NC30 revving issues

[Jarlly]

Hi guys

2 issues with my vfr400. Firstly, if she stands for a few hours, when starting off there's a 50% chance she'll bog down and not rev properly. Seems to be worse if I sit at idle (at lights) within the first minute of starting, and if she's fine at 3 minutes, she'll be fine for the rest of the ride. When she's bogging down it feels like I'm running out of fuel, very jerky, no consistent power, and going to full choke normally helps. When she gets through it, it's like a switch has been thrown and she's happy from then on, full power restored with no repetition until I don't ride her for a few hours.

Second issue is that she won't rev past 13k. If I hold her at 13k then after about 10 seconds she'll 'break through' and rev up to 14.5 with no issues. I can let her drop back to 14k and then rev back up to 14.5, but if she drops down to about 13.5k then there's no power and the revs will just drop to 13k where she'll start backfiring again. But this is the really odd bit- once I've 'broken through' once, she'll get through it easier the next time on the same ride. After ~5 redline runs she'll rev cleanly through the whole range in multiple gears. Until I stop riding for a couple of hours.

I asked a mechanic and he recommended I drain the carb float bowls because it might be water in the float bowls. Only thing was that when I put the carbs back on and went to start the bike again I was on reserve- could this have caused problems? Any other ideas?

I've replaced the vac tube, opened up the fuel tap and checked it (no obvious issues), and serviced the carbs according to the thread on here (the float bowls were fairly dirty, 115/118 jets fitted, one pilot screw all the way in which I corrected). The air filter is dirty but not awful, spark plugs are in unknown condition, and the reg/rec gives a slightly low voltage (13v with load at revs).

Any help gratefully appreciated.

[Dynamohum]

As you say the charging voltage seems a bit low would be worthwhile going through the system checks as explained on the sticky on here first as that can cause running problems. Assuming carbs are clean and set up filter clean might be worthwhile changing plugs checking coils leads also if you've done everything else.

[Neosophist]

is that with lights on?

with lights off id expect a little more.

Anyhow, brushing that aside, could have a sticking float causing the carbs not to fill up properly?

is snokely and rubber flap fitted ok?

[Jarlly]

Dave, do you mean Rick Oliver's charging system checks? Can't see the sticky. I have run through those and alternator output is great, no voltage drop from RR to battery, just a low voltage. Have an R6 RR in the post....

Snorkel and rubber flap doesn't look to have been molested. Also worth mentioning I get the same issues at idle and at speed.

Would a sticking float be obvious? All appeared to move cleanly when I had the carbs off, but I wasn't trying to spot a sticking float.

And that voltage is with lights on. 13.4 without. Oddly if I go to full beams and heated grips it goes up to 13.7V??!

[Dynamohum]

Sorry should have said in the documents section.

[Jarlly]

So, today my indicators stopped working. Great. New flasher on order! At the same time as bodging the indicators to display solid instead of flashing (better than nothing!) I fitted the R6 Reg/rec, though am still having the same issues. On the bright side, battery voltage is looking much more promising.

I noticed that if I'm at 14k with the revs dropping and I open the throttle, absolutely nothing happens until she drops to 13k (ish) at which point I get power again. Also, I can hold her at 13k, give her more throttle and get a definite engine note change (gets deeper with more throttle), both of which seem to imply it's not a fuelling issue.

The things that I'm planning to look at are:

Coils/HT Leads
Is there a way to check my coils or should I just buy some replacements, swap them out and see what happens?

CDI
Is this likely to be the culprit? There's one on ebay for £70 and 30 day no quibble return. It'd only cost me a fiver return postage to fit the new one and see if that makes a difference.

Spark plugs
I already have a replacement set, and have ordered a socket so I can take the old ones out! I think this is unlikely given my symptoms but worth doing nonetheless. Especially considering I've no idea when they were last changed.

Any other ideas? Where would you start if you were me?

[Cammo]

I wouldn't buy anything until you've done checks on it all, you'll just be throwing money away guessing what the problem is. It could be a few different things, you'll need to go through a process of elimination to find the culprit.

You can check the coils and leads to see if they're in spec, instructions are in the haynes manual.

Other things I'd be looking at are the condition of the battery, fuel tap vacuum line, clean and set the carbs, and make sure the 2 cdi plugs are connected properly.

[Jarlly]

Battery is good (I swapped the unknown one I got with the bike for the good one from my old bike!). Fuel tap vacuum line is new, carbs are cleaned and set, and the 2 CDI plugs are properly connected.

I'll do the checks that the haynes manual gives on my ignition circuit. They look to be fairly comprehensive so I'm still hopeful! If nothing turns up I'm wondering about sticking floats. I've noticed she normally reacts quite well to bump starts, and the bum slam might have persuaded the floats to un-stick. With a bit of luck something will turn up sooner rather than later!

[Jarlly]

One of my HT leads was giving me a resistance of 100k, the others 5k. Hmmmmm.....

[Neosophist]

One of my HT leads was giving me a resistance of 100k, the others 5k. Hmmmmm.....

dirty connection seomwhere.

I dont think any of your ideas above will be the issue. You never know they could be but dont sound like it.

Sounds like fuel / air intake issues to , you had any dyno runs? youll soon see by the fuel mixture if its starving itself.