Modern Vespa

Just passed 40K on NelliBee, my 2013 GTS, woohoo! Here's the scenario: after an hour long ride on secondary roads, I stop for fuel which takes, what, 5 minutes. After saddling back up, I push the start button. The starter just begins to crank, then, like a skipped heartbeat, it hesitates, then continues. Engine fires right up.

On a couple of occasions, the starter didn't even start to crank, just nothing. But on the second attempt it cranks, maybe with the hesitation, and the engine fires right up.

Brought the battery in, less than a year old and still under warranty, it tested just fine. Meter testing of the scooter on start, etc, all within normal ranges.

So, the starter hesitates when the engine is hot, never when starting cold. I have yet to meter it when hot, that's next. Any clues here to what might cause the starter to hesitate?

Thanks for your attention.

Could be the starter relay starting to play up.

In order of likelihood - brake light switch dirty (very common), starter relay pitted contacts (your's is about the right age to exhibit this), starter button switch failing (pretty rare), starter brushes worn (even more rare).

The brake light switch is easy to test - use the other brake lever!
The starter relay is a generic auto 80A relay - not that expensive to test by replacement.
The starter button can be replaced, headset dismantling required.
If the starter brushes are worn, they can be replaced, but better to replace the whole starter as bearings and bushings are likely to have worn as well.

Jimc, thanks for the info! Here is some new diagnostic information: I ran the scoot until warm and then shut it down and let it sit for a few minutes. I then metered the volts when I turned the key to start (without pressing the start button) and could hear the fuel pump pressurizing the system. While the fuel pump was humming, voltage dropped to 8.8 volts! When the fuel pump stopped (again before pressing the starter button), volts shot back up above 12 volts.

There is no precipitous drop in volts when the fuel pump is pressurizing the system with a cold engine. I'm zeroing in on the fuel pump motor as the culprit. Thoughts?

I really, really doubt it's the fuel pump - if the battery is fine I can't think of a failure mode that would let it take a huge (60-70A?) current without giving out some smoke along the way, plus the wiring couldn't deliver that.

So I suspect a poor connection to the battery - corrosion inside the eyelet crimp perhaps? Or even that the battery has a failing connection inside that is affected by heat.

But looking at the wiring diagram when key is turned on the fuel pump energizes to prime the system, voltage at battery drops to 8V, once primed pump stops and battery voltage recovers to 12v.

The only equipment on that 15A ckt are the fuel injector, fuel pump and HT coil. HT and FI energizing controlled by the ECU with only fuel pump connected directly to 15A ckt to grnd.

I would think if corroded connection or failing battery internally it would be showing up elsewhere. I believe it is a new battery.

Thanks for the continuing attention to this. Yes, I considered that it might be the battery, so I brought it in to be tested. It tested just fine, being less than a year old, but the shop swapped it for a brand new one under warranty, anyway. I trickle charged it over night to make sure fully charged. Same issue with new battery installed.

Plus the jump back to 12+V from 8.8 when the fuel pump is finished pressurizing the system.

All other systems seem to be running without a hitch. Voltage readings for starting cold, and idling, and with some throttle are all within normal range.

I also understand that if a fuel pump fails when the impeller (or whatever it might be called) gets stuck, it can blow a fuse. This suggests that when the fuel pump motor is struggling, it might increase the voltage draw? I'm guessing at this, I don't really know what I'm talking about

Keep us posted as the next step is to replace the fuel pump. Not overly complicated or expensive but can be a PITA to get the tank out.

And i say not expensive when you just replace only the motor/pump assy at ~$40 vs the entire assy at ~$300.

What I don't get, is that the current required to drop the battery volts to 8.8V is huge - as much as the starter stall current, 60A or so. A fuse should blow! I also can't see how the pump motor can draw that much current - if there's a short circuit inside it the pump wouldn't turn at all.

The voltage measured could drop to about 8.8 volts if there is a high resistance between the point of measurement and the internals of the battery. Where was the voltage measured? At the battery terminals or somewhere else?

I would ensure that the battery terminals are clean and tight, and if that doesn't fix it I would test the battery under load once again.

When were the valves last checked? Don't know for sure if that's the cause, but valves out of adjustment can make a hot engine hard to start.

Agree but looking at the circuit and the symptoms presented, the fuel pump seems to be the only logical choice.

My information has it that a new pump motor been ordered and should be installed by Thursday. Replacement is a Kemso.

All will be known then. Should make for a good discussion over a beer somewhere!

Voltage was measured at battery terminal the first time, and by an installed volt meter the second. Both measured a drop below 9 volts.

Where is the installed voltmeter?

FTFY.

The installed volt meter is on a switched line in the leg shield. But it read just as the voltmeter read at the battery terminals when the fuel pump was humming. And it also indicated the immediate rise to 12+ v as soon as fuel pump stopped.

All other readings are within normal limits.

Again, this is the second good battery, this one fresh off the shelf, swapped out under warranty just for luck, and put on a trickle charger overnight to make sure it was fully charged. It had exactly the same readings as the first. Since I installed this battery yesterday, I made sure the terminal connections were clean and tight.

The starter relay was the first part I replaced when this problem started. I've replaced the starter relay, installed a new starter, and installed a new battery, so far. All had zero effect on the issue at hand. Even purchased a bonafide multi-meter to replace the Horrible Fright $5 one I had been using.

Can't wait until Thursday to hear how the fuel pump replacement went and what the results are.

What brand of battery? It sounds like it's dropping a lot when the pump primes.

So, the engine starts and runs, but after a moment's hesitation when it's hot. How far from normal is this, really? Where's the problem?

When were the valves last checked? Valves out of adjustment can cause problems starting or running when the engine is hot.

So not just the fuel pump but the starter as well?

I wonder how good a connection the negative from the battery is making to chassis, AND to the engine/transmission. Corroded terminal?

I'd like to see the battery voltage measured at several points when the issue arises. At the battery, and between chassis and the battery +ve, and between the engine and the battery +ve.

There's still an errant high resistance somewhere - by high it may only be 1 ohm, but 4A through that and you've lost 4 volts.

Here is the latest on the hesitation at start for NelliBee, my GTS. This morning I loosened, cleaned, and re-seated the two grounds to the chassis. They were tight, but the area and the bolts were pretty grimy. Then I spent a better part of the day trying to reproduce the hesitation in a variety of ways, but NelliBee wouldn't cooperate, no, it refused to be anything but a perfect starter.

I started it cold, let it warm up until the cooling fan kicked in, took it for a good ride, and, well, just kept starting it, probably 20 times during the day. There was no untoward voltage drop when the fuel pump was running, and no hesitation when pushing the start button. So, was it a grimy ground all along? Heading down to Whiskey Dick this weekend feeling pretty confident that NelliBee will be great.

Thanks for the advice.

Sounds like a winner - well done.

Those chassis ground connections are so easy to overlook, they get taken for granted. And it took quite a while for me to think about them as a possible cause.