my bike is sick :(
April 20, 2010 2:30 PM   Subscribe

unstable idle on my sport bike

after being in super slow moving traffic on my bike (3 mph +/-) the idle gets really erratic. normal is ~1200 rpm, but it will drop 500 or so then return, drop, return etc - sometimes dangerously low to dying when i come to a stop. died 4 times as stop lights yesterday.

under normal use, it idles just fine.


its a 99 CBR f4.

I suspect the problem is electrical, but who knows?

Ive had some electrical troubles in the past. the reg/rec died last season (one of the diodes went out) and i replaced it. the wiring harness was a complete mess - with some parts replaced with stereo wire (!!). I had that replaced entirely earlier this year. Is it possible that some bad wiring nuked some electrical components that will need to be replaced?

last time i had it serviced for unrelated issues (new engine side cover) the shop informed me that my battery was holding a charge, but had no life left - said that it would hold the charge but would only live through a couple ignition attempts. So i had them drop in a new one. this particular shop is kinda crooked so A) the battery may have been fine and they wanted to charge me for a new one, or B) they may have not actually put in a new battery (as a responsible owner, i should know the serial on my previous battery so i could verify that, but i didn't)

that battery was only a year old so it seems like there may be something sick/dying in my charging system that is chewing through batteries.

ideas? suggestions? lines of inquiry i should pursue? completely alternate possibilities? its not fuel injected so perhaps the strain of stop and go is monkeying with my carbs?

[yes, i know that there are specific sites/forums for these questions, but mefi is not without its motorcycle enthusiasts so i figured id ask here first]
posted by nihlton to Travel & Transportation (10 answers total)
I don't have too much to say, but your battery has nothing to do with the idling of your bike (I'm 99% sure). If I had to guess, which I am, I would say the problem has to do with the airflow (This is my #1 guess), the carb/fuel injection or both.

Also, I'm basing this off of working with dirt bikes, but I think they're essentially the same.

Hope everything works out.
posted by DoublePlus at 2:59 PM on April 20, 2010

the trick is - why would the behavior be isolated to slow traffic? my air box should always pull air in and drive it to the carbs the same, no matter what the rest of the bike is doing.

im thinking electrical because the spark plugs may not be delivering enough for combustion?
posted by nihlton at 3:08 PM on April 20, 2010

Well, here's what I was thinking. When you're moving at a normal speed you're giving the bike gas which makes up for air it's not getting. I would suggest cleaning out your carb, it wouldn't be too much work.

I don't really know though, I actually haven't worked on a bike in about two years, haha. Just putting in my two cents.
posted by DoublePlus at 3:19 PM on April 20, 2010

No idea. Airflow doesn't make sense to me but maybe fuel flow. Maybe your jets are a little clogged.
posted by chairface at 3:23 PM on April 20, 2010

just found this:

sounds like my problem :)

so for posterity - seems i might want to look into getting an after market regulator/rectifier, or find a more clever way of dissipating heat from it.
posted by nihlton at 3:41 PM on April 20, 2010

pretty good run down:
posted by nihlton at 3:49 PM on April 20, 2010

While it's true that electrical output is at a minimum at idle, if you're reliably producing 13.1 VDC or better across the system, your bike should at least idle steadily, not crap out regularly. You may be suffering a bit of confirmation bias because of your prior issues with that bike's electrical system, but if you're having intermittent idle issues, and yet seem to be getting decent voltage from your charging system, you really need to set the electrons aside in your troubleshooting basket a bit, and consider other reasons why things are intermittently wonky. Upgrading electrics is gonna cost you a couple hundred, to several hundred bucks, and unless you can reliably correlate the engine dying with electrical events like the fan motor coming on, spending that money and time may just be a waste.

Along with some other posters above, I bet your idle issues have to do with your airbox/carbs/vacuum, too. Possibly a minor vacuum leak developing in rubber carb mounts. The '99 F4 had a fairly complicated airbox that was supposed to balance airflow to all carbs, and improve low end throttle response by keeping vacuum high through mid-range, but if any of the filter element or baffles get clogged/loose, you can get some funky symptoms, that temporarily seem disconnected from the rest of the engine performance. But at idle, air flow is as minimal as it will get, and your carbs on their restricted idle circuit, so, in standard troubleshooting method, you sort of have to start there, and demonstrate that your problem isn't airflow/fuel related, before moving on to electrical issues.

And usually, electrical issues result in temporary performance drop out at starting, or at speed, when the electrical components are under relatively more stress, than before such defects show up as idle problems.
posted by paulsc at 4:03 PM on April 20, 2010 [1 favorite]

Have you verified that the cables to the battery terminals are firmly attached?
posted by Mike1024 at 4:39 PM on April 20, 2010

ITs probably a long shot, and I don't know much about this bike (I am only ever owned dual purpose bikes) but it is possible that you might have a problem with bike overheating in slow traffic. Maybe not enough to give you a warning light or anything but enough to push marginal electrics over the edge. It is even possible you have a bad ground somewhere in the cooling fan (assuming it is liquid cooled) that is causing some kind of intermittent short in the electrical system. I would first make sure that you don't have any vacaum problems though starting with a good seal at the airbox/carbuetor junctions and then into the intakes. I would also check to make sure you don't have a gummed up idle jet? (once again my carb experience is limited to single barrel carbs where there is only the one carb for the one cylinder).
posted by bartonlong at 4:51 PM on April 20, 2010

When you're moving at a normal speed you're giving the bike gas which makes up for air it's not getting.

This shows a fairly fundamental lack of understanding of how carbs work, actually. You don't give the bike any petrol/gas at all. Only the air through the carb does - it draws in fuel to match the air flow through it according to the size of the hole in the jets. I can't see it being related to air flow through the carbs at all.

At idle, it is unlikely to impossible that you will get a variable air supply (for the air flow to be causing the erratic idle) but it is possible that you will get an erratic fuel supply. Check your fuel filter and lines to be sure that flow is not being affected. When was the last time you changed your filter? Clogged lines or (worse) clogged idle jets (super tiny passages in the carb) could cause poor idling that the standard mechanism for idle control on the bike can't cope with compensating for.

Vacuum is a more likely/usual culprit. Look for rubber hoses about 5/16" OD or smaller from the inlet to the carb and vice versa and check for cracking or splits; especially at the joins or where they pass anything hot.

So cleaning the carb is basically a no-lose situation. A carb that old could probably do with it anyway. When that is done, it's likely that any split vacuum hoses will show themselves during removal, and even replacing them all would only be (literally) a few bucks. Whenever I have had any vacuum hose issues, I'll buy a small roll of it (10 feet or so) and just replace the lot. They're so much grief compared to the cost it's not worth any kind of extended problem solving. Once you have established good fuel flow and no loss of vacuum (this is what the engine uses to balance against to know where the right idle speed is) then you can look for other causes.

It is extremely unlikely to be electrical from the symptoms you have described. The battery incident is likely to be a red herring - it didn't sound a convincing reason for replacement to me. It's not possible to detect a battery that is 'a few ignition cycles away from failing'.
posted by Brockles at 7:02 AM on April 21, 2010

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