Topic: Main jet size in a 1971 CB750K1

[Yamahawk]

I am pretty sure the jets I have are too large. I pulled my carbs today to check the float levels and jet sizes, as you never know what the PO has done in there. The float bowls didn't look too bad, but the mains were 125's in all 4 carbs. The pilots were 40, so that is good, I believe, but I do not know any other work that may have been done on the engine. It has been running cheap pods, and I believe that's why the PO put in 125 mains. Do you think these are too large? My plugs are really dark and sooty, but it runs fairly well on the highway.
Charlie

[ekpent]

 Responded in the 'what did you do' section also but all the k1's I have had all had 120 mains and brass floats.

[Yamahawk]

Mine has brass floats, and 125 mains. I will check the float levels, what exactly should be good? I have seen 25mm, 24mm, etc. but if I remember, Hondaman had a recommendation for a certain level that would get rid of the rick condition along with DP7EA plugs... for a hotter burn? Also, I will check the o-rings in the tops of the carbs to see if they are in good shape.
Charlie

[PeWe]

Mine has brass floats, and 125 mains. I will check the float levels, what exactly should be good? I have seen 25mm, 24mm, etc. but if I remember, Hondaman had a recommendation for a certain level that would get rid of the rick condition along with DP7EA plugs... for a hotter burn? Also, I will check the o-rings in the tops of the carbs to see if they are in good shape.
Charlie

I set my floats to 26mm, K6 carbs to be used with my K2 build. K1 cylinder and head, 61 mm cruzinimage pistons.
125 mains look like pods and/or rather open 4-1.
My stock CB750, airbox w paper filter, 4-1 was 115 mains, max 120.
Pods need more, maybe needles at 4th instead of stock  3.
Check hondachopper.com / garage for carb and float adj.

[Yamahawk]

PeWe, are the K6 carbs still the round tops with the screw rings? That's how mine are, stock K1 carbs. i will set the floats at 26mm and try that. I know that something is too rich for the bike, could be float levels but won't know until I check them. I have 4-2 MAC exhaust with the large  lake pipe type side cut mufflers. I don't think there are any baffles in those, as it has a nasty growl when you rev it up lol. I am going to change the pods, or glue the rubbers into them, as they are sooooo loose that you can wobble them around and that tells me they leak air instead of pulling it through the mesh filter on the pod. I have actually pulled the filter off the boot by just trying to remove them from the carb bell, and the clamp was very loose.
Charlie

[HondaMan]

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.

[Yamahawk]

The K1, depending on which Factory made it, had either 120, 115, or 110 mainjets. It went like this:
The first K1 form 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.

Thanks, Hondaman,
   My build date is 12/70 on the neck label, so would that put it in the 2nd class with the 'old' factory and newer cams, or still the 1st build? Was thinking that I read somewhere that 11/70 was cutoff for better cam timing, or 1st ones...
Charlie

[Yamahawk]

I reset the float levels to 26mm and straightened the brass floats so they were a parallel pair each, I believe the PO used the gasket base to set the float level, as they were all at least 2mm lower than they should have been. So, now on to jetting!
Charlie

[PeWe]

My K6 carbs look like the older ones except for plasric floats.

[Johnie]

Do those look like OEM brass floats? I had so much trouble with those leaking I got rid of them all. In fact, if I get a bike with them I change them out rather than have an issue down the road.

[HondaMan]

I reset the float levels to 26mm and straightened the brass floats so they were a parallel pair each, I believe the PO used the gasket base to set the float level, as they were all at least 2mm lower than they should have been. So, now on to jetting!
Charlie

In the #657A carbs, one (brass) float is always staggered 1mm from the other, with the higher float set at 25mm-26mm, the other at 24mm-25mm. These heights are also measured from the little edge on the inside of the bowl's gasket. This combination of settings made the legendary low-end torque of the K1 bikes (and early K2 like mine, with their "A" series carbs), but it also increases plug fouling issues in city riding unless the air screws are leaned out. I run these at 7/8 turn instead of 1.0 turns out, to compensate. But, this setting can also make it slightly cold-blooded on cold mornings.

A better, less troublesome, setting with today's fuels is to set them at 26mm and 25mm using that inner ledge, and use lesser octane when riding in town: this is because our modern fuels burn far slower than the gas of the 1970s era. I typically run low-grade when strictly in town, midgrade for commuting (short freeway runs) and premium only when hitting the interstates.

