Topic: re-jet carbs

[Albany]

hi I have a cb750K 1978 that im trying to adjust. last weekend i put a new carb kit in adjust the flot, timing and put new carbs holder. The carb are running on the original setting main jet 110. the bike it self is running with the original air box and the original pipe, thye only modification ( that the guy that i bought it from did) is the pipe are empty but still have the cover on the end of it. after set all that stuff up and went for a ride the spark plug came out to lean.

My question here is, I found out that i need to rejet the carb but I'm not to sure how big should I go? and I'm wondering since I sync the carb already if I jet bigger do i need to re-sync it?

[flybox1]

First, welcome to the forum.
Aftermarket jets are known to cause many tuning issues.  IF you have to buy new jets, only buy OEM Keihin. 
www.jetsrus.com is a good seller

Yes, you will be lean if the previous owner removed the baffles.
You will also be lean if you are still using the stock #35 keihin pilot jets.
You will need #40 Keihin pilot jets,  and probably an adjustment of the needle height (1 shim).
I highly doubt you will need to increase your main jet size.
Once done, you can fine tune idle mixture at the fuel screw. Under the carb @ the bowl edge.
Probably somewhere near 1.25 turns out.

You will need to do another vacuum sync of the carbs once you pull the needles to install the shim.
see this...  http://forums.sohc4.net/index.php/topic,71245.msg791919.html#msg791919

 

[Kickstart]

Can you post a picture of the spark plug/plugs?

I'm wondering if you're actually running lean, especially since you're still using the stock airbox.  How does the bike run?  Any backfiring on deceleration, or is it hard to start when cold?

And yes... welcome to the forum!

[Albany]

thank you for the warm welcome

Yes i do have some backfire in deceleration but only if the engine is revving high this is some pic of the spark plug after I've done around 100km with it and the gas screw been turn 2,5 turn out
 
the bike doesn't have much problem to start when is cold but i need to put the chock take it out and put it back on an so on and so fort and i need to bring the idle higher to be able to keep it running until he get warm and after i can bring it back down to normal.

[flybox1]

the bike doesn't have much problem to start when is cold but i need to put the chock take it out and put it back on an so on and so fort and i need to bring the idle higher to be able to keep it running until he get warm and after i can bring it back down to normal.

Typical result , and exactly how the bike should work, but, ride your bike, get it to operating temperature, and set your idle.
You shouldnt need to touch the idle at all after this.
Just use the choke to start it, and ONLY take the choke off when the idle speed starts to rise (when parked) or once you get on the road for a few minutes.  leave the idle adjustment screw alone 

Only the top plug looks OK...the rest are lean.
@ 2.5 turns out, you are well past the ability for the fuel screw to effect mixture.  #40 pilots, and you'll be right.

[brewsky]

Did you remove and clean the push in slow jets?

[Albany]

 

Just use the choke to start it, and ONLY take the choke off when the idle speed starts to rise (when parked) or once you get on the road for a few minutes.

That might be a problem when i actually have the chock on the idle is going down and down untill the bike stop, hopefully after i change the idle jet the problem will be resolved.

My carbs were already clean, and I did not look at my slow jet, I'll have to look at it

[Kickstart]

Just use the choke to start it, and ONLY take the choke off when the idle speed starts to rise (when parked) or once you get on the road for a few minutes.

That might be a problem when i actually have the chock on the idle is going down and down until the bike stop, hopefully after i change the idle jet the problem will be resolved.

My carbs were already clean, and I did not look at my slow jet, I'll have to look at it

With the slow screw turned out 2.5 you should be running rich at idle - may explain why you can't leave the choke closed for long.

When you say you "put a new carb kit in" - what specifically does that mean? Did you change out any of the jets/needles?

"My carbs were already clean, and I did not look at my slow jet" - I'm assuming you didn't remove the slow jet or clean out the slow jet passage (not an issue if it is actually clean, but might be a good idea to confirm).

I actually think those plugs look OK, at least the first and last one.  I was actually going to say I think the first one looks a little rich.  Especially if you're running regular pump gas with 10% ethanol.  The middle two may be a little lean - hard to tell in the picture. Flybox probably has more experience/knowledge than I have though.

It's also hard to tell just looking at the plugs what part of the throttle band you need to address unless you do a plug chop.

Was there a performance problem you were trying to address (flat spot in the throttle response, engine running hot?)... how did you determine you were running lean?

