749 won't start when hot. Why? - ducati.org forum | the home for ducati owners and enthusiasts

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Old April 14th, 2014, 08:28 PM   #1
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749 won't start when hot. Why?

I turned off the engine to fill up on gas and when I went to restart, the starter turned really weakly like the battery was dead (LiFePO4 battery). The bike went through its normal pre-start routine. The voltage reading on the instrument panel was normal, but it felt like the starter wasn't getting enough juice and the bike would not start. No error codes on the readout, and the engine temp was ~215F. I let the engine cool to 130F and it still wouldn't start. I trailered the bike home, and the next morning it started right up. The voltage reading before I started the bike was the same as when it didn't start the night before. Any idea as to what is going on?
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Old April 15th, 2014, 01:25 AM   #2
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It could have been a loose connection or a bad starter motor. I've seen both issues cause the same problem.

I'd first test it by checking all the connections and getting it hot once more, but in a place not far from your pad.

Bad starter motor will crank very slowly, but the dash will be lit normally, showing perfect battery voltage. Usually if you've got a bad battery (not enough current) the moment you hit the starter button, it doesn't have enough current to keep the dash and ECU running, so it kills both of them and triggers a full reset.

Usually however, bad starters don't come back to life very easily. But perhaps yours is different...

Worst case scenario is hydrolock. You've got too much compression in the cylinders due to a head gasket leak. I've seen that happen on these motors before, but only on clearly blown motors. But the symptoms do fit potentially something wrong there.
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Old April 16th, 2014, 05:23 PM   #3
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Worst case scenario is hydrolock. You've got too much compression in the cylinders due to a head gasket leak. I've seen that happen on these motors before, but only on clearly blown motors. But the symptoms do fit potentially something wrong there.
Thanks! I thought it might be hydrolock, but can a hydrolocked engine clear itself just sitting in the garage overnight? Bike runs fine now.
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Old April 16th, 2014, 07:29 PM   #4
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Thanks! I thought it might be hydrolock, but can a hydrolocked engine clear itself just sitting in the garage overnight? Bike runs fine now.
Ohh yea, the moment it gets cold everything seals back up again.
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Old April 17th, 2014, 01:55 AM   #5
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hydrolock is definitely worst case!

something like leaking injectors will do it. did you try cracking the throttle a bit?
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Old April 17th, 2014, 05:57 AM   #6
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did you try cracking the throttle a bit?
What will cracking the throttle do? Do you mean when the bike is running?
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Old April 17th, 2014, 07:29 AM   #7
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^^ no he means when it doesn't start and you are trying to get it started. When you are trying to turn it over give it a little bit of throttle and see what happens.
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Old April 17th, 2014, 08:09 AM   #8
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Ohh yea, the moment it gets cold everything seals back up again.
Great. Besides leaky injectors, how can fluid be getting into the cylinder? If I pull the oil plug and find gas in my crankcase, will that tell me how it got there? Thanks for the help!
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Old April 17th, 2014, 09:53 AM   #9
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just try cracking (opening) the throttle first.

leaky injectors are a common/inexpensive problem. if you're holding down the start button with the throttle open and it still wont start, then the problem may be more serious/expensive.

to be clear, i dont mean doing this when the bike is cold. get it hot, kill it for 10 minutes, then give it a shot. if cracking the throttle while you're kicking it over brings it to life, and it runs perfectly fine otherwise, it's almost certainly leaky injectors.
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Old April 17th, 2014, 09:57 AM   #10
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Great. Besides leaky injectors, how can fluid be getting into the cylinder? If I pull the oil plug and find gas in my crankcase, will that tell me how it got there? Thanks for the help!
My initial thought was a bad head gasket on ONE of your cylinders. But a stuck injector which just drips slowly, could for sure cause the same symptoms if the bike is super hot.
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Old April 19th, 2014, 09:07 AM   #11
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Does it make any abnormal noises when you hit the starter button or as it stops?
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Old April 21st, 2014, 06:49 AM   #12
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Does it make any abnormal noises when you hit the starter button or as it stops?
No abnormal noises.

The engine temp was about 215F when I shut it off and it wouldn't start again. Yesterday, I went for a ride (engine temp got up to 215F at one point) but I let the engine cool down to 195F before shutting off. Then it started up just fine.
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Old October 14th, 2018, 07:30 PM   #13
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Did u find ur problem I am having the same thing going on
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Old October 16th, 2018, 03:41 PM   #14
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sounds like heat soak. hot bike stopped no air flow, heat from the engine boils fuel in the fuel lines on the back head so when you go to restart the injector has no fuel only vapour to inject. happens quite a bit here in aust with high ambient air temps and hot bikes, just have a break and let it cool a bit and give it a go. I once had to use a watering can on my old 750ss to cool down the carbs and fuel lines (44c day)
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