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  1. #1
    CB750 Enthusiast Chuck Hahn's Avatar
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    No top end power

    Bikes a 79 CB 750K. I rebuilt the carbs with all new accelerator pump, new rubber plugs over the one jet ( dont know its proper name) thats long and comes to a rounded end and sticks into the bore of the carbs, dipped the bodies in Berrymans for 24 hours. Ran a few cans of carb spray into all the openings and lots of compressor air as well Took a fine wire and poked out all the holes in all the jets.

    So heres my problems. I set the float hts at the 15.5MM that manual calls for but 3 and 4 will still start dripping out then overflows. Yes, they are new rubber tipped float needles as well. Whats the HIGHEST I can set a float at and have it not be lean due to fuel being too low in the bowls???

    Second, it wont idle down like a hanging idle if theres air leaks at the boots but some starter fluid sprayed on them doesnt change the RPMs so I think its not air leaks I got a Carbtune pro and then sync is very very good...all 4 almost exactly the same. I put new NGK plugs in and after it ran a few minutes i took them out. 1 and 2 looked OK, but 3 and 4 had white ashy look on the bent electrodes. Manual says a possible cause is clogged jets so I yanked the carbs yet again and nothing was clogged in any of the jets. Seems wierd that all 4 bodies got the same dip time and cleaning procedures and two are showing fine plug color and two are ashy white...what gives??

    And third is the timing. Looking at the marks on the advancer there are 2 marks close together with an "S" to the left of them Then the "F" just to the right of the 2 close together marks. If I am understanding the manual correctly, when you put the timing light on the indicator at the top of the cases falls between the 2 close together lines?? Can someone further explain where the indicator falls because the online manual i have is kinda hard to see clearly exactly where I am supposed to get the point of the indicator to point at.

  2. #2
    CB750 Enthusiast Chuck Hahn's Avatar
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    Think maybe i should redip carbs 3 and 4?? Anything i can use that stronger than Berrymans carb and parts dip??? Just in case theres something super hard that Berrymans hadnt been able to remove in the original 24 hr dipping??? By the way the bike had sat for 14 years and has 39,519 miles on the speedo.

  3. #3
    CB750 Guru amc49's Avatar
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    Compression test to make sure you don't do exactly like everybody else and spend money on a dead engine. Nobody, and I mean NOBODY ever sets valves on them and lack of top end is the first sign of it, the engine refuses to rev out fully. The valves burn super easy on them and nobody ever figures it out, then they work on carbs until it drives them crazy and still no result. It's some kind of male macho testosterone based stupidity thing...........

    If you did not break the carbs apart and off the rack you have ruined some parts inside that absolutely do not tolerate carb cleaner in them.

    Set the floats correctly, all the rigs in the world won't fix pitted needle seats.

    You time exactly on the F mark (firing). 2 places, 1.4 and 2.3.

  4. #4
    CB750 Enthusiast Chuck Hahn's Avatar
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    Unlike some, I already have done the valves first off. i will get out my compression checker though. I fully broke down the carbs....this isnt my first time. My first time with a CB750 yes..but FAR FAR FAR from my first set iof carbs I have ver rebuilt.

    As for the "F" mark..my question is just EXACTLY which one of the SEVERAL lines on the advancer is the correct one to line up to. I am thin king its the RIGHT line of the double set. You know, the two lines with the S on the left and the F on the right. Im thinking the right one is F. The manuals picture isnt very clear as to what is the EXACT line to use. This is the manual from dotheton.......

    http://www.dotheton.com/forum/index.php?topic=44399.0

  5. #5
    CB750 Enthusiast Chuck Hahn's Avatar
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    Fixed. Had nothing to do with carbs or compression. I swapped in a known good set from a 78 KZ650 and problem solved. I took the timing light and just held the trigger as i went from wire to wire and found a lot of intermittent flashing going on. So i swapped coils and it all went away.

