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Old 04-09-2013, 04:50 PM   #1
Highwood OP
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Location: Banff, AB
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R100/7 High Idle after long warm up

Bike:1977 r100/7 43K Kms (no bean can, no electrics, mechanical advance)
Bike starts well with choke, idles at 900
After 2-3 km warm up, choke off, idles about 1100
After 20-30 minutes of aggressive throttle, idle "sticks" at 2500-3000
If I drag the rear brake to less than 2500 rpm, bike returns to normal 1100 idle
If I turn off the key and restart, bike returns to normal idle
Carbs have been rebuilt (all gaskets/o-rings/jets/needles/floats etc.). Mix, idle and balance set by 55-year-old, factory-trained bmw tech.
Timing: seems good, but not personally verified by me, but the engine and bike are smooth and robust, no flat or dead spots in the rev range.
Valves: good, properly adjusted including valve lash.
Heads: retorqued and pushrod seals re-sealed
Cables: in good repair
Intake gaskets: no leaks per test with wd40, gaskets fresh in the last two years.
Mechanical advance: carb cleaner, varsol dunk, silicone lube - seems ok, though "up-down" sloppy? But I have nothing to compare against.
So, why does the bike idle high after 30 minutes of riding, but not after 10 minutes?
Given I can drag the brake, force down the rpm mechanically and then it idles correctly or just turn off the key and restart and it idles correctly, that would seem to indicate a stuck advance mechanism. But, given I just cleaned and lubed the hell out of it, that seems like it can't be the problem.
I have googled "site:www.advrider.com high idle r100" and read for the last two hours. I have read Snowbum's stuff. (Clearly brilliant, but he needs a good editor and to stop using bracketed after thoughts. As you will note, they ruin whatever flow may have been generated.)
I am now more confused after the long discussions of jets, needles, choke gaskets (though they are actually an enricher circuit) 323 jets, in is richer, turn mixture out, too rich, too lean, leaking vaccuum, etc.
What I know is I can't recreate the symptoms in the garage. High idle sticking only happens after 20-30 minutes of spirited riding. Bike pulls and runs great, pre and post high idle sticking. I can't tell it is going to do it until it does it, but it does it every time after the bike has been run hard.
Please help, and if it isn't overly annoying, perhaps a step-by-step process to diagnosis and repair.
I'm in Canada. The weather is still dodgy (-15C this morn, snow yesterday, +4C now).
I promise to report back precise findings as I am able.
I have a full rack of wrenches, feeler gauges, a timing light and a multimeter. I was raised on a ranch and I can fix stuff, but I am not an airhead or any other kind of guru. Just an eager, cheap, middle-aged guy that wants his airhead to return his affection.
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Old 04-09-2013, 05:42 PM   #2
hardwaregrrl
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Didn't read that the diaphragms were replaced?

hardwaregrrl screwed with this post 04-09-2013 at 06:09 PM
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Old 04-09-2013, 06:48 PM   #3
photorider
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I hear you. It has all the symptoms of a stuck advance, no?
Have you tried looking at it with the cover off?
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Old 04-09-2013, 07:07 PM   #4
ME 109
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Back to normal after turning the engine off/on sounds like stuck advance.

I like how the pros leave it up to us clueless to cover all the possibilities that end up having nothing to do with the problem.
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Old 04-09-2013, 07:08 PM   #5
Highwood OP
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Diaphrams

Diaphrams were replaced with carb rebuild, a year ago in February.
I have not inspected the advance while running with the cover off. I feel like I'd be tempting fate if I rode around for half an hour with the cover off.
I'll get some heavy mitts and see if I can take the cover off next time it happens.
What does a stuck advance feel like? Mine wasn't that physically dirty and the weights seem to move freely. I can grab some pix if that would be helpful.
Thanks
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Old 04-09-2013, 07:14 PM   #6
Kai Ju
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you don't have to ride it to see if the advance sticks, just start it up.
make sure you pull the ground lead before you pull the cover so you don't fry a diode.
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Old 04-09-2013, 07:35 PM   #7
Highwood OP
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Ran with Cover off

Just disconnected the battery, removed the front cover, reconnected the battery and fired it up.
The advance weights seem to my eye to be moving correctly (spin out at higher revs, retract as revs lower).
Please keep in mind, the bike is now almost stone cold and I used the choke to start it.
Cold idle was about 950 rpm.
Each side or each weight of the advance seems to feel the same; that is they retract - spring back - with about the same amount of force.
There was a wee bit of white smoke when reving it high.
Any other ideas?
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Old 04-09-2013, 07:55 PM   #8
Highwood OP
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Timing

Just ran my induction timing light.
At idle is seems to be a few degrees below "OT" There is a wee paint fleck and a line in the inspection window.
At high revs, it seems to advance to another convenient dot of paint.
I don't know precisely where it should be timed, but it seems to be lining up on some properly marked bits so I take that to mean that it is timed correctly.
Any other good ideas I can test in the garage?
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Old 04-09-2013, 08:59 PM   #9
brittrunyon
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Have you read this thread?

http://www.advrider.com/forums/showthread.php?t=876626

I dealt with the same issue.
You might get some new ideas or insights.

