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Engine starting and idling problem on a 5th gen VFR


htx

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So, the most problematic VFR returns yet again with another woe. After a clutch and oil change, I assembled everything back and decided to start the bike up, see how it's running (and if the clutch behaves correctly, which it did). Started right up and I left it to warm up a bit (up to ~60 degrees Celsius), before blipping the throttle. All was well until I released the throttle and the bike immediately died. I tried starting it afterwards and it was a bitch to start - couldn't get it going for over 15 minutes. Afterwards it was much of the same - runs OK, blip the throttle - dies. If I blip the throttle very slightly and release it, I can see the RPMs drop down beneath the normal idle speed and then return back to normal. So, any ideas? Does it sound like an electrical issue? Keep in mind the bike has a new battery, new regulator/rectifier, rebuilt stator, new cables from stator to reg/rec to battery, new master fuse holder + fuse. Thanks!

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Little bit of update: increased the idle RPM to about 1500 and it seems to have kinda solved the engine stalling issue, but the bike keeps hunting for idle pretty wildly (the needle jumps from 1500 to 2000 RPM), which is a bit disconcerting. Also with the current setting, the idle actually approaches 2000 RPM, while the engine heats up, which I don't like. The way I see it, the causes could be:

1. Fuel pump not creating enough pressure for the injector system or dirty fuel filter. Holy crap, this bike's fuel filter is expensive!

2. Dirty injectors or throttle bodies.

3. Vacuum leak from one of the hoses near the throttle bodies.

4. Ignition coil/s not creating enough voltage for a clean combustion or spark cable issues (sparks are new, though).

 

Which of these seems more likely and how can I test whether the issue is coming from there?

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Check for faults:

 

Did you have this bike running well before this? Were you in the airbox since then?

 

Check that sensors in the airbox are connected and (I can't recall) if there are identical connectors that can be cross-wired (TPS/MAP?). The same with the ignition coils to the correct spark plug. It might just be that a feedback signal is missing due to a bad connection or something is feeding back the wrong signal from a crossed connection. If you just got this bike you have to be extra careful to throw out assumptions and check connections from start to finish,  validating that the prior owning did not insert a jerry-rig.

 

If the bike had not yet been running well and all of that is good then yes you might need to use the specs in the service manual to troubleshoot the items  you listed with a multimeter, checking fuel flow, etc. 

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Checked for faults - there weren't any. The bike did lower its RPM beneath idle before, though it didn't stall. Only time I lifted the tank was to change the air filter and spark plugs and that was last year. Despite its rough idle the bike runs well, has a smooth power curve and doesn't cut off.

I'll probably have to start isolating and diagnosing it piece by piece. Does the service manual describe how to measure the fuel flow and ignition coils with off-the-shelf instruments, or would I need to get special tools for that?

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Yea what Grum said, it may be that simple. I read into it that it was not running well, but if it is just idle, the starter valves are the idle control vacuum circuit. The manual tells how. You may find a youtube video showing a 6th generation procedure for reference, but note the vacuum specs are different so get those from the manual. Yes, for future reference the factory service manual yields some specs such as resistances/voltage/etc. and procedures, although occasionally helpful hints that an OEM certified tech might know are left out. A lot of people here can fill those in though. 

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