  6. #6
    CB750 Addict Dave's Avatar
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    The real F mark

    In the following photos you can see the two marks above the F. The right one is the mark you align the pointer on the case to as the second (B/W) photo from the 79-83 service manual shows.

    Click image for larger version. 

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    96% of the time I edit my replies so check back for updates if it isn't at least an hour old

  7. #7
    CB750 Enthusiast Chuck Hahn's Avatar
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    cool. The F is a lot farther to the right of those two double lines on my advancer. Certainy not directly underneath the line by any stretch on mine.

  8. #8
    CB750 Guru amc49's Avatar
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    Two sets of double lines, you put the mark in the middle of the pair you are using, the one set way to the left is for 6000+ rpm and full advance. You set the one and run the rpm up to check the other, it being right if the initial is right and the centrifugal advancer working right. The F mark is correct but the engines lack compression even when new and you can easily bump the timing up a couple extra to make it run a bit better. I routinely bump it a bit on a bike with a header and other mods and they love it.

    Swapping coils fixed flooding carbs and a hanging idle???

    I'd be checking the ohm of those strange to the bike coils, they need to be at least 3 ohm resistance as the Honda ignitors will quickly fail if they are less. Thinking that KZ used points instead of electronic pickups to trigger. A known weak point with the ignition modules.
    Last edited by amc49; 02-04-2018 at 10:33 PM.

  9. #9
    CB750 Enthusiast Chuck Hahn's Avatar
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    The hanging idle was from when the coils reignited. The engine stills sucks gas into the jugs so nit has to be burnt off..right!! The dripping carb turned out to be a hairline crack in the overflow tube. Think i posted a wanted add for a 3/4 bowl somewhere. Compressions ere from 135 to 155 on a stone cold engine. carbs were on the bench so they ere sucking as much air as is possible when testing.

    I have read all the threads and your "standard" answer of compression may hold true..but NOT ALWAYS!!!!

  10. #10
    CB750 Guru amc49's Avatar
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    Well of course I can be wrong but I have to play the numbers. I used to buy and sell early DOHC based on that alone.

    Coils 'reigniting'? You can't do that to have idle hang, something else was wrong there. The coils 'reignite' in normal use anyway and you don't see any of that then do you? And by changing them you are saying they weren't igniting to begin with. Improper ignition LOWERS rpm, it cannot increase from that. How every car crank sensor on the planet tells a PCM that the engine is missing, the crank slows down in between ignition pulses.

    The aircuts can easily cause a hang if leaking.

    'The engine stills sucks gas into the jugs so nit has to be burnt off..right!!'

    Uh, no, the plugs not properly firing hot enough then foul out. The unburned mixture wets them and passes out into the exhaust pipe still unburned to then ruin the cat if you have a car. These DOHC run fine with a weak OEM electronic ignition but get the fuel off a bit to the rich side or drop the compression and they foul easy as spit. All plugs in normal use need to be fired and heated on 100% to stay running clean, once the temp falls off they will foul quickly to miss then cut out altogether.

    No harassment intended at all but the physics of it are what they are.
    Last edited by amc49; 02-05-2018 at 12:52 PM.

  11. #11
    CB750 Enthusiast Chuck Hahn's Avatar
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    What I know is this..

    It wouldn't rev over 4500 RPMs and was surging and trying to catch after any throttle after that.
    The bowl was just dripping pout the overflow tube.... not a steady stream like a fuel level too high situiation.. this was the cracked overflow tube.
    When I took the timing light and moved from wire to wire 2 and 3 would flash then cut out for 2 or 3 seconds and flash again and the engine acted up.
    The idle would come down but not below around 3,000

    I swapped out the coils and it revs great, the idle falls back to 1200 where I like it, and its all good now. This is what happened plain and simple. I don't know why but it is what it is..done.

  12. #12
    CB750 Guru amc49's Avatar
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    Sorry I peeved you...........the lower you can get the idle toward 1000 the less chance of idle hanging, they tend to try to do it with higher idle speeds, a CV carb thing. It can of course be hard to get that low with an older engine.

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