Topic: Carb Question

[Harper]

Hi Everybody,

I've been rebuilding my carbs (Kei Hin, for CB 750K5, with 4x1 exhaust), and am about ready to put them back on, But I want to make adjustments while I have everything apart.

Currently: Somebody in the past had changed the carb settings, I think to accommodate an aftermarket four into one exhaust. Now, Main Jets are 130 (120 is stock), the clips are set one notch below middle (that is, the needle is raised one notch. Center notch is standard), and the spark plugs are Black and Sooty and Dry (running rich?).

So, to correct the problem, would it be better to drop the needle a notch or two, or to change the main jet to 125s or 120s? Or is there no standard advice, and better for me to just try one and adjust?


thanks,


Harper

[Hondell]

Yea it seems almost every one acts differently. I tried the mainjet first but it was the needle back to stock that fixed my all round richness.

[ekpent]

Stock main on a 75 750K is actually a #105. 70-71 used a #120.

[Harper]

Hi Ekpent,
The Haynes Manual says K5 mainjet is 120, as does this site: http://www.salocal.com/sohc/tech/carb/asmpg_mgs/jetgraph.htm

I think I'll try lowering the needle before reducing the jet size

thanks

[jtsmith]

Ekpent is correct. The 75 is not 120. 73-74 was 110, and the 75 was reduced to 105 for emissions.

http://www.hondachopper.com/garage/carb_specs/carb_specs.html

Some of those charts out there group 70-76, but there we changes every year or two to the jets

Cheers,
jt

EDIT:

The link I posted shows an important piece of info, the casting number on the carbs. That is really how you should set the jets (I think, I am sure I will be corrected if wrong!    )

[8 Track]

I have a k6, and I (think) am running 115's and stock needles in the center position.  Black and sooty is rich, but unfortunately, you don't know what range is rich.  To get an accurate plug reading, you have to do a wot pull on clean plugs.  Anyway, to get to that 115, I started with my 105's, bought a pin vise and started drilling.  Ride for a day or two, read the plugs and I kept going bigger and bigger until I couldn't tell any improvment anymore.  Then, I bought three sets of jets, right on, one under and one over (well, four jets!).  When you drill, the jet is now actually bigger than the drill you used, and they're not an accuate match anymore.  But, it gives you an idea of where you are.  I found that with the 120's, my performance didn't improve, but my mileage went from 45 to 41 mpg.  Yes, I'm that anal that I checked.  That was four years ago, the bike runs like a rocket ship and the plugs still look perfect.  My four into one is sort of quiet, so we're looking at normal backpressure.

Like mentioned earlier, put the needle back to the stock location.

[TwoTired]

 The exhaust back pressure will likely be most significant at 3/4 to WOT.  This is where the main jet dominates mixture.
The 1/4 to 3/4 throttle positions are controlled by the slide needle height and taper.
Pilot jet circuit mainly controls mixture at idle to 1/4 throttle position.
So, you need to determine at what throttle setting your spark plugs gather soot, to determine what to adjust.

While poor jetting can be responsible for sooty plugs, so can a dirty/old restrictive air filter, behaving as partial choke.  You may just save yourself a lot of adjustments, by going to a new air filter.

If someone else has been inside the carbs, there are other possibilities.
1- jets altered by abrasive cleaning or drill bits.  This doesn't change the number stamped on the jet, obviously, but it changes the fuel feed characteristics.
2- There have been many threads noting that aftermarket replacement carb kits include parts that have different dimensions than the stock ones, most notable the carb slide needles (which is the primary "cruise" fuel metering device).  Some have cured their "rich" running issues by reverting back to the original genuine stock needles.

Finally, the carbs are stamped to indicate their original set up number and application.  See here for a guide:
http://www.motorcycleproject.com/motorcycle/text/specs.html

Of course, these settings and applications only apply to stock unmodified bikes (air filter, engine, and exhaust).  But, it does give knowledge about what they were, and therefore what's been meddled with since new (by comparison to what you have).

[8 Track]

Yes, thank you TT.  I left alot of specifics out and I think you covered it.  Carb troubleshooting can get very detailed and tricky.  The best thing to do is to read until your eyes bleed, then read some more.  It's hard to argue with those stock carb settings, but you've got to make sure everything is unmolested new jets and needles.  And a new air cleaner.  And plugs.  Ignition.  Timing.  *sigh* it goes on and on.   

[Harper]

Hi everybody,

Thanks for the advice!
The air filter is clean, I think it was replaced just before I bought the bike (beginning of last fall.)
I did notice a couple weird discrepancies and extra parts between the rebuild kits I got and the stuff I was pulling out of the carbs. So I anticipate a lot of fiddling.
Twostrokecrazy: Nice detail work, most of that went over my head. I wish I was that meticulous, or at least knew what I was doing

I still don't understand about the mainjets for 75s being 105s. I dont have any frame of reference, but wouldn't there have to be some sort of extreme change for the carburetors to be jetted 25 sizes larger? (since I was running on 130s before)

thanks

Harper

[TwoTired]

If you are going to re-jet the carbs, begin with a brand new, known squeaky clean, air filter.  That is the base line you need to tune to, not something that has already trapped and bunch of dust and is part way to being "restrictive".
IMO

[8 Track]

If that was more than you want to take on, since jets aren't that expensive, put a set of 115's in there as a baseline and work from there.  It's a good starting point.  Get that air cleaner tt suggested.  That number on the jet is the hole size, in inches- 115= 0.115".  But a .115 drill won't fit.  Also, they go in intervals of five- 105, 110, 115, etc.  The previous owner likely didn't know what he was doing and just went over rich with the jetting.  My p.o. thought that leaving one jet loose and one jet laying in the bowl was acceptable.  I've read on here that some guys were putting as big as 140's in their carbs, but they had hopped up 836's, air pods and good headers.

Aren't carburetors fun?    Once they're right, they're great!

[Harper]

Twostrokecrazy: that sounds like a doable plan. And I didn't know the the sizes corresponded with an actual measurement, how handy!

I'll keep you updated on the tuning process, thanks for the help!

Harper