Idle speed

[Gibson]

Hey guys, anybody out there able to get a reliable 1200-1300 rpm idle? It says 1200 +-50rpm in the service manual. I cannot get a reliable idle below about 1450 indicated.

 

[5twins]

Yes, I'm able to use 1100 to 1200 quite reliably. I could go even lower if I wanted but that's not supposed to be good for these motors, too little oil circulation and pressure. I thought the 1200 sounded a bit "busy" when I first got my 650 but I've grown used to it. I guess I was just used to my other bikes which idle down a little lower, like around 1000. When I first started riding the bike, I did try setting the idle lower but would get random stalls at stops. It really likes and needs the 1100 to 1200 setting.

Your '76 should have a linked carb set. Are they synced perfectly? It also takes a bit of fooling around to find that perfect speed setting where it won't idle too fast when hot or too slow (to the point of stalling) when cold. It can be done, just like the elusive perfect clutch adjustment.

 

[Gibson]

Thanks. It must be a vacuum leak or low compression. Will have to check both. Carbs are in sync and jetted perfectly. Also timed ignition, lubed advance shaft and checked valve clearance. It will just cough and die at the most inconvenient times at 1200.

 

[retiredgentleman]

Have you measured the battery voltage when idling at 1200? If voltage is too low then the ignition gives a poor spark. Speaking of ignition, what are you using. Check in a darkened garage, to see what colour the spark is. It should be blue not yellow.

 

[gggGary]

Gibson what year? Early carbs formed a drop of gas on the idle port hole, it would get big and fall off creating one rich cylinder load then the drop would start building again. Thanks to 2M for revealing the SB about this. I think he problem was cured for good with the 73 TX650, somewhere around there.

other possibilities a leaky choke plunger seal, float level, float leak.
Might want to cross check the tach reading if your ear isn't used to the XS650 pulse.

Darkened garages can reveal erratic high voltage leaks also. spark plug caps, condensers, an inline spark gap can show up "missing" sparks.

 

[Gibson]

Thanks Retired. The voltage makes sense. All stock 76 electrical system. I know the stock regulator is a problem, but on start up I have 12v. It starts easy enough...

 

[retiredgentleman]

Thanks Retired. The voltage makes sense. All stock 76 electrical system. I know the stock regulator is a problem, but on start up I have 12v. It starts easy enough...

With the engine idling at 1200, its best if you have 13 to 13.5 volts. Make sure your brushes are at least 3/8" long so that the alternator's rotor is getting the field current it needs.

Some lads still get by with the stock rectifier and regulator,and more power to them.(Pun not intended). However, I always recommend on 70 to 79 years, going to a modern 3 phase rectifier, and a new VR-115 regulator. Also wise to dump that #1157 bulb in the tail/brake light, and change over to an LED type tail/brake light.

 

[5twins]

All stock charging and electrical stuff? Does that mean plug caps and plug wires too? If so, I would change those out pronto. The original caps can and do go bad. They also had a rather oddball and high resistance rating when new, something like 8 or 9K ohms. These stock ignition systems were weak to begin with. They don't need 9K ohm plug caps stealing some of what little power they have. With points, you can use non-resistor caps. I recommend these .....

 

[Gibson]

Have solid copper wire, no resistors. Thanks.