Inconsistent Idle

Gpaws1968

Gpaws1968

XS650 Enthusiast
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Hi all, I will be working on the bike in winter, I have a new coil and plug leads, and I will pull the carbs.

I took it out yesterday for a 50-mile ride and in the beginning after a difficult cold start the idle was 1000 to 1500 RPM but by the end of the ride it was racing at 3000 RPM, it's also backfiring.

1981 XS650 Special 2 SH, no modifications
 

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I imagine that the carb guys will check in, but, chances are the stock EPA jetting is a touch lean and your carb set needs a sync adjustment. There is the possibility that you have an air leak also. Check carb boots and vacuum lines to the petcocks.
Verify you ignition timing is correct or 2 or so degrees retarded.
 
Sounds like your carbs need a good going through and cleaning, maybe some new o-rings as well. The hard cold starting may be due to the choke feed jets being plugged or partially plugged. The chokes won't work if they are and that makes cold starting difficult. The choke feed jet is located off to the side at the bottom of the bowl. If there's gunk in the bowl, it will get sucked in and plug the jet when you put the choke on .....

Choke Jet.jpg


The jet is pressed in place so can't be removed for cleaning. You need to flush and blow it clear with carb cleaner and compressed air. The area below the jet can also get packed with gunk so you may need to dig that out. To check if the jet is clean and clear, do this .....

ChokeJet2.jpg


There is a tiny o-ring on the bottom of the mix screw that seals it in it's hole. If it's gone bad, the bike won't idle right .....

Mix Screw 80'-on.jpg


An old original is pictured here on the top screw. It's all flattened out and deformed, and needs replacing .....

MixScrewsCompared.jpg


From the factory, the mix screw hole is capped over with a metal plug .....

mixtureadjustmentplug.jpg


You'll need to remove that to get the mix screw out, and you must remove the mix screw and it's various parts (spring, washer, o-ring) to do a proper carb cleaning.
 
I've been working on the Yamaha carbs, I got a rebuild kit from Mike's XS, went one size up on the main and pilot, and cleaned the carb in an ultrasonic cleaner. I got it all put together and put them on the bike with rough settings, it started the first kick and could rev it straight away and after a couple of minutes, I pushed the choke in and ran great. It's a different bike.
However I finally got the bike put back together and took it for a spin and again it ran great, but it is still racing at idle 2000 or 2500 RPM or if I turn the idle down at the next stop sign it will probably stall, I can't figure out why, do I need to go richer in the idle jet, richen or lean out the fuel mixture?
 
Gpaws1968 said:
I've been working on the Yamaha carbs, I got a rebuild kit from Mike's XS, went one size up on the main and pilot, and cleaned the carb in an ultrasonic cleaner. I got it all put together and put them on the bike with rough settings, it started the first kick and could rev it straight away and after a couple of minutes, I pushed the choke in and ran great. It's a different bike.
However I finally got the bike put back together and took it for a spin and again it ran great, but it is still racing at idle 2000 or 2500 RPM or if I turn the idle down at the next stop sign it will probably stall, I can't figure out why, do I need to go richer in the idle jet, richen or lean out the fuel mixture?
Click to expand...
No. Don’t mess with the jets. You have an air leak or you’re grossly out of sink. Your bike should run just fine on stock jetting.
 
These are old bikes and they don't compensate for motor temp differences like modern bikes do. You need to set the idle speed when the engine is fully warmed up and hot. It will idle slower, maybe even want to stall, when first started and cold. That's just the way they are. So, if you set it so it will idle when the engine is cold, the idle speed will climb and "race" once the motor is hot.

The idle speed spec for these is 1200 RPM and that's what you want to use. Don't set it any higher or the motor may start in on the ignition advance curve and start speeding up on it's own. It's sort of a self-feeding problem - as the ignition begins to advance, the RPMs climb, which causes more advance and more RPM increase. The motor can end up "racing" at 2000 to 2500 RPMs all on it's own without you even touching the throttle.
 
Last edited:
jetmechmarty said:
No. Don’t mess with the jets. You have an air leak or you’re grossly out of sink. Your bike should run just fine on stock jetting.
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5twins said:
These are old bikes and they don't compensate for motor temp differences like modern bikes do. You need to set the idle speed when the engine is fully warmed up and hot. It will idle slower, maybe even want to stall, when first started and cold. That's just the way they are. So, if you set it so it will idle when the engine is cold, the idle speed will climb and "race" once the motor is hot.

The idle speed spec for these is 1200 RPM and that's what you want to use. Don't set it any higher or the motor may start in on the ignition advance curve and start speeding up on it's own. It's sort of a self-feeding problem - as the ignition begins to advance, the RPMs climb, which causes more advance and more RPM increase. The motor can end up "racing" at 2000 to 2500 RPMs all on it's own without you even touching the throttle.
Click to expand...
It's doing it when it warm
 

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Yes, that could simply be because you set the idle speed when it was cold, fast enough so it would stay running. But, once the engine warms up, the engine is going to naturally speed up. You need to set the idle speed (at around 1100 to 1200 RPM) on a fully warmed up engine. Set it on a cold engine and it will run too fast once warmed up.

When the engine is cold, you may need to keep it running by cracking the throttle a hair or blipping the throttle. As the motor warms up, the idle speed will gradually increase.
 
As far as the jetting goes, one up on the mains is good even on a totally stock machine. These BS34s came set up very lean from the factory. But you may not need the larger pilot jet size. To make the idle circuit just a little bit richer, you can reduce the air jet size instead. This doesn't have quite as big an effect as changing the pilot jet does. With it, you can achieve richness levels in between pilot jet sizes. So, with the stock 42.5 pilots, reduce the air jet (135 stock) a size or two (132.5 or 130). This will make the idle circuit a little richer, but not as rich as going up to a 45 pilot jet.
 
5twins said:
As far as the jetting goes, one up on the mains is good even on a totally stock machine. These BS34s came set up very lean from the factory. But you may not need the larger pilot jet size. To make the idle circuit just a little bit richer, you can reduce the air jet size instead. This doesn't have quite as big an effect as changing the pilot jet does. With it, you can achieve richness levels in between pilot jet sizes. So, with the stock 42.5 pilots, reduce the air jet (135 stock) a size or two (132.5 or 130). This will make the idle circuit a little richer, but not as rich as going up to a 45 pilot jet.
Click to expand...
My XS650SJ service manual lists two sizes for the main jet, depending upon where in the book I’m looking. I run with the larger of the two. My air jet is reduced as well. Many thanks @5twins .
 
OK, everyone, I have just found the culprit the vacuum tube that goes to the petcock was disconnected at the tank so I didn't notice it, just took it for a spin, and made a couple of adjustments to the hot idle and bang, she runs beautifully. Thank you all for your help and kudos to those who were saying it was a vacuum leak.
 
With the vacuum petcock not connected to a vacuum source, it wouldn't flow any fuel, except in the "Prime" position. That position bypasses the vacuum function and just flows like a normal petcock. And yes, with the vacuum hose disconnected, that would create a pretty big vacuum leak, lol.

Keep an eye on your plug color because like I said, a one size larger pilot than stock may be too much for an all stock machine. If the stock pilot is too lean but one size up is too rich, then "fine tune" the circuit with the air jet. This is a nice feature on the BS34s that many aren't aware of and don't take advantage of.
 

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