Having a bit of an issue with my Dad's 2001 GS500. We bought the bike several years ago and it has over time started to run more poorly... it's still been rideable but has been losing power at certain RPM's and has for several years now been sputtering somewhat at around 2k RPM. I decided it was probably time to clean the carbs (this bike has the newer style Mikunis) so I took them apart, cleaned out the rust, cleaned all the jets and openings, replaces the float bowl gaskets, inspected the diaphrams (they looked fine), and put them back together and on the bike. I don't have a manometer so I did a mechanical sync using drill bits. Mixture screws were 3.5 turns out on each carb (please confirm if this is correct). I put the carbs on without the air filter so I could see how the bike runs... it idles just fine and seems to run fine at higher RPMs, but as before, one of the carbs "sneezes" or spits sometimes at around 2k RPMs, which it appears is when the slides start to move. In my case it seems to just be the left carb that's doing this and as it sneezes, you can actually see a mist of gas coming out of the two lower openings in the carb body. Here is video/audio to compensate for my crappy description :cool:
https://www.dropbox.com/s/i7cojgd6bbf6zmm/VID_20140817_123728440.mp4
Any ideas on what might be causing this? Does it sound like the carbs are out of sync (I tried playing with the sync screw in both directions and it was still happening)? Perhaps it's the air/fuel mixture, or some other adjustment I'm not aware of? Or could it be something unrelated to the carbs such as a spark or valve issue? Thanks in advance!
When last did you check valve clearances?
I have th same thing on mine... Every once in a while right as I open the throttle it "sneezes" or pops out of the carbs... Also sounds like the left side. I know that my valve shim clearances are tight so I'm thinking that it is allowing a valve to momentarily hang open on the compression stroke?? Idk I will report back after I get the shims changed.
I've heard this termed 'lean misfire' on here. For some folks, just turning the idle adjustment screw was able to sort it, for others, it may take some carb work such as cleaning the pilots or even increasing the jet size. I had it on my '04, and it didn't go away until I did some carb work, which included rejetting for an aftermarket exhaust and a carb sync - But I did a bunch of things all at once, so I'm not sure which specific thing got rid of it.
Here are a few threads detailing the issue:
http://gstwins.com/gsboard/index.php?topic=66633.msg801785#msg801785
http://gstwins.com/gsboard/index.php?topic=66853.msg804812#msg804812
Halfway through this thread HPP8140 brings the issue to the table
http://gstwins.com/gsboard/index.php?topic=46920.msg815085#msg815085
- Bboy
Thanks for the quick replies. I haven't checked valve clearances on this bike at all (it's currently at 10,700 miles). Probably should do that.
Quick question about the idle mix adjustment screws. On these newer style Mikuni carbs, how many turns out is the factory spec? Also, which direction would I need to turn the screws to enrich/lean out the mixture? I've heard that the carbs are tuned lean from the factory for emissions purposes so I was wondering if enriching the mixture might help the issue and make the bike run better in general.
Many thanks!
Mine used to do it when cold without choke. It is a combination of lean jetting, retarded ignition and (probably) tight valve clearances. I recently reset all of my clearances on the LARGE side of things as per another thread as part of a Major Service. I have only ridden the bike three times since then but have not noticed the carbies "popping" as they sometimes used to do. My carbies have never been touched since new and the ignition/exhaust are standard. It was never a problem so I never bothered to do anything about it.
As I said, times are early, but it seems to run smoother with the wide valve clearances.
I hope to get out for a 250km run tomorrow if the rain stays away.
Macka
Used to be OK and getting worse ? Float drift, air filter geting dirty, exhaust leak, intake leak, jets begining to plaque up etc etc.
Set floats, clean the filter and check for intake and exhaust leaks and clean carbs are remedy for these..
Cool.
Buddha.
Thanks for all the replies. I don't know if it makes any difference, but the airbox is not on the bike currently... any chance that the sneezing will go away once the airbox is put back on the bike? I think the valves may be a factor, but the annoying thing is that the sneezing is worse now AFTER I took the carbs apart and cleaned them. The diaphragms looked good and I changed the little o rings as well as the carb bowl gaskets. The only o ring I didn't change is the one around the main jet... maybe those are leaking?
I thinks that the air box and filter may help you as it will provide a small intake restriction and slightly richen the mixture.
I serviced a Yammie VStar 650 a while ago and took it for a test ride around the block with the air fliter still off the bike and it was absolutely "gutless". It would barely rev to 4500 rpm. With the air box and filter in place it was back to normal. The carby jetting is set with the known "standard" intake and exhaust restirctions. An EFI engine will not suffer the same problem as the ECU largely adjusts automatically.
Most of those O rings should be OK. Airbox may or may not help the issue. So basically follow the fundamentals man, it may just need cleaning and adjusting to spec.
2k sneeze may not be lean misfire, it may be rich. Is it acting up when you open throttle or on steady throttle ?
Cool.
Buddha.
Thanks. If I rev it up quickly past 2k, I don't think it acts up then. It happens most when I rev it slightly up to around 2k and hold it there.
Quote from: pudzley on September 04, 2014, 09:39:57 AM
Thanks. If I rev it up quickly past 2k, I don't think it acts up then. It happens most when I rev it slightly up to around 2k and hold it there.
What else can I say ... dont hold it @ 2k ... this aint no Harley, 2K is where my idle is set @, maybe it sneezes too. I would give it some baby Robitussin. Cos its my baby and its got a cold ... lol ... no idea... but 2K is idle circuit and 0-1/8th throttle is Pilot jet. Maybe you ahve a little bit of a transistion spot there where its over rich.
Cool.
buddha.