[flybox1]

With the slow screw turned out 2.5 you should be running rich at idle

With stock (lean on purpose by HONDA) #35 jets and an open exhaust, I dont think so.
(Past 2 turns there is little to no mixture change.  If you need to go richer, pilot jet change is needed)
A few things we are still unsure of:
What model carbs are these?  PD42b?  PD41a?
Did new 'kit' pilot jets get put in?  What size is in there now? (35's?)  And are they clean?
Is the emulsion tube under the main jet clear of all blockages?
What is the status of the accel pump, and fast idle cam?  Are they clean and functioning? (both have an effect on idle)

[Albany]

I have no idea which model of carbs they are, how can i find out? the only way that I'm saying that is lean are from the spark plug, the bike does not have any  performance problem, the acceleration pump is new I change it and its working, but i have no idea what is the fast idle cam, and how to make sure its working?
the pilot jet is a #35, I haven't look at them when I open the carbs, but the carbs it self where clean ( I'll have to look at it when I'll open the carbs)
when I install the kit i change the needle, needle seat, gasket and o-rings but i didn't change the main jet because i change it last summer, the kit did not come with a new pilot jet.

one more thing that i remember now is that when i start it cold and i have the chock on the bike will start running slower and slower and when i put the chock off the rpm come up for couple second and will start going down as well, and when i put the chock back on the rpm will go up and will go back down and so on and so fort. I don't know if that can help or not, but I'm just putting it out there.

[Kickstart]

I have no idea which model of carbs they are, how can i find out? the only way that I'm saying that is lean are from the spark plug, the bike does not have any  performance problem, the acceleration pump is new I change it and its working, but i have no idea what is the fast idle cam, and how to make sure its working?
the pilot jet is a #35, I haven't look at them when I open the carbs, but the carbs it self where clean ( I'll have to look at it when I'll open the carbs)
when I install the kit i change the needle, needle seat, gasket and o-rings but i didn't change the main jet because i change it last summer, the kit did not come with a new pilot jet.

one more thing that i remember now is that when i start it cold and i have the chock on the bike will start running slower and slower and when i put the chock off the rpm come up for couple second and will start going down as well, and when i put the chock back on the rpm will go up and will go back down and so on and so fort. I don't know if that can help or not, but I'm just putting it out there.

Oh boy... that's a lot of different modifications.  Not to scare you, but usually you want to try to make one change at a time so you know which change made a difference.

Also, generally speaking, old used OEM needles are better than the ones in the aftermarket kit.  It's my understanding the aftermarket needle taper isn't quite right and makes tuning the carbs a little more challenging.  if you still have the old ones, assuming they're not trashed/bent, you may want to put them back in.

I'm going to back down and let Flybox take the lead - he clearly has more experience with this, and with your specific year.

[Albany]

no worries I'm already scared haha

[flybox1]

Look ok the #4 end of the carbs....the model should be stamped there.
if you used any brand of jets other than genuine Keihin, your bike will be difficult to tune.
we cant do much more until thos is resolved.  40 pilots and 110 mains.  Both Keihins.
keihin jets are stamped with a K .

Ill see if i can video tue fast idle cam and post it.  Its proper function makes starting easy.

[flybox1]

Just shot this.
These carbs have a bunch of moving parts that all must work together to make starts effortless, and acceleration easy.
Jetting-  Keihin 40/110's are what you need.

Accel pump -  squirts fuel into the carb throats upon twisting of the throttle.  PD carbs need to be slightly lean at idle, and coupled with the fast idle cam, will give you the right starting air fuel mixture conditions.

Fast Idle cam - The principle of the FIC is to increase the slide height when the choke is pulled (to allow for extra vacuum - you will see in the vid below) as well as closing the butterfly valves, to create a rich mixture for startups.
Taking the bike off choke when the bike warms opens the butterflies back up.

Idle set screw....need to be set once the bike is warm, and left at this position, so when the bike warms and the choke is taken off, the bike can idle at this set rpm.
When you pull the choke, the fast idle cam will need to hold a 2500+/- rpm during idle.  Idle will start to rise from here as the bike warms, which is your sign to slowly start removing the choke.

have someone slowly pull your choke, and see if your slides lift slightly.  If they do, good. You probably wont need to adjust this.  Just be aware of what they are doing and once your bike is warm, slowly pull the choke...the bike rpms should rise.  if they do, your FIC tests good.
Sorry for the dirty carbs, these are an old junk set used for parts. 

[Albany]

Thanks for the video that's explaine a lot
but the sit you gave me does not sent to canada (for my idle jets) do you know any good site that sent up here in the great whit north haha.

[flybox1]

www.siriusconinc.com

[Albany]

Thanks a lot for all the help I'll have to wait for my new jets to get here and do the work on it.

One more question how can I figured if I need to shim the needle? what type of problem will to bike have that will give away the need of raising the needle?

[flybox1]

do the jets first, and then go for a ride.  tune the mixture screws for a good idle (plug color)
you'll have to do a 2/3 throttle position plug chop to see if shims are needed.