Topic: CB750 K0 Carb Tuning

[Simpson]

Hello All,
I've been lurking on this site for many many months. Its an amazing resource for tech information and people who truely appreciate classics. I guess I've starting to come realize lately I'm switching forums from another bike site. Posted a introduction back awhile. Just wanted to get some thoughts from the general crowd on my CB750 behaivor.

http://forums.sohc4.net/index.php?topic=49447.msg524435#msg524435

I posted a issue comment:
-Very cold blooded, will not idle smooth until running temp
-slight fumble in acceleration 1-3k
-slight idling fumble, very slight

These are classic symptoms for a lean condition.
I've been through everything from intake to the air box in extreme high detail.
Since then I have resolved the issue but the fix was to my surprise.
Changed to #42 slow jets and #125 main jets. So you may ask, "yeah so..."

My bike is completely bone stock and meant to be a an exact peice straight from a 1970s showroom.

The air box, air filter, exhaust and other major contributors are new or like new.

All connecting tubes and seals are tight!

Could it be from worn carb slides or leaky air box to creat this condition?

Is it because I live right on the coast?

Ideas?


Thanks and I look forward getting to know the group better

[Johnie]

Most of these bikes are cold blooded.  My KO is the same way, but as it warms up it gets much better.  Did you sync it at low idle and then at around 3,000 RPM.  You have to do both.  My KO is running #40 slow and #120 jets.  Idles around 1200 - 1400 depending on how it feels that day.  You have to give these 40 year old bikes some slack. Sometimes they have arthritis...

[Simpson]

Hi Johnie, Yeah I've done a pretty good dynamic/static sync on the carb. Most of my complaints were the stumbling that came from the 1-3k range and additional vibration. The engine seemed to smooth out after going more rich. Just seems weird for a stock setup.

[Spanner 1]

Not weird, IMO, if you live and ride between 6ft. and 100ft. above sea-level and find that your bike runs better than stock with bigger jets...stock carburation had to deal with variable elevations so a median jetting was fitted.....as I understand it......

[scottgarland]

I may be repremended for this but if you are still running the four cable system you may want to consider the K1 two cable pus/pull set up for drive ability and save your other carbs and stuff for a 100 point display bike, just a thought.

[Johnie]

I may be repremended for this but if you are still running the four cable system you may want to consider the K1 two cable pus/pull set up for drive ability and save your other carbs and stuff for a 100 point display bike, just a thought.

I had considered putting K1 carbs on my KO too.  Just could not bring myself to do it.  I spent a lot of time and $$$ keeping all the OEM parts on this bike.  Even chroming the original parts rather than buy new.  Just could not do it.  Finally after dinking around with it I have it running real nice.  Never a miss or burp.  Lucky I guess, but persistance pays off.

[phactory]

I am going to be tuning my K1 REAL soon and hope that it doesn't exhibit ANY of these problems. One of the main reasons I chose a K1 over a K0.

Phil

[Simpson]

I may be repremended for this but if you are still running the four cable system you may want to consider the K1 two cable pus/pull set up for drive ability and save your other carbs and stuff for a 100 point display bike, just a thought.

I had considered putting K1 carbs on my KO too.  Just could not bring myself to do it.  I spent a lot of time and $$$ keeping all the OEM parts on this bike.  Even chroming the original parts rather than buy new.  Just could not do it.  Finally after dinking around with it I have it running real nice.  Never a miss or burp.  Lucky I guess, but persistance pays off.

Same here... I just couldn't change the original design of the K0 bike. At the risk of being corny, it's a part of the beauty of the machine. Additionally I have too much money involved for ir not to work. All needles, jets, floats, plugs or clips are new from Honda. It all adds up. Runs great now.... after tons of shooting and 100 test rides around my nieghborhood.

[Simpson]

Holy crap, I just found out the air box is suppose to have a large rubber gasket (sits in between the clam shell enclosure). I don't have one installed. Could this be the reason for my #125 mains and #42 slow jets?    

Thoughts?

[Duke McDukiedook]

Sure, if air is bypassing your filter then that could create a lean condition.

[Johnie]

Keep in mind that gasket is for the K1 air box.  It will not fit for the KO box.  Believe me, I tried it.  I had to buy a K1 just so I could use my extra air box gasket...   My parts book shows no gasket for the KO.

[Simpson]

Keep in mind that gasket is for the K1 air box.  It will not fit for the KO box.  Believe me, I tried it.  I had to buy a K1 just so I could use my extra air box gasket...  My parts book shows no gasket for the KO.

Really? So, there is no gasket for the K0? I'm not sure looking at the micro fiche from cnsnl, bike bandit (they show a gasket but do not list one).
 
It seemed plausible because I could not get my air filter to fit properly in the cases without pealing off one of the foam gasket material on it. The two gaskets on each side made it too thick and I could not close the filter case shut all the way. I discussed it with "experts" I knew at the time and they had no idea. This was before becoming a member here.

Anyone have this issue with the K0?

Then of course i've re-jetted it now. Perhaps I'll hold on the gasket and new air filter purchase.  

[Johnie]

I think Andy with the KO had the same issue. Maybe he will chime in. But my parts book shows that gasket starting with the K1. I just did not look at the book before I bought it. I used one on my K1 & K4, but those air boxes are different in style from the KO. I am not sure what you mean by "2 gaskets" keeping you from closing the air box. The only gasket material I have is the foam at the top of the air filter. Are you using an OEM fliter there?

[Simpson]

I think Andy with the KO had the same issue. Maybe he will chime in. But my parts book shows that gasket starting with the K1. I just did not look at the book before I bought it. I used one on my K1 & K4, but those air boxes are different in style from the KO. I am not sure what you mean by "2 gaskets" keeping you from closing the air box. The only gasket material I have is the foam at the top of the air filter. Are you using an OEM fliter there?

Yeah this is a genuine Honda air filter! Came in a sealed bag. Also keep in mind this is my first CB750. It has a sealing foam gasket on the top and bottom side. i could get you a picture in the future it it helps. Man this is strange. It made the over all air filter thickness too big so the enclosure had a small crack all the way around it. scrapped it off and it fit together perfect.

[Spanner 1]

Correct me if I'm wrong, but these airfilters are all the same part# for K0 thru' K8 bikes and the airboxes were modified along the way.....regarding the bike sucking non-filtered air due to lack of foam gasket, it might suck 5% of air thru' the upper and lower case mating surface if not tightly seated together...but these motors suck a large volume of air which must be supplied thru the filter...would not cause a need to up the main to #125....IMO.

[Simpson]

Correct me if I'm wrong, but these airfilters are all the same part# for K0 thru' K8 bikes and the airboxes were modified along the way.....regarding the bike sucking non-filtered air due to lack of foam gasket, it might suck 5% of air thru' the upper and lower case mating surface if not tightly seated together...but these motors suck a large volume of air which must be supplied thru the filter...would not cause a need to up the main to #125....IMO.

69-78 are the same air filter I think...

[AshimotoK0]

Don't know if this is an issue but is your Airbox a repro one from LPM/David Silvers/CSNL. If so, for some reason,, the mouldings are slightly smaller than the stock item. I know this because I managed to get them to sell me the lower half only  and it was narrower than the top and I had to buy the whole repro item in the end. I think you have to 'shave' some foam off for the filter to fit.