Post Rejet Blues

[ElevenDollars]

I bought a '94 gs500 a few months back and I recently decided to rejet. There's a K&N lunchbox installed from the previous owner so I went from stock to 140 mains and 40 pilots. I also put one #4 washer on each needle. Turned the air screws 2.5 times. Pretty much just following the suggestions from the wiki.

I put everything back together and took her for a quick ride around the block and first thing I noticed was that when I try to take off from a stop the bike acts like it has almost no power and really chugs for a few seconds. No matter how much throttle I apply it doesn't seem to make a difference although everything seems fine after I get up to speed.
At this point it was practically unrideable because trying to get going at a traffic light/stop sign was a (probably funny looking) exercise in frustration.

After messing with the airscrews for a few days with no luck I decide on a whim to take the washers off of the needles. Another trip around the block and I am super pleased with myself because the dead spot seems to be gone. I noticed at this point that maybe the bike is running a little lean because the bike is taking just a little bit longer to return to idle RPMs than usual. Probably no big deal, I'll turn out the air screws another half turn or so and it'll be great.

The next morning I turn the air screws out another half turn and hop on the bike to go to work. When I first start it up everything seems perfect. It starts up easy with minimal choke and the RPMs snap quickly back to idle when I release the throttle.

I hop on the freeway and make my 20 minute commute and realize that something is terribly wrong. I exit the freeway and pull in the clutch and the RPMs take FOREVER to drop to idle. Probably like 5-7 seconds to go from 7k down to 1.5k.

I let it sit while I was at work and same story on the way home. When I turn it on everything seems good but after riding for a bit it hangs like crazy.

I tried putting the washers back on the needles but the dead spot just returned and RPMs still hang after getting it warmed up. I tried putting a second washer on each needle just for the hell of it and it was the same story. I tried all sorts of different configurations with the air screws and no dice.

It seems like maybe there is an air leak somewhere, but I don't understand why it seems fine at startup and only gets bad when it's warmed up. I also don't get why shimming the needles is giving me problems going from stop ---> go.

Sorry for the long story.

Short version: Rejet bike, now after it's warmed up it takes forever to return to idle RPMs. Also, shimming the needles make bike lose power when starting from a stop.

[lucas]

I had basically the exact same symptoms when one of my ignition coils died.  Check to see if one of your exhaust down-tubes is cold (<200 degrees F instead of >500).  Check that you are getting spark on both sides.  If one side is dead or has an intermittent misfire then swap the positions of the coils, spark plugs, and plug wires one at a time to isolate which part is bad.

I've read on this forum about ignition coils that work while the engine is cold and then stop working when the engine warms up.

[fakejimmymorgan]

Check for leaking o rings under the black slider caps but also in the intake boots. It WILL be something stupid with these bikes it's always the case. Check electrical connections are clean and making good contact.


Sent from my iPhone because I like owning a phone that works reliably.

[ElevenDollars]

I had basically the exact same symptoms when one of my ignition coils died.  Check to see if one of your exhaust down-tubes is cold (<200 degrees F instead of >500).  Check that you are getting spark on both sides.  If one side is dead or has an intermittent misfire then swap the positions of the coils, spark plugs, and plug wires one at a time to isolate which part is bad.

I've read on this forum about ignition coils that work while the engine is cold and then stop working when the engine warms up.

I checked and it definitely seems like the left side is not as hot as the right side. I replaced both spark plugs and when that didn't make a difference, I swapped the coils. Still no dice. What do you mean by the plug wires?

Check for leaking o rings under the black slider caps but also in the intake boots. It WILL be something stupid with these bikes it's always the case. Check electrical connections are clean and making good contact.


Sent from my iPhone because I like owning a phone that works reliably.

I sprayed some starter fluid around the carbs while idling and found a leak on one of the intake boots!! I took off both of the intake boots and used gasket-maker to make everything airtight. Sprayed some more starter fluid around and it seems like the leak is gone, but nothing seems to have changed as far as how the bike runs.   

[lucas]

The black spark plug wires are a separate part from the coils.  Although they look like they don't come out of the coils you can pull them out.

You should use/borrow an infrared thermometer to get quantitative measurements of your exhaust tubes.  Also, mark your coils "left / right" or " 1 / 2" and maybe take a picture.  I had a dead cylinder and I mistakenly thought I swapped the coils, I must have gotten confused or distracted.  When the dead cylinder didn't move with the swap I was sorely confused for many, many weeks banging my head off the walls trying to diagnose what turned out to be a bad coil.

