Topic: UK 750K2 pilot jets

[38rudge]

Hi

I am based in the UK with a 750K2.

It is fitted with a set of 567A's with #110 main jets and #40 pilot jets. Needles currently in middle, third position. Float levels at 26mm.

Due to a couple of pots running weak with the air screws around 1/4 to 1/2 turn out I want to try a size up on the pilot jets to #42. To date I have not been able to find any reason as to why No's 1 and 4 are running weak as I can find no external leaks etc. Needles have been raised due to original black sooty plugs and all air screws around 1/2 turn out.

However, I am having difficulty in the UK finding any?

The original jets, I believe, are 28mm long with a 16mm body. The body dia is 5.5mm and the tail dia is 2.0mm with a 3.5mm mounting thread.

With reference to the attached, all that is available in the UK are the N424-22's and the N424-24's.

The 22's are the correct length but wrong tip dia and mounting thread and the 24's are 4mm longer at 32mm?

The question is would the 24's at 32mm fit in the first place and would they work correctly? What are the implications of 4mm longer jets? I assume that the extra length is all in the body?

Thanks

[magnum56]

I am by no means an expert, however when I read that trouble is associated with 1 and 4, (or 2 and 3) my first instinct is always drawn to ignition problems rather than carburation issues.  Those cylinder combinations share the wasted spark.

Just putting the thought out there.

Good luck.

[38rudge]

I am by no means an expert, however when I read that trouble is associated with 1 and 4, (or 2 and 3) my first instinct is always drawn to ignition problems rather than carburation issues.  Those cylinder combinations share the wasted spark.

Just putting the thought out there.

Good luck.

Thanks for the reply.

Have electronic ignition fitted and a new replacement pair of coils fitted. So whilst that is no guarantee that neither is the source of the problem at this stage I do not feel that the ignition is the problem?

[HondaMan]

Most of the time, this sort of mis-mixing is due to the overly hard rubber hoses between carbs and head, and/or the use of "pod" type air filters.
If you have the option to change those hoses, it will go a long way toward fixing mix troubles. You can also use the Oetiker clamps in 50mm size, but be careful to not overtorque them, as they can easily crack a hardened hose. If you have 'pod' type filters, you will not be able to get clean sparkplugs, ever, because these destroy the carb's ability to mix in the lower 1/3 throttle range, altogether.

The #657b carbs on a K2 should be set at #110 mainjets, #40 pilot jets, 3/4-1.0 turns out on the air screws, and the needle in either the middle position or the 4th groove down (from the blunt end), depending on whether it has HM300 pipes (former) or HM341 pipes (or in the latter case, 4-1 or 4-2 pipes).

Oetiker clamps are available here:
http://www.aircraftspruce.com/catalog/appages/oetiker.php?clickkey=23818
Use the 050-9 size for this bike.

[38rudge]

Thanks for the detailed reply Hondaman.

The carb rubber hoses are new and it has a std airbox fitted with new filter. Std DS replacement 4 it 4's also fitted.

So current position is somewhat frustrating and confusing. Hence the desire to fit some next size up, #42. pilot jets as it would at least help identify if it is a pilot circuit issue?

Are you able to advise with my original question as to if you can use the N424-24 jets which are 32mm long compared to the std 28mm ones which are seeming unavailable in the UK?

[HondaMan]

Thanks for the detailed reply Hondaman.

The carb rubber hoses are new and it has a std airbox fitted with new filter. Std DS replacement 4 it 4's also fitted.

So current position is somewhat frustrating and confusing. Hence the desire to fit some next size up, #42. pilot jets as it would at least help identify if it is a pilot circuit issue?

Are you able to advise with my original question as to if you can use the N424-24 jets which are 32mm long compared to the std 28mm ones which are seeming unavailable in the UK?

Ah, a clue: did you buy the hoses from Honda (OEM)? If so, they are undersized on their OD, which causes them to leak vacuum even when the clamps are fully screwed tight. I had this happen on 3 engines now (one was my own!) that used new Honda hoses since 2013. I switched to using the Oetiker clamps at the head side for that reason. The clamps seem to just barely fit at the carb side, but their spigot is slightly larger OD as compared to the head side.

There's no trouble running with LONGER pilot jets, but there is one caveat: the stock ones are set to the same length (bowl depth) as the mainjet, with the hope that when you run out of gas (before reserve) they both run out at the same time. Most of these carbs actually run out on the mainjets first, the pilots slightly later: when you're running on the hiway, this provides a few moments of reduced power between full speed and maybe 30 MPH before it quits altogether. With longer jets, it will run longer at the lower speed, but for the length difference, there shouldn't be much other change.

There are some differences between, for example, Mikuni and Keihin jets: the entry angle (on the bowl side) is different between them, and the fluid metering hole itself is a different length. This slightly changes the metering: if you take a Mikuni #100 jet and put it in the mainjet of this Kehin carb, in place of a Keihin #100 jet, the Mikuni runs about 5%-7% leaner, approximating a #93 or #95 jet. This is due to those differences, so maybe keep that in mind while tinkering? We see this happen with the aftermarket jets in some of the carb kits we are getting these days: the entry angle is shallower than the ones from Keihin. This makes them run leaner by a little bit.

[38rudge]

Thanks for the reply and info about the OEM hoses. I had a similar problem with my GT750 in that when the clamps were fully tightened the hoses were still loose.

I overcome that by placing some thin aluminium strips, the same width as the clamps, under the original clamps and that gave enough to clamp them. Will try the same on the Honda hose clamps.

You say that it is only the head ends that are undersize on the O/D?

[HondaMan]

Thanks for the reply and info about the OEM hoses. I had a similar problem with my GT750 in that when the clamps were fully tightened the hoses were still loose.

I overcome that by placing some thin aluminium strips, the same width as the clamps, under the original clamps and that gave enough to clamp them. Will try the same on the Honda hose clamps.

You say that it is only the head ends that are undersize on the O/D?

Yep, the head size is the smaller one. And, your 'fix' will work: I had mine like that for almost a year before I got the Oetiker clamps, because Aircraft Spruce was out of stock for about 9 months. They are the only outlet I have been able to find who will sell them less than a whole case at a time. I used some 0.020" thick brass, cut into strips about 5/16" wide, wrapped around the inlet tubes and held in place with a piece of tape, then put some silicone on the hose so I could get it to slip over without ripping off the tape. It was real gummy to remove when I got the clamps, but it worked fine!