Idle speed slows to near stall

[Tony Smith]

Barry

I have a thought.
If you are brave, once the revs start dropping, pull your spark plug leads one at a time which will at least tell you if both cylinders are involved of if it is only one.

If there is a dramatic difference I would take a long hard look at the idle circuit of the offending carb, but, if your R65 is a twin coil model - swap the coils and see if the problem "moves" first.

If the fault doesn't move, there is nothing wrong with your coils and if there is nothing in the idle circuit then your problem is with the trigger mechanism, or just maybe an air leak at the intake trumpets. The one minute to onset may be instructive in that is about the time it would take a cold intake side of a head to heat soak when stationary, but that is a very long shot, I will go with wither a crook coil, a crokk idle circuit in one carb or a crook trigger unit.

[Barry]

Barry

I have a thought.
If you are brave, once the revs start dropping, pull your spark plug leads one at a time which will at least tell you if both cylinders are involved of if it is only one.

If there is a dramatic difference I would take a long hard look at the idle circuit of the offending carb, but, if your R65 is a twin coil model - swap the coils and see if the problem "moves" first.

If the fault doesn't move, there is nothing wrong with your coils and if there is nothing in the idle circuit then your problem is with the trigger mechanism, or just maybe an air leak at the intake trumpets. The one minute to onset may be instructive in that is about the time it would take a cold intake side of a head to heat soak when stationary, but that is a very long shot, I will go with wither a crook coil, a crokk idle circuit in one carb or a crook trigger unit.  

....mmm on the basis of the old saying "90% of all carburetor problems are electrical"   I guess I shouldn't rule anything out. I have a points ignition amplifier fitted which has never given trouble so far.


A little more information though.

After finding that slight idle enrichment had made the symptoms marginally worse not better I returned the mixture screws to their original position and then turned them a further notch in to weaken the mixture. After an extended ride in this morning it was definitely better and idled steadily at 1000 RPM for longer.  

I'm just about to run on reserve which is no time to be attempting to fine tune the carbs even though I know now a fully service and tune up is the way forward. I'm still curious to understand what brought this problem on. Idle mixture settings don't drift they can only be affected by other things like blockages, valve clearances and float levels. There is no doubt this is a subtle effect as I'm seeing changes from turning the mixture screws by a tiny amount. Even though I checked the float levels I'm wondering if one of the floats is dodgy. One of these days I'm going to get around to drilling the float bowls to add a temporary sight glass.

In the mean time the engine is running as well as ever on the move and I'm still getting high 60's MPG (Imp) so there is not much wrong.  

[Luca]

I returned the mixture screws to their original position and then turned them a further notch in to weaken the mixture. After an extended ride in this morning it was definitely better and idled steadily at 1000 RPM for longer.  

is the air filter clean and clear and giving your carbs all the air they yearn for?

[Barry]

It's not new but looks clean enough. I doubt it would trouble the amount of air that flows at idle but I could take it out to see if it makes a difference.

[Barry]

An update of what I've done although it's not fixed yet.

The closed up exhaust valve theory as the cause of falling idle speed had me worried in case I had bigger problems so I did a quick check and they hadn't moved so that was ruled out.  

I decided to follow up on the idea of closing the gap between the carb and cylinder head. Having a smooth inlet tract without steps or gaps seems like a logical ideal but closing the gap by having the carb make metal to metal contact with the cylinder head inlet stub would defeat the object of rubber mounting which is there to provide vibration and thermal isolation.  At some point in Airhead history BMW changed the specification of the inlet rubber to cure a problem with fuel frothing due to vibration at high revs so the issue must be real enough.

Some carb inlet rubbers have a ring moulded in for this very purpose but in the absence of that I made up spacers from 3 mm thick cork gasket sheet. The carbs are pushed up against the cork ring when remounted so I now have a smooth unbroken inlet tract. Did it resolve the idle problem - No but  it's something I had been meaning to do for some time anyway.

[Mike V]

Barry...

See attached PDF below.

Any success?

[Barry]

Thanks Mike,

I did already look at that SI and I checked the valve clearances which were spot on.

Any Success ?

Well no in fact it's worse although that's probably because it's now so cold I have no chance of getting the engine fully up to temperature until the spring.  

Symptoms are the same - perfectly smooth and steady idle which after a period of time starts to fall off. Only difference now is the time period is getting shorter. The bike otherwise starts on the button, performs perfectly and is still returning 65 MPG so there can't be  much wrong. I thought about resetting the idle mixture from scratch but at the the current temperatures I think it's not a good idea to try and tune carbs in the winter. I'm going to live with it until the spring service. The only thing that will bug me is if in the end I fail to understand the mechanism that's at the root of this.
  

[Barry]

OK I fixed it.

Contrary to intuition it was an excessively rich mixture that was killing the idle.  

Although I had checked the float levels and they seemed fine it turned out one of my floats must have been a marginal sinker.  The bike ran great and there was no incontinence from the Bings - not a drop but at idle the fuel level must have gradually risen enough to flood one cylinder.  With new floats fitted it idled fine at the end of my run this morning.

What lead me to investigate the floats was the way the bike was starting without choke. It always did start without choke in the summer and at any temperature down to 10 Deg C but this winter it would start at even lower temperatures. That couldn't be right even though I was still getting 65 MPG.

