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'82 CB750SC / Difficult Starting & rough idle

NoIdeaWhatImDoing

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Hello All,

...Its important to state for the record that i am not an experienced motorcycle mechanic...

82 cb750SC NH. Got the bike free from a friend. It sat in his garage for a year-ish. He cleaned up the carbs but didn't rebuild them. When I hauled it to my house, it wasn't running, carbs were off, but bike in reasonably good shape for its age and recent known neglect. Gas tank was too far gone (rusted to holy hell), and unfortunately was discarded.

Known issues and my attempts to correct; (condensed cliffnote version)
-Bought another OEM '82 750SC NH gas tank on ebay. Removed rust from inside. Stripped old paint, which revealed rust pitting in the lower fronts and rear of tank. Still working to resolve. Tank is currently at a shop and i'm awaiting their prognosis

-Oil change
-new plugs
-New battery
- Cleaned carbs thoroughly and installed on bike. Gas leaking out of several places. Brought carbs to local Honda moto dealer to have rebuilt. Got them back, still leaking out of #3
bowl. Took apart and found dried crust (??). Scraped out and cleaned. Re-installed. No more leak. Still not at all sure about correct vacc tube connections.
-Eventually got it started, and was rideable. Sounded okay, although I have little mental basis of comparison. Starting the bike was and remains difficult. I mean cranking and cranking and cranking and resting resting and resting then cranking again for a couple minutes. With varying degrees of choke. Have to stay close to the throttle for the first few mins to regulate the idle. Once warmed up it stabilized and idled on its own. Now there were days when it would start and quickly stabilize on its own and seem ok, and other days it just wouldn't cooperate, not start, or run very rough and stall. Keying engine off, then restarting again soon after or while still at temp, it starts right up. But only at OT. Cold starting is always very ugly. Usually a day or two would elapse between starting.

My outlook going forward;

The bike has now been sitting about two months with gas in the carbs. I dont have much faith in the shop that rebuilt the carbs. I don't yet have a permanent fuel tank solution. In
a perfect world Id have the space and the time to methodically disassemble, check, clean, replace every last piece of the bike. It would take me over a year to do that, especially since id be learning while doing. I want the bike ready for this coming season (Im in New York). So i'm considering bringing it to a shop...against every fiber of my being. Ask them to fix all issues mentioned above, and give the rest of the old gal a once-over. Even though i'm quite scared of how much this will cost, its seeming like my best option. I'd be very interested to hear the opinions/experiences/harsh truths of the CB750 community.

I'm just winging it here buds...I thank everyone for taking the time to read this erratic and poorly thought-out description : )
 

jpdevol

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Most of what you describe is likely just carburetor issues (plugged pilot circuits, synch & mix adjustments, etc.). It sounds like the previous shop didn't actually do anything - at least did it poorly. The carbs don't often need "rebuilt" - not much wears out - they just need a thorough cleaning (ultrasonic), the rubber bits replaced and everything properly adjusted. You need a competent shop or learn to do it yourself.
 

Biebs

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WOW finding a shop that will work on older bike can be a problem. Most dealers will not work on older bikes. Sounds like the carbs need cleaning or maybe just run some gas through them with carb cleaner. WD40 works as a carb cleaner spray in engine while running. Just a note these bikes are cold blooded and hard to start in cold weather / choke setting and being a veteran user of carbonated bikes is a help. Nothing really hard just some analyzing rough running. Synching the carbs is an easy process if you have correct tool ie mecury sticks or vacuum gauges.
 

NoIdeaWhatImDoing

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Most of what you describe is likely just carburetor issues (plugged pilot circuits, synch & mix adjustments, etc.). It sounds like the previous shop didn't actually do anything - at least did it poorly. The carbs don't often need "rebuilt" - not much wears out - they just need a thorough cleaning (ultrasonic), the rubber bits replaced and everything properly adjusted. You need a competent shop or learn to do it yourself.
I think its a combination of carb and vacuum. When reinstalling carbs, I found it difficult to marry the engine, carbs, and airbox all back together perfectly. Im not satisfied with the alignment / fitment of carbs into the boots and clamps. Ill post some pictures...

Supposedly they cleaned them submerged in what they referred to as "The Soup" , and then replaced seals and such. I only brought them the carbs. They didnt have the bike to properly sync it as well. I will likely try it myself first before hauling the bike over to them. It runs, im almost there. Hopefully nothing else major is lurking that my untrained eyes and ears cant recognize yet.......theres also the lack of the gas tank lol
 

NoIdeaWhatImDoing

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Most of what you describe is likely just carburetor issues (plugged pilot circuits, synch & mix adjustments, etc.). It sounds like the previous shop didn't actually do anything - at least did it poorly. The carbs don't often need "rebuilt" - not much wears out - they just need a thorough cleaning (ultrasonic), the rubber bits replaced and everything properly adjusted. You need a competent shop or learn to do it yourself.
Learning to do it myself is the goal. I plan to bring it back to its clean, new, original condition. Lots of body work too. I also need the side fairings.
 

