Author Topic: Highway stumble/loss of power  (Read 3172 times)

Offline qwerty123

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Highway stumble/loss of power
« on: June 22, 2017, 01:44:25 PM »
Hello all,

I'm experiencing an infrequent stumble or loss of power at or above 60mph/ 4000rpm. Engine sound changes, I lose power,  and opening the throttle further does nothing.  Closing the throttle, even for a short time,  fixes it.

My first thought was fuel starvation,  so I disassembled and cleaned the petcock and screen, as well as replacing the aftermarket paper fuel filter.  I also confirmed the fuel overfill vent isn't clogged and tested with the gas cap loose--still occasionally get the stumble.

It happens in the wet and dry,  but doesn't appear to happen with a full or nearly full tank. I mostly notice it about 150-200 miles into a tank.  I've been measuring low 50s miles power gallon.

Spark plugs looked fine.

Any ideas? Thanks much!

Matt

Offline Barry

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Re: Highway stumble/loss of power
« Reply #1 on: June 22, 2017, 05:02:22 PM »
It's a tricky one as I think you have already done a good diagnostic. 

While it does sound like a lack of fuel, the intermittent nature is odd and starts me thinking about ignition but then it doesn't do it on a full tank which pushes the thought process back to fuel again as the mixture will be very marginally rich with a full tank and a potentially higher fuel flow rate.

Some ideas. Assuming it's safe, when it does it again shut the engine off quickly, close the fuel taps and coast to a halt. Whip the float bowls off to see if the levels are low. If they are then you know it's fuel but not yet why. Given the extra head pressure of a full tank solves the problem, instinctively my first action would be to ditch the fuel filter if only as a temporary test.
« Last Edit: June 22, 2017, 05:10:00 PM by bhodgson »
Barry Cheshire, England 79 R45

Offline Bob_Roller

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Re: Highway stumble/loss of power
« Reply #2 on: June 22, 2017, 05:22:47 PM »
Two things I can think of, low fuel level in the float bowls, and holes in the carb diaphragms .
'81 R65
'82 R65 LS ?
'84 R65 LS
'87 Moto Guzzi V65 Lario
'02 R1150R
Riding all year long since 1993 .
I'll give up my R65, when they pry my cold dead hands from the handlebars !!!!!

Offline qwerty123

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Re: Highway stumble/loss of power
« Reply #3 on: June 22, 2017, 06:44:53 PM »
Thanks for the thoughts. My mind is now running to ignition as well. A couple more pieces of data:

1) I rode the bike through a deluge a couple days ago, and the last few hours has been the only truly dry air it's seen in a while

2) There is a sucking / maybe slight popping upon closing the throttle at higher revs (as the revs come down) that I don't remember from earlier in the bike's life

3) I can see fuel in the clear plastic fuel filter when it happens

4) It occurs to me that the lack of a problem after a fill-up could be down to the bike getting a few minutes to cool as it's filled and I stretch, rather than the pressure from a full tank

5) I might describe it as a stutter rather than a stumble, in terms of noise. It does happen only a little / a blip that could've been a bump in the road, but when it's fully happening it's a clear change in engine note as well as no power.

The notable binary nature of the problem (rather than smooth in and out of it, it's happening or it's not) makes me think ignition (any way to test that?), but the fact that it really only happens at highway speeds in the middle of an extended run (i.e., when fuel usage is highest) makes me think fuel / air / mixture...




Offline Bