Idle speed slows to near stall

[Barry]

Very unusually I got stuck in some road works today on the way home from work which meant the engine was idling for long periods. Normally the bike starts instantly runs perfectly and I have a very stable idle when warm but also normally the engine isn't asked to idle for very long perhaps 2 minutes maximum.   Today after being stuck in traffic the idle speed started to fall slowly almost to a stall and had to be caught with the throttle.  It would then idle fine for a while before slowly falling again.

I have some theories to explore but wondered what you guys can make of it.

[Ed Miller]

If I'm at a stop I usually blip the throttle anyway, every 30 seconds or so, "like the Harley dudes do" as Oak said.  It's to keep my timing chain lubed.  Or I shut the bike off.  I almost forgot about that.

I think if you want to tune your bike to idle for a long time, adjust your idle speed when it's warm enough to start to stall like you describe.  

[Bob_Roller]

Where do you have the idle speed set at

When I first got my '81 R65, the idle was set at 850 rpm from the factory .

It was not very ' happy ' about sitting for extended periods of time at idle, 3-4  minutes in traffic, stop lights, etc ....., it exhibited the same thing you described .

I increased the idle speed to 1300 rpm and never had the issue again .

[Tony Smith]

Bob

I currently have a cylinder head temperature probe (sparkplug washer type) so that I can monitor things while I finalize adjustments on my rebuilt engine.
 
Now I know that my engine is a bit tight, and will remain so until the rings break in fully, but the temps I am seeing during even brief periods of idle once the engine is up to temperature are quite thought provoking.
 
The reason I wanted to monitor things closely is that my model was a "lean burn" model in Australia, I have used the low compression pistons and I am hoping to use 91 octane unleaded fuel - hence my interest in both knock and temperature.
 
Presently I am observing that once the engine has reached operating temperature that idling for more than about 2 minutes is starting to produce numbers that are a bit worrying (nudging 280~300 Celcius).

The temperature drops amazingly quickly once moving, a tribute to how well sticking the cylinders out in the airflow actually works, but it was designed for a different era of higher octane fuel and not too much stop~go high density traffic.

I will be monitoring for a bit longer, as soon as i get it registered I am going to go on a long run which will amount to about 400km and involve highway, hill climbing, extended running at high altitude and transiting a number of small towns. I will be very surprised if this does not only go a long way towards running in the engine, but also greatly reducing temperature. If it does not, I will have to start enriching the mixture across the board, a prospect that does not fill me with joy as in modern terms the R65 gets lousy mileage anyway.
 
Might have to take a closer look at the fuel injection project another member is running.

[Barry]

Where do you have the idle speed set at

When I first got my '81 R65, the idle was set at 850 rpm from the factory .

It was not very ' happy ' about sitting for extended periods of time at idle, 3-4minutes in traffic, stop lights, etc ....., it exhibited the same thing you described .

I increased the idle speed to 1300 rpm and never had the issue again .  

Idle speed is set at 1000 -1050 RPM fully warmed although this being the heavy flywheel model it will idle very steadily at lower speeds too like 800RPM before it's fully warm. This is not about an erratic or unstable idle. The idle speed slows smoothly down to 600 rpm and would probably stall if I didn't then catch it.

It's not a major problem as I do not normally need to idle for long periods and I would in any case blip the throttle occasionally to aid lubrication.

I'm just curious as to what's going on.  I do have the idle mixture set weak i.e. as weak as it will stand on the Bing Manual principle.  If we assume for minute the problem is being caused by further weakening of the mixture then the question is how would it get weaker ? Seems that could only be if the fuel level is not being maintained when the bike is static as compared to when the bike is moving.

I'm quite certain that the fuel level measured when a bike is static on the stand cannot be the same as the fuel level with the bike bouncing down the road. The difference may not be much but common sense suggests that the float mechanism cannot maintain perfect control of the level under all conditions. Someone has even been able to confirm the change in mixture this produces using wide band O2 sensors. I think his comment was "there is a big difference between O2 readings on the stand & on the road when riding around"

So I'm wondering if this slight fall in fuel level after a long period at idle is tipping me over the edge of too weak an idle mixture. The test would be to try and recreate the falling idle speed then shutdown and whip off the float bowls to check the level.  I won't be able to do that in a traffic jam but we'll see.  

Tony,

It did get a lot hotter than usual. When I got home the whole of the main engine casting was glistening wet and I was thinking what the **** is that. Then I realized it was just the coating if waxoyl recently applied that had melted which it wouldn't normally do during the winter.
 

[Barry]

It did it again on the way home tonight.  This time the road works queue was quite short as the smart people had learned to use a different route. With the shorter wait the engine could not have overheated. It  Idled perfectly at a steady RPM for a minute or two and then started it's slow fall off in revs.

When I got home I let it idle on the drive with the same result.  It's odd that it will idle for a time at perfectly steady RPM before something happens to cause the revs to begin the slow. I whipped the float bowls off and the levels were close to the scribed lines I have in there.

Maybe it's just the onset of cold weather. I think the next step will be to tweak the mixture screws out a little to richen things up. I've developed a  precise way of doing this which is reversible. I cut a short length of plastic tube which is a tight push fit over the mixture screw boss. I then align a fine saw cut I've put in the end of the tube with the screwdriver slot. That marks the exact position of mixture screw so that it can be set back where it was if needed.

