engine RPM hanging up

[madjack56]

My 1976 XS650C is running a Pamco electronic ignition and stock (re-built) carbs. It runs perfectly, but tends to resist returning to the proper idle RPM when warm. I have explored every possible cause I can think of and have come to suspect that the spark advance is hanging up. I've installed two new springs on the weights, but it still fails to slow down as it should. If I slowly let out on the clutch while holding the front brake, it will drop down to the correct idle and stay there. Anyone have any thoughts or know where I can get some heavier springs for the advance weights?

 

[weekendrider]

Have you considered the throttle shaft seals?

Other springs I have tried tend to open to quick or too late. I gave up and sent them to 5twins for his consideration. There was a thread on this on the old 650Rider site.
I don't remember a exact solution or outcome.

 

[5twins]

I tried a couple of those WE and they're not quite right. In fact, one is in use on a local guy's 650. When he got it, it only had one spring, lol. It doesn't work quite right but it doesn't seem to bother him. There's a bit of a lag or flat spot just off idle. Once the RPMs climb enough, the advance snaps to full. I managed to score a couple sets of N.O.S. originals off eBay and hope to be able to talk him into a set. They were only $10 so he should go for it.

Madjack, you mentioned new springs but did you service the advance rod? It needs to be greased and most haven't been since they left the factory.

 

[madjack56]

The rod has been greased. It operates smoothly but I still think the springs are a tad too light.

 

[pamcopete]

madjack56,

The other potential cause for idle hover is un synced carbs. One of the carbs is right on the edge of working or not working, so when it decides to work, the engine speeds up.

 

[madjack56]

The carbs have been synched

 

[5twins]

Well, the usual "fix" is to stiffen one spring a bit by clipping a loop off it. Clip the "hook" off one end then bend out the next loop to form a new hook. Just do one spring only, that's usually enough. Obviously, this is a "no going back" mod. If you still have the old springs, experiment by clipping one of them.

 

[gggGary]

It will idle when cold? Hook up the timing light and watch the advance cold and hot.
One thing that will point at a carb issue is where the idle stop screw is set. If your screw is extended much more than this,



you have some carb work to do yet.

 

[TwoManyXS1Bs]

On my BS38's, the idle stops are about 1/3 turn from fully closed.
Wonder if that's the same on later carbs...

 

[gggGary]

2many that wouldn't be the carbs with rejiggered throttle plates and choke plungers???????





Just a few more pics from my bikes that are running well.

 

[koendd]

have you checked for air leaks?
had the same issue with my CB750 a while back...

Had an airleak between carbs and the head..

 

[funky]

check the "t" piece between the carbs for leakage. are your mixture screw set right. it could be many thing my bike idles low when starting cold but hold 1200 rpm when warm.
when you balance your carbs make sure they are both the same hold them to the light to make sure the butterflys are closed then blow through them like your playing a trumpet this will highlite any leakage ... any slight gap will increase when carb bodies get hot. also mixture screws must be set right. with washers spring and rubber seal in place... screw them in slowly until they bottom out then 3 1/2 turns out...
all circuits in the carbs checked to be clear and clean ...float levels to spec also. make sure floats are fully bouyant.
check breather pipe from petcock if you have one. or while the bikes running spay alittle break cleaner on the carb seals and air intake seals if she speeds up you have a leak.
best of luck ....

 

[TwoManyXS1Bs]

2many that wouldn't be the carbs with rejiggered throttle plates and choke plungers???????

Haha, rejiggered. New throttle shaft bushings, throttle plates aligned, rebuilt choke plungers and seats, tryin' to undo 44 years of life? I reckon so.

 

[XSLeo]

funky, he has a 76, the 76 didn't have the tee between the carbs, it used two fuel lines with a rubber crossover tube.
The 76 used only 1.5 turns out on the mix screws.
My best guess is the advance springs are weak. Thus not pulling the advance closed. When you use the clutch to pull down engine rpms, the weak springs can then pull the advance closed.
Leo