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  1. #1
    CB750 Member tdubsy's Avatar
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    Just rebuilt my carbs still a few issues looking for some advice 1978 k8

    Hey Everyone,

    I just rebuilt the carbs on my 1978 K8.

    I had an issue with a vacuum leak before so while changing the boots I used the opportunity to buy rebuild kits for the carburetors.

    -replaced all the jets apart from the slow jet (as a new one wasn't included in the kit), Gaskets, O-rings, Floats, float valves, and 3.5/4.5mm lines between carbs.
    - confirmed same sizes of replacement jets also match oem spec for stock airbox
    - Did not change to the new needles that were in the kit
    -Checked the float height and adjusted all to within .5mm
    -Bench synced carbs

    So far the bike seems to run a touch better as there is no obvious vacuum leak and my RPM is constant.

    BUT I still cant get the bike to idle under 3000 rpm when warm. Also now choke has to be on to start when warm as well.

    Am I correct in thinking the bike is too lean? had set the pilot screw 1.5 turns out as per spec. Would turning it out a bit help me or should I try swapping the needles?

    Thanks!!!

  2. #2
    CB750 Member Mars's Avatar
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    Quote Originally Posted by tdubsy View Post
    Hey Everyone,

    I just rebuilt the carbs on my 1978 K8.

    I had an issue with a vacuum leak before so while changing the boots I used the opportunity to buy rebuild kits for the carburetors.

    -replaced all the jets apart from the slow jet (as a new one wasn't included in the kit), Gaskets, O-rings, Floats, float valves, and 3.5/4.5mm lines between carbs.
    - confirmed same sizes of replacement jets also match oem spec for stock airbox
    - Did not change to the new needles that were in the kit
    -Checked the float height and adjusted all to within .5mm
    -Bench synced carbs

    So far the bike seems to run a touch better as there is no obvious vacuum leak and my RPM is constant.

    BUT I still cant get the bike to idle under 3000 rpm when warm. Also now choke has to be on to start when warm as well.

    Am I correct in thinking the bike is too lean? had set the pilot screw 1.5 turns out as per spec. Would turning it out a bit help me or should I try swapping the needles?

    Thanks!!!
    Was this bike running before? Was it sitting a long time?

    There are more experienced people who might be able to help you, but I just had this issue with a 1979 (so maybe not exactly same carbs?). I got my bike firing but if I let the throttle down below about 3K it would die.

    My issue, and it sounds to me like your issue, was blockage of the idle circuit. My Clymer manual didn't mention cleaning this at all, but my bike sat in a shed for 20 years so it wasn't just routine maintenance to fix. To clean the idle circuit I blew carb cleaned through every little hole I could and then used compressed air as well. I mostly did through where the idle mixture screw goes after taking it out. To force air through the whole idle circuit I used a finger to block the little hole on the outside of the throttle plate while blowing air/cleaner. When all carbs seemed to flow the same with carb cleaner spurting out of each hole similarly I figured it was good I put everything back together and it worked.

  3. #3
    CB750 Member tdubsy's Avatar
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    The bike was running before but also had problems getting an idle RPM under 2500. I made sure to give the idle circuit a blast with carb cleaner and air when I took them apart. and I soaked everything in pinesol for a few hours as well.

    Before I could get the bike down to 1500 RPM but it would have no power to pull off so I had to keep it around 2500. Now as soon as I adjust the idle screw to take it down below 2700 the rpms just start to drop and the bike dies.

  4. #4
    CB750 Member tdubsy's Avatar
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    OK so new developments.

    When I came down to the bike this morning I noticed that the carbs drains had leaked a decent chunk of gas over the floor- No signs of gas on the gearbox so its all coming out of the drain. Does anyone know what would cause this?
    Also the bottom of my airbox was wet with gas? Is gas somehow flowing backwards into airbox and leaking out the bottom?

    I brought the mixture screws out a quarter turn, I can now Idle at 1300rpm but bike bogs down when given throttle. So the useable idle is now currently down to 2200 RPM
    I also realized that cylinder 4 isn't firing since rebuilding the carbs. Once I get this firing I feel that things will start to look a little better.

    What I consider a useable Idle is when the RPM is stable and I am able to roll forward on the bike using only the clutch on flat ground without using the throttle.

    Current issues I am facing
    - Leaking gas out of drain hoses when off
    - New carb boots seem slightly longer causing the airbox to be pushed back just far enough to not allow mounting holes on frame to line up
    - Cylinder 4 not firing Cold exhaust
    - Cylinders #2 and #3 exhaust temp seem similar while #1 is noticeably hotter
    - Getting a useable Idle below 1500 RPM
    - Bike has trouble starting from electric start while I can easily kick start. (this is a new issue)

    If anyone has any Ideas let me know what you think

  5. #5
    CB750 Enthusiast mrtelec's Avatar
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    when you cleaned carbs did you check to see if the accelerator pump was squirting gas into the carbs through the brass nozzles in the intake? did you take the idle jets out and clean them with a .010" cleaner ( a high E guitar string works great for this) ? and did you do the clear tube test on your carbs to see where the floats are set to? the overflowing is probably a float adjustment, the cold cylinder could also be a stuck float needle. put a charger on the battery and see if the starter works better on a fully charged battery.

