Cranks but won't start when hot

Bike ran perfectly last riding season. Failed to start once late last year, waited a bit, then it started fine.
Now it's doing it regularly, usually after a fuel up. Which I would guess is perfect time for maximum heat soak.
I have to wait 15-20 minutes then it starts. (Never fails to crank- hot or cold)
Also has a part throttle stumble/surge/buck when at operating temperature, don't know if that's related.

What I"ve checked;
All grounds, fuses and connectors (except those under the tank)-cleaned inspected and greased.
Looked for arcing plug wires (have NGK boots already)
Checked valve clearance (spot on)
Balanced TBs and reset TPS
Reloaded Beetle Map
Observed TPS reading in Guzzidiag while operating throttle (seemed smooth and linear)
Tank vent is clear
Replaced both coils with good used spares-no change
No active or stored error codes
Bike runs great at WOT at full temp.
Thanks in advance

It sort of sounds like vapour lock (fuel gasing off in the pump) but is that even possible with an in tank pump?

Might be worth testing the injectors when the engine is hot.

Also try opening the tank and equalising the pressure. Does it start then?

Might be worth testing the injectors when the engine is hot.

Just pull them to see if they're spraying?

Also try opening the tank and equalising the pressure. Does it start then?

Tried that Pete, sorry should have mentioned that. Tried so many things it's hard to keep them all straight!
I should also mention that the ambient temperature has not been unusually warm, 65-70F.
Thanks for the reply

Has it still got the charcoal can on it? If so disconnect it and plug the ports in the manifolds and see if that fixes it.

Patlikestolean wrote:

Just pull them to see if they're spraying?

You can use GuzziDiag to test them. It fires them a fixed number of times. You could check spray and volume.




Also, +1 for Pete's idea re the charcoal canister.

Charcoal canister has been off since before the problems started (sorry-again)
I'll test injectors this weekend.
I've got parts coming to replace the fuel filter and submersible hose- anything else I should check/do
while the tank's off?
Thank you very much for the help- I do appreciate it!

It would be very interesting to know if the Secondary Injection Relay (29) is picking up, it feeds power to the fuel pump, injectors and coils. If you can hang a 12V lamp on the red wire somewhere. [You must be registered and logged in to see this link.]
Perhaps wrap a small wire around pin 3 of the relay to a lamp with the other terminal connected to chassis.
It's one of the first things I did on my new bike, put a permanent lamp on each relay, if ever it fails to start I lift the seat to see what the lamps are doing.
I spent over 50 years in electrical and related trades, I still cannot tell if a wire is alive just looking at it.

Well I finally did more testing (life gets busy!) I checked the injectors, spray pattern looks good,
and the volume was the same cold and full operating temperature (approx. .9ml for 50 squirts).
I back probed the coil + and it is lit solid while cranking during a will not start event, so the secondary injection relay seems to be doing what it's supposed to do. I also checked for spark with an inductive tester while cranking and coils are firing. While bringing it up to temp it is running worse now too. Occasionally cutting out, especially if I just crack the throttle. Not stalling completely, just stumbling.

I would be looking at the spark leads next perhaps they are tracking to ground or you may have gaps in the core.
A simple test you can do is measure Ohms from the cap to chassis, at a guess 8,000 Ohms, it may be different but both sides should be identical.
This test includes the cap, HT leads and the coil secondary.
If it has the nasty carbon core HT lead I would replace it with copper core.
Much has been written about the plug caps, I don't know if that is an issue with the GRiSO?
You say you changed the coils, perhaps change the originals back in just in case you are adding to the problem.

Have you checked airbox for a rats nest? Exhaust plugged? Low compression?

I checked the HT leads for arcing last night and they looked fine, checked resistance just now, 4,900 both sides. They are solid core. I gave them a good wiggle/stretch while testing and they stayed steady.
Plugs are iridium and have 5k miles, I'll replace them just to eliminate the possibility.
Compression test is next, I'll also check for blocked intake/exhaust.
I've got the parts to replace hose and filter in tank, will do those asap.
I don't want to change too many things at once, I'd like to know what fixes it!
Many thanks to all who replied, Yous guys are great!

I also checked resistance cap to chassis, RH was 8,150 and LH was 8,350. Is this too much difference?
Thanks

Patlikestolean wrote:

I also checked resistance cap to chassis, RH was 8,150 and LH was 8,350. Is this too much difference?
Thanks

No it doesn't seem bad what I am looking for is one with an open circuit indicating a gap in the lead, you shouldn't see that with solid core cable.
I don't know where the difference would be, probably in the cap.

Actually the way you list it the difference is in the coil, since you have a couple of spares how about measuring them, I think you might have to measure from HT to the primary

Checked my original coils- exactly the same at 3,420 ohms each. Which would put my total at 8,320.
Haven't re-installed them yet.
Noticed original coils are marked "Champion" while spares I have in bike now are marked "Marelli".

Interchangeable from a variety of manufacturers - quite common in Europe

I tested compression- 190 psi both sides.
Whats the best way to determine if the exhaust is stuffed?
My understanding is there is a catalyzer in the pretzel. Do I need a flex scope to inspect?
Thanks, Pat

The pretzel is cat free. All cats live in the muffler.

Good to know-Thanks!

Unless you have a Japanese import. Then there's also cats in the headers.

Ahhhh...so! I’ll check it out tomorrow

I checked the exhaust today and it's clear- and no cats.
Installed new plugs but no change. I'll change hose and filter in tank tomorrow.

Why I was really looking for the status of the secondary injection relay.
It relies on pulses from the timing sensor without which you won't get spark or fuel but that seems ok so I'm out of ideas.
One last thought, you have spark and fuel, is it possible you don't have fuel pressure due to a leak or worn out pump/bad regulator, I have no idea how you could attach a gauge to check that.