I was just reading on an old thread about hot starting trouble on a Fatboy.

Could someone please tell me why if a bike spins over and coughs but doesn't start too good when hot, but improves if you start it whilst the fuel pump is still buzzing, shows up a possible mapping problem.

Surely it makes no difference if the fuel pump is still buzzing or not??

Anyway if someone could enlighten me that would be great.

 

If the bike starts while the pump is still spooling up but won't when the system is pressurised, I would agree with the duff map diagnosis.

The Engine Control Module contains a Map of how the fuel/air system should be modulated in response to throttle position and the input of various sensors located throughout the powertrain. The Map will be assuming that the volume of fuel flowing into the system will be X (X being whatever the pressure/flow rate is when the system is fully pressurised). I suspect that the Map in your bike is assuming that rate to be less than it is, so all the calculations the ECM makes will actually be far too rich.

But because the pressure is less as the fuel pump begins to spin, for a brief moment the fuel pressure is "correct" and the ECM is able to start the bike using the inaccurate Map it's using.

 

okay, thanks for that, like I say still learning,

I understand that, but;

I have just had the correct map flashed at my dealer, it was running on a 96 stage 1 thanks to my spanish dealer instead of 103 stage1, so I am confident it's got the correct map
and,
I had the rain cover over the air filter, does this make a significant difference to air flow?

Thanks in advance

 

Depending on what tuner you have, these maps are infinitely adjustable. And just because one map works perfectly on one bike, it still may need slight adjustments for another.

 

Okay then, so it looks like I'm heading off to see Bob Farnham and his dyno.

Or a Harley dealer with a dyno, which don't exist here because they all say, you don't need a dyno, HD have already done all the work and you just need a map.

Thanks again for your knowledge.