[Mike V]

Barry,

My experience with my '81 is: "Richer = Happier".  

I also have found a bit higher idle in the range of 1000 to 1100 rpm to limit stalling, as others have suggested.  My '81 with stock jetting is on the lean side in my opinion from the factory so I've richened the idle mixture to help with a smoother and more consistant transition to mid-range.  I'll admit my bike's idle seems to have a mind of it's own from time to time in regards to ambient conditions and general conditions.  I've stopped chasing the ghost.

At the time of my restoration it was a big problem with stalling but I found the sintered bullet type in-line filters were not doing the job catching the minutia of red lead primer bits from the tank interior (a story for another day).  The smallest of particals getting past the sintered filters were adversly affecting the carburetion.  Since changing to the paper (ugly) filters the big problems have gone away.

Have you checked valve settings lately?

-Mike V. / San Diego

[Barry]

Have you checked valve settings lately?
 

That I haven't Mike. Based on previous experience I would be greatly surprised if they had moved even a thou since the last service but you never know. If the mixture doesn't sort it I will go back to basics and start with valve clearances and timing.

The thing is most of the usual culprits would produce an erratic idle straight off.  What is surprising in this case is that it idles perfectly for a minute or two before starting to slow. I'm convinced the clue must be in that fact.

Even if richening up the mixture cures it and I suspect it will, I'd still like to understand what's going on. It's as if there is a small reservoir of fuel being used up over this brief period and when it runs out what remains in my case is the relatively weak idle mixture which is borderline and insufficient to support a steady idle.

Here's a crazy theory that might fit that idea  The inlet tract is not perfectly smooth as there is a narrow gap between the carb and the head where the diameter increases to the ID of the rubber connector. In normal running that gap will tend to fill with a small volume of condensed fuel. When I stop and let the engine idle this reservoir of fuel is adding a little richness to the idle mixture so I get a steady idle. When it runs out the idle mixture becomes too weak and the RPM starts to fall.  Not sure how I would prove that except by eliminating the gap but it's there for a reason to provide vibration and thermal isolation to the carb.

[Mike V]

Barry,

This is a debate that could go on for weeks.  I can think of a bucket-full of possibilities. In my small and parallized mind I would contribute this dynamic idle issue to be due to the endless variables of having normal asperated internal combustion engines, fed by carburetors (not EFI) that have very high operating temperatures with varrying qualities of fuel. When I think of our Bings - I think of them as simple mechanical valves that respond to atmospheric pressure, vacuum and throttle position with no analytical means of compensation of what the engine actual wants/needs. Then we could debate about volumetric efficiency in regards to intake pressures, pulse, port condition, etc.  These are pretty simple tractors that will operate in a lot of less than perfect conditions.  But it runs great - right?

Does that make any sense? I'm going to closely follow this thread because I know you're on a mission and I'll bet you'll find what the cause is, as you normally do.

I would first check basic settings, and make sure you have no blocked idle ports, then .....

-Mike V. / San Diego

[Barry]

Does that make any sense?  

I think you might be saying I could be asking too much to expect an old tractor to hold a perfect idle indefinitely.

We shall see but I might not get a chance to test it again before Monday.

[Mike V]

Does that make any sense?  

I think you might be saying I could be asking too much to expect an old tractor to hold a perfect idle indefinitely.

We shall see but I might not get a chance to test it again before Monday.

Barry,

You said it in less words than I did.  

I'm also saying if anyone can - you can.

-Mike V. / San Diego

[Barry]

Quick update.

I turned out the mixture screws one notch (screwdriver slot width) and rode the bike to work this morning.  To be fair it wasn't a real test as in only 4 miles and not a full 10 mile warm up. Same result though- it idled nicely at 950 RPM for a minute then slowed to a stall.  

I need to take the long route home and try again.

[Luca]

FWIW, mine behaves similarly for the first ten miles now that I have the idle down around 1K instead of 1150.

I'm also running plugs that are one step cold.  For such a short commute you could try one step hotter as the weather cools, and running 10w40 if you've got 20w50 now.

Seems like if you get it to idle nicely for a 4 mile ride to work it would idle way too high after a proper warming up.

[Barry]

I hadn't thought about 1 grade hotter plugs - that's a good idea for the winter.  I'm already on 10W40 and it does increase the idle speed compared to 20W50. I'll be changing to 5W40 soon when the weather gets colder. It's past 6.00 pm here and it's been dark for an hour but the temperature outside is still 15.3 Deg C.

I'd normally expect 950 rpm after 4 miles and that's what it did. Fully warm would be 1000 - 1050.  The thing is it will idle reliably even slower if I choose to set it that way. Well at least it did.

I took the long way home and tried again. This where it got stranger still and confirmed a suspicion I had this morning. With the engine close to properly warmed up it stalled even quicker.  Still idled steadily at 1000RPM but the revs fell away after only 30 secs.  So richening the idle mixture has made it worse !

This may be a case of waiting until I do a full service in the spring and tuning the carbs again from scratch.

At least I enjoyed a spirited ride on the way home. There won't be too many more days of 15 Deg C and bone dry roads this year .

[Ed Miller]

At least I enjoyed a spirited ride on the way home. There won't be too many more days of 15 Deg C and bone dry roads this year .

Sad sigh.  And when I get a nice one like today I'm more likely to bicycle!  Which I did.