SOHC.co.uk Forums > CB500/550
Carb tuning with all original brass!
K2-K6:
--- Quote from: DomP on August 14, 2026, 11:27:28 PM ---Look quite stretched!
*before cleaning!
--- End quote ---
Definitely, and likely to cause a problem set like that. Honestly feel you're chasing your tail with other consideration if this is not resolved.
K2-K6:
The technical view in respect of this should give reason why its problematic.
The combustion event is a "burn" over specific time period (not explosion) which largely remains the same for most engine conditions.
That should be timed to reach peak expansion at 12 degree after top dead centre, which gives most advantageous push on the piston and conrod in applying the highest force to crankshaft, hence most torque developed.
The static timing is set to give this series of events at idle rpm.
As crankshaft speeds up, then it moves more degrees as rpm accumulate...... in the time frame of that combustion is "fixed" in it's own time frame ..... meaning that the crankshaft gets further and further (15, 20, 25 ish) past the ideal 12 ATDC figure, essentially wasting the combustion energy ultimately to give less torque.
The ignition advance curve has to "chase" that moving target if its not to lose its potency, it should produce it's most available power by trying to hold onto that ideal 12 degree target. 0
If the advance is delayed, as these springs, when shortened will do, then torque won't rise as it should. Not a flat spot as we ordinarily get (lean running, leading to missing combustion) but holding torque down/suppressed until the ignition is eventually allowed to get where it should have been.
Then it should do as described in lost thread, give a two stroke type step up in output and take off into higher torque output.
Notably, the CR 750 ignition deleted the ignition retard unit for reliability , and because there's not need to run it below about 2,500 rpm. It's just a straight 36 degree static setting as I understand it. I can't see logical reason in suppression of early ignition advance as this spring cutting targets.
It needs to get up and advanced as soon as practically possible in generating the power expected from this engine.
deltarider:
The good news is that within a fraction of a second your engine already makes 3000RPM and IGN is fully advanced.
DomP:
Agreed Nige, I'll look no further until I sort this. Delta, the problem is in traffic etc im right in that problem range
K2-K6:
--- Quote from: DomP on August 15, 2026, 12:52:25 PM ---Agreed Nige, I'll look no further until I sort this. Delta, the problem is in traffic etc im right in that problem range
--- End quote ---
It's something that's not readily appreciated in its effect, on carburettor equipped engine.
There's ignition retardation used in more controlled car system, reduction of torque during full throttle upchanging for automatic gearbox, also in place for traction control systems. Both of these to alleviate torque output in facilitating easily without more dramatic fuel cut method.
On a car with distributor, the timing can be moved while idling to observe the torque produced rising and falling.
The acid test of course is to see if it makes a difference here in resolving your issue.
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