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  1. #1
    CB750 Member Red_Teisco's Avatar
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    Could high idle after warming up be caused by worn jet or improperly adjusted float?

    I have a 1981 cb750 with about 18,000 miles on it. I've had this bike for a few months now and I've been trying to crack down on a high idle issue for a while and I can't seem to figure it out. I've replaced the boots from the airbox to the carb and from the carbs to the engine and I've sprayed starting fluid all around while it's running and didn't hear even the slightest increase in RPM. The bike idles at 1100 RPM cold even when I let it idle for a little bit to warm up.

    But as soon as it gets down the road it wants to stay around 4000 - 3000 RPM. If I shut it off it then start it back up it down might go to 2500 RPM. Could it be that they're out of sync? Could it be the floats or jets? Why is it only when the bike gets going down the road? I've exhausted my knowledge.

  2. #2
    CB750 Guru amc49's Avatar
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    No to both questions.

    If you have the idle speed sped up to hold engine at a stone cold idle with the throttle off, that is wrong, you set idle speed for hot engine only and suffer the cold result. A cold idle setting will be way too high at hot idle then it races up. Normal on any engine on the planet. Keeping engine running while cold is a biker learned art.

  3. #3
    CB750 Member Red_Teisco's Avatar
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    So I should try backing off the idle? Maybe check to see if the throttle cables have enough slack?
    And I thought it could be the float because of the cylinder 1 carb that is leaking gas

  4. #4
    CB750 Member Red_Teisco's Avatar
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    but now that I think about, the leak and my issue likely have no correlation.

  5. #5
    CB750 Guru amc49's Avatar
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    See below, the machine double-posted on an edit..........

  6. #6
    CB750 Guru amc49's Avatar
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    Well you need to fix the flood leak, ethanol likely causing it if they use it in your local fuel and common.

    Adjust the idle speed with the engine hot and then do what it takes to keep engine running when cold, it is a rider skill. The choke linkage has a fast idle feature that can help with that, I for one always used a throttle lock mechanism like a Vista-Cruise cruise control, it makes it super easy to crank cold engine up, set the cold idle with the throttle and then lock it with the Vista and adjust as needed and then flip it off when engine is warm. You basically have two setups to set idle speed then, it saves burning fingers under carbs.

    https://www.chapmoto.com/vista-cruis...iABEgIdTPD_BwE

    Best add-on ever invented.

  7. #7
    CB750 Member Red_Teisco's Avatar
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    Ok, thank you for the advice. I will post an update in about a week.

  8. #8
    CB750 Member Red_Teisco's Avatar
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    The bike ran well before I replaced all the boots. These boots were dry rotted. The airbox to carb boots were worse than the carb to engine boots. Could it have been that the bike was adjusted by the previous owner to run with these air leaks? I'm just throwing other things out there as food for thought.

  9. #9
    CB750 Guru amc49's Avatar
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    '...the bike was adjusted by the previous owner to run with these air leaks...'

    It could only run worse. The more the air intake leaks back of carbs the less the slides open up and that is a sure performance killer. Many of them do not run right with owners thinking they do. Much of it begins with lower compression because nobody ever sets the valves and then they burn to lower power, yet people still think they are running right. You lose a solid 10-15% power before the engine ever begins to act up in a way that tells you something is absolutely wrong with it. By the time the engine actually changes note to sound like something is wrong; well, you lost the edge long before that.

    The airbox shares the vacuum induction pulses to lower pressure in the box, the slides then lift more and bike runs better. Leaks drop that and then it doesn't run as well, why pods on them often mess things up even worse.
    Last edited by amc49; 11-03-2019 at 10:58 PM.

  10. #10
    CB750 Member Red_Teisco's Avatar
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    Should I look into adjusting the valves after this carb issue has been settled? Winter is coming and I'll be snowed out here in about a month so I'll have plenty of time. I have a service manual for this bike. I'm much more of a mechanic when it comes to cars bc there are things like alldata and shopkey. It would be nice if such a thing existed for motorcycles. "Professionals" in my area are limited when it comes to motorcycles so there's no one to get good advice from

  11. #11
    CB750 Guru amc49's Avatar
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    Man, in my book it matters not if it is a bike or car or battleship or atomic reactor. If one has the knack for mechanical then jump into it, just jump advised. You can think and nobody else has that over you, the most complicated devices on the planet were made by ordinary guys like you and me. Past that, the big difference is reading, the big limitation to most people getting ahead.

    On the valves, set them at .005" instead of the book spec of .003", use .004"-.006" as the running range and looking for .005". The setting job then lasts much longer. The valves can and will burn at .002" which is one side of the Honda range, the .002" is not real, the cams move around too much and a reading of .002" is effectively zero. The valves on these tend to close up toward zero because they were heat treated for low lead fuel and no lead at all lets them recede quickly. Especially the exhausts.

    FYI, I have kept 3 and usually 4 cars running at all times for over 45+ years and never paid ever for any car repairs. So far anyway. I eat OEM service manuals for lunch to do it though. No formal training in any of it. Just an urge to not spend megabucks.

  12. #12
    CB750 Member Red_Teisco's Avatar
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    I sympathize with that urge. Cars are one thing. Motorcycle maintenance, however, feels like a lost art almost. Why else would a website dedicated to asking and answering questions on how to fix just one model of bike even exist if not for this lack of available knowledge?

    I will take a feeler gauge and my manual to the valves once the carb does its job properly and if I can get the valve cover off. I tried to take it off once just to have a look and it didn't want to come off at all so I let it be before I damaged it.

  13. #13
    CB750 Guru amc49's Avatar
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    If the valve cover has never been off they can seem to be glued on.

    Go here and register for free and then go to the technical documents section to get the OEM service manual in sections.......

    http://www.cb750c.com/modules.php?na...iewtopic&t=301

  14. #14
    CB750 Member Red_Teisco's Avatar
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    Way ahead of you bud. Printed this out a month ago lol. This site had the PDF for the manual posted. The manual now sits on the seat of the bike.

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