Topic: Carb setting question after rebuild

[pburress]

I will be rebuilding the carbs on our '75 CB400F.  My question is, how do I set the fuel/air mixture after I put it all together?  Do I set them at two full turns out and then not mess with them anymore?  That's what someone said but it just doesn't seem right!  I have vacuum guages on the way, and a carb  adjusing tool.  Maybe I'm just missing something.  I've rebuilt and tuned carbs before but not in sets.

Thanks,

Phil

[Jimray23]

If u havent removed them yet, turn them back in till barely seated and make sure u count the turns it takes to seat each of them. Then just put them back where they were.

--------------------------------------------------
Jim
1979 kz1000b ltd
    stage 3 cams
    pod air filters
    4-1 open exhaust(no muffler)
    full rewire
    dyna-s ignition
    dyna 3 ohm coils

[flybox1]

Set them to stock turns out as indicated in the honda manual.
Adjust as needed after you get the bike running to get the idle air fuel mixture correct
 

[TwoTired]

Use the stock settings in the shop manual, if you have the original exhaust and air filter design installed.

If you made exhaust and intake changes.  Then you have a much more interesting odyssey ahead of you.

You use the vacuum gauges (4) to balance the carbs for equal draw.  Main jet sets the W.O.T. mixture.  Slide needle position sets the midrange mixture.  And the Pilot screw sets the idle mixture, as well as affecting the throttle response from idle behavior.

[pburress]

I haven't changed the muffler but the PO did- he said it was original specs for what it's worth.  I have rebuild kits on the way from 4into1 and I'm hoping that the jets are the same!  The bike's unmolested but who knows if the jets have been messed with in a bike this old (1975). 

So i guess another question that may clear things up is, once I get the carb together, what external adjustments are there besides idle speed?  Is the top screw that I will be turning to synch the carbs the only air mixture screw?  If that's the case then my question is answered.

Thanks! 

[TwoTired]

If he changed the muffler to a type other than stock, the jets are simply wrong, because the back pressure is different.
Pods change the vacuum level in the carb throat. And that pressure determines how much fuel is drawn through the metering orifices.  Orifices need to change diameter to accommodate the different vacuum levels so they can deliver the same fuel quantity as before.

Syncing the carbs involves raising or lowering the slides so they all draw the same vacuum.  It is not a mixture screw.  On the side of the carb is a pilot or idle circuit air bleed adjuster.  This sets idle mixture, as well as how the engine responds to throttle twist.

I recommend you download the Honda shop manual.  It outlines what adjustments can be made to the carburetors.
 But, it does assume the bike is in stock configuration.

[flybox1]

If he changed the muffler to a type other than stock, the jets are simply wrong, because the back pressure is different.
Pods change the vacuum level in the carb throat. And that pressure determines how much fuel is drawn through the metering orifices.  Orifices need to change diameter to accommodate the different vacuum levels so they can deliver the same fuel quantity as before.

+1
Helps when the add all pertinents in the first message   

rebuild kits are only worth the soft parts that they come with.  the jets they come with area shiit.
buy OEM keihin jets from siriusconinc or jetsrus.com

[pburress]

thanks flybox1.   Is there way to tell the condition of the existing jets with a visual inspection?

[Jimray23]

Usually if their not clogged or beat up, their good

--------------------------------------------------
Jim
1979 kz1000b ltd
    stage 3 cams
    pod air filters
    4-1 open exhaust(no muffler)
    full rewire
    dyna-s ignition
    dyna 3 ohm coils

[Jimray23]

If the po changed the exhaust and tuned the carb for the exhaust, the jets will still be right unless u change something else. If the carb was good before cleaning, it still is. All u have to do, is make sure the floats and air screws are right.

--------------------------------------------------
Jim
1979 kz1000b ltd
    stage 3 cams
    pod air filters
    4-1 open exhaust(no muffler)
    full rewire
    dyna-s ignition
    dyna 3 ohm coils

[TwoTired]

Can you visually measure the difference between 0.016 inches and a hole drilled to 0.020 inch?

I can't. I use drill bit shanks to make such measurements. 

You can use your eyes to see if light can pass through them.

[flybox1]

thanks flybox1.   Is there way to tell the condition of the existing jets with a visual inspection?

