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Discussion Starter · #1 ·
Hey guys, glad to see the forum is still alive!

When I get my 73 CB450 up around 70mph it starts to bog and hesitate a bit.
I cant get it any faster than that.
Could it be point flutter? not enough fuel? air leak? weak coils?
Ive racked my brains. Previous owner had the same probs.


I Set the timing and valves. Bike has new points, new chain & front sprocket, new petcock/filters, hose, spark, new plugs,
good compression, tightened the advance springs
thanx for any help /ideas.
 

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Sensei
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Does it FULLY advance by 3500 RPM?......( Index between the advance lines)
Did you clean the carbs, and adjust float levels?.............SYNC the carbs?
What kind of pipes and airboxes are you running?.....
AND...what should have been the first question.... What are the compression readings?( hot, WOT)
 

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Discussion Starter · #3 ·
Bike has 7000 miles on it. Stock exhaust and air boxes. rubber is good. squirted carb cleaner around carb rubbers -no leaks.
Carbs were synced manually, not by gauge, carbs have been rebuilt and twice cleaned, floats set to spec in manual.

-compression was checked when bike was cold -wot not hot. (I'm still learning) but comp was still pretty close to where they need to be according to what I've read on here. dont remember the #'s exactly.

The timing check was static as I didnt have a gun. I did not check for full advance @ 3500 rpm.
that will be something I have to read up on here and in the manual and learn to check.

Bike otherwise runs great and has great power at all speeds under 70 but who wants to go 70 all day?.

Here's a pic of it
 

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Sensei
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Bike looks great!...I still suspect compression, 160 PSI (which would be good on my chevy,) is the MINIMUM spec with a 15lb (10%)variance between cylinders... I suspect yours is around 135/140...maybe less....... If you get full [email protected], you are ok....If not, you should have only adjusted one spring.....
 

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Did you time the ignition with a neon timing light? If the advance goes beyond the maximum advance marks, it will produce the effect you describe. The springs only control the rate of advance, not the limit. Too rich a mixture has the same effect as not enough fuel: Start with clean plugs and run down the highway for several miles at the problem speed. Kill the motor, disengage the clutch and come to a stop. Check the plugs. Your coil output could be weak - the faster you go, the shorter the saturation time for the coil's magnetic field. Weak coils are sensitive to throttle - they misfire on acceleration. With too much advance, your motor will go flat at a certain RPM. Accelerating hard, you won't get past it. Accelerating gradually, same thing. Power drops off around the same RPM in any gear. The drop off is more abrupt than not enough advance - that will kill your power, too - but it will just feel like a motor that doesn't have a lot of power, not one that gets up to a certain point and won't go higher. Ignition or timing problems will usually let you get up to a certain speed and stay there. fuel starvation will probably get you up to a certain speed, but you won't be able to hold it there; you'll slow down - unless the problem is too small main jets. Even then, that's usually a gradual absence of power at a higher speed than seventy. The 450 with standard gearing will get up to slightly over 100 MPH with #130, which is the smallest main usually used (some early bikes used #125).
 

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Discussion Starter · #6 ·
I will do a hot comp test tommorow and do a bike kill and read the plugs. I need to borrow my friends timing light and hit the manual to check the timing correctly.
I have a set of new Yamaha xs650 coils that I remember someone here switched their Honda over to.
Maybe I will do that as well. Kerry, and 66 thanx for the help. I will investigate the things you suggest and let you know how I make out. unfortunately we have rain in the forecast for the next week so it may be a bit till I post my findings.
 

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I had the same problem and spent ages (several months) to find that the cam timing was out, when I was sure it was spot on. The timing marks on the 450's can be very hard to see, check very carefuly and I bet that is what you will find. Allan
 
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