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1975 CB500T
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Discussion Starter · #1 ·
Hello, I recently got my honda cb500T road worthy, but I am scratching my head regarding performance at high throttle/rpms.
Power is good and smooth at low throttle range, but when I'm gunning it, power pulsates and does not feel consistent.
I just took out the slides and swapped sides to mitigate a hanging throttle/idle issue, but I am stuck as to what to do next.

Any advice is greatly appreciated! Thanks all
 

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I had the same problem a while ago a step up on the main jet fixed it. Try a larger main jet, the original is 145 make sure it has not been changed.
 

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1975 CB500T
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Discussion Starter · #4 ·
Stock air filters but I replaced the paper element with foam and I can check the fuel chain for any errors. Would uneven tubing length between the petcock and carbs matter?

@450roo how much did you step up your jet size? And where do you live/altitude? I am in Los Angeles sea level
 

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Uneven tubing is normal for these bikes, and will not create any problems. Replacing the thick paper media with foam, however, will probably change the air pressure profile in the carb, often causing the engine to be leaner. Bumping up the mains a bit will help when the CV piston rises most of the way (so, at or near WOT), but to get the tune best when the needle jet is in control, which is most normal operation, you really need to change the needle jet size and the taper of the needle; very hard to do outside a tuning shop. So, bump the main jet and sync the carbs for open throttles, and hope for the best.

Or, you can partially cover the foam filters to bring the flow resistance back near what the paper filters provide; you will have to experiment with how much you need to cover the foam with something like cheesecloth layers.
 

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1975 CB500T
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Discussion Starter · #7 ·
i am reading that anemic power at high rev means a rich mixture so a smaller main jet (smaller number right?) is the course of action -- can anyone confirm? should I take my spark plugs out and check the color or something?
 

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If you want to check your plugs, put in fresh, then run it for several miles at the speeds it feels week, maybe 10, then pull the clutch and use the run/stop to kill the engine at the same time and pull off to a safe place. After it is cool enough, read the plugs for color. They will tell you if it is rich or lean.
Yes, a very rich mixture will make it run poorly, too, but you described a condition that sounded like it was leaning out with higher fuel demand.
 

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Stock air filters but I replaced the paper element with foam and I can check the fuel chain for any errors. Would uneven tubing length between the petcock and carbs matter?

@450roo how much did you step up your jet size? And where do you live/altitude? I am in Los Angeles sea level
Also at sea level.
Just realized that I have mislead you, I reduced the jet size from 145 to 135. Did his by checking the mixture with the chop test. Increase the speed to the miss occurs then pull the clutch and turn off the ignition. Pull the sparkplugs and check to see if rich or lean (white or black) The only problem was finding a long straight road with no police presence.
 

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Stock air filters but I replaced the paper element with foam and I can check the fuel chain for any errors. Would uneven tubing length between the petcock and carbs matter?

@450roo how much did you step up your jet size? And where do you live/altitude? I am in Los Angeles sea level
Sorry I have miss lead you. I decreased the jet size from 145 to 135 .
 

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1975 CB500T
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Discussion Starter · #11 ·
BREAKING NEWS

I found the fix
There are two small holes in the stock airbox that have little rubber nubs to plug them.
I bought my airboxes off eBay because the bike had pods when I bought it.
The airboxes were missing one of two plugs and I glossed over it thinking it wouldn't be a big deal.
I recently tried taping up the intake to constrict airflow since the theory was running lean.
But after I threw all that aside and plugged up the holes (see pic)
SHE RUNS BEAUTIFULLY
Good throttle response through entire rpm range and full throttle

Automotive lighting Automotive tire Wheel Hood Tire
 

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1975 CB500T
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Discussion Starter · #13 ·
Anyone know if these nubs on there on purpose to tune airflow? Or is it just acting as standoffs for the plastic cover?
 

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1970 Honda SL175
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WOW! Hard to believe that little hole could screw up the running of the bike!

OP... Try looking up the part number/name... That may give you an indicator what the function is.
 
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