Topic: cb500 spark advance problem

[wjustinleigh]

I've been fighting this for a while - I have 72 cb500 that just had a complete top end rebuild and has a dyno ignition but I cannot fix the timing.  I have a timing gun and can get it set correctly per this thread http://forums.sohc4.net/index.php?topic=50533.0 but once the bike gets warm it stops advancing/retarding correctly.  It both stays advanced while in neutral with no throttle and when starting from a stop it hesitates / almost refuses to move to advanced - resulting in having to rev it way up just to get the bike moving.  Once it revs up it will stay advanced and drives fine.

I pulled out the advance mechanism and shortened the springs per a thread here, which seemed to improve the issue when the bike is cold, but once it's been around the block the issue returns.  I also don't know why that would cause the start from a stop issue.

Any thoughts?

[HondaMan]

If the 500/550 engine has too much advance, it falls flat on its face until about 4500 RPM. That's one good clue...

The Dyna S ignitions have a [long] history of being somewhat temperature-sensitive. The gap distance between the magnet pole in the cam and the pickup(s) on the plate must be very close to the Dyna spec, or it will be affected in 2 ways:

1. It will act very cold-blooded when starting.
2. It will tend to "hang" at above-normal spark advance, due to poor turn-on, when hot.

Most of the time, the gap is too big when these symptoms occur, so check the distances on yours, and make sure they are identical for both the left and right side.

Another problem they have: sometimes they sent the wrong advancer cam out with the package. The hole in the center of the cam is too big on these, and it lets the cam wobble. When it does, it tends to mechanically "stick" when decelerating from full advance, which also makes the engine "hang" at high RPM. Make sure yours isn't "wobbly" on the spark advancer?

[wjustinleigh]

Wow - advice from Hondaman, I don't think you can get any closer to the source of knowledge of the sohc ignition than that.

Skeptical of the Dyna, I swapped it out for my points and but it behaved very similar.  When doing the bench test light it took a lot of adjustment to get both sets to light up on the 'F' mark but I got it pretty close and moved onto the timing gun.  Starting cold it's always been a harder start than I think a 500 should be, but it did fire up and on the 1-4 set it looked pretty good, throttle response seemed good.  Then as it got hot the problem returned - would go to advance and not fall back to the t/f set of marks.  By the time I moved to the 2-3 cylinder test it was next to impossible to set the timing as it would run continuously at advance.  Sometimes I could blip the throttle or cut the idle down enough to get it to go back to the t/f marks but it would then start to die and adding throttle sent it back into advance. 

Other symptoms - it didn't seem consistent, seemed to be moving the marks around too much.  The light was missing cycles too.  Maybe I have a cheap gun but once the engine was hot there would be multiple cycles where my light gun wouldn't light.  It was raining outside today so even with the exhaust pointed out of the garage enough was coming back in I was starting to feel asphyxiated so I gave in for the day.

Here are my thoughts on what could be wrong, would love any input:

- My timing is so far off it's just a cluster, I don't have a ton of experience with setting timing so this is a real possibility
- The coils are shot
- Worst case - the timing belt has a problem

I have a set of coils from another 500 I'm working on so once I get some oxygen I'll try throwing those on to see if it's better.

[HondaMan]

Wow - advice from Hondaman, I don't think you can get any closer to the source of knowledge of the sohc ignition than that.

Skeptical of the Dyna, I swapped it out for my points and but it behaved very similar.  When doing the bench test light it took a lot of adjustment to get both sets to light up on the 'F' mark but I got it pretty close and moved onto the timing gun.  Starting cold it's always been a harder start than I think a 500 should be, but it did fire up and on the 1-4 set it looked pretty good, throttle response seemed good.  Then as it got hot the problem returned - would go to advance and not fall back to the t/f set of marks.  By the time I moved to the 2-3 cylinder test it was next to impossible to set the timing as it would run continuously at advance.  Sometimes I could blip the throttle or cut the idle down enough to get it to go back to the t/f marks but it would then start to die and adding throttle sent it back into advance. 

Other symptoms - it didn't seem consistent, seemed to be moving the marks around too much.  The light was missing cycles too.  Maybe I have a cheap gun but once the engine was hot there would be multiple cycles where my light gun wouldn't light.  It was raining outside today so even with the exhaust pointed out of the garage enough was coming back in I was starting to feel asphyxiated so I gave in for the day.

Here are my thoughts on what could be wrong, would love any input:

- My timing is so far off it's just a cluster, I don't have a ton of experience with setting timing so this is a real possibility
- The coils are shot
- Worst case - the timing belt has a problem

I have a set of coils from another 500 I'm working on so once I get some oxygen I'll try throwing those on to see if it's better.

Check the primary side of the coils with an ohmmeter: they should be 4.4 to 4.5 ohms. If they are more than 4.7 ohms, they are done for: test this value between the Black-White and Yellow (or Blue) leads. Also, give a tug to the spark wires to see if one might come out: if so, the coil is history. The copper spark wire is spot-welded to the coil's wire up inside, not a repairable thing. Sometimes they corrode apart, just form age. This can cause an erratic or weak spark, especially after the engine heats them up.

When they "go bad" this way, they work better when cold than when hot. The timing lights often go "out" when the engine is revved up, too, as the spark voltage dies down too much. The stock coils peak at about 4500 RPM (in voltage), and are a little less at idle and 9000 RPM. When they go bad, they die off by about 5000 RPM.

[wjustinleigh]

I didn't have an ohmmeter handy, but in inspecting the connections between the wires and the coils they look pretty bad to me.  Some had a very stiff connection where the wires entered the coils - which seemed correct - others hung down like spaghetti and it looked like the rubber was the only thing that was keeping the wire connected to the coil.  Between the two sets of coils I couldn't put together a decent set so it looks like I'll be waiting on some parts to arrive before I can report back.  I actually heard one wire to coil connection snap while winding the wire out of the frame while taking the coils off.  Those seem pretty brittle, I'm surprised it's not a bigger problem.

Thanks for the advice.

[calj737]

If you do purchase new coils, be certain to get 5.0 ohm units. Many people (myself included) have bought new Dyna coils and gotten 3.0 units by mistake.

[wjustinleigh]

If you do purchase new coils, be certain to get 5.0 ohm units. Many people (myself included) have bought new Dyna coils and gotten 3.0 units by mistake.

Thanks, I did see that on one of these forums and ordered a set of Dynatek 5 ohm coils (DC8-1).  Will post back this weekend if it solved the problem.

[wjustinleigh]

I got the new coils installed with new plugs and what a difference.  It had always taken a bit of effort to start but now it starts on the first flick of the button or one kick.  Performance is way better too.  In the past all four pipes got hot but it clearly was missing something as it's way better now.

Still have some advance issues though, it's better but it still often goes to advance and stays there.  I can turn the idle way down and get it to return from advance but it struggles to stay going at low idle with the idle that low.  I can also force it out of advance while I'm stopped by putting it in gear and engaging the clutch slightly.  It sounds great, no clunkiness, smooth acceleration, just has the problem of staying on advance so I sound like I'm ready to drag race while I'm waiting at a traffic light.  I tried it with both the dyna ignition and the stock points - performs almost exactly the same.

I had the carbs cleaned and rebuilt at a shop here in town that's pretty good, but I don't know what kind of jets it has in there, nor am I a carb expert.  I'm thinking I have too much gas at advance and too little at low idle.  Does anyone know if that's reasonable and what I might look at in the carbs to adjust? 

Right now it's running on a dyna ignition with dyna coils and the stock airbox.