Cb450 back fire, no acceleration

73CBnoob

73CBnoob

New Member
Having problems with my 73 cb450, Everytime I hit about 3500 RPMs it starts to back fire and bog down like crazy. Won't let me accelerate at all, I'm not sure if it's running Rich or what. The old owner said the carbs were just rebuilt but idk if they were adjusted properly. Iv owned the bike for about about a month or so. The left side spark plug is completely black, the right one is just black at the top. Any help would be awesome.

Thanks


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Black = too rich, or bad timing, or oil slipping past the rings or....

You have learned lesson#1 The previous owner, regardless of who they are (family member, friend etc.) usually didn't tell you the truth or didn't know what they were doing. Check everything yourself as it's your butt on the line now.

Do you own the manual or have you downloaded the PDF version?

Check float heights first, but also when you open the float bowls, what jet #'s are in there? Compare them to what the stock ones SHOULD BE.

Exhaust stock? No? Search for jetting your particular bike on the internet. Many opinions, so that's fun.
Intake (air filter) stock? No? See above.

Change spark plugs right away just in case they were crap from before hand and it's just fouled plugs from them running it on choke forever or something.
Welcome, and let the fun begin!
 
MotorbikeBruno said:
Black = too rich, or bad timing, or oil slipping past the rings or....

You have learned lesson#1 The previous owner, regardless of who they are (family member, friend etc.) usually didn't tell you the truth or didn't know what they were doing. Check everything yourself as it's your butt on the line now.

Do you own the manual or have you downloaded the PDF version?

Check float heights first, but also when you open the float bowls, what jet #'s are in there? Compare them to what the stock ones SHOULD BE.

Exhaust stock? No? Search for jetting your particular bike on the internet. Many opinions, so that's fun.
Intake (air filter) stock? No? See above.

Change spark plugs right away just in case they were crap from before hand and it's just fouled plugs from them running it on choke forever or something.
Welcome, and let the fun begin!
Click to expand...

I am going to go threw the carbs this weekend, I own the clymer book but I was told they aren't any help. I don't know much about these older bikes. To be honest this is my first motorcycle so I'm in the learning process. I will check all of those things thank you I bought 1 new plug the other one was fine and now that one is black and the other one if fine. So it must be the left barb that's not right. Where is the PDF that I can download ?


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Excellent info! So let's make sure your jets are stock and the PO didn't mess with them. I'm assuming you've got a few settings off. Float height is likely. Float needle spring could be stuck in or out too. And we'll need to verify what your mixture screws are set at. They could have messed with those too.
 
MotorbikeBruno said:
Excellent info! So let's make sure your jets are stock and the PO didn't mess with them. I'm assuming you've got a few settings off. Float height is likely. Float needle spring could be stuck in or out too. And we'll need to verify what your mixture screws are set at. They could have messed with those too.
Click to expand...

Thank you ! I will be sure to check these things as well, I appreciate all the help


Sent from my iPhone using DO THE TON
 
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