VFR 800 FI - fails to start

This VFR 800 FI from 1999 served me well for 10,000 kilometers (total 35,000) and always started right away.
Now suddenly the engine does not start anymore.

I did a number of checks already:
- Battery loaded, starter turns fine
- Spark plugs > a bit gasoline wet, sparks are fine
- ECU error memory > no errors
- Fuel pump > runs for 3 seconds when ignition is ON & does pump fuel
- Air filter removed and jump start spray in the inlet nozzles > Motor runs for a second
- Tank emptied, fuel pump removed & checked for dirt > all clean

What is the most likely cause of this?
How to narrow down on the fault?

 

Check if plugs from 3 and 4 are wet. This might mean you have a ruptured FPR diaphragm, this will dump excess fuel into Cylinders 3 and 4 via the vacuum hose. Remoce the FPR vacuum hose, check for any fuel weeping from the vac hose fitting when going through the Fuel Pump prime phase at Ignition switch ON. Check that the vac hose looks dry.

 

@Grum - thanks for the hint.
My VFR runs again - and I don't know why :-(
I cleaned all plugs, checked ignition timing, loaded the battery etc.
Re-assembling the air filter housing with a dozen hoses and plugs .. it's really a puzzle, and IMHO no fun.
Than the engine started right away as if nothing ever went wrong.
But what remains is an unsecure feeling because the fault was not found and might reappear any time.
Basically the ignition adds a lot of complexity - and possible faults - to the VFR, so now I'd rather recommend the RC36/2 or maybe CBR 900 SC33 for anyone looking for a reliable older Honda sports bike, or the latest carb models of other brands.

One question, just to clarify for other readers - where is that 'FPR diaphragm' - in the fuel pressure regulator?
And the fuel would come through the hoses that you'd need to synchronize the starter (#3 and #4) - or which 'vac hose' do you mean?
There are ~15 hoses under the tank, at least they have their number printed on them.

 

Keep us in the loop. Maybe something wasn't connecte quite right and you actually fixed it.

 

of all the things to check, ignition timing is the least likely to be a problem.

 

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You may have had a flooded start situation! Cleaning all the plugs may have just been enough to get you over that hurdle!

Really important with Fi bikes, my own VFR will do it and I know of a VFR1200 where a guy was tearing his hair out trying to get his bike started for days, a friends CBR600RR also did it.............. Fails To Start.
The issue is - If you start your engine then stop it fairly quickly, Before allowing the engine to warm up a bit, then within the next few days trying to start the engine will be very difficult, this is because the very rich mixture of the first start remains in the cylinders and not burnt or evaporate off because the engine remained cold, now when you try to start again days later with a cold engine, again the ECM injects a rich start mixture into a cold engine now affectively flooding the engine and refusing to start, this condition can remain for days and is the reason the Owners Manual has a flooded start procedure.
So, whenever starting the engine ALWAYS let it at least warm up a bit before switching Off.

The Fuel Pressure Regulater (and its Diaphragm) hangs off one of the fuel rails! I assume you have the Service Manual? It has one vacuum hose that divides off to cylinders 3 and 4. Follow the hose and you'll see what I mean, and check the Service Manual.

 

Yes the FRP is the fuel pressure regulator, the round diaphragm device on one end of the fuel rail. Basically as the vacuum in the intakes increases, the fuel pressure has to drop to maintain a steady relative pressure. There is a vacuum hose which operates a diaphragm in the FPR and changes the fuel pressure. If the diaphragm ruptures, this will pull raw fuel down just that vacuum hose.

 

Ok, so we are not talking about the vacuum hoses printed "3" and "4" that serve for syncing; I misunderstood Grum in that.
Now I see that hose going down from the regulator.
In the case the membrane is ruptured - we'd remove the hose from the regulatur - than fuel should come out when the pump runs - right?

Just tested that - lifted the tank front, unplugged the vac hose with long radio pliers, turned on the ignition so the pump works > no fuel comes out - so my regulator membrane is ok.
Ok, so that's a worthwile addition to the test procedures in the the manual (which wants us to use a pressure gauge with a special adapter).

@Grum "rich mixture of the first start remains in the cylinders" - I did consider that after the first start failure > unscrewed 4 spark plugs, loaded the battery and tried again the next day. So any fuel should have evaporated by then.

 

My conclusion is now that the reason for start failure was a combination of low battery voltage (during start) and some other minor problem that I (accidently) fixed by dis- and re-assembling the parts around the airbox.