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Hello to all, first off I have a 1980 cb 750c with uni air pods and a mac 4 into 1 exhaust. I was wondering if anyone knew what the size of the main jet should be. I had a 102 and bumped up to a 108. The problem is that when im riding on my tach it stops at around 6,000. Any help would greatly be appricated, thanks in advance!
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it does not accelerate beyond 6000? Or when in nuetral it won't rev beyond 6000? Also the main jet from 102 to 108 seems like a big jump. How was it running with the 102? Does it blow smoke and when you pull a plug is it black. Most likely you are too rich that is if it really related to your jetting and not something else.
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When i accelerate it wont go beond 6,000 and then i obvisouly have to shift, ill check on the black smoke and the plugs i didnt check that yet, but as far as i know it doesnt blow black smoke. i was also thinking that it could be the rubber boots, they may be dry rotted or cracked, i cant see anything with the naked eye but that doesnt mean anything. thanks
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how did it run before the uni filters and jet work? Did you get adjustable needles? See if it won't accelerater you are either too rich or too lean, but since you increased the mains I would guess its too rich. Try dropping to a 105 or whatever is available that is close to that size. If it improves your problem but doesn't eliminate you can keep going down mains until you find the one that gives you the best top end. Once your main seems to work right time to work on the secondary mains if you have them, then needles, then slow speed/ idle mix. The correct way to rejet is to work your way down from the mains until your bike runs well everywhere. With out the correct testing equip (dyno, and gastesters) its all based on trial and error so be prepared to take the carbs off alot. Its pointless to mess with needles and anything below mains until your top end is right. As far as cracked boots its more likely you would notice an issue like that in you low end since it would reduce vaccum the bike would want to die or stumble at idle.
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It didnt have any air filters on it at first, and the revs were actually less than they are now. thats why i was thinking that i was in the right direction. The secondary jets are whatever stock is, i cant remember off the top of my head right now. The mixture screws are set at what the manual says its suppose to be a turn and a half out.
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I should have asked about the filters being on. I made this mistake a while back. These carbs don't work right with out the vacuum from the filters. So how does it run now? Rev smoothly?
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It revs pretty smoothly, it sometimes will rev up and then take a while to come back down to idle, but doesnt happen all the time. Would it be a problem that the crank case breathers are just sitting on the top of the trans with no filters? seeing how there isnt a air box anymore.
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that wouldn't make the revs stay up though as far as I know. One likely cause is float height is out of adjustment. Some of the cb's had not adjustable plastic floats and some have a metal and plastic float that is adjustable. The adjustable ones need to be set right. If they are too high, or even if one carb float is too high it will continue to pull gas in even after the butterflies are closed causing a delayed return to idle. Also when you say delay how long does it take? I have noticed my cb does not return to idle in what I would say is fast but I wouldn't say its slow either.
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i havent rode it in a while seeing how im repainting my tank and doin an oil change and doin some carb work. but thte weird thing is that when the bike is cold i tach it up and it drops back down but if i ride it for a while and it warms up on a warmer day it will take a little bit for the revs to come back down. From taking the carbs apart i put them on the bench and it seems like all the floats are sitting at the same height weather its the right height or not i dont know. all i know is that i rebuilt the carbs with the rebuild kit from part unlimited and it had the little piece that the float. so i have not a clue what the problem is.....
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well if the floats are all plastic, and there is no metal on them at all they aren't adjustable and your float height is fine. If there is a metal tab the pushes on the needle then they are adjustable and you should probably check them. This could also be a balance issue, you may need to have the carbs balanced. Also check to make sure your idle screws are 1 and 1/2 turns out from lightly seated all the way in. These all affect slight tuning they do not have much affect on the mid range to top end. Also because of your mods you may need the mixture screws 2 turns out but I would start with the 1 1/2.
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yeah as far as i know they are non adjustable, when i open them back up i will look and see i dont remmeber. i have the mixture screws out 1 and a half turns, i will turn them out another half of turn and see what happens. thanks for the help i appricate it!