[Yamahawk]

I reset the float levels to 26mm and straightened the brass floats so they were a parallel pair each, I believe the PO used the gasket base to set the float level, as they were all at least 2mm lower than they should have been. So, now on to jetting!
Charlie

In the #657A carbs, one (brass) float is always staggered 1mm from the other, with the higher float set at 25mm-26mm, the other at 24mm-25mm. These heights are also measured from the little edge on the inside of the bowl's gasket. This combination of settings made the legendary low-end torque of the K1 bikes (and early K2 like mine, with their "A" series carbs), but is also increase plug fouling issues in city riding unless the air screws are leaned out. I run these at 7/8 turn instead of 1.0 turns out, to compensate. But, this setting can also make it slightly cold-blooded on cold mornings.

A better, less troublesome, setting with today's fuels is to set them at 26mm and 25mm using that inner ledge, and use lesser octane when riding in town: this is because our modern fuels burn far slower than the gas of the 1970s era. I typically run low-grade when strictly in town, midgrade for commuting (short freeway runs) and premium only when hitting the interstates.

What i get is, I reset the factory stagger on the brass floats, when I didn't need to. Well, it couldn't hurt, all the floats that are replacement probably don't have the staggered 1mm in the pair. Also, the measurement reference i used is from the Hondachoppers info, here is a picture of where they reference the 26mm measurement for the floats. Is this correct? And, do you think the 125 mains are too rich?
Charlie

[HondaMan]

I reset the float levels to 26mm and straightened the brass floats so they were a parallel pair each, I believe the PO used the gasket base to set the float level, as they were all at least 2mm lower than they should have been. So, now on to jetting!
Charlie

In the #657A carbs, one (brass) float is always staggered 1mm from the other, with the higher float set at 25mm-26mm, the other at 24mm-25mm. These heights are also measured from the little edge on the inside of the bowl's gasket. This combination of settings made the legendary low-end torque of the K1 bikes (and early K2 like mine, with their "A" series carbs), but is also increase plug fouling issues in city riding unless the air screws are leaned out. I run these at 7/8 turn instead of 1.0 turns out, to compensate. But, this setting can also make it slightly cold-blooded on cold mornings.

A better, less troublesome, setting with today's fuels is to set them at 26mm and 25mm using that inner ledge, and use lesser octane when riding in town: this is because our modern fuels burn far slower than the gas of the 1970s era. I typically run low-grade when strictly in town, midgrade for commuting (short freeway runs) and premium only when hitting the interstates.

What i get is, I reset the factory stagger on the brass floats, when I didn't need to. Well, it couldn't hurt, all the floats that are replacement probably don't have the staggered 1mm in the pair. Also, the measurement reference i used is from the Hondachoppers info, here is a picture of where they reference the 26mm measurement for the floats. Is this correct? And, do you think the 125 mains are too rich?
Charlie

Well, certainly the 125 is too rich, unless it is one of the Keyster version of these jets: then it is closer to being a #117.5 in real life. Their #110 jet works out to run about #107 or so, too.

Look next to the notch that you have outlined in your picture: toward the inside of the gasket there might be a thin ledge about 3/16" long, right next to the floats. This is the "inner" reference I mentioned above, and was only referenced in the early Honda manuals for the K0 and K1 Old Factory carbs. This inner ledge disappeared during the #657B series carbs, though (which have plastic floats). The plastic floats are synonymous with the 26mm float level so often quoted, and that level is referenced from the notch you have circled in the picture, for plastic floats.

On the staggered floats (brass) the float tang is also angled as if mimicking the staggered floats, with its high-side edge "pointing" toward the higher-side float. This is so consistent in the early carbs that I cannot believe it was a mistake: all brass-floated 657 and 086 carbs, including those found in the 750F0 bikes, are set like this.

I'd suggest using a #105 mainjet, genuine Keihin, or else a #110 knockoff clone jet (107-ish) as the starting point, particularly with today's fuels. My (very early) K2 came OEM with #110 mainjets and has always been on the rich side: by February of 1972 these same carbs, with brass floats, were coming thru with #105 jets instead, all else being equal.

Today's midgrade, ethanol-laced gas burns at close to the same speed as the 1970s premium grade fuels, too. So, consider running that instead, if the plugs are pretty dark. I run low-end gas for in-town work, just to help keep the plugs cleaner.

