Hot Start Issue Correction Idea - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 8 (permalink) Old 04-24-2020, 11:34 AM Thread Starter
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Hot Start Issue Correction Idea

From what I've heard, the hot start issue has to do with the coil voltage dropping during starting, causing a weak spark.
I can attest that riding with 11.5 volts made for an inconsistant spark at best!

I have an AGM battery, and I did the direct coil wiring (but haven't tested it yet).
The AGM battery has much less voltage sag during cranking, as compared to a traditional battery.
However, the voltage still drops quite a bit.
It still backfires now and then on hot starts, and is marginal at best.

I though, what if the ignition had it's own capacitor, charged through a diode?
During starting, the cap would hold the ignition voltage high while the battery voltage drops.
This should completely eliminate all starting issues, and may even provide stronger sparks while running.

The ignition takes a calculated 0.4A at 1000 RPM and 3.5A at 8500 RPM.
A diode like a SBR10U45SD1 can pass 4A with only 0.35V drop, with a max current of 10A.
A 12V 8.3F can be had on Ebay for $20, and should handle 30 seconds of cranking without issue, and recharge when you release the starter.
To be clear, the starter still pulls from the battery and not from the capacitor.


'89 VN750 Bought stock 4/12/18 @ 17805.9 miles $800. Wrecked once. 27454.5+ miles

Completed modifications:
Removed mufflers, straight pipes out of GB. 110/90 Front Tire. Pick-up coil mod. Shindengen MOSFET FH020AA r/r. USB Outlet + Voltmeter. Repainted Metallic Blue w/ new tank bages. Ear Shave (wo/ coasters) 40/140 NO shims 45MPG. MCCT conversion.

Planned modifications:
Custom LED turn signals. Windshield. Highway Pegs. (Possible) ignition adv. Tuxedo Mod. Ignition coil mod.
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post #2 of 8 (permalink) Old 04-24-2020, 01:04 PM
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My theory is, it has to do with r/w wire from the start circuit to the CDI. Low voltage there may cause the CDI to alter the ignition timing, leading to a backfire.

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post #3 of 8 (permalink) Old 04-24-2020, 01:18 PM
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Good idea.
I don't know a ton about circuit design, but you might need to add a resistor in series with the capacitor to prevent a short scenario when turning the ignition 'on' when the capacitor is in a discharged state.

1992 US-nonCA * tuxedo mod * r/r relocated

Vulcan slang for newcomers
Service Manual Download
Wiring Diagram + annotations * still undergoing revisions, but still more useful than the generic one. Let me know if you find mistakes or think something additional should be added
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post #4 of 8 (permalink) Old 04-24-2020, 03:49 PM Thread Starter
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Quote:
Originally Posted by Thorn View Post
Good idea.
I don't know a ton about circuit design, but you might need to add a resistor in series with the capacitor to prevent a short scenario when turning the ignition 'on' when the capacitor is in a discharged state.
If you put the switch between the capacitor and the ignition, then it's no different (from the switch's standpoint) from how it operates now.
If you put the switch between the battery and the capacitor, you end up defeating your emergancy cut-off switch, as the ignition would continue to operate for many seconds while the capacitor drains!!!

You would need a resistor whenever you connect it to the battery, to prevent a massive arc.
It would probably be a good idea to use the resistor to drain the capacitor too.

I drew up (in paint) a circuit diagram.
I added a DPST switch. In the left position, the diode is bypassed but the resistor is in line.
This allows charging/discharging the cap with current limited to V/R (~2A with 6 ohm, ~6A with 2 ohm).
Simply put the switch in this mode before connecting to the battery.
Also put it in this mode when disconnecting, and short the left end to ground to drain the capacitor.

In the right position, the diode is in line and the resistor is bypassed.
Once connected, leave it in this position.
This minimizes the voltage drop for full coil voltage at all times.
The cut-off circuit (neutral switch/kickstand switch/Emergancy stop switch etc) are all shown towards the right side, just before the coils.
If the direct coil mod is used, that is also where the relay would sit in the circuit.
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'89 VN750 Bought stock 4/12/18 @ 17805.9 miles $800. Wrecked once. 27454.5+ miles

Completed modifications:
Removed mufflers, straight pipes out of GB. 110/90 Front Tire. Pick-up coil mod. Shindengen MOSFET FH020AA r/r. USB Outlet + Voltmeter. Repainted Metallic Blue w/ new tank bages. Ear Shave (wo/ coasters) 40/140 NO shims 45MPG. MCCT conversion.

Planned modifications:
Custom LED turn signals. Windshield. Highway Pegs. (Possible) ignition adv. Tuxedo Mod. Ignition coil mod.
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post #5 of 8 (permalink) Old 04-24-2020, 05:00 PM
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I wonder if it might be easier to use a DC-DC converter? Power both the coils and the ignition box from it.

https://www.mouser.com/ProductDetail...qSriRj3fYeg%3D

'99 Vulcan "Victoria"
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post #6 of 8 (permalink) Old 04-27-2020, 11:56 AM Thread Starter
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I thought about that. It's a bit more expensive than just a capacitor, but it would be an even better solution. The converter would still need a capacitor, but a much smaller one (10,000 uF vs 5 F). This could also ensure proper voltage in the case of a charging issue like I had last year where my running voltage was < 12V at idle. It would at least allow me to get home without mis-fires.


'89 VN750 Bought stock 4/12/18 @ 17805.9 miles $800. Wrecked once. 27454.5+ miles

Completed modifications:
Removed mufflers, straight pipes out of GB. 110/90 Front Tire. Pick-up coil mod. Shindengen MOSFET FH020AA r/r. USB Outlet + Voltmeter. Repainted Metallic Blue w/ new tank bages. Ear Shave (wo/ coasters) 40/140 NO shims 45MPG. MCCT conversion.

Planned modifications:
Custom LED turn signals. Windshield. Highway Pegs. (Possible) ignition adv. Tuxedo Mod. Ignition coil mod.
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post #7 of 8 (permalink) Old 04-27-2020, 04:17 PM
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without going over the datasheet (just a quick look at the page in the catalog), it wont work.. 12V output is too low. gotta remember nominal operating voltage is 13.8 for the electrics in the bike.

2005 VN750

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post #8 of 8 (permalink) Old 04-28-2020, 08:37 PM
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It is common to wire a "horn" relay into the starter button circuit.
Start button is the trigger, coil gets direct power from the batt as long as the start button is depressed.

Idk why Kawasaki decided to bundle all it's ground wires to a single point on the frame.
Prone to failure, or at least amperage drop.
I tend to ground everything to the frame as close to the load as possible.
Bonus, you can eliminate a third of the wiring harness bulk this way.
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