Topic: CB500 Engine Power Problems after warm-up

[19thSTCB]

Hey all,
I have a '72 cb500 that I have been tinkering with for the last year.  I finally have it up and running but I'm having a weird problem.   The bike runs best for the first 5-10min of operation and then it acts like it has no engine power.   I would say the power output drops by almost half.  It still runs but is weak.  It doesn't stall or cut off but just runs as if something is restricting air or fuel.  The carbs have been cleaned (many times) the tank is clean and creamed, points checked, valves set....ect. 

How could it run fine and then lose power output after 5-10min? Why would it show up after warming up?

Thanks.

 

[mlinder]

What do your plugs looks like? And, are you sure it's firing on all 4 after you lose power?

[TwoTired]

What do your plugs looks like? And, are you sure it's firing on all 4 after you lose power?

+1 
and...
have you retained the stock exhaust and air filter?
Has your tinkering changed the internal parts of the carbs?

Are you sure the mechanical advance is operating properly?

Hows the oil supply?

[19thSTCB]

What do your plugs looks like? And, are you sure it's firing on all 4 after you lose power?

What would be the best way to check this?

[19thSTCB]

What do your plugs looks like? And, are you sure it's firing on all 4 after you lose power?

Forgot to answer first question.  The plugs look normal and all the same, nothing weird on them and they are new.

[19thSTCB]

What do your plugs looks like? And, are you sure it's firing on all 4 after you lose power?

+1 
and...
have you retained the stock exhaust and air filter?
Has your tinkering changed the internal parts of the carbs?

Are you sure the mechanical advance is operating properly?

Hows the oil supply?

The exhaust is aftermarket and its a 2-1 (1 and 2 into one pipe and 3 and 4 into the other) but I did a carb sync after they were put on.  They were the pipes that were on the bike when I got it (not originals), I just bought newer ones (b/c old ones were rusted bad) but same pipes.  Im, I colorado and the bike has been here for 15yrs so the jets should be correct?  I have not changed anything internally with the carbs.

Sorry still learning about everything....What is the mechanical advance? :-)

Oil is new and good.

Have

[TwoTired]

Mechanical advance changes the ignition timing when the engine RPM is increased.  It is behind the points plate.  You can check it with a strobe timing light to see if it is working.  If not, then surgery.

Check your spark plugs against this chart.
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

What air filter are you using?  Is it clean as new?

Is oil getting to the top of the engine behind those tappet covers?

[19thSTCB]

Mechanical advance changes the ignition timing when the engine RPM is increased.  It is behind the points plate.  You can check it with a strobe timing light to see if it is working.  If not, then surgery.

Check your spark plugs against this chart.
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

What air filter are you using?  Is it clean as new?

Is oil getting to the top of the engine behind those tappet covers?

Cool, I'll check the mech. advance, tappet covers and re-check the plugs and get back to you (probably won't get to the mech. advance till next week, I have to borrow a strobe timing light)  The air filters are aftermarket cones and look like a K&N filter that is no longer new "red" looking but not totally caked in dirt either (if that makes sense).   I'm not sure the brand as they came on the bike when I got it.