GT550 idle at 3-4k and won't come down

[derekstudt]

I just put a new battery in my GT550J. It starts right up now, but it idles high...around 3-4k and doesn't want to come down until I put it in gear with the clutch in. After I ride around for a while and come to a stop, it will idle around 1500-2k, but after a while it will randomly idle back near 3-4k.

I've set both the air screw and idle screw according to what my clymer says...1 1/4 turns out on the air screw and 1 1/2 out on the idle screw. Adjusting either of them doesn't seem to make a difference.

If I run it with 2 of the spark plugs pulled so it's only running off of 1 firing cylinder, I can adjust the idle screw to where it will idle smoothly at 1100RPM. After adjusting all 3 that way and running off all 3 spark plugs, it idles at 3-4k.

Any ideas/help is greatly appreciated.

[pearljam724]

Idle air adjustment screw settings are a baseline recommendation from factories. Air screw adjustments can be different for a lot of people. Depending on the elevation where you live, current moisture in air, jetting, air filter, cylinder wear, fuel to oil ratio, etc. How old are your intake boots ? If an idle hangs as you described you're running lean. That can be caused by air screws incorrectly adjusted, leaks around boots or air box, jetting, incorrect idle adjustment, carbs not being synched. You can check for intake leaks by spraying starter fluid in and around suspected areas while the bike is idling. If idle increases after you spray a given area. That's where your air leak would be. The thing that most people overlook is throttle cables being on a bind or incorrectly adjusted. Check cables first. You have to have all the slack taken out of each end of each throttle cable. With the required amount of minimum slack suggested by the manufacturer or your slides will not open and close properly. This is very important in addition to the slides being in synch to one another.

[derekstudt]

Thanks pearl jam.

Intake boots are in good shape. I'll try the starter fluid idea and check all my cables to be sure they're not snagged. I'm pretty sure it's not the cables, but I haven't checked them yet either.

I've followed the manual in syncing the carbs by lining up all 3 dimples in the slides as the diagram said. This also helped ensure the oil is being injected properly as well. Is there something else I could be missing with syncing the carbs?

If it turns out it's just air/idle screw adjustment, and I'm running lean, am I correct in thinking I need to turn those out, not in?

[Weirdbeard1000]

If the mixture screws are on the air box side of the carbs they are "air screws" they adjust the amount of air allowed into the idle circuit and to make the mixture more rich you would turn them clockwise. A little bit goes a long way with these. Usually you don't want to go more than a 1/4 turn at a time.

If the mixture screws are on the engine side of the carb, they're called "fuel screws" and adjust the amount of fuel allowed to pass into the intake tract. They work opposite, clockwise for leaner and counterclockwise for more rich. They typically are on newer bikes. Usually they are adjusted 1/2 a turn at a time.

Hope this helps!

[pearljam724]

You're welcome, bud. Weirdbeard made a correct point. The easiest way of synching the slides you have to have carbs removed from the bike if you're not using a manometer. Manometers are not necessary. Looking into the bellmouths. Use a homemade gauge feeler. Anything can be made into homemade gauge feeler as long as it's solid, will not bend easily and is not tapered. Use a very small diameter solid tube for example as your gauge. Completely close the slides with your idle knob. Slide your gauge into the bottom of the bellmouth, resting on the bottom of the intake. Being very sure it is laying flat against the bottom of the mouth all the way through. At this point, turn your idle knob causing the slides to slowly open. Open them, until your homemade feeler gauge barely slips under the slide with minimal drag. You adjust the synch of all three slides with synchronizing screws to be sure that they open and close at the same exact distance or time using your feeler gauge. After they are synched, reset idle with idle knob. This is a simple way of synchronizing slides that has proven to be very accurate. If you've adjusted your timing recently. There's also a chance that your timing is slightly too advanced. The largest possibility would be your cables being incorrectly adjusted via the nuts on the pulley end. If the cables are correctly adjusted and the idle is not set too high. You can hear the slides slap the bottom of the carbs when you release from a full throttle when the bike is not running. The throttle has to snap back from a open to closed position on it's own if the cables are adjusted correctly. Oil pump adjustment can also effect slides closing far enough or not on some models. Not sure about yours.

[Cliff]

Check for any air leaks in the rubbers between the carbs. and cylinders. An air leak (running lean) will raise the idle! Two ways to check for a leak at the intake rubbers. With the bike running at idle, use an UNLIT propane torch! Open the valve on the torch and aim the tip at the rubbers. Check around all the connections. If the RPM goes up you found a leak. Or use a garden hose with the water running at about 1 gallon per minute. Again, with the bike running, soak the intake rubbers with water. If the idle drops, you found a leak!

[pearljam724]

Cliff, what's your mileage at now bud ?

[Cliff]

Cliff, what's your mileage at now bud ?

On the GT550, 165,000-170,000 miles. Not sure, I'm on my third speedo! The GT750, 2-3 miles!! I've only gone around the block a couple of times so far on her!

[pearljam724]

Cliff, what's your mileage at now bud ?

165,000-170,000 miles. Not sure, I'm on my third speedo!

How many crank and top end rebuilds ? You're my hero !

[Cliff]

One crank rebuild, just to see if I could do it myself at home. A couple sets of rings, just for shits and giggles. Replaced a cracked piston once. One rebore because, stupid me, I ran her out of injector oil and seized her! Speedos wear out faster than engines!

[derekstudt]

Update:

I spent some time making sure the slides were in sync last night. One of them was a little off so I adjusted it.

A buddy of mine happened to stop by this morning while I was attempting to tweak the air/idle screws. He's been rebuilding large farm tractor engines since he as 10 - no exaggeration! He spent less than 10 minutes adjusting the carbs and got the bike to idle perfectly. I took it for a spin around the neighborhood and have never had this thing running so smooth and strong!

I learned a ton about carbs by watching what he did.

Next week is new tires and brake shoes, then I'm going to really get some miles in.

Thanks for the advice in where to look. I'm really thankful for all the great people on this forum and your willingness to share your knowledge.