NoIdeaWhatImDoing

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WOW finding a shop that will work on older bike can be a problem. Most dealers will not work on older bikes. Sounds like the carbs need cleaning or maybe just run some gas through them with carb cleaner. WD40 works as a carb cleaner spray in engine while running. Just a note these bikes are cold blooded and hard to start in cold weather / choke setting and being a veteran user of carbonated bikes is a help. Nothing really hard just some analyzing rough running. Synching the carbs is an easy process if you have correct tool ie mecury sticks or vacuum gauges.
I do not have those tools. Can you recommend a good store or website for bike related tools and test equipment?
 

Biebs

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I have used DennisKirk for years. I have the old Mercury tool - very dangerous. When syncing the carbs #2 is you baseline / reference carb. Very satisfying once you master the procedure.

 

Old_Crow

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I have used DennisKirk for years. I have the old Mercury tool - very dangerous. When syncing the carbs #2 is you baseline / reference carb. Very satisfying once you master the procedure.

https://www.vintagecb750.com/ also has quite the selection of special tools required for servicing our bikes.
 

NoIdeaWhatImDoing

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https://www.vintagecb750.com/ also has quite the selection of special tools required for servicing our bikes.
I have used DennisKirk for years. I have the old Mercury tool - very dangerous. When syncing the carbs #2 is you baseline / reference carb. Very satisfying once you master the procedure.

Most of what you describe is likely just carburetor issues (plugged pilot circuits, synch & mix adjustments, etc.). It sounds like the previous shop didn't actually do anything - at least did it poorly. The carbs don't often need "rebuilt" - not much wears out - they just need a thorough cleaning (ultrasonic), the rubber bits replaced and everything properly adjusted. You need a competent shop or learn to do it yourself.
I thank you all for taking the time to share your knowledge and experience. Very much appreciated : )
 

Bmb2492

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My bike had sat for 20 years, I'm a mechanic by trade so I couldn't let just anyone work on the bike. I rebuilt the carbs, changes the plugs, bought a battery and it started right up. For $30 on Amazon you can get a carb rebuild kit. New jets, new pilot screws, everything. And synching the carbs isn't as hard as people portray it, if you understand the premise. I don't think it's a vacuum issue because I used a Gatorade bottle for a fuel supply. I imagine it's the pilot screws. I would be more than happy to walk you through the process if you decide to rebuild them. It's worth a $30 carb kit to get the knowledge and satisfaction of hitting the starter one time and hearing it fire up
 

NoIdeaWhatImDoing

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My bike had sat for 20 years, I'm a mechanic by trade so I couldn't let just anyone work on the bike. I rebuilt the carbs, changes the plugs, bought a battery and it started right up. For $30 on Amazon you can get a carb rebuild kit. New jets, new pilot screws, everything. And synching the carbs isn't as hard as people portray it, if you understand the premise. I don't think it's a vacuum issue because I used a Gatorade bottle for a fuel supply. I imagine it's the pilot screws. I would be more than happy to walk you through the process if you decide to rebuild them. It's worth a $30 carb kit to get the knowledge and satisfaction of hitting the starter one time and hearing it fire up
Well thats a relief to have a mechanic on the case. The carbs were rebuilt by a honda bike shop....and when I got them back they were still leaking gas, which I since fixed. But this cranking and cranking with no start thing is driving me nuts! Even when the bike did start, it was still very difficult. Also, I broght the carbs to honda, just the carbs, no bike. they said they did a default sync but of course couldn't tweak it without the bike. Im sure that remains an issue.

And thats very kind of you to offer that. I think I will take you up on that! with this hard/no starting thing
 

Bmb2492

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I had the gas leak problem when I rebuilt mine because I used the old floats. The sides were wore where they are plastic and not metal and adjustable. Bought new floats and that fixed the leaks. If you get a chance, send me a picture of your carbs if you can get a decent angle of the bottom.
 

Bo82CB

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I just rebuild carbs on may 82 CB 750. It really is not that bad but you have to take your time. I am in process of bringing my bike to proper running condition after sitting for a while. Here is step by step manual how to do it yourself
Make sure to check your valve adjustment which is as I learned very important for this bike, .005 for intake and exhaust!
 
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NoIdeaWhatImDoing

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I just rebuild carbs on may 82 CB 750. It really is not that bad but you have to take your time. I am in process of bringing my bike to proper running condition after sitting for a while. Here is step by step manual how to do it yourself
Make sure to check your valve adjustment which is as I learned very important for this bike, .005 for intake and exhaust!
Thats great info! Thank you!
 

NoIdeaWhatImDoing

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I just rebuild carbs on may 82 CB 750. It really is not that bad but you have to take your time. I am in process of bringing my bike to proper running condition after sitting for a while. Here is step by step manual how to do it yourself
Make sure to check your valve adjustment which is as I learned very important for this bike, .005 for intake and exhaust!
I especially like the part in the list of chemicals needed that says "kona pipeline porter or another adult beverage (optional)" hahahaha. Speaking my language. Although im a hard liquor guy. I'll have to adjust that step.
 
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