the 3rd (middle) notch. They went to the 4th notch with the advent of the HM341 exhaust pipes, which are more restrictive at higher RPM (above 4000 RPM, mostly). Those notches are slightly more than 1% adjustment.

More often in the 675A 657A carbs (which are the normal ones for the K1) the brass floats, combined with the OEM extra-softly-sprung float-bowl checkvalves, cause the bowls to run deep while the bike is in motion: deeper than when sitting still. This was widely known back in the day, but is lost lore today. If the carbs have the modern stiffer checkvalves, this is somewhat less so: if the floats were also changed to the Keihin plastic floats then they will act more like the 657b series carbs, which piddled less from their overflow tubes on hot summer days while parked in the sun.

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.

[newday777]

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?

Dan
Here is a cutaway of a Keihin slow jet so you see what they look like inside(someone posted the picture some time ago and I saved it)

[KickstandDan]

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.

[Jerry Rxman Griffin aka MuthaF'er]

While it is by far easier to change main jets... the main culprit will be your needle position. PITA to keep removing and reinstalling carbs but don't take the easy way out. This is what the majority of your riding will be using. It does overlap on low and higher rpms. Consider getting this right first and out of the way. Now with all said, a dyno run or 2 with the sniffer will tell you your exact air:fuel ratio and which way to adjust.