[Yamahawk]

Ok, checked out the emulsion tubes, and I believe they have been modified with .039" holes drilled in the 4 holes farthest from the main jet, on opposite sides.(2 on each side) There are 12 holes altogether in the tube. The float needles are solid brass or steel, no  viton tip, and have slight grooves which I polished with 0000 steel wool.
The carbs have no numbers except 7A IL on the inlet side flange. They have 125 main jets that seem to have HUGE holes in them, perhaps they have been drilled... not sure. No K logo for Keihin on the jets. There is no inner flange to measure the float levels, like Hondaman was stating, at least from what I see, just the gasket area, and the 'notch' in my previous picture. I am thinking on replacing the float needle and seat in all 4 carbs, what do you think?
Charlie

[HondaMan]

Ok, checked out the emulsion tubes, and I believe they have been modified with .039" holes drilled in the 4 holes farthest from the main jet, on opposite sides.(2 on each side) There are 12 holes altogether in the tube. The float needles are solid brass or steel, no  viton tip, and have slight grooves which I polished with 0000 steel wool.
The carbs have no numbers except 7A IL on the inlet side flange. They have 125 main jets that seem to have HUGE holes in them, perhaps they have been drilled... not sure. No K logo for Keihin on the jets. There is no inner flange to measure the float levels, like Hondaman was stating, at least from what I see, just the gasket area, and the 'notch' in my previous picture. I am thinking on replacing the float needle and seat in all 4 carbs, what do you think?
Charlie

Ah, a clue...the "7A" carbs came from a 1975 CB750F0 bike, or from a K6 made at the same factory and time, where these got intermixed (like the wiring harnesses...).

Do the float valves have the little spring-loaded tips? They should have...

The upper emulsifier holes have been modified if they are 0.039", as they were originally 0.0375" in all the carbs after the K0, except I found some of 0.035" in the K4 during the "657B" and "086a" series carbs that advertised bikes with 50 MPG, a temporary marketing ploy American Honda implemented for a while.

This hole-size mod was very common, though, to reduce plug-fouling problems on the K0-K2 bikes, in particular. The early bikes were jetted too rich to appease Honda's Warranty Department accountants who worried that Americans would burn these bikes down on our 70-80 MPH interstate hiway system (before the speed limit was dropped to 55). They did not cover sparkplugs, though...so, to clean them up we would drill these holes out during the warranty period (90 days) and then drop the mainets down a size after that.

[Yamahawk]

Ok, checked out the emulsion tubes, and I believe they have been modified with .039" holes drilled in the 4 holes farthest from the main jet, on opposite sides.(2 on each side) There are 12 holes altogether in the tube. The float needles are solid brass or steel, no  viton tip, and have slight grooves which I polished with 0000 steel wool.
The carbs have no numbers except 7A IL on the inlet side flange. They have 125 main jets that seem to have HUGE holes in them, perhaps they have been drilled... not sure. No K logo for Keihin on the jets. There is no inner flange to measure the float levels, like Hondaman was stating, at least from what I see, just the gasket area, and the 'notch' in my previous picture. I am thinking on replacing the float needle and seat in all 4 carbs, what do you think?
Charlie

Ah, a clue...the "7A" carbs came from a 1975 CB750F0 bike, or from a K6 made at the same factory and time, where these got intermixed (like the wiring harnesses...).
These carbs might be due to the newer cylinder and head on the K1 cases. I am not sure if there has been any engine work done, but the cylinder base gasket is new looking. It would be nice to figure out if there had been a big bore kit and cam in there, hehe.

Do the float valves have the little spring-loaded tips? They should have...
Yes they do!

The upper emulsifier holes have been modified if they are 0.039", as they were originally 0.0375" in all the carbs after the K0, except I found some of 0.035" in the K4 during the "657B" and "086a" series carbs that advertised bikes with 50 MPG, a temporary marketing ploy American Honda implemented for a while.
Well, not sure of the exact size, as I used my digital mic to approximate the size, but they are definitely larger than the two holes under them. I estimated .039" but could be .035"...

This hole-size mod was very common, though, to reduce plug-fouling problems on the K0-K2 bikes, in particular. The early bikes were jetted too rich to appease Honda's Warranty Department accountants who worried that Americans would burn these bikes down on our 70-80 MPH interstate hiway system (before the speed limit was dropped to 55). They did not cover sparkplugs, though...so, to clean them up we would drill these holes out during the warranty period (90 days) and then drop the mainets down a size after that.

I am not sure whether I should replace the float needles, the seats are probably fine, what do you think of the rubber (viton) tipped needles? Or, should I spend the big bucks and buy 4 new needles and float seats? They have the brass floats too, did the 7A carbs come with brass floats? I am going to get some genuine Keihin jets, I will drop them down to 115 or 120 size, and see what they do.
Thanks for the help. Hondaman!
Charlie

[seanbarney41]

If you are going to use those pods, stick with the 125's...