OEM keihin jets will be stamped with a K and the jet size.
Many issues with aftermarket jets...thru-hole sizes, emulsion holes and overall lengths, etc... are inconsistent.
If yours are not stamped with the Keihin logo, know that you might run into the tuning issues many here have experienced.
No worries, though, you'll need to re-jet based on your intake and exhaust anyway 

[pburress]

thanks flybox1.   Is there way to tell the condition of the existing jets with a visual inspection?

OEM keihin jets will be stamped with a K and the jet size.
Many issues with aftermarket jets...thru-hole sizes, emulsion holes and overall lengths, etc... are inconsistent.
If yours are not stamped with the Keihin logo, know that you might run into the tuning issues many here have experienced.
No worries, though, you'll need to re-jet based on your intake and exhaust anyway 

If the exhaust was replaced with and "original specs" replacement as stated, why will I necessarily have to re-jet?   And why to do you say "based on your intake and exhaust" when the intake hasn't been mentioned? 

Thanks,  Phil

[flybox1]

Non-OEM exhaust will not have OEM back pressure. 
Don't believe everything your PO tells you. 

Pictures and description of your exhaust, please. 

...and what jet size/brand are in there now?

[Jimray23]

These carbs came from the factory with mikuni jets. Mine still have them, i pay double for them. But they are the only actual factory replacement.



--------------------------------------------------
Jim
1979 kz1000b ltd
    stage 3 cams
    pod air filters
    4-1 open exhaust(no muffler)
    full rewire
    dyna-s ignition
    dyna 3 ohm coils

[flybox1]

Keihin carbs with Mikuni jets?

[pburress]

Non-OEM exhaust will not have OEM back pressure. 
Don't believe everything your PO tells you. 

Pictures and description of your exhaust, please. 

...and what jet size/brand are in there now?

I haven't got the carbs off the bike yet, much less determined that I have any issues with jets at all.   I do appreciate the advice.  Once I get into this I will post questions if I think the jet may need to be changed.

My question, about the fuel/air mixture, has been answered, I think.  LOL

[flybox1]

Stock Keihin jets for the 400F are 40 pilots, and 75 mains.
IF, whatever exhaust your PO put on is close to stock back pressure, then jets should be as stated.

SOME 400f owners have attempted to tune out the common idle richness by going to 38 pilots, just so you are aware if those are found.

[pburress]

The rebuild kits' jets are 75 and 40.   Would I be able to eyeball the difference between the jet sizes on the bench if my current ones are different that stock?

Got the carbs out last night and got # 1 rebuilt.  I should be able to get the other three done tonight and put it back together.   Then we'll see if my low RPM bogging issue is solved... 

Thanks again for the advice.

[pburress]

I got the carbs rebuilt and got it back together.  Synced them with the new gauges and it turned out good.  Unfortunately it didn't solve my problem of it bogging while under 3k on the tach.  I will address that in another thread.

One final question:  On the needle you can change it's length depending on where you put the retaining clip.  Should I try changing this to see if it affects the low-rpm bogging issue?

[Deltarider]

Stock Keihin jets for the 400F are 40 pilots, and 75 mains.

P. 17 of the CB400 Parts List shows that Honda also listed #38 and #42 for the slow jet and #72 and 78 for the main jet. http://www.honda4fun.com/pdf_parts_list/pdf_spac400/CB400_2.pdf

[HondaMan]

The 1975 version of the 400F had #75 mains, the later bikes had #78 mains. The '75 version can benefit from the #78 mains, if the float levels are accurately set.

[pburress]

Thanks guys.  So if the exhaust breathes freer than stock, would I move up to a higher number on the jet sizes?

When I rebuilt the carbs made sure the floats were all right at 21-22mm so I think they are right.

[flybox1]

Thanks guys.  So if the exhaust breathes freer than stock, would I move up to a higher number on the jet sizes?
Yes, but some jetting changes might be less than you think.  Always start with IMS adjustments first, then pilot, then needle clip, then main jet.
When I rebuilt the carbs made sure the floats were all right at 21-22mm so I think they are right.  Dont you want to be SURE they are right?     The clear tube test takes out all the guess work

[pburress]

Thanks flybox1.  The manual says to set the IMS at 2 turns out plus or minus a half turn.  I checked at 1 1/2 and 2 1/2 with little to no change in my bogging issue.    So following your suggestion I would next try a larger pilot jet.  Whenever I find the time to get the carbs out and opened up again, I may have more questions!