tighten air screw for fast idle when hot?

[d.bradley]

Hey folks,

I'm tweaking my '83 450T trying to get it running properly. When the bike heats up (city riding for about 10-15 mins) I found the idle was creeping up to 2000-2100rpm. No good. It idled nicely when cold (without choke) so I figured the whole idle was set a little fast.

I've sprayed WD40 on the carb seals and get no throttle response from that (good!), so I cranked down the air screw a lot, and it seems to have settled the idle on a fully hot bike to 1400-1500rpm. Obviously performance at speed is still fine, idle is smooth enough.

Am i going to foul up the plugs or run into any problems having done this? Is this the proper way to set the idle speed?

Love this forum...what a great reason to own a GS

[doctorgonzo]

I have a bit of that. When bike comes off choke, idles right at 1100, but when it gets hot, goes up to around 2000.

[BassCliff]

Hi,

From what I've learned, this symptom usually indicates an intake air leak, possibly the intake boot O-rings. The WD40 spray trick is not terribly reliable and catches only the worst of air intake leaks.

Is your air filter properly maintained? Airbox sealed? Carb boots good? Keep us informed.

Thank you for your indulgence,

BassCliff

[Nessism]

Lean idle mixture causes the speed to go up. Most likely cause is carb boot o-rings leaking. Could also be pilot screws too lean. If you haven't changed the intake boot o-rings I'd do that first - if they aren't leaking now, they will be soon enough.

[d.bradley]

Well I've tightened the air screws on both carbs, and things have calmed down on idle to just under 1500 at full operating temperature. I'm getting a little bit of exhaust noise (poping, not sure what the proper term is) while engine braking, but otherwise things seem fine.

[seuadr]

i'm having a similar problem, and having just replaced the o rings and inspecting the boots ( which were still pliable and seemed fine ) and sealing the airbox, i've come to the conclusion that the pilot jets may need ajusting. i have the CV style carbs, and i'm not sure, nor do i have my manual handy till this weekend, do i want to "tighten" or "loosen" the pilot to enrichen the mixture? i started at 2 and 1/2 out, i'm going to try it out with 1/4th turn increments, but i'm not sure if i want to turn them clockwise or counter to richen. anyone?

[mcloud]

LOL, same problem here.
New intake boots and orings, sealed the airbox per tutorial with new air ''hoses'', dipped and rebuild carbs with new orings.
Cold, when off the choke, idles around 1000 rpm.
When riding, idle goes up to 2500.
Jiggling with the throttle doesnt do much.
What adjustment am I missing?
Haynes doesn't adress in troobleshooting section.

[Nessism]

Open the pilot screws to about 2.5 turns from lightly seated and then adjust them further after the engine is warm to achive the highest idle.

Sync could cause the idle to wander but most likely cause is lean mixture. Intake boot O-rings and carb o-rings need to be fresh for best results.

Good luck.

[chef1366]

Out of sync carbs in my experience surge at idle.

[seuadr]

Open the pilot screws to about 2.5 turns from lightly seated and then adjust them further after the engine is warm to achive the highest idle.

Sync could cause the idle to wander but most likely cause is lean mixture. Intake boot O-rings and carb o-rings need to be fresh for best results.

Good luck.

ok, but do you turn the pilots clockwise or counter clockwise to enrichen the mixture?

[Skateguy50]

what is an acceptable range?

When I start up I can back off the choke fully in about 10-15 seconds and the bike holds an idle right around 1k, then when its warmed up at full temp it holds idle around 1.5k. Im happy with it but is that a normal range?

[Nessism]

ok, but do you turn the pilots clockwise or counter clockwise to enrichen the mixture?

Out is rich, so that's counter clockwise. Most stock GS's run best with the screws set at roughly 2 - 3.5 turns, if you are outside that range, something is most likely wrong.

[johnbtrask]

Re Idle speed increase as engine warms

Assuming that there are no leaks in the intake tract between carbs and head, and the bike starts ok without the choke then your mixture is set too rich.

The choke enriches the mixture to compensate for fuel that condenses on cold intake walls, once the engine reaches operating temperature it should idle and run at around 1000 RPM without the choke.

If your engine starts without the choke then when the engine gets warm your mixture is definitely going too rich. Make sure that it is well warmed up before attempting to make any adjustments to the carburetors.

There is a really good description at this site about setting up these carbs.

http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html

[DimitriT]

You need to adjust the pilot screws and the throttle stop knob (under and behind the carbs) to get the right mixture and amount for your idle. If you think you've eliminated all the air leaks and your carbs are clean start with the idle mixture screws.

Keep in mind that those screws are the fine tune adjustment for the amount of fuel you are allowing into the motor at idle. At these small throttle settings, the amount of air you are allowing is controlled by the throttle knob. You need to adjust both to get the right balance of air and fuel.

If your motor surges as it warms up that means it is running too lean. This is because the ideal air/fuel ratio depends on the motor's temp. As it gets hotter it becomes more efficient at burning the leaner mix. You need to enrich the idle mixture.

Try backing out the idle mixture screws 1/2 turn. This will probably raise the RPMs while the motor is cold but it will have no effect while the motor is hot. Now reduce the air by turning the knob so that the RPMs fall into the 1000 to 1100 RPM range.

This enriched idle mixture will cause the motor to run stable when it gets hot: no surging. The mixture is rich enough so that a hotter motor is not more efficient. At the same time a cooler motor will run better and come off choke a little sooner.

You need to find that balance where the mixture is rich enough to prevent the surging but not too rich to cause the plugs to foul. Another useful trick is to set the idle mix screws using the "highest" RPM method:

* warm up the motor
* set the RPMs to about 1500 - if that's hard to do then you are way out of adjustment and need to follow the above steps to get there
* turn a single screw (slowly) until you notice the RPMs drop slightly- you are now at the lower end of the mixture range to allow the cylinder to fire on idle
* turn the same screw out about 1/2 a turn - this is usually the middle of the range

You may find that the middle of the range is a bit lower or higher - you can find the middle by turning the screw out more until its running too rich. The RPMs will again drop. This is the upper end of the range. You want to set the screw halfway between the lower and upper ends. It doesn't have to be exactly half way: there's a working range which seems to do fine for all conditions. I keep it on the leaner side for fuel economy. You may get smoother cold running if you keep it on the rich side.

[seuadr]

Assuming that there are no leaks in the intake tract between carbs and head, and the bike starts ok without the choke then your mixture is set too rich.

The choke enriches the mixture to compensate for fuel that condenses on cold intake walls, once the engine reaches operating temperature it should idle and run at around 1000 RPM without the choke.

If your engine starts without the choke then when the engine gets warm your mixture is definitely going too rich. Make sure that it is well warmed up before attempting to make any adjustments to the carburetors.

There is a really good description at this site about setting up these carbs.

http://www.factorypro.com/tech_tunin...m_engines.html

thanks for the link. i've ajusted my idle pilots out and it's a much happier machine. i think i still have a little tweaking to do as it still hangs at around 1500 for a few when the throttle is comming down, but MUCH better than it was!