I had started a thread on float weights so 'll report back there on what the new ones weighed.

[MichaelSydney]

Happy to hear you have found a solution. I think we were all getting as frustrated as you with the mystery!

[Barry]

Thanks Michael,

I should explain my main learning point from the whole exercise which is:

The method of setting float level at 24mm in the float bowl is fundamentally flawed for anything other than new floats or floats known to weigh the same as new.

The actual operational fuel level which has an impact on the mixture is close to the top front edge of the bowl due to displacement of the fuel by the submerged part of the floats and by the castings and jets. Because of differences in buoyancy,  setting float levels by depth of fuel in the bowl will give different  operational fuel levels depending on float weight. A heavy  float can be adjusted to give say 24mm of fuel in the bowl but it will still produce a different operational fuel level than a new float adjusted to give 24mm depth of fuel. The reason is simply because a heavy float displaces more fuel and raises the operational fuel level above what a new float would.

To give a specific  example:

You have one new float weighing 12.5 grams adjusted to give 24mm of fuel in the float bowl.

You have another old float weighing 16 Grams adjusted to give 24mm of fuel in the float bowl.

Place the new float in the float bowl and it will displace the 24mm of fuel increasing the level within a few mm of the top edge of the bowl.

Place the old float in the other bowl and because of it's reduced buoyancy it will increase the 24mm of fuel by approx.  2mm beyond the level that the new float did. When the bowl is then offered up to the carb the 2mm difference is increased even further to 3 or 4mm. That difference in fuel level will have a big impact on mixture strength .

I’m not saying you can’t use old floats that weigh more than new ones. Even at 16 grams they do not sink and produce an incontinent carb (at 17+ Grams  they probably do). What I am saying is you can’t judge the operational fuel level by measuring the depth of fuel in the bowl at the normal quoted figure. The 24mm often quoted is only valid for new floats and would need to be substantially reduced for heavier floats.

If I had to set up old floats again by the depth of fuel in the bowl method as an approximation I would reduce the 24mm figure by 1mm for every additional gram that the float weighed above new ones.  So I would adjust a 16 gram float to give to 19.5mmmm in the bowl. Had I done that I bet it would have resolved my idle speed problem. But what I also learnt was that I needed new floats anyway and you can only know that by weighing them.

[Mike V]

Barry,

How old were those floats?  Any idea how long they were exposed to fuel?

-Mike V.

[Barry]

8 years Mike


That's not too impressive is it.

I now have an accurate bench mark for the weight of the new floats so I will be able track any changes. Hopefully the new ones are more resistant to ethanol.

[Mike V]

I've gone that long on my /7 flat tops.  Just as a mention - the flat tops have been a pair of the most stable Bings I've experienced.  While I'm always fiddling with the R65's Bings (domed). But that was due to debris entering the body and ports.  Come to find out.

I find this interesting only because I've always experienced smoother and more consistent operation on the 81 R65 with a bit richer idle mixture with OEM jetting, which is lean out of the box.  Even though my normal procedure of mixture balance is to find that sweet spot then turn the mixture screw clockwise (lean) a screwdriver blade width. I seldom mess with the settings after initial balance unless things get problimatic. But any of my adjustments after static have been in the direction of richer.

And I've never set my float level with measuring the fuel bowl volume.  Maybe I'm a hack or lazy, but what works for me is to use a clean section of fuel line, blow lightly into the line and listening for the air to stop when lifting the bowl and it becomes parrellel to the carb base.  But, I change my floats pretty regularly, maybe every 3 or 4 years.  Maybe I should drop my bowls and do a measurement as a check.  My biggest problems with the Bings have been caused by poor filtering or deteriorating o-rings.

Interesting thread.  What are you using to measure the weight of the floats?  I've always wanted a decent scale in the shop for numerous things.

An Airhead friend just discovered the poor operation of his carburetion was due to old floats that measured 6 grams heavier than new ones.  Probably not a coincidence.

[Barry]

Now that I've experienced new floats and the quite dramatic leaning of the mixture that they have brought I can understand your inclination to a richer idle mixture.  The thing is I was experiencing a rich mixture without realising it because I trusted the depth of fuel in the bowl method of setting the float level was giving the correct operational fuel level. I now know that it wasn't. That perhaps is why I was inclined to lean off the idle mixture setting to compensate. I say I trusted that method but really I didn't and have long wanted to drill a carb bowl to devise a sight glass method of setting the level. I've just been reluctant to start drilling holes in the original bowls. If I ever come across a spare bowl it will get done.

I've only ridden the bike to work and back twice since fitting the new floats and it is noticeably more cold blooded, needing choke to start and taking 2 or 3 miles to warm up and run properly. I know this is how it should be but I'm not sure I like it after being able to just hit the starter and ride.  I think I'll richen it up for that reason and another. I've learnt that you can tell if an engine is running weak just by the noise it makes. The tone of the engine changes with a richer mixture to something more mellow.

I took the floats into the lab at the school where I work and weighed them on a very sensitive scientific scale which reads to 0.001 grams. It's the type that has a glass enclosure over the scale to ensure air movements don't affect the accuracy.