Hey Im new here
I have an issue with my 1983 R100 CS. Its start fine with choke and idles fine when its cold at about 1100 rpm. without choke ofc.
But when it gets hot after about 15 min. of driving it idles about 2500 rpm. If i force it down with the clutch it just rises to 2500 again. Do u have any suggestion what's causing this?
i have checked for air leaks and didnt find any. i also checked that the choke and thottle on the carbs is going all the way back, and they do.
possible causes...
Vacuum leaks, timing advance sticking, carb passages dirty, sticking cables, mixture adjustment off, incorrect timing, and the list goes on and on.
IF all the usual suspects are accounted for...Back off both throttle stop screws until you achieve the correct idle speed WHEN FULLY HOT. Should be between 950-1050 rpm. It should stall until fully warmed up if you totally shut the throttle; This is a perfectly NORMAL airhead thing you'll have to get used to.
Never warm it up with the choke 'ON'. Turn the choke 'OFF' as soon as it fires up and maintain sufficient rpm to prevent stalling with the throttle. You should ride away as soon as the bike is started, just take it easy for a mile or so until it warms up and blip the throttle a bit so it doesn't stall if you have to stop.
Mechanic from Hell "I remember every raging second of it...
My bike was on fire, the road was on fire, and I was on fire.
It was the best ride ever!"
Welcome to the forum! Since you were able to pull the RPM down with the clutch but it rose back to the 2500 level, that suggest to me that it's not the advance weights sticking. My first thought, after checking for air leaks, is that some carb adjustments were made with the engine/carbs not at operating temperature. I would go for a ride to warm things up. Then back at home, and working quickly, put the carb settings in their nominal positions - idle mixture, idle speed screw, slack in throttle cables. With a fan blowing air over the engine, start the bike and proceed through the three stages of setting idle mixture and associated idle speed and finally throttle cable tension just off idle.
To be honest i haven't looked too much at the ignition since it got electronic ignition, i only replaced the sparkplugs.
The bike haven't run for 6+ years before i bought it 5 months ago. So i think i will order a gasket and membrane set for the carbs and get it cleaned and adjusted. I'm also considering that the guy who sold it might have adjusted the carbs, since it probably was running quite bad from the long standstill.
Btw, the bike runs and pulls great.
As GSPD said, possible vacuum leak.
I had a similar problem with my Mystic.
Found the collar threaded into the head that the carb mounts to got loose when the head got hot. Loosen the clamp on the carb on the elbow side when it’s hot and see if the carb twists easily and the rubber connector to the head moves with it. Just one of many potential problems. I’m assuming the carb diaphragm is not the problem as it wouldn’t run well. The slides wouldn’t lift well or evenly.
Doesn't help when there are multiple possibilities. Here are some signs to help target the cause.
1. Leaks. Supported if you have exhaust "popping" when backing off. If the exhaust has been off recently? Also, the input flange at the head can be loose.
2. Carb tune. If the carbs have been tuned very recently before this high idle issue started...suspect tuned when engine not hot enough.
3. Cables too tight/caught up/deteriorating. When the bike is idling ok, turn the bars lock to lock slowly, then a little quicker. See if it elicits a rise in the idle. Once again, maybe a recent tune has reset the cables with inadequate freeplay? It doesn't need much, some is important. Inspect the routing and the cable condition.
I ordered a gasket and membrane set for the carbs.
The exhaust do pop quit alot when i back of and i will definitely take a look at the intake flange, i have completely overlooked that possibility.
Regarding sticking advance timing i guess that i shouldn't be possible with electronic ignition.
Ewad wrote: ↑Thu Aug 26, 2021 1:34 pm
Regarding sticking advance timing i guess that i shouldn't be possible with electronic ignition.
On your '83 model, the trigger for the ignition is electronic (instead of points) but the advance mechanism remains fully mechanical.
The weights spread with centrifugal force to advance the timing as rpm increases.
It is not uncommon for these weights to erratically stick because of weak springs or grunge build-up.
Don't take anything apart yet, get a timing light and check the timing through the inspection hole near the dipstick;
The marks on the flywheel can vary by year and model; typically you should see a "S" (slow) at idle speed, and F (fast) or Z(zoom?) at over 3000rpm.
Mechanic from Hell "I remember every raging second of it...
My bike was on fire, the road was on fire, and I was on fire.
It was the best ride ever!"
Ewad wrote: ↑Thu Aug 26, 2021 1:34 pm
Regarding sticking advance timing i guess that i shouldn't be possible with electronic ignition.
On your '83 model, the trigger for the ignition is electronic (instead of points) but the advance mechanism remains fully mechanical.
The weights spread with centrifugal force to advance the timing as rpm increases.
It is not uncommon for these weights to erratically stick because of weak springs or grunge build-up.
Don't take anything apart yet, get a timing light and check the timing through the inspection hole near the dipstick;
The marks on the flywheel can vary by year and model; typically you should see a "S" (slow) at idle speed, and F (fast) or Z(zoom?) at over 3000rpm.
Thats just incredible information. i was so convinced that there wasn't any advance mechanism. I will definitely take a look at it this weekend.
R100 CS High idle when hot
