Trying to isolate the culprit of a high idle speed on my '79 425L!

[Guest]

I'm getting my bike running after a rebuild (new seals and gaskets, honed cylinders, new rings, cleaned carbs and air-filter, a few damaged parts replaced), and at first couldn't get the bike to idle without some throttle. After some GSR forum reading I went after any air-leakage and ended up patching a few muffler holes (PO laid the bike down a few times), as well as fixing a leak in the air-box cover's seal. Now the bike is idling on its own... but at about 5,000 RPM, and it rose to 6,000-7,000 when I shoved a rag into the air-intake under the seat to limit the airflow. Very little change happened when I adjusted the carb air-mixture screws from 3 turns out (manual default is 1 1/4 turns out) all the way to full closed.

This is just my guess (obviously not sure or I wouldn't be asking for everyone's advice!!!), but does this mean she is running a bit rich since the speed of the idle got worse as I decreased the air flow? Maybe a PO put in larger jets to make up for the air-leaks that I just fixed... requiring very "different" carb adjustments? Maybe this means more air leaks, that I've missed (I hope not), or unbalanced carbs (all other tune-up things are done, but don't I need a steady idle before I can balance carbs?), or any other of the dozen things that could be wrong :-P

I'm still a newbie at this, but I've been figuring this out one slow step at a time, and the input of everyone here has always helped my get unstuck! Thanks!

[chris032188]

Check the simple things first - The throttle cable have some slack in it? Does the idle adjustment screw have an effect on the bike if it has one?

[tom203]

Yes, first trying backing down idle stop screw. Did you bench sync the carbs? The throttle plates will appear almost fully closed on the idle stop screw- open too much and you get sky high idle .

[Fraphead]

No slack in throttle cables... I just adjusted when putting on the carbs to have a nice snapback and just a hint of free play. The idle adjustment screw closed some, but it didn't seem to have much effect. When I twist the throttle the engine responds by revving, and since many peoples excessive air leaks seemed to have the symptom of stumbling or engine dying when they use the throttle, I was thinking that was a sign of the air leaks being under control.

[Fraphead]

Yes, first trying backing down idle stop screw. Did you bench sync the carbs? The throttle plates will appear almost fully closed on the idle stop screw- open too much and you get sky high idle .

What is a bench synch? Is it just using calipers and feelers to get close while the carbs are off the bike? I just reassembled them and set the air-mixture screws to the manual default of 1 1/4 out (again, ended up trying out to about 3) and did the trick I heard here to make the idle adjuster screw make contact, then give it 3 turns in. I figured everything else was done on the bike. Would it be a good idea to pull the carbs off again to do the bench synch?

[jeffgs]

When the idle rev's switch the petcock to prime and pull the vacuum line to it. If it drops it may be pulling fuel from the petcock and straight into the carbs

[Fraphead]

When the idle rev's switch the petcock to prime and pull the vacuum line to it. If it drops it may be pulling fuel from the petcock and straight into the carbs

Ding ding!!! If I pull the vacuum line from the carbs to the petcock the engine runs almost right where I want it to (close enough to where minor adjustments could get it right), regardless if it was set to Prime, On, or Reserve. However... I'm not really sure what that means. I don't want to ride it like that if it is just a way to diagnose the issue... is the petcock the issue? It was super leaky when I put gas back in the tank after the winter/spring/most-of-summer rebuild, so I pulled the petcock out, cleaned, lubed, and put back together. Leaking stopped, but maybe it still has issues.

[hjfisk]

Get a new petcock

[Fraphead]

Is this starving the engine of fuel making it run lean? Or is it dumping so much uncontrolled into the carbs to make it run rich?

[Fraphead]

Another question... is getting the OEM petcock best? Or should I just make sure it fits, and get a newer, shiner, fancier petcock?

[jeffgs]

You need to pull the oil dipstick and smell for fuel as well as the plugs which should have a sooty, carbon ring. Just make sure.

I had both issues. When I fixed the petcock issue I still had leaky air box boots aanndd carb hangers so the petcock fix dropped the rpms to 3500.

I replaced mine with a new Suzuki petcock as well as the hangers and boots. Parts Outlaw has treated me well. You do not need to buy the seal seperate like I did, it will come with one. Set the idle after it's warm only or it will climb once it's warm. It will be real low cold but I use a touch of choke anyways

[jeffgs]

Another question... is getting the OEM petcock best? Or should I just make sure it fits, and get a newer, shiner, fancier petcock?

You can buy a brand new Suzuki part. I would not use the one that fits approach as you may change your normal and reserve hieghts.