Topic: 1978 CB750K Main Jet Replacement

[JimsCB750]

My bike has a 4 into1 header system with the OEM airbox.   Air filter was new last fall.   Engine has run rich since I bought it last fall, even after removing the pods and putting the original OEM air filter box back in place, no difference.   You can see and smell black smoke when riding behind the bike when shifting.   Yesterday, I removed the original #110 main jets and replaced with #105 jets.   Idle jets are original #38, I believe.    I made sure the carbs and passages were clean while apart.  The engine is running way better through the whole throttle range with the #105 main jets.   Starts up good with no choke.   No black smoke when shifting with the new 105 jets.   Still has some exhaust smell when riding behind it but not nearly as bad as it was with the 110 jets.   

However, it still runs rich at idle with some black smoke.    The choke blades are opened properly.   The rubber carb intake boots were replaced several months ago.   All four air/fuel screws underneath are 1-1/2 turns out.    I would like to deal with this.  Which way do I turn them to lean it out?   Any advice on setting these screws and dealing with the rich idle issue would be greatly appreciated!   At what point would I drop from #38 jets down to #35 idle jets?  On a side note,  I'm still trying to find a 4 into 2 or 4 into 4 exhaust system for it.   Thank you for your input and advice!

[minimo]

This is a good reference for any carb issues:

http://forums.sohc4.net/index.php?topic=5410.0

Generally, with air mix screws:

If your airscrew is on airbox side of carb, turning out will lean mixture; opposite if airscrew is on engine side of carb.

Always a good idea to adjust one system at a time, usually matched evenly with the inline fours, and take note before moving onto the next system. And a good practice to adjust the easier thing before deep-diving into other more labor-intensive issues.

Good luck!


Sent from my iPhone using Tapatalk

[maxheadflow]

K8 are fuel screws. Turn them in. 

[HondaMan]

My bike has a 4 into1 header system with the OEM airbox.   Air filter was new last fall.   Engine has run rich since I bought it last fall, even after removing the pods and putting the original OEM air filter box back in place, no difference.   You can see and smell black smoke when riding behind the bike when shifting.   Yesterday, I removed the original #110 main jets and replaced with #105 jets.   Idle jets are original #38, I believe.    I made sure the carbs and passages were clean while apart.  The engine is running way better through the whole throttle range with the #105 main jets.   Starts up good with no choke.   No black smoke when shifting with the new 105 jets.   Still has some exhaust smell when riding behind it but not nearly as bad as it was with the 110 jets.   

However, it still runs rich at idle with some black smoke.    The choke blades are opened properly.   The rubber carb intake boots were replaced several months ago.   All four air/fuel screws underneath are 1-1/2 turns out.    I would like to deal with this.  Which way do I turn them to lean it out?   Any advice on setting these screws and dealing with the rich idle issue would be greatly appreciated!   At what point would I drop from #38 jets down to #35 idle jets?  On a side note,  I'm still trying to find a 4 into 2 or 4 into 4 exhaust system for it.   Thank you for your input and advice!

By chance: have you recently replaced the points and condensors with Daiichi parts? This was EXACTLY what my normally healthy-running 750K2 did in 2016 when I installed a Daiichi plate to see what everyone was complaining about. The condensors are bad, out of the box, and they make the spark extremely erratic and very weak, if the bike will run at all. I could not ride 10 miles without fouling a set of brand-new sparkplugs, either.

---AND - putting the (30-year-old) TEC condensors back in immediately and completely fixed the 'fuel' problem. Nothing else was done (except a 4th set of sparkplugs was also installed that same time).

[JimsCB750]

In regard to the ignition question, my bike has a Dynatek electronic ignition.    I now have the air/fuel mixtures screws 1/2 turn out and it still has black smoke at idle.  It's way too rich at idle.   The black smoke clears up when driving, revving and shifting with the new smaller #105 main jets.   I will pull the air filter and see what happens but that filter was new last fall and this problem has been persistent since I bought the bike last fall. 

How close to fully closed can I go with the air/fuel mixture screws, now at 1/2 turn out?   After that, what should I look at next?    The emulsion tubes on all 4 carbs are clean and free of gunk.   I can see clearly through the tiny holes on all 4 tubes.  Thank you for the input and advice!

[goodtryer]

Have you tried moving the clip on the needles to a higher slot so the needle is lower?

[brewsky]

No needle clip adjustment on K8, only shim adjustment possible with stock needles.

Stock slow jet is #35 (push in). Did you pull the slow jets and clean including the side passages?

[goodtryer]

Didn't realize that. Mine came with adjustable needles and I hadn't questioned it. Must have been swapped in before I got it.

Cheers.

[maxheadflow]

I'd say the chances are that you running rich down low is due to fuel being sucked out of the needle // main jet orifice.  You mention the needles have clips which means the needles are not stock.  It's possible that the needles don't match the motor  / needle jets / emulsion tubes.   

[brewsky]

In regard to the ignition question, my bike has a Dynatek electronic ignition.    I now have the air/fuel mixtures screws 1/2 turn out and it still has black smoke at idle.  It's way too rich at idle.   The black smoke clears up when driving, revving and shifting with the new smaller #105 main jets.   I will pull the air filter and see what happens but that filter was new last fall and this problem has been persistent since I bought the bike last fall. 

How close to fully closed can I go with the air/fuel mixture screws, now at 1/2 turn out?   After that, what should I look at next?    The emulsion tubes on all 4 carbs are clean and free of gunk.   I can see clearly through the tiny holes on all 4 tubes.  Thank you for the input and advice!

You should not have to close the mix screw in that much, something else is off......

It is really important to pull the slow jets and make sure all the holes are clear. Also blow compressed air thru all carb and jet passages to make sure they are clear.

[JimsCB750]

At some point soon, I will pull the carbs and go back through the slow jet passages.    I talked to a bike shop about it yesterday and was told the same thing as brewsky wrote, sounds like a problem with clogged up passages in the slow jet configuration.   I also read that you end up pulling the carbs 5 to 6 times on CB750's before you get everything right.   I'm halfway there!!!   It runs better and better every time I pull them.   On the next carb takeoff, I should be able to reinstall in the carb rack and the air box assembly with my eyes closed!!!  Thank you for the input and advice.  This is a mighty fun bike to drive.

[JimsCB750]

Update:   I pulled the carbs and focused on measuring the float heights since this is not something that I looked at when cleaning the carbs.   A local Honda expert mentioned float heights to me when I told him it was running rich.   Carbs 1,3,4 measured 17 mm or more.   Only carb #2 was close to 14.5 mm.    All 4 carbs are now adjusted to 14.5 mm and look nice and parallel to the carb housing.   I'm confused, shouldn't my engine have run lean with those 17 mm settings? 

There is a smooth pipe sticking up on the accelerator pump row next to Carb #2 that would take a 3/16" hose.    Nothing is attached to it.   Is this a vent?   Thank you for the input!