Topic: How to Calculate Advance Ignition Timing?

[DaveBarbier]

I posted this in the high performance section but thought I’d get more exposure here.

So, a little background. I’m making the Dyna S work on my CB650 by a little machining. That’s done and the bike runs well, BUT not as much power as when I had the C5 ignition. I’m chalking that up to my advance ignition. The C5 advance is 40° and the stock 650 is 28.5° and the Dyna of course uses the stock advance mechanism.

Now, my Honda manual says advance ignition happens at 28.5° but when I test it on my degree wheel it actually shows 38.5°. That makes sense if they calculate advance ignition by adding the 10° idle ignition to the advance ignition number. Like, they mean 28° FROM idle ignition. H.lanooy shed light on that for me, thanks dude.

My question is, is there a standard how ignition advance is measured? If the C5 says 40°, is that a true 40° BTDC or is that 40° from idle timing which would mean it would show 50° BTDC on my degree wheel?

I plan on bringing the bike to my local dyno to see where the best advance timing is anyway, but I thought that more knowledge on the subject is better .

Last question. Obviously if I just clock my plate then idle ignition will also be affected. Is there an easy way to keep idle around the same but only adjust advance if I’m using the stock advance mechanism? Like, bending or grinding down the ears which stop the weights?

[bryanj]

Having worked on these for over 40 years the NORMAL workshop standard for valve and ignition timing is total btdc. You would be given an idle figure then a total figure.
As to your second query i can see no easy way of incraesing max advance whilst leaving idle alone using a standard mechanism.

[DaveBarbier]

Thanks for the reply Bryan. So you’re saying that if Honda says the advance timing for the 650 is 28.5°, then when I have my degree wheel installed and having found perfect #1 TDC with a piston stop, when I spin the motor to where the advance hash marks line up with the witness mark on the case I should see ~28.5° on my degree wheel? Because I see 38.5°.

[BenelliSEI]

Dave..... you should see 28.5 on both the advancer and your degree wheel. Does the TDC mark on the advancer line up with your degree wheel when #1 piston is a true TDC (determined by your stop)? On a CB750 the advancer is doweled to the crank. Yours the same? Any damage or movement there? I’ve seen variances on my CB750 advancers and this always drives me a bit crazy!

On another project (1975 cb750F) I used a Power Arc C5 System that removed the std. advancer completely. I mounted a degree wheel and fixed pointer and only after I set the timing with my light and the engine running, was I satisfied with the results.....

[DaveBarbier]

Dave..... you should see 28.5 on both the advancer and your degree wheel. Does the TDC mark on the advancer line up with your degree wheel when #1 piston is a true TDC (determined by your stop)? On a CB750 the advancer is doweled to the crank. Yours the same? Any damage or movement there? I’ve seen variances on my CB750 advancers and this always drives me a bit crazy!

On another project (1975 cb750F) I used a Power Arc C5 System that removed the std. advancer completely. I mounted a degree wheel and fixed pointer and only after I set the timing with my light and the engine running, was I satisfied with the results.....

Yup, the T lines up with the 0 on my degree wheel. Also 10° lines up with the F hash marks on the advancer. Advanced is in good shape, there’s about 2° of wiggle in the advancer but that’s about what my 550 was. And yes, there’s a dowel pin that mates with a hole in the crank.

All actually seems totally fine, just that the numbers don’t match up to what I’m seeing in the book. Very strange.

[ekpent]

  On a 750 and smaller 500/550's the T mark is to set the tappets and the F is fire for timing if that makes any difference.

[DaveBarbier]

  On a 750 and smaller 500/550's the T mark is to set the tappets and the F is fire for timing if that makes any difference.

Same with the 650. T is TDC, F is idle ignition fire and the double hash marks are advance ignition fire.