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100 Mile Ride

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I went on a 100 mile ride Sunday on my 1980 CB750K. The bike has 19500 miles and just recently had the Carbs Cleaned and rebuilt. The main Jets and Secondary Jets were not relaced because the guy doing the work said they were fine. The Carbs were also Synced afterwards. I also just finished adjusting the Valve Clearances to .005" on all intake and exhaust valves. The idle is set at 1000rpm and the bike idles fine. Acceleration is normal and smooth until 4000 to 5000 rpm; at that rpm there is a surging type miss feeling and if you throttle past 5000 everything is fine again. I have checked for vacuum leaks at the Carburetor to intake boots and found nothing suspect. Before the carbs were rebuilt the bike set for 4 to 5 years thus the reason to rebuild them.
I am at a loss as to what could be the problem. I was told to add a little choke and see if it made a difference and it seemed to but I was still not able to hold it at 4500 rpm which is about 55 to 60 MPH, it wanted to accelerate with the choke out a little. The situation happens in any gear at that rpm. I have stock Air box.
Any suggestions would be greatly appreciated. I posted before about this same problem but have not been able to find a solution.
I hope someone can point me in the right direction.
Thanks!
 
I have an 81 DOHC and I think mine does the same thing. If I'm not mistaken, these bikes had a small range mid-RPM that was a weak spot. I notice in first gear but rarely notice it in any other gear.
 
Start with some sea form in the gas as a cleaner. Are you using 91 octane fuel?? Fresh fuel??



No mention of spark plugs or wires are they new??
Spark plugs and wires are not new. I do use 93 octane gas and it is new.
Do you just pour the Seafoam in the gas?
 
This is definitely not normal, and I'm wondering whether it's a delay in the raising 1 or more of the carburetor slides. Perhaps the guy who went through the carbs missed something.
The transition points between pilot jet and main jet, and then needle jet, are supposed to be smooth as the slides all rise. If one of them doesn't move along with the others it could produce your symptom. It'd be easy enough to miss a dirty spot or a casting flaw in one of the four carbs when working on them.

My other guess might be an oddly-behaving ignition timing advancer. Our bikes have a mechanical advance driven by centrifugal motion which is mounted to the left side of the crankshaft. The manual specifies that timing light advancer tests be performed above 6,000 RPM, but it's possible that the advancer is slightly sticky and not opening smoothly when it first begins to work around 4,000 RPM. That wouldn't necessarily feel like a misfire but it would feel like a significant dead spot.

I'd lean towards carbs since work was recently done on them. Not to demean the guy who worked on your carbs, but I'm a mechanic myself and we all make mistakes.
 
Thanks for the help,
Aren’t the slides controlled by vacuum?
Also it wouldn’t be a fuel mixture issue would it? When the carbs were worked on one of the fuel mixture screws twisted off. It was drilled out and tapped to clean the threads out. Every since then that screw seems a little loose in the threads. Not sure if it seals completely or not.
 
I bought these carbs off eBay 8 years ago, I took the bowls off and checked the floats, closed them up and put them on and the bike ran perfect until I let it set. Now I’m kicking myself every time I ride it for not taking care of it.
 
A loose mixture screw is likely only influencing/impacting the idle. If your slides are not lifting like Sense suggests, it would behoove you to take the tops off the carbs and make sure those slides lift and fall smoothly. When I took mine apart months ago one of them did not fall all the way down. Instead it got stuck half way and a little push made it fall down. I took a maroon scotchbrite pad and lightly cleaned around the piston edge where I thought it was hanging up. After a few minutes it would fall normally. Maybe you have something similar going on.
 
A loose mixture screw is likely only influencing/impacting the idle. If your slides are not lifting like Sense suggests, it would behoove you to take the tops off the carbs and make sure those slides lift and fall smoothly. When I took mine apart months ago one of them did not fall all the way down. Instead it got stuck half way and a little push made it fall down. I took a maroon scotchbrite pad and lightly cleaned around the piston edge where I thought it was hanging up. After a few minutes it would fall normally. Maybe you have something similar going on.
Only thing I would add is, if you do take carb tops off do yourself a favor and get some aluminum polish and hand polish all sliding parts. I know you can’t do carb bores due to polish deposits may be left behind but carb slide, vacuum piston and cap can be done.
I have been sounding like broken record repeating this all the time. Did this to my carbs and throttle response is amazing…!
 
