SOHC.co.uk Forums > CB500/550
CB500 runs lean and doesn't want to pull through
Oddjob:
What about the wasted spark Nigel, could he be referring to that spitting back on the open cylinder?
Yoshi823:
A few years ago I bought a nice condition CB550K3 from someone relatively local and it had a lean running problem. I made the point that I didn't know much about bikes & that it would have to be put in to a bike shop to be fixed.
In reality, I had a good idea where the problem was from the start & once home I pulled the carbs & replaced all four of the inlet manifold 'O' rings (part #14 on the fiche) as one was snapped & the other 3 didn't look in good enough condition. For what they cost it wasn't worth looking for cheap alternatives & having resolved the lean running it pulled in a similar way to the wife's CB550F2.
https://www.cmsnl.com/honda-cb550k3-four-england_model14671/partslist/E02.html#.Yhp5PrunzIU
K2-K6:
--- Quote from: Oddjob on February 26, 2022, 03:42:50 PM ---What about the wasted spark Nigel, could he be referring to that spitting back on the open cylinder?
--- End quote ---
I was thinking that through too Ken, but to me it reinforces that it's not ignition rather than causes doubt .
Taking the piston on power stroke, as it comes to bdc and looses any useful drive pressure the exhaust valve opens, then with piston coming up pushes spent gasses out past valve (wasted spark is arriving anywhere from static time to full advance on this stroke) but critically before the inlet opens. There's effectively no available fuel to ignite present, and so the spark is truly just wasted. Bringing more ignition advance to this scenario just moves the spark further down the approach and further away from any inlet opening time, and so should have less effect not more if were present.
The same sequence ends, at TDC, with valve overlap but only after the spark has gone out. Increased overlap does bring compromise though (but not controlled by the spark) because the inlet tract is exposed to exhaust pulsing which with the right design of exhaust can aid cylinder filling. This is a dynamic situation though and with more overlap in the cam will have best performance at higher speeds, giving a classic "cammy" engine that fits-and-spats until that point (often around 4,500rpm) when the dynamics clean up the exit and entry to the cylinder and the motor takes off.
Honda made the cam timing to run clean at 10,000tpm but restricted the cam to best give a conservative wildness and make it run sweetly all the way down to very civilised tickover. Quite a balancing trick to do in reality, with very basic design. I still think Honda were peerless in four stroke design during that period.
deltarider:
--- Quote from: Yoshi823 on February 26, 2022, 07:08:20 PM ---[...] I made the point that I didn't know much about bikes & that it would have to be put in to a bike shop to be fixed.
In reality, I had a good idea where the problem was from the start [...]
--- End quote ---
............
(no comment)
K2-K6:
Question for the OP, have you changed the fuel tap, fuel lines or installed fuel line filters on the bike recently? We've seen a few examples of restricted fuel flow that can do this in preventing the carburetor from refilling as it should and caused the bike to run lean, not go up into higher rpm.
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