If you found that the cold exhaust tube moved when you swapped the coil you would need to pull the wires out of the coils and swap those to make sure you don't have a simple bad wire before buying a new coil.

Have you checked that you are getting sparks on both cylinders?  Pull the spark plugs and pop them into the spark plug wires.  Hold the end of the spark plug near or touching the engine and watch for an electric arc while you crank the engine with the starter motor.

Don't be shy, getting zapped with a spark plug is not painful.  Just hold it by the rubber part, keep it grounded against the engine and you won't feel a thing.

[ElevenDollars]

The black spark plug wires are a separate part from the coils.  Although they look like they don't come out of the coils you can pull them out.

You should use/borrow an infrared thermometer to get quantitative measurements of your exhaust tubes.  Also, mark your coils "left / right" or " 1 / 2" and maybe take a picture.  I had a dead cylinder and I mistakenly thought I swapped the coils, I must have gotten confused or distracted.  When the dead cylinder didn't move with the swap I was sorely confused for many, many weeks banging my head off the walls trying to diagnose what turned out to be a bad coil.

If you found that the cold exhaust tube moved when you swapped the coil you would need to pull the wires out of the coils and swap those to make sure you don't have a simple bad wire before buying a new coil.

Have you checked that you are getting sparks on both cylinders?  Pull the spark plugs and pop them into the spark plug wires.  Hold the end of the spark plug near or touching the engine and watch for an electric arc while you crank the engine with the starter motor.

Don't be shy, getting zapped with a spark plug is not painful.  Just hold it by the rubber part, keep it grounded against the engine and you won't feel a thing.

Decided to swap the coils again today and marked em with a screwdriver so that I could keep track of which was which. No luck unfortunately. The left side was colder before and after the swap.
I also dug out my multimeter and tried this : http://gstwins.com/gsboard/index.php?topic=19546.0 and all readings came back good.

I also took your advice and tested to confirm that I am getting spark on both sides.
When I took out the plugs, first thing I noticed is that the left plug has little bit of black sooty stuff on it and the right one is kind of white on top. I got spark from both plugs, though the spark on the right side was more purple and the spark on the left side was just white/blue. Dunno if that means anything but there's definitely spark.

Something interesting I noticed: I can start the bike with only the right side spark plug installed and the left plug removed, but I cannot start the bike with the opposite configuration - unless I open the throttle. If I have the left plug in and the right plug out the bike will start and run as long as the throttle is open, but it won't idle.

Similarly, I can remove the plug wire from the left side plug while idling and nothing changes, but if I do the same to the right side plug, the bike dies. If I open the throttle and then remove the right side plug the bike will run until I close the throttle.

It seems like the left cylinder is not firing at idle?

At this point I'm guessing the issue lies in the carbs somewhere so I took them off again. Everything looks really clean in there. No clogs or build up or anything that I can see, but I sprayed some compressed air through pretty much everything I could find to spray it through just in case.

I did find when I removed the air screws that one of them was missing the tiny washer between the o-ring and the spring. Dunno where it could have gone, considering I've never taken them out before.  I can't imagine that would be the source of all of this, but I'll try and track down a new one before I put the carbs back in the bike I guess. Not sure if I can get one locally or if I'll have to order a new air-screw. I'll check around tomorrow.

Thanks for all the suggestions so far, I really appreciate your guys' help.

[lucas]

If you can run the engine on the left cylinder with the throttle wide open that seems to indicate that you are getting spark and also fuel through the main fuel circuit.

So something is up with the pilot circuit... I've heard it on this board that teeny tiny bits of grime or rusty can foul a pilot jet.  There are orifices in the carb body itself that the fuel passes through to get to the intake air.  You can try cleaning those out.

Hey something you might try is putting your old pilot jets back in.  That might help isolate the issue.

Can't wait to hear the solution to this.

[The Buddha]

Couple of points to make. I didn't read all your posts - just skimmed em.

1. K&N with stock pipe is hard to rejet. If you cant go bacck to airbox and paper, go to a slip on.
2. I think 140 is still too small, maybe 145 or 142.5.
3. Your float level may be off. Especially one higher than the other.
4. Some fuel passage is blocked. (or more likely the air screw passage is wallowed out by someone gorilla tightening the air screw.
Cool.
Buddha.