[ekpent]

 Only genuine Ko and K1 carbs came with brass floats. If or when you take out the 10mm head brass float needle seat and you see a little plastic fuel filter/strainer stuck in the end you definitely have later Fo carbs. As for buying new needles and seat if the little spring tips are still working fine I would try them out and replace if they leaked later. I have had good luck with good originals and they can be changed even after the carbs are mounted without too much hassle if you have finger dexterity 

[Yamahawk]

Only genuine Ko and K1 carbs came with brass floats. If or when you take out the 10mm head brass float needle seat and you see a little plastic fuel filter/strainer stuck in the end you definitely have later Fo carbs. As for buying new needles and seat if the little spring tips are still working fine I would try them out and replace if they leaked later. I have had good luck with good originals and they can be changed even after the carbs are mounted without too much hassle if you have finger dexterity 

Now, that's interesting.. the fellow who built this bike evidently pieced it together with parts from a few different years. If these are 7A IL carbs and they didn't come with brass floats, that means he swapped parts from the original 654A carbs on the 1971 CB750K1, and made it work.. someone else said it looked like there was a later model cylinder head and jug on it too. Now, I have noticed what looks like a newer base gasket (black or dark grey) on it, and the head isn't K1, i don't believe. I am wondering if he took the upper end off of a K6 or F0 and stuck it on the 1971 cases. The VIN matches 1971 on the case. It is curious... The holes in the 125 main jets look huge, more like a 300 main in a Mikuni hex jet would be. I will have to get out my drill set and see what size these really are. The bike runs well, but it is really rich and blackens the plugs easily. It is hard starting cold too, and most times have to spritz some ether in the carbs to light her off. That needs to be remedied. But once it has started and run, it starts easily. Oh well, this is the fun of owning an older bike!
Charlie

[HondaMan]

Well...the F0 bikes with 7A carbs also have brass floats: for some reason, they were revived for a while then. I actually have one of those bikes, parked on my patio at the moment: it was built 3/75. The F1 that was in my shed until last month has plastic floats in its carbs. It was built 11/75. Both are (or were, until I got them) virgin bikes, with less than 7k on the F0 clock and 13k on the F1.

[Yamahawk]

Well...the F0 bikes with 7A carbs also have brass floats: for some reason, they were revived for a while then. I actually have one of those bikes, parked on my patio at the moment: it was built 3/75. The F1 that was in my shed until last month has plastic floats in its carbs. It was built 11/75. Both are (or were, until I got them) virgin bikes, with less than 7k on the F0 clock and 13k on the F1.

Hmmm then where is the reference point on the F0 carbs for setting the float level? Is it the notch like was circled in red in the Hondachopper article's picture? That is where I set the 26mm level at. The carbs seem to work well, they are just impossible to start cold without starting fluid sprayed in the pods. After they are warm, they work great. I am thinking the pilot circuits need cleaning, and then this might not happen. The emulsion tubes have a small bit of sediment in them, from sitting, and I will clean them up too. The main jets concern me, as their holes are huge to my eye, for a 125 main jet. I think I will get some better quality pods, these hardly hold onto the rubber spigots and can be pulled off of them easily. Looking at the UNI pods, 4 of them are around $40 and are a foam filter instead of a cheap fabric K&N clone.
Charlie

[750FourK1]

The K1, depending on which Factory made it, had either 120, 115, or 110 mainjets. It went like this:
The first K1 form 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.

A small question for Hondaman

In the last case for the New Factory K1's with 110 mainjets at which notch would the needle be please? Mine is on the 4th from top.

[onepieceatatime]

The K1, depending on which Factory made it, had either 120, 115, or 110 mainjets. It went like this:
The first K1 form 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.

A small question for Hondaman

In the last case for the New Factory K1's with 110 mainjets at which notch would the needle be please? Mine is on the 4th from top.

From the Honda Service Spec Manual:
If you have carb model B750A or 7A, 3rd groove from the top
If you have carb model 086A, 657A, or 657B 4th groove from the top.

[750FourK1]

OK, so mine are in the right place, many thanks!

[HondaMan]

The K1, depending on which Factory made it, had either 120, 115, or 110 mainjets. It went like this:
The first K1 form 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.

A small question for Hondaman

In the last case for the New Factory K1's with 110 mainjets at which notch would the needle be please? Mine is on the 4th from top.

From the Honda Service Spec Manual:
If you have carb model B750A or 7A, 3rd groove from the top
If you have carb model 086A, 657A, or 657B 4th groove from the top.

Yep, and the B750A and 7A needle jets are thicker in diameter until 1/4 throttle, too. They have a different part number, 2713107 (F1) or 271301 (F0), from the other carbs you mention, 271301 (086a) or 27102 (most), or 27302 if a 1978 CA bike. These numbers describe the thickness of the top of the needle and where they start to taper, and how fast they taper. Somewhere I have their magic decoder ring, but itis a visible difference, not hard to tell them apart. And, they are etched with the numbers, next to a tiny "K" just below the bottom groove.