Topic: Help cure Lucile's cough - '73 CB500

[metaphysique]

Hey there, needing some help with an issue from all of you experienced folk. Many thanks in advance and another thanks for all of the posts that have helped me over the years til now.

Bike:
1973 CB500
Stock airbox / jetting
Dyna S ignition - Dyna Coils
Mac 4-1 Exhaust
Ricks Reg/Rect
LED lighting all around
Scorpion battery - holds charge no problems @ 13.5v

12,000 miles when purchased
16,000 miles ridden in past 4 years
28,000 miles currently.

I did a Valve job / cleaned pistons + chamber / replaced head gasket about 8,000 miles ago when I noticed the head weeping onto my boot. Ran great afterwards, more power, no leaks.

Always serviced on time - probably too often - never had any major issues.

Issue:
Runs perfectly, fires right up/idles, plenty of power...then, consistently at 10 or so miles, 1 of the cylinders cuts out - bog sound, loss of power but still runs. Guessing 3 cylinders still firing, 2 wouldn't carry me and accelerate. ( suggests cylinder instead of coil? ) Sometimes while riding it to safety it begins firing again and runs perfectly, until - again - it doesn't. Very inconsistent after it begins happening.

Testing:

Started with a voltage test:

Found an overcharge on the battery which I rectified the voltage drop at the ignition switch. (Grimy and warped connection - cleaned and leveled with sand paper)
Issue persists

Found coils shorting to frame - could have happened from my removing and installing when testing coils. Anyhow I insulated connections, no longer shorts.
Issue persists

Purple sparks across all four plugs.
Ignition is timed and triggering properly.

Coils
1-4   4.8   17.48
2-3   4.8   17.49

Coil leads all 0 ohms

Plug Boots all <11 ohms
9.07
8.3
9.39
10.93

Compr.
102 PSI
102 PSI
99 PSI
102 PSI

Plugs
Tan+Wet(oil - amber and doesn't air dry)
Light Tan
Light Tan
Light Tan

Valves readjusted

Hypothesis:
Compression is a little low (tools/age who knows), but consistent across all four cylinders. Didn't bother with the wet test as all cylinders were fairly consistent. So, probably not rings, right?

Only thing I can think of is a sticking valve at cylinder 1? What do you all think? Anything I'm missing?

Some sympathy photos:

[scottly]

Hey there, needing some help with an issue from all of you experienced folk. Many thanks in advance and another thanks for all of the posts that have helped me over the years til now.


Runs perfectly, fires right up/idles, plenty of power...then, consistently at 10 or so miles, 1 of the cylinders cuts out - bog sound, loss of power but still runs. Guessing 3 cylinders still firing, 2 wouldn't carry me and accelerate. ( suggests cylinder instead of coil? ) Sometimes while riding it to safety it begins firing again and runs perfectly, until - again - it doesn't. Very inconsistent after it begins happening.

Sounds like a classic case of a failing Dyna S. If so, it will get worse as the temps go up. This may be the first case this Spring.

[metaphysique]

I was definitely suspicious of the ignition first. I'll try putting the stock ignition back on the bike and see how it runs.

I've heard the coils go bad but hadn't heard of the ignition being terrible too.

Thanks for the point in the right direction.

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[scottly]

Be aware that Diachi (sp?) points also have a bad reputation; use TEC or Denso parts.

[Deltarider]

Although ignition is (always) the first suspect, make sure fuel flow is not hindered. Inline fuel filters can do that! With the appropiate length and diameter of the fuel lines (17 and 28 cm resp., ⌀ 5,5 mm) it is practically impossible to route them wrongly.

[metaphysique]

Thanks everyone, much appreciated! Stock ignition goes back on today. Will update in a few hours.

[strynboen]

i had the same problem.
..after running hard..im Loose one cylender..and after running softly, for some minnuts ..it came back  running fine Again
.it vas fuel relatatet
..first i took my extern filter off..and got the fuel line direktet more straight to the kaburettr
..and then i kleande the cap ventalasion hole ..so no vaccum come in the tank..
but better emty the tank-  tap-cap-  fuel line- kaburettors...then all is komplet gone through..

[scottly]

Plugs
Tan+Wet(oil - amber and doesn't air dry)

Tan+Wet indicates a lack of spark, rather than fuel starvation. If the switch to points doesn't correct the problem, the coil is the next suspect.

[metaphysique]

Thanks!

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[metaphysique]

@Deltarider @strynboen thanks for the hints.

When I initially couldn't find the problem where I suspected it, I went through everything including tearing down the carbs.

Took the bike out for a test ride tonight with the points, 10.5 miles and no issues... Tomorrow in the heat is the real test.

I do prefer the electronic ignition, something about it felt crisp and strong. What do you all think of the Pamco/Daytona/Dyna(current)

Thanks again everyone.

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[metaphysique]

Thanks Calj

I'll probably just replace the plate, I inherited this one that failed but my coils are new.

Also, while researching I came across a post about advancing the stock ignition 3 degrees. After putting the points back on I was less than satisfied with the performance. I tried the advancement, went with 2 degrees to be safer. WHAT A DIFFERENCE in low end torque, but it hangs a touch when warm...may back it off 1 degree and see how it behaves.

Thanks again everyone.