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I bought my first bike, a 80 CB750K. I have not been able to ride it all summer due to charging problems which I have narrowed down to a shorted out rotor... Hopefully. Thing is electrosport wont have my part for more than 10 weeks from the date I ordered it. Also the bike has very little power. I rode it home 90 miles barely able to reach 60 MPH. It will only Tach up to 6000 RPM with no load. If I pull the plugs while running .. one at a time each one will kill the engine leading me to believe they are getting spark. The #1 and #2 cylinder on the header never get hot though. Why would both of these cylinder not be firing. Is my charging issue and spark issue related? Any help would be appreciated.
Thanks
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ive got an 81 CB750 custom with the exact same issues. ive cleaned and scrubbed and blown the carbs to death. the air filter is clean, the compression is 147, 146, 145, and 146 across the board. the guy i got the bike from said he had a problem with the charging thing too.. is this common on these bikes? but i have spark on every plug, they are all gapped right, but just like yuors, i cant get any real power out of it. it seems to fly up to RPM with no load, but under a load, its sluggish to say the least. i know you need RPMs with these bikes to go as is their nature, but right around 5-6k, the revs just fall of the face of the earth.. what gives? anyone? help us!
Signed-
lost boys in loathing...
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First check the compression to eliminate internal problems. Clean or replace plugs and gap them at this time.
Next make sure the wires are going to the proper cylinders. 1 and 4 go to the left coil and 2 and 3 go to the right coil.
If this is correct and all 4 plugs are sparking then while it is idling shoot some gas or starter fluid into one of the carbs on the affected cylinders and see if it starts firing on that cylinder. If so it's carbs or leaky boots.
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im certain you are describing most of those ideas to the primary poster. as i have checked all but the vaccume lines since im not so sure how to verify which is good or bad. lol.. assistance in this dance for instance? ive also cleaned the top vaccume actuated needle valves on the carbs via removing the top caps of the carbs.
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My 83 CB1000 is doing about the same thing and I'm confused. I just put new plugs and left coil pack in. That didn;t help. Let me ask you this (I want to help you and I want you to help me): if you rev it up in nuetral does it act just fine?
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No it does not act OK in neutral if thats what you are asking. Even with no load it will not tach above 6000 RPM and is very sluggish at low RPM. If you crack on the throttle hard it wants to stall out. Thing is if you pull each plug wire separately while running it will affect idle. Plugs are new. Wires are not visibly cracked. They are in the correct locations. I am guessing it is fuel delivery. When I pulled the 1 and 2 plugs they were wet and black. I have brand new intake boots. The other boots look fine. I am going to try the starting fluid later today and see what happens.
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my bike revs nice and high nice and fast, but under a load ( ie- starting in first gear) you have to be very gentle with the clutch and very heavy on the gas. i have noticed that the left side pipes dont ring once i reach a certain throttle/load threshold. that is to say, you can hear all pipes ringing with function until you reach a certain threshold on the gas, then the left pipes just seem to die. the exhaust gets cold out of that side of the pipes, but the right side stays hot as it exits the pipes.
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POE: my CB1000 is acting the same way. It idles fine, revs up fine in nuetral, and then when you put it in gear and head down the road mine loses all its balls around 3000 rpms.
Last weekend I replaced the left side coil pack (because I thought four wasn't firing), put in new plugs, and changed the oil. None of that helped; so don't waste your time thinking about doing that.
What repairs have you made? What do you suspect? We can figure this out togather and hopefully save a little money and time in the "trial and error dept."
Thanks, Dave
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i have rebuilt the carbs, the bowls, the jets, the top vaccume diaphrams and needle valves have all been cleaned. the vaccume lines look a lil dry, but looking on the inside of them, they all look like shiney good functioning rubber. between the fights with my wife and trying to run down a (brand new problem) POURING leak of gas with my other bike and a massive oil leak in one day (lol damn the luck!), i will test the spark at high rpms from the left coil pack. otherwise i might try the ignition module. thank god for free spare parts bikes. i have a 79 cb750 mostly complete. i think i till star poppin and swappin if the spark runs away at high idle.. i have to check that too.. listen closely to your bike as you slowly turn up the revs.. does one side or the other seem to sound less "poppy"? also you can put your hand behind the mufflers to see if one side starts blowing cooler at high rpms ( not suggested while actually driving!) lol..
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i will say that these bikes were disigned without the aftermarket stuff, so i put very little faith in the potential that thing would help us.. but way to look for answers bro!
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as a matter of fact i do.. i have both of them..
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If you have time, switch out your CDI units and let me know what happenes. My friend had a similar problem and he said a new battery fixed his problem. ...that just doesn't make a lot of sense to me.
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neither of you have charging problems like me? Could this be related to my other problem?
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mine also has (had- i sold the bike last night) the charging problem as well.. it seems to be a fairly recurring problem with these bikes. the stator is easy to get to, 3 bolts on the right hand side cover.. only problem is the lead time on the part.. up to 10 months from some companies. but make sure you buy it new or from someone trustworthy. its hella easy to replace it, i dont know how expensive. but www.bikebandit.com is the cheapest place ive found to get the majority of my parts for my bikes.. good luck man!
.. it seems that our firing issue is also fairly recurring with these bikes. 9/10 times its a vaccume problem as there is a rail on the bike side of the carbs that interconnects the carbs. make sure that all lines are in good shape and connected. there are a few postings on this forum with similar problems with the same bikes.
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Let me share my experience. It might not be the carbs. I've had a very similar, if not the same problem.
Does the battery charge? If ti doesn't that it might be electrical parts, such as CDI, recitifier, etc...
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Have you rebuilt your carbs yet? If you have not, that would be the first thing I would check into. From my experience: 9 out of 10 times its the carbs. Hope this helps< Dave
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Well I am going to wait until I receive the rotor from electrosport. I dont want to try to mess with the carburation to find out the lack of power from the battery was causing my firing issue. I hope this part comes in soon, so I can move on to the next problem.
Thanks,
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Hey, guys!
I'm having the same difficulties with my Super Sport and was both shocked and relieved to find this thread come up as the first hit from my obscure search of "1980 CB750F cylinders 3 and 4 cold"!
I know it's been a while, but did you ever happen to come to a solution? I'm at my wits end...
Token
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