Cb750 possible carb issue. Need help!

J

jinsonphoto

Active Member
1978 Cb750k Possible carb issue? No power in first gear.

hey guys, it’s been awhile since I’ve been on here. I’m having an issue with my CB 750. A little context I did the cafe thing, got the bike going, cleaned the carbs, replaced suspension and pretty much everything to get it going and rode it without problems for several years. However due to life stuff the bike ended up sitting for about two years without being ridden. I would start it from time to time and ride around the block. Also I kept it on a battery tender.

Now just recently I decided I was to start riding again. Added fresh gas to it, started it up and the bike won’t make it up the hill out of my drive way.
It’s like it doesn’t have any power in first gear. It will run but it won’t make it up the hill!
So I thought I would ask for help here before I took the carbs off and started potentially causing other problems trying to solve this problem.
I’m def not a mechanic, but I did do most of the original work myself.

one other thing to note is that I believe one of the carb floats was getting stuck and started to leak gas. A little tap with a mallet fixed that.

any help would be greatly appreciated!
 
I would start with the carbs. If they had fuel in them when it was left, by now that will probably have caused all manner of issues. They are a total PIA to remove and refit if you have the stock airbox, but that way you will know that they are clean and ready to run.

If you need to procrastinate before you go into the carbs, you could start by draining the tank and cleaning the fuel tap. While the tanks is off, may as well replace the plugs and test for sparks on all 4 plugs.
 
teazer said:
I would start with the carbs. If they had fuel in them when it was left, by now that will probably have caused all manner of issues. They are a total PIA to remove and refit if you have the stock airbox, but that way you will know that they are clean and ready to run.

If you need to procrastinate before you go into the carbs, you could start by draining the tank and cleaning the fuel tap. While the tanks is off, may as well replace the plugs and test for sparks on all 4 plugs.
Click to expand...
O


teazer said:
I would start with the carbs. If they had fuel in them when it was left, by now that will probably have caused all manner of issues. They are a total PIA to remove and refit if you have the stock airbox, but that way you will know that they are clean and ready to run.

If you need to procrastinate before you go into the carbs, you could start by draining the tank and cleaning the fuel tap. While the tanks is off, may as well replace the plugs and test for sparks on all 4 plugs.
Click to expand...
Ok thanks teazer. I too thought that it seemed like a carb issue but I wanted to get some insight just in case it sounded like another obvious issue.
I kicking myself now for letting it sit!
 
No point in kicking yourself. It is what it is and we have to deal with what it is and not what we wish it had been.

We all do stuff like that, especially with draining fuel.

I don't want to do that yet because I might get a chance to ride it on a warmer day.
Damn it's cold this winter and I am too busy to ride the bike
Wow. It's way to cold to be messing with the bike right now. I'll do it when it gets warmer.
Oops. How can it have been so long since I rode it.

Sound familiar to anyone?
 
Sounds pretty damn familiar to me. Ethanol fuel certainly puts the corrosion in the bowls, which is usually aluminum oxide, and clogs the hell out of jets. Good chance that could something going on. Plus, a good chance your accelerator pump isn't spraying fuel either. IMO, those carbs need to come off and get cleaned whether this is your issue or not. They at least need to be inspected, and likely cleaned. I do this to myself all the time.
 
teazer said:
No point in kicking yourself. It is what it is and we have to deal with what it is and not what we wish it had been.

We all do stuff like that, especially with draining fuel.

I don't want to do that yet because I might get a chance to ride it on a warmer day.
Damn it's cold this winter and I am too busy to ride the bike
Wow. It's way to cold to be messing with the bike right now. I'll do it when it gets warmer.
Oops. How can it have been so long since I rode it.

Sound familiar to anyone?
Click to expand...
teazer said:
No point in kicking yourself. It is what it is and we have to deal with what it is and not what we wish it had been.

We all do stuff like that, especially with draining fuel.

I don't want to do that yet because I might get a chance to ride it on a warmer day.
Damn it's cold this winter and I am too busy to ride the bike
Wow. It's way to cold to be messing with the bike right now. I'll do it when it gets warmer.
Oops. How can it have been so long since I rode it.

Sound familiar to anyone?
Click to expand...
Yep that’s pretty much how it went for me!
 
irk miller said:
Sounds pretty damn familiar to me. Ethanol fuel certainly puts the corrosion in the bowls, which is usually aluminum oxide, and clogs the hell out of jets. Good chance that could something going on. Plus, a good chance your accelerator pump isn't spraying fuel either. IMO, those carbs need to come off and get cleaned whether this is your issue or not. They at least need to be inspected, and likely cleaned. I do this to myself all the time.
Click to expand...
Right on I think you guys are right. I’m just going to have to bite the bullet and pull them off and clean them.

Quick question. Do I need to sync the carbs after cleaning if they are synced well now? I honestly can’t remember because it’s been so long since I did the original carb work.

Thanks again for your alls
 
Sync will be good, although you can get them pretty close by observing where the butterflys cover the bleed holes. Heat thos insulators up and they will be much easier to release the carbs. Also, take a picture/make a drawing of ALL hose connections and move the carbs to the side to disconnect/reconnect the throttle cables.
 
teazer said:
No point in kicking yourself. It is what it is and we have to deal with what it is and not what we wish it had been.

We all do stuff like that, especially with draining fuel.

I don't want to do that yet because I might get a chance to ride it on a warmer day.
Damn it's cold this winter and I am too busy to ride the bike
Wow. It's way to cold to be messing with the bike right now. I'll do it when it gets warmer.
Oops. How can it have been so long since I rode it.

Sound familiar to anyone?
Click to expand...
pidjones said:
Sync will be good, although you can get them pretty close by observing where the butterflys cover the bleed holes. Heat thos insulators up and they will be much easier to release the carbs. Also, take a picture/make a drawing of ALL hose connections and move the carbs to the side to disconnect/reconnect the throttle cables.
Click to expand...
pidjones said:
Sync will be good, although you can get them pretty close by observing where the butterflys cover the bleed holes. Heat thos insulators up and they will be much easier to release the carbs. Also, take a picture/make a drawing of ALL hose connections and move the carbs to the side to disconnect/reconnect the throttle cables.
Click to expand...
Oh thanks for the Great tips! I’m looking forward to cleaning these carbs now.
 
jinsonphoto said:
Oh thanks for the Great tips! I’m looking forward to cleaning these carbs now.
Click to expand...
You have to be the first one that I've ever heard this from. Use patience, attention to detail, don't cut corners or skip any steps, use ONLY JIS screwdrivers. Be resigned to the fact that you will probably have them on-and-off at least three times. Make up an aux fuel supply filled with >91% isopropyl alcohol to test the floats BEFORE re-installing the rack.
 
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