  6. #6
    CB750 Member tdubsy's Avatar
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    Quote Originally Posted by mrtelec View Post
    when you cleaned carbs did you check to see if the accelerator pump was squirting gas into the carbs through the brass nozzles in the intake? did you take the idle jets out and clean them with a .010" cleaner ( a high E guitar string works great for this) ? and did you do the clear tube test on your carbs to see where the floats are set to? the overflowing is probably a float adjustment, the cold cylinder could also be a stuck float needle. put a charger on the battery and see if the starter works better on a fully charged battery.

    Yes pump was squirting and I replaced the Idle jets with new ones
    I will try the clear tube test although I manually set the floats with a calliper when I had them apart.
    I don't think it is the battery I just tried starting the bike and it was impossible with the electronic starter and with the kick start it took me a good 10 minutes to get going - Before I could get it on the 1st or 2nd kick from completely cold

  7. #7
    CB750 Addict Dave's Avatar
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    Quote Originally Posted by mrtelec View Post
    did you take the idle jets out and clean them with a .010" cleaner ( a high E guitar string works great for this) ?
    I disagree with you on this. Never stick anything in a jet. The slightest damage to the venturi edge will make it stop functioning correctly. Visually inspect them with a magnifying glass. if plugged soak in carb cleaner (the real stuff that comes in a can with a parts basket inside) and blow out with air. If you can't unplug it that way, replace it.
    Anyways, it sounds like the idle circuit to me as well. Did you save and install the restrictors into the new lines between carbs? (you can find posts here on making new ones)

    Gas in the airbox is a sure sign of a float valve not closing. Test the float valves too. Once I had a brand new float needle with a small burr on the edge of it that prevented it from centering in the seat hole, which ofc prevented it from stopping the flow of gas. I filed the small burr off and now the valve works fine.

    Clean the parts however you do it, put the needle in the seat, put the seat to your lips and blow. Can you blow any air through it? (see the cleaning part 1st LOL)

    Study the carbs and figure out the entire idle circuit. You should be able to blow air into one place and feel it coming out the other. (assuming you have a compressor and rubber tipped air nozzle)
    96% of the time I edit my replies so check back for updates if it isn't at least an hour old

  8. #8
    CB750 Member tdubsy's Avatar
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    I was able to blow spray carb cleaner and compressed air through everything. I didn't notice any clogs in the idle circuit or anywhere else. I had all cylinder firing before I rebuilt the carbs so I will check the floats first.

  9. #9
    CB750 Enthusiast mrtelec's Avatar
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    "I disagree with you on this. Never stick anything in a jet. The slightest damage to the venturi edge will make it stop functioning correctly. Visually inspect them with a magnifying glass. if plugged soak in carb cleaner (the real stuff that comes in a can with a parts basket inside) and blow out with air. If you can't unplug it that way, replace it. "

    I soaked my carbs jets and housings in the gallon can real carb cleaner w/ basket for a good soak (overnight) then used e string on low idle jets and one of the accelerator pump jets in one of the carbs. (had to take it out and press it back in) Bike runs great, so I'll agree to disagree with you on this one :-) . the e string is the proper size for the 78 carbs.

  10. #10
    CB750 Enthusiast mrtelec's Avatar
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    getting the floats and needle seat working first, then recheck idle jets by pulling them out and seeing if all your holes are clear. ( I shot carb cleaner through jet to see if it was actually getting through while jet was in my hand) then a good synchronizing should do wonders. what exhaust are you running? if it has a 4 into 1 you may want to adjust the mixture screws out a bit.

    I found the clear tube method helped me with the floats as they will show you what's going on with your carbs, I set my floats up properly but had a sticky needle on one and a hairline crack on one of the bowls from someone tightening the drain screw a half turn past breaking which had that overflow dripping one drop every four seconds.

  11. #11
    CB750 Member tdubsy's Avatar
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    Quote Originally Posted by mrtelec View Post
    getting the floats and needle seat working first, then recheck idle jets by pulling them out and seeing if all your holes are clear. ( I shot carb cleaner through jet to see if it was actually getting through while jet was in my hand) then a good synchronizing should do wonders. what exhaust are you running? if it has a 4 into 1 you may want to adjust the mixture screws out a bit.

    I found the clear tube method helped me with the floats as they will show you what's going on with your carbs, I set my floats up properly but had a sticky needle on one and a hairline crack on one of the bowls from someone tightening the drain screw a half turn past breaking which had that overflow dripping one drop every four seconds.


    I was doing the same thing with the carb cleaner as well when I rebuilt them. Im using the stock 4 to 4 exhaust.

    Just checked the floats with the clear tube method. Carb 4 was absolutely empty
    1 and 2 were perfect 3 seemed empty as well strange considering I wasn't having an issue.

    Could 3 and 4 be sending their fuel over to 1 and 2 considering I'm on the side stand leaning the bike over to the left.

    Im going to look over everything again this weekend. Is there anything I should be looking for that could cause the bowls the be empty?

    Thanks

  12. #12
    CB750 Enthusiast mrtelec's Avatar
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    I did mine either on the center stand or a jig I made for the bench that allows me `to flip them to any angle to set them while being level I used a temporary gas supply from a old gas leaf blower fuel tank. If they are tilted I would think they would give you a bit of a skewed reading from the gas going to one side of the carb it would push that side float up moreso. It looks like you have a stuck needle or float. did you try tapping the side of carb #3 and 4 with the handle of a screw driver very lightly to see if it unsticks. you may want to change number 4 needle with one of the new ones you got in the kit. When doing that check the seat to make sure its nice and clean too.

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