Good Luck,
Britt
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Old 04-10-2013, 12:55 AM   #10
190e
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Idle Hang up

Sticking advance mechanism is a possibility but idle hang up can also be carb related even when they have been set up by a pro. In fact is much more often carb settings rather than the sticking advance.

1100 RPM is a touch high which can start the transfer ports flowing some mixture. Try turning the idle stop screws out by very small equal amounts to drop the idle speed to 1050 RPM.
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Old 04-10-2013, 01:14 AM   #11
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Quote:
Originally Posted by 190e View Post
Sticking advance mechanism is a possibility but idle hang up can also be carb related even when they have been set up by a pro. In fact is much more often carb settings rather than the sticking advance.

1100 RPM is a touch high which can start the transfer ports flowing some mixture. Try turning the idle stop screws out by very small equal amounts to drop the idle speed to 1050 RPM.
+1 They all do that when the idle is set too high and/or the mixture screw too lean. Idles at around 900rpm damn near stone cold? Of course it idles fast when it's hot. Idle speed all depends on what 'a lot of throttle' is to you. If I want my bikes to idle at 1000rpm after they have had 'a lot of throttle', I need to set the idle idle speed to where they won't idle at all after a couple of km's. If I had my idle speed adjusted to idle around 1100 rpm after 2-3 km's warm up my bike would fast idle like a mofo once it got good and hot. They all do that. It's an air cooled engine.
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Old 04-10-2013, 01:35 AM   #12
ME 109
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Quote:
Originally Posted by 190e View Post
. Try turning the idle stop screws out by very small equal amounts to drop the idle speed to 1050 RPM.
It is easy to be caught out by too high idle screw setting.
2-3000 rpm is beyond what I've experienced from too high setting tho'

Backing the idle screws out is a damn good place to start.
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Old 04-10-2013, 03:15 AM   #13
Plaka
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Quote:
Originally Posted by Highwood View Post
Just ran my induction timing light.
At idle is seems to be a few degrees below "OT" There is a wee paint fleck and a line in the inspection window.
At high revs, it seems to advance to another convenient dot of paint.
I don't know precisely where it should be timed, but it seems to be lining up on some properly marked bits so I take that to mean that it is timed correctly.
Any other good ideas I can test in the garage?
um...learn what the timing marks mean and make sure it is doing the correct thing at the correct mark.

If you think the weights might be sticking it is trivial to lube them, So do that, Then you know they aren't sticking. If recently lubed, unlube them.

Make sure your throttle lock screw isn't set.

You say it doesn't idle high after 10 minutes. You don't say 10 minutes of what. 10 minutes of hard riding? Easy riding? idling in the driveway?

What kind of oil do you have in it?

What did the mechanic who worked on it say?
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Old 04-10-2013, 03:33 AM   #14
Plaka
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Quote:
Originally Posted by Highwood View Post
Diaphrams were replaced with carb rebuild, a year ago in February.
I have not inspected the advance while running with the cover off. I feel like I'd be tempting fate if I rode around for half an hour with the cover off.
I'll get some heavy mitts and see if I can take the cover off next time it happens.
What does a stuck advance feel like? Mine wasn't that physically dirty and the weights seem to move freely. I can grab some pix if that would be helpful.
Thanks
You always disconnect the battery before removing the front cover.

You can run it with the front cover off. The points like to stay clean tho, avoid, water, oil and lots of dust. Clean them with alcohol after running open. With it off observe the resting position of the weights. Then rev. it with the timing light pointing at the points and watch them do their thing.

Try holding it on cruising rpm in the driveway until it gets as hot as it does with your "hard" riding. What happens? This train of thinking goes along the lines of a thermal issue.

Springs can be weak, advance shafts can be worn, etc. As a rule an electronic ignition and never screwing with points again is a good investment. All the systems That I know of that replace points allow you to convert back on the roadside if you need to.

You can also have a sticky carb slide/cable/butterfly. When you shut it down you not only close the throttle but the vacuum stops. A finger lifting and releasing the slide says much...but do it with the bike hot.
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Old 04-10-2013, 07:22 AM   #15
SOLO LOBO
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One more check for something simple:

Push down on the arms of each choke where it enters the carb. I had a bike with a similar behavior and found that one chock wasn't returning to the full "off" position and was hanging up.
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Quote:
Originally Posted by Stagehand
your bike is suitably dirty. Well done.
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