Long-term problem: misfire at 1400-1500rpm

[Meuryn]

Hi everyone!

I've had a long-running problem with my GS; it has a tendency to misfire at the 1400 to 1500rpm range. Giving the bike a tiny amount of throttle to reach those rpms causes an audible bang or clunk, coinciding with a 200 to 300 drop in rpm. There is no such misfire when coming off the throttle at higher rpms and letting the engine fall through the problem rpm range.

I'm in the process of disassembling the carbs, and will thoroughly clean them - my theory is that one of the pilot jets / idle circuit components is blocked, causing a misfire in one or more cylinders.

All the screws holding on the top bowls are seized and the carbs are currently at the mechanics awaiting their removal, so I've got a few days to mull over my approach to carb cleaning / what to look at in particular.

I'm no expert mechanic, and even less so when it comes to carbs, so are there any other solutions / causes I should be considering?

Hope someone can help, thanks in advance for your help!
Cheers,
Meuryn.

[radodrill]

It would help if you provided the year of the bike, mileage, location (country) in which the bike was originally sold (when new), and what (if any) modifications have been made to the intake or exhaust.  Also, what pertinent services (e.g. valve check/adjustment) have been done recently.

Since these bikes are carbureted, the intake and exhaust can make a big difference on how the bike runs.  Also, there are different carbs for the 01+ vs the 89-00; and the needles/jetting varied based on the market sector it was delivered to.

Any idea where the noise is coming from? (top of the engine, crankcase, muffler, etc)

[Meuryn]

Hi there

The bike is a UK '02 E model, with 30,000 miles on the clock. Valve clearances were checked about 8000 miles ago, and the bike has a Delkevic stainless can attached to stock headers (this one: http://www.ebay.co.uk/itm/Suzuki-GS500E-F-89-02-350mm-Oval-Stainless-Steel-Silencers-Exhaust-Kit-/271214616592?pt=UK_Motorcycle_Parts_13&hash=item3f25a6a010)

It's hard to tell where the noise is coming from, it seems to be best heard near the carbs themselves, although I wouldn't rule the noise out as coming from the cylinders (might just be the typical misfire sound?). The noise definitely isn't coming from the muffler or the headers.

The jetting is completely stock, so is the air filter and all other parts, BUT I'm planning on fitting a K&N air lunchbox, and rejetting to 20/62.5/140 to get some more performance out of the bike. The problem has been there since I bought the bike in April 2013, although the owner neglected to let me know. It doesn't look like the carbs have ever been opened and cleaned before, judging by the way the screws were seized absolutely solid.

Hope that helps, and thanks for your reply
Meuryn.

[BockinBboy]

Check this thread out... it sounds like the same issue, or at least very similar... turns out a few of us have experienced what is in that thread too.

http://gstwins.com/gsboard/index.php?topic=66633.0

The solution was as simple as adjusting the idle mixture screws.

- Bboy

[Meuryn]

Oh wow, that does sound very similar! Is there a thread / guide on resetting and changing the idle mixture and other mixture screws? Can't find much on the wiki.

Thanks for the reply!
Meuryn.

[radodrill]

All the new info certainly makes it sound like the idle circuit is running lean.  As I said before intake/exhaust changes can make a big difference;  less restrictive exhausts and air filters result in a leaner mix, unless you rejet to compensate for the increased air flow.  Sometimes you can get away without rejetting when changing the intake or exhaust, but it's best to rejet for any change; when changing both a rejet is a must.

For now you may be able to get away with simply adjusting the mixture screws until you get the LB installed.

You might want to give this thread a thorough read; it has a lot of TLDRs on rejetting the 3-jet carbs.
http://gstwins.com/gsboard/index.php?topic=66638.0

[Meuryn]

Hi again everyone,

Thanks for all the replies!
Got the carbs open today and oh boy is there a LOT of rust settled in the float bowls, and more or less everywhere else. I'm guessing this is all coming from my tank, is there anyway to prevent rust from getting into the carbs?

Cheers,
Meuryn

[radodrill]

clean the inside of the tank; some people even coat the inside (IIRC with POR15) to prevent additional rust issues.

[Meuryn]

Just got round to pulling out both idle air mixture screws, and found that one didn't have a washer or an o-ring at the end of the screw and spring where the other one did. Going to try and locate the replacement parts now, hopefully this solves the problem...

[1990dtgl98]

I have the exact same problem and was just about to post a video!

Anyways, I changed the pilot and mains (to 20 and 132.5 I believe... Thanks buddha!).

Still does it.

Think I should turn the air mixture screws out another turn (to 4), the opposite way and turn them in to 2 turns?