This is definitely not normal, and I'm wondering whether it's a delay in the raising 1 or more of the carburetor slides. Perhaps the guy who went through the carbs missed something.
The transition points between pilot jet and main jet, and then needle jet, are supposed to be smooth as the slides all rise. If one of them doesn't move along with the others it could produce your symptom. It'd be easy enough to miss a dirty spot or a casting flaw in one of the four carbs when working on them.

My other guess might be an oddly-behaving ignition timing advancer. Our bikes have a mechanical advance driven by centrifugal motion which is mounted to the left side of the crankshaft. The manual specifies that timing light advancer tests be performed above 6,000 RPM, but it's possible that the advancer is slightly sticky and not opening smoothly when it first begins to work around 4,000 RPM. That wouldn't necessarily feel like a misfire but it would feel like a significant dead spot.

I'd lean towards carbs since work was recently done on them. Not to demean the guy who worked on your carbs, but I'm a mechanic myself and we all make mistakes.
Thanks I'll look into the carb slides. Sounds like it could be a problem.
I did check the timing before the ride and it was good. Not sure about the Advancer?
 
Only thing I would add is, if you do take carb tops off do yourself a favor and get some aluminum polish and hand polish all sliding parts. I know you can’t do carb bores due to polish deposits may be left behind but carb slide, vacuum piston and cap can be done.
I have been sounding like broken record repeating this all the time. Did this to my carbs and throttle response is amazing…!
As long as you properly bathed the whole carb in solvents afterwards you could totally polish the bore. I think the bigger problem is that CV carbs depend heavily on the air velocity and vacuum level through the venturi. Hand-polishing or porting can make it behave differently than intended -- and that's especially problematic in carbs with metal diaphragm-less slides like ours. It would take a much more advanced tuner than myself to properly port/polish a set of DOHC CB750 carbs and then tune their air jets and slide holes to compensate.

Your static timing wouldn't have to do with this -- it's dynamic ignition timing and has to be checked with a timing light at 6,000RPM. You can't fully access the mechanical advancer to check if it's stuck unless you remove the left side cover from the engine. It uses a pair of movable weights with springs and high crankshaft RPM causes the weights to pull away from the crank, pushing the timing mark forward.

It's probably carburetor-related though if I had to guess. Let us know what you find inside those carbs. Could even be a partially clogged main jet.
 
As long as you properly bathed the whole carb in solvents afterwards you could totally polish the bore. I think the bigger problem is that CV carbs depend heavily on the air velocity and vacuum level through the venturi. Hand-polishing or porting can make it behave differently than intended -- and that's especially problematic in carbs with metal diaphragm-less slides like ours. It would take a much more advanced tuner than myself to properly port/polish a set of DOHC CB750 carbs and then tune their air jets and slide holes to compensate.

I was referring to OP intention to pull the caps, vacuum pistons, slides and leaving carbs mounted on the bike thus if attempted to polish bores, deposits will be left in the carb without possibility to clean them up properly. Just to clarify, I didn’t spend two hours polishing one cap or vacuum piston…
I spent few minutes bringing up smooth surface, feeling polished area with my finger for imperfections. Trust me it didn’t take long nor did I remove excess material anywhere.
I am not an expert but common sense is if you have two soft metal pieces sliding against each other and there is no lubricant, to eliminate as much friction as possible and to improve operation, sliding contact area of both bodies should be polished!
 
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Well, I did polish the slides on all four carbs and the inside of the caps as well. While it did make a difference in the Throttle Response it did not help the issue I'm having between 4000 and 5000 RPM's. I did put Sea Foam in the gas as suggested by Berttp. The days to ride are getting scarce here it southern Indiana.
I'm open to suggestions. I guess this winter I'll pull the carbs and go through them and take a shot in the dark. I'm not sure what to look for, What still puzzles me is the bike runs perfect and smooth up to 4000 and after 5000. It does it in any gear at that point.
Thanks to all!
 
Question for anybody: is there an air circuit that might need to be cleared/cleaned? If it happens in all gears, I'm seeing an air or vacuum issue.
 
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