Carb tuning with all original brass!

[DomP]

Following on from my deleted thread I have been fiddling (for want of a better word) with my carb tuning.  Carbs have been balanced but are still a little unstable but as good as I could get with a Morgan carbtune.  I get a good rev drop and stumbling when seating each pilot air screw so that suggests the pilot circuit is clear I believe?  When warmed up after a 20 minute ride the revs are still a little unstable and im still noticing a flat spot and it struggling to raise the revs from 2500 up to 3500, after that it is very much happy to rev which is where it is most used really, its just awkward at junctions and in traffic.

Is this still just a case of me adjusting the air screw from the 1.5 turns out im using?

[K2-K6]

Did you get the advance springs sorted out (thought it was in the "infamous" thread) yet.

Some crude analysis may help initially. Go to 2 turns out airscrew and judge if better or worse than now.

[DomP]

No, not managed to sort the springs yet, cant find any, would giving them a slight stretch help?

[K2-K6]

No, not managed to sort the springs yet, cant find any, would giving them a slight stretch help?

Likely yes, it would help to stretch them as it should be most prominent in this range if they're too tight.

[DomP]

I wouldn't be at all surprised if this is some of the issue, one is has a few coils less than the other

[K2-K6]

They're not that difficult to make, and can be done with minimal tools.

You'd need piano wire (basically spring steel) in same gauge as original.

Bend a right angle into one end, enough that you can form a loop like the original (just leave straight for now) clamp that tail along a small bolt that is sized to go inside the original spring coil bore ..... now wind the other part of the wire around the bolt to form the coils. Count how many you need to make factory spec.

At the end of coils, form another tail at right angle to coils. Then you can bend both tails into the spring ends needed to attach.

Finally, heat to cherry red (just a gas ring, camping stove type will do) and drop into cold water, this will harden the spring steel to make it  ..... spring.

Don't stretch it, as it now needs tempering to remove brittleness. Heat again to cherry red and just let cool without intervention.  Leave for a day, then temper again .... should now be usable.

[DomP]

Thats brilliant info, thanks I'll look for some spring steel and try it, nothing to lose!

[DomP]

From looking through Google images it seems they have 9-10 coils when new

[DomP]

Look quite stretched!

*before cleaning!

[deltarider]

This whole 'sloppy springs' thing is another made up or should I say exaggerated problem. As far as I remember it has been more a CB750 thing. Yes, they can stretch overtime, but heat annealing? Really, at such low temperatures? Here's what you can do if you do suspect weak springs, which can cause a bit of fluttering idle. A stroboscope will help you determining it. But be aware that a fluttering idle can have more causes! When fully advanced, there's no problem and that's where your bike is most of the time. If you have the advancer out, press lightly with your finger sideways to the spring to estimate the play. If you find them too sloppy, you can tweak the loop which goes around the post a bit. This after you have lifted the cam ofcourse to relax the springs (see pics). I wouldn't experiment with an advance taking place in higher RPMs than Honda has prescribed. So far I have not read one post in the international forum saying that that has lead to an improvement, not one, not on a CB500/550. Realise that springs on a TEC and a Hitachi advancer differ in length.

[K2-K6]

Look quite stretched!

*before cleaning!

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]

Agreed Nige, I'll look no further until I sort this.  Delta, the problem is in traffic etc im right in that problem range

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.