78 XS650E Running rough. Idle rises to 3k when warm.

[Chicken_Feet]

Hi team,
I need your helps. My bike started running rough up in the mountains, back firing, spitting. I thought it was altitude sickness so I was playing with the fuel mix screws trying to get it to idle properly but it turns out it was the points. I have replaced them and set them to factory spec. As I have done with the fuel mix screws, 2 1/4 turns out.

At this point, the bike starts with the choke on but revs like crazy until i drop the drop the choke. But then any throttle kills it. As it warms up it, the idle rises to +3k at which point the throttle works. When I open the choke the idle drops to a normal(ish) level. The bike is completely stock so I wouldn't think I am running lean unless there is an air leak. I have ordered some new intake boots to be safe but spraying card cleaner on the current boots didn't change the idle.
Any wisdom to impart? I'm at a loss.

thanks
Mat

 

[retiredgentleman]

Have you checked that the ATU shaft and fly-weights move freely. Open them up and release.............do they pull back in smartly? The BS38 carbs use an O-ring on the needle jet. If that O-ring is worn out, it allows unmetered air/fuel mixture to flow. Air leakage is a very common cause of high rpm hovering...............that could be throttle shaft seals or even a screw on the side of the carbs leaking air.

 

[5twins]

It could be one of several things, or a combination of them. Your idle speed may be set too high for starters. The ignition begins it's advance at maybe 1500 to 1800 RPMs. As it advances, the engine speed increases. That increasing speed makes it advance even more. It's sort of a self feeding problem. The idle can climb up to 2 or 3K on it's own without you even touching the throttle. So, set the idle speed to 1200 when the bike is hot.

You may also need to service your advance unit and advance rod. The advance rod has to be kept greased so the advance unit will rotate back and forth between full advance and full retard easily. If it's dried out, the advance unit may be hanging or sticking in the full advance position, or maybe not turning to full advance like it should.

 

[Chicken_Feet]

Great, thanks for the tips guys!
I just got the new springs and will grease up the rod after a good clean!
Hopefully new points, new advance springs and grease, new condenser and a tolerance check on all components will sort the issue!

Maty

 

[5twins]

If you grease up the advance rod, you may not need the new springs. Do that 1st and see.

 

[Chicken_Feet]

Greased up the advance weights which worked a treat. Is there any special trick to removing the advance rod itself?
So having the high idle fixed another problem has arisen. The same issue i had originally.
It was running great for about 20 minutes until it got warm then it started missing on one cylinder. I can figure it out. Maybe bad coils and condenser. More investigation is needed.

 

[5twins]

I remove the advance rod from the points side. I leave the points cam in place and remove the little disc from the advance side that ties it into the advance unit. You may have to hold the points sets open because there's a little lip on the bottom of the points cam that can catch on the points rubbing blocks.

When the mis-firing starts, observe the points sets while the bike idles. Is the one for the missing cylinder arching and sparking like crazy? If so, that's an indication that it's condenser has gone bad. But it's also common for coil problems to only show themselves when the coils get hot.

 

[Chicken_Feet]

Great, thanks for the tips!
Ill have a look on the weekend!

 

[Chicken_Feet]

I remove the advance rod from the points side. I leave the points cam in place and remove the little disc from the advance side that ties it into the advance unit. You may have to hold the points sets open because there's a little lip on the bottom of the points cam that can catch on the points rubbing blocks.

When the mis-firing starts, observe the points sets while the bike idles. Is the one for the missing cylinder arching and sparking like crazy? If so, that's an indication that it's condenser has gone bad. But it's also common for coil problems to only show themselves when the coils get hot.

So, Replaced the coils and condenser. One problem solved. It doe not start missing when warm now. Huzzah!
But, the points are arching at low revs. What would be causing that? I have set the gaps to factory spec. past 2-3k revs it doesn't arch any more.

 

[5twins]

Some minor sparking of the points is normal so you're probably OK. I'm not sure what caused your misfire. Bad condensers or bad coils could both do it. The points need lots of attention. You'd best inspect their condition every 500 miles or so. Check the faces for pitting. It's not unheard of to get new bad condensers. They need to be electrically matched to the system or the points can pit badly and quickly. Sometimes they're not.