Rough idle, dies a lot , rev locking 3-4k rpm

[Slantnose]

Hello, if anyone could give me some possible insight to what’s going on with my bike that would be much appreciated.

My 1980 CB 750 has been running terribly, I’m able to get it to start but has a very very weak idle even with the choke all the way on. In addition to this, it gets rev locked around 3-4k rpm and is shooting a ton of black smoke out the back. I just changed the spark plugs, had the valve springs adjusted, and I know it has fresh 92 in it. The carburetors are brand new and only have about 300 miles on them, they are Keihin racing carbs. I did not install them I had a shop do it but who knows they could have done a bad job and left something loose or whatnot. Also, there is already a bunch of carbon buildup on my spark plugs that literally have less than 100 miles on them.

I am just getting into bikes, so while I do have a solid amount of basic knowledge about them, I am still a novice. Any help would be greatly appreciated.

 

[brettp]

You're running fat (aka rich). Whoever installed your carbs didn't tune them. It's likely your pilot and main jets are wrong. And because of that, likely your needle is off.
You need to look into how to jet your carbs. You can do it yourself, you just need to be willing to get started.

 

[Loose Chain]

Brett has good points. Did the bike run okay after the shop installed the carbs? Has it been sitting for a long time, giving the fuel a chance to go bad in the carbs? Why racing carbs? What do you have for air filters? What exhaust are you running?

 

[Slantnose]

Thank you both for your insight,

The shop that installed them did a bunch of stuff to the bike, here’s a list of everything they installed.
All new professional wiring harness, push button controls, LED lights, Dyna coils, Motogadget ECU, Vance and Hines exhaust, Keihin CR 31 racing carbs, Shorai battery, fresh EBC clutch and springs.

As for air filters, I’m not 100 %sure what make they are but I can check tomorrow, bike has not been sitting for any longer than a month-ish so I don’t think the gas has gone bad at any time.

Unfortunately I am currently at college and don’t have much or any amount of tools/garage/space to work on the bike. Do you think I would be able to jet the carbs if I was just doing it in the street? Or would it be worth it to take it to a shop instead? Not really sure how difficult the process is.

Again thank you guys for the help

 

[Loose Chain]

Carb work should not be done on the street. Have you talked to the shop that did all the work? I would suggest you start there.

 

[Slantnose]

Yeah didn’t think so haha, unfortunately I am very far from the shop that built it at the current moment. I’m thinking I will take it to one in town

 

[brettp]

Carb jetting takes time. You could do it in a parking lot but I would be afraid of losing parts on the asphalt, wind blowing things around, rain, etc. It would be a last resort thing. But your jets definitely need attention. Good luck!

 

[Loose Chain]

Yeah didn’t think so haha, unfortunately I am very far from the shop that built it at the current moment. I’m thinking I will take it to one in town

How did it run when you first got it back from the shop?

 

[Slantnose]

How did it run when you first got it back from the shop?

it ran okay, managed to get it from Seattle to Portland but felt a bit off, inconsistent power but strong idle and easy startup

 

[Slantnose]

Carb jetting takes time. You could do it in a parking lot but I would be afraid of losing parts on the asphalt, wind blowing things around, rain, etc. It would be a last resort thing. But your jets definitely need attention. Good luck!

Okay, thanks for the info!

 

[brettp]

It would take time for plugs to foul, not surprised you got home. But after they are fouled, you have to fix the problem.

 

[Loose Chain]

Do some asking around you area to find a shop or an independent mechanic that is familiar with these bikes and Carburetors. A lot of shops now don't even know what a carburetor is. The mechanic gets this glazed over look on his face when you mention "carburetor" like it's from another dimension.

 

[Slantnose]

It would take time for plugs to foul, not surprised you got home. But after they are fouled, you have to fix the problem.

Gotcha

 

[Slantnose]

Do some asking around you area to find a shop or an independent mechanic that is familiar with these bikes and Carburetors. A lot of shops now don't even know what a carburetor is. The mechanic gets this glazed over look on his face when you mention "carburetor" like it's from another dimension.

Hahaha alright, I’ll call around and see what I can find. Thanks again