high idle after ride?

[26667]

So among all the other issues I'm figuring out, with carbs completely rebuilt, synch'd and balanced w/in an inch of their very lives, after riding for a bit, when I down shift, brake and come to a stop, the bike is verrrry slow to reduce rpms, and won't really come back to where the idle had been set ( right about 1000rpm). But rather stays a couple hundred rpm or so higher.

I put in new spakplug wires, checked the coils. Yesterday we put in new floats and float needles and checked a re-re-checked the level of fuel in the bowls. It's still vibrating the footpegs at way too many different speeds, ( the first symptom I keep noticing after new or cleaned plugs) and doesn't really have that sweet spot around 4200, and I bet in an hour when I look at the plugs again, it's still fouling the left sparkplug. Phoo-ee!

What the heck do you think is up? Are the idle and the plug fouling neccesssarily related?

Elec ign, but timing light says the adv is slightly ret. at speed, good at idle.

 

[20774]

High idle is usually one of two things...carbs synched when not fully warm or the mechanical advance unit is hanging. Electronic ignition - what type? Dyna III (uses the old advance unit) or Boyer (which has a built in advance curve). Can you get the RPMs to come back down to normal by slowly letting the clutch out to the point of just beginning engagement which pulls the engine RPM down to say 8-900? If you can do that and the RPMs stay there, then it's likely the advance unit. You'll need to disassemble and put some silicone or similar lubricant on the pivot points within the unit. Otherwise, I'd be sure the carbs were synched under warm conditions.

Could be the carb slides are hanging up. You might want to pull the air tubes and see if you tell if the slides are coming back down completely. Some models, not all, had springs on top of the slides to help force them back down.

 

[yaktackler]

Choke?

I had a choke on one of my carbs on my /6 that would hang up from time to time. This gave me many of the same symptoms that you are experiencing.

 

[88bmwjeff]

If you're using the original mechanical advance, then the following website should help.

http://w6rec.com/duane/bmw/advance/index.htm

 

[26667]

so near and yet...

Thanx, you guys. Just checked the plugs and they're both carbon fouled. Not as bad as prior to the new float needle and floats, but still. And L a bit more than R.

Called Bing they said the fuel level should be 1/2 in from the top of the bowl, which is a tad less gas in the bowl than the 19-24 mm range I've seen other places.

Choke cables are a hair short so I've been pushing the mechanism itself down w my finger till I can re adjust the cables properly

There's 1/2 a turn or more on the mix adj screw between so rich it stumbles and so lean it stumbles. I wonder if by leaning it out as much as possible rather than in the middle of the two extremes, that might help.

I dk what kind of ignition it has. PO installed it. The "amplifier" mounted to the top of the air box says "CAMTRON." But If I'm not seeing the F mark at about 3000 rpm, it's off, right? I see two parallel lines at idle, but no effing F.

 

[boxermaf]

It does sound like the ignition isn't quite advanced enough, and I'd suspect that theweights in the advance mechanism were sticking
except for your carb symptoms of running too rich. I am not at all familiar with the CAMTRON product you mentioned, so I guess I am not much help, cept that I'd recommend adjusting the mixture down a bit
closer to the too lean end, put in some new plugs, and see if you can't find a way to advance the timing just a bit.

 

[10564]

just ran into the same problem on a 78 r80/7, turned out to be a cracked exhaust valve. run acompression check and see what you get.

 

[26667]

don't EVEN tell me that! just did a top-end w new exh valves, guides, seat etc.

i'm convinced it's fuel delivery or spark. I just can't find the one final last thing to "fix."

I don't really believe we made a mistake, but I suppose tightening a lock nut the adjuster might've slipped a little and left an inlet valve too tight.

(Suddenly as I think about it, the inlets hadn't moved at all so we left them alone. So much for the "too-much-gas-not-enough-air" theory.)

When we took the spark plugs out for the 500 mi valve adjust, head torque and oil change, they were clean as a whistle. Ten miles later, it's running rough. I'm gonna re-check the valve clearances tomorrow and while I have the front of the engine off, look at the advance unit and see if I can figure out which are the two screws to loosen and what exactly it is I'm turning anti-clockwise(according to Clymer) to advance the spark.

$5 says I can make it not start at all.

 

[sumran]

$5 says I can make it not start at all.

I would never bet against you!

 

[20774]

Called Bing they said the fuel level should be 1/2 in from the top of the bowl, which is a tad less gas in the bowl than the 19-24 mm range I've seen other places.

Bing's measurements are from the top of the bowl down to the fuel level. The 19-24mm measurement is from the bottom of the bowl up to the fuel level.

 

[boxermaf]

As some have indicated, a high idle can also be caused by an air leak in the rubber 'hose' between the carb and the head, but then you usually have a cylinder that appear to be running more toward the lean side, and you indicated that those were running rich.

Sometimes, if the high RPM idle is due to a slightly sticky advance mechanism, one can shut the engine off when the engine is idling too fast. and then restart it - the sudden decel/accel of the crankshaft can
sometimes "unstick" the weights from their advanced position. If
after restarting the bike idles at the normal RPM (might have to try
shutting off/restarting a couple times to see if it will resolve itself) then it
is very likely that they problem is related to sticking advance mechanism.