'06 Yami T8 4 stroke ign question

[bobwinkel]

My sons T8 seems to be firing both plugs at the same time.Common sense tells me this isn't right. Any ideas why?

[rodbolt17]

its broke.
Ill give ya 5 dollars for it.
actually???
its SUPPOSED to do that, if it does not then its really broke.
its called waste spark ign.
simple,cheap and effective.
quite a few small engines use it.
if you test futher you will find the spark polarity is different as well.
the engine really doesnt care that one spark occurs on an exhaust stroke as long as the other occurs on a compression stroke.

[bobwinkel]

'06 Yami T8 4 stroke ign question

its broke.
Ill give ya 5 dollars for it.
actually???
its SUPPOSED to do that, if it does not then its really broke.
its called waste spark ign.
simple,cheap and effective.
quite a few small engines use it.
if you test futher you will find the spark polarity is different as well.
the engine really doesnt care that one spark occurs on an exhaust stroke as long as the other occurs on a compression stroke.

Thank you for your quick response.
Darn! So much for my diagnostic skills.

Full story: Has been used all winter for bird hunting. Now time for salmon fishing so engine lost power - #1 cylinder dead - ocassional backfire through intake - new plugs - cleaned intake and flushed carb with cleaner - replaced in line fuel filter - compression equal at 115 psi - new #1 plug slight carbon and damp, not wet - timing on - normal advance - both plugs seem to fire strong.

Any ideas?

[rodbolt17]

do a leakdown test.
then check the ign timining on BOTH cylinders.
should stay the same.
then its back to fuel issues.
the reason for leakdown over compression testing? several.
one its way more accurate.
two I am lazy.
a lot of the small F yamaha motors used an automatic decompression device to relieve the strain on the starting mechanism.
usually I dont feel like going to the parts or service manual picture book to see which one does or does not.
3 goes back to 2, a leakdown test is simply faster.
you will need to check ign timing at both idle and wot.
test propeller YB1625 will be a great testing asset.

[bobwinkel]

Was reading elsewhere in the forum and came up with a question. Does the Yami T8 4 stroke use one carb or two. I have been assuming one carb.
Thanks

[bobwinkel]

More info. Same problem

Still fighting the dead cylinder problem. Tested ignition system. Found following voltages:
CDI output should be 140V reading 30V
Charge coil output should be 112V reading 8 volts
Pulser coil output should be 7V reading 2mv

Motor will run normally, then drop cylinder, then run normally erraticly. Hesitation on opening throttle. Removed flywheel and checked for loose/broken wires. Checked and tightened all connections including grounds. Coil tested good at shop.

Which comes first? Does the charge coil energize the CDI which is triggered by the pulser or is there some other sequence?

As usual I am in over my head. Any suggestions welcome!

[rodbolt17]

to test charge coil,pulser coil and CDI output voltages takes either a digital meter with a DVA adapter or an analog meter capeable of reading voltage PEAK output.
most meter are averaging or RMS.
RMS is about 70% of peak.
we dont need RMS or working voltage we need PEAK voltage.
CDI stands for Capacitor Discharge Ignition.
when a capacitor discharges through the ign coil primary windings it does it in milliseconds.
the stator charge coils,not to be confused with stator lighting(battery charging) coils,simply charge a capacitor.capacitors act like a battery, they can store DC and pass AC voltages.
so we now have a charged capacitor( little battery) sitting there.
how do we discharge the capacitor into the ign coil primary windings?

we are going to add a Silicone Controled Rectifier(SCR) this acts like an electronic on/off switch.
its voltage con*****ed and current controling.
the SCR is on the capacitor discharge side between the cap and the ign coil.

so the cap is charged and waiting.
along comes the SCR gate signal(pulser input) now the SCR is turned ON and remains ON until the current through it drops below its on threshold.
then it turns off awaiting the next ON trigger signal on the gate.
you need to REREAD your manual.
pay close attention to LOADED and UNLOADED.
the CDI assy DOES NOT step up any voltages,infact it will typically drop about 10V.
unloaded means the device unplugged from the input.
loaded means all circuits are attached and operating.

at 30V peak CDI output it simply wont run.
not enough voltage to generate a sufficient magnetic field in the ign coil primary windings to collapse and induce a voltage in the secondary windings.

what type Multimeter are you using?

but you simply wont see,nor should you expect to see, higher CDI output voltage than charge coil input voltage.
ever.

thats kinda basically how your ign system generates spark
the timing of that spark is done using some extra things such as inverters and R/C time circuits.
as all your readings are consistantly low I assume your using an incorrect meter.

do you have an analog meter ?

[bobwinkel]

Used an analog meter and found the following AC readings:
CDI 1.4V
Charge Coil 80V
Pulser 6.5V

Sorry to be such a pest but I don't have access to the motor. Son lives about 70 miles away and doesn't have internet access so I pass the info back and forth by phone.

Based on the above readings, it would seem the CDI is failing/failed. Would this be correct or is it the charge coil?

[rodbolt17]

Charge coil inputs and pulser coil inputs are A/C.
CDI output is DC.
standard meter wont work correctly but it will show intermittant failures.
the CDI output is a very rapid peak from 0V to about 120 volts in a few milliseconds.
most meters are dampened and only show a 70% average or RMS voltage.