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I have the same problem with my CB750KZ, after removing the carbs and cleaning all the crap out of the float chamber. Since replacing the carbs, it revs up to the redline with no problems at all but coming off the throttle it seems as if the butterly`s are not shutting immediately. Its annoying to say the least, but as these carbs on the DOHC model are a swine to get off and back on , I`m prepared to live with it, as once its running it just fine. Have just learned to shut the throttles off a little sooner then before.
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I have the same problem with my CB750KZ, after removing the carbs and cleaning all the crap out of the float chamber. Since replacing the carbs, it revs up to the redline with no problems at all but coming off the throttle it seems as if the butterly`s are not shutting immediately. Its annoying to say the least, but as these carbs on the DOHC model are a swine to get off and back on , I`m prepared to live with it, as once its running it just fine. Have just learned to shut the throttles off a little sooner then before.
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Hi TS750c,
Well i have some good news and some bad news for you.The good is i am having the exact same problem with my 81 CB750K uni filters minus the 4 into 1.The bad is i haven't figured it out yet but one thing i am sure of is that it is not a jetting problem,more like it is a carb trait that can only be corrected by reinstalling the stock air box.Too bad i don't have the stock air box or i would try it.I've done the same things you have with the same results.I've even removed the slide springs against my better judgement and nothing changed.I'm thinking of trying stiffer slide springs thinking that with more airflow that the uni filters provide that the slide is opening too soon and causing a lean condition.If i come up with something i'll post it here.
Hopefully i'll talk to you soon!
liljohnnie69 
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Save a lot of headache, and find an airbox....Or Change carbs....These carbs need a lot of vaccum to work right, and the pods kill em.
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I fine that 2 ~B~ TRUE, the stock airbox works best. I have 78~K8 & 78~F3,I try rejetting.   There is a way to make them work! Just havent found it yet  RIDE SAFE  LEERIDER
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THey work great on a non-vacuum-slide carb...
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My bike is stock standard just as it was when I bought it new in 1982 from my Honda dealer. So everything the factory put on my bike is still there. Nothing has ever been removed or modified, so even with the factory fitted airbox, the throttles are a little on the slow side when closing the throttle. It doesnt throttle down as quickly as it used to, and I dont seem to be able to find the cause. Have just learned to live with it in the hope it will sort itself out one day
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I'm also having this problem. When I first rescued this (honda cb750c, 1980) bike it had set for 4 years in a backyard covered. In the begining I could only run hotter NGK D7EA spark plugs now. after six months and cleaning the carbs serveral times tighting up the timing I can now run the regular D8EA plugs. I have the mix down to a half turn. and still my idle is sluggish comming down and I'm still a little on the lean but not bad.... I have noticed when I come to a stop if I rev up and let off it drops quickly. So I agree that newer/stiffer slide springs might correct this problem ... Looking into finding newer springs today ... I will get back with my results...
Thanks this sight has helped a lot....
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I had numerous problems with my carbs when I first bought the bike because of rust and paint in the gas tank. I learned a lot about those carbs overhauling them and cleaning them numerous times. I believe that assuming your throttle cable linkage and return spring are all functioning and the bike takes time to wind down the rpm's then it is very likely to be the vacuum piston, chamber, needle etc. Those must all be spotlessly clean in order for the piston to travel up and down as it controls your acceleration and deceleration. I suggest using using a carb cleaner, air compressor and clean lint free rags to clean your carbs...especially the vacuum piston side. Make sure you are using good filter(s) out of the tank to the carbs. I actually have 2 in line, one being a paper filter. I also took off the vacuum hoses that control the throttle response and blocked the one vacuum port. You will fine that most remove all that crap and the carbs run better.
Those cheap pods came with my bike and everyone told me to get rid of them and get an airbox. I actually bought an airbox but haven't put it on yet due to the fact the bike (1979-CB750F) is running great. The former owner put in 105 jets when he did the pods, although I'm not convinced that was necessary. I used the K&N cleaner/oil to clean the pod filters. After getting the gas tank clean, good filters in line, and the carbs spotless clean and functioning smoothly the bike ran great. Of course I also did all the valves set to .005, replaced the rubber boots holding the carbs on. and did carb synchronization with the vacuum guages. I've had all the problems discussed in this thread...and have fixed them all.
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