Topic: Carb Issues 1977 Honda cb750K

[lhemrick]

We have discussed this issue in the past on a thread I started over float height. I was given some suggestions and have just now gotten around to trying some things. I have a 77 CB750K with coffin top carbs running pods. The exhaust is aftermarket  2 into 1 with about half the stock baffle in it. Originally I just kept going up on the main jet to try to stop it from hickuping when you try to hold the throttle at about 1/8 open and cruising between 45 and 60. The suggestion was made to go up on the needle setting which I have done twice. It has helped the issue but the plug tip is still burning white (lean). Now the bike won't take the full throttle and handle anything over 4k of rpm without gurgling and the spark tips are sooted black (rich). I'm currently running #35 slows #127.5 mains and the needles in the third slot from the top. I think I know what I need to do but am looking for other opinion's.

Thanks!

[flybox1]

#35 is way too lean for a pilot jet.
40's at a minimum.  42's might even be better with your pods.
This is just for your 1-1/3rd throttle position.

for the over 4krpm, your jet size might be fine, but your needle is probably clipped too low, giving you that over rich condition

[flybox1]

road speed has nothing to do with what jet is being used.  its all about throttle position.
 if you're anywhere in the 0-1/3rd twist, you're completely in the pilot jet and just starting into needle jet/clip position,


First suggestion. while youre waiting for your 40 pilot jets to arrive  drop your needles lower by raising the clip one position.  recheck your stumble and sooty plug condition at over 4k rpm.   also see how the bike reacts to a handful of throttle at say 5000rpm...does it pick nicely and all the way to near redline?  those main jets should be real close to correct....
If it acts and reacts well above 4k rpm, you can focus on the lower throttle response and plug deposits with 40 pilots and fuel screw positions...

[lhemrick]

Well that's what I was thinking myself. Where is the best place to purchase the pilot jets? These are the push in type they do not screw. I've already looked on Ebay and pickings are slim and the Parzilla or Honda website I usually use for my parts do not even list them for sale.

Thanks!

[flybox1]

jetsrus.com for original keihins.

http://www.jetsrus.com/a_jet_kit_street/honda_750_CB750F_1978.html

[lhemrick]

I'm gonna put the needles back to where they were to get the top end back. Now the bottom situation will get really worse so if #40 is the min what do you think about going to the #45? I think it needs a really big jump to run right.

THANKS!

[flybox1]

lhemrick
slow don a bit.  make one SMALL change at a time and then ride to feel, and see(plug deposits) results.
Just move your needle clips up one slot.  thats all.
Then go ride.

Dont add new pilot jets until you get the richness out of the upper throttle positions.

45's most likely will be too rich.  you wouldnt be able to dial down your mixture screw enough to compensate.
40's at a minimun.  with 42's you could still use your fuel mixture screw to help dial in that 0-1/4 throttle mixture

[lhemrick]

Yeah thats what I gonna do. Already moved the needles back where they were and I'm gonna put em back on and ride it again. I really appreciate your help and I'll post the results tomorrow.

Thanks!

[lhemrick]

Finally it's tomorrow! Ha! I ordered some 40 push in slow jets to replace the 35's. and installed them this morning. Its running better but were still not there. The plug tips aren't near as white at 50 1/8 throttle (which is where the hesitation starts) as they were before. I'm running 127.5 mains, 40 slows, the needle clips in the second notch from the top and the mixture screws one turn out.  I can't find anyone who has these push in jets in 42's. Should I try adding a little more fuel with the mixture screws?

Thanks!

[flybox1]

mixture screws out will richen up idle.
if its still lean at 1/4 - 1/2 throttle, drop the needle clip one more position.

if your mixture screw and needle clip adjustments cannot richen you up enough(out of adjustment range) for throttle positions under 1/2, then go to 42's
 
jetsrus.com has Keihin 42s
can you post up an idle plug chop picture?

[lhemrick]

I've already tried dropping the needles one slot and it ran to rich at 1/4 to half throttle. It was bogging really badly so I put them back. Now could it be that the main jets are to big?

[flybox1]

Because of the overlap of the fuel delivery sources in our carbs, the most accurate way to tune is to start with the main jet, get it right, first, then move to needle clip position, then pilot jet, then mixture screw.  Each with its own plug chop or chops to dial it in.

how are you testing for rich/lean?  plug chop or seat of your pants 'feel' ?
if you are unsure of your main jet, concentrate on that first.

[lhemrick]

I'm plug choppin. It will run like the wind as long as your on the throttle hard. It just want cruise with a hesitation every so often. Its a lot better since I went from the 35 to the 40.

[flybox1]

complete an accurate chop with new plugs on a hot engine and replicate that hesitation and keep it there for good deposits.

I got some lime green painters tape and cut thin 2mm strips about 10mm in length.  I placed these on the throttle grip right next to the control for reference while i was riding. 
1 strip on my R control to measure against, 1 strip @1/4 turn, 2@ 1/2turn, 3@3/4turn, and a WIDE strip at full. Really helped to be able to take a quick peek at my grip and see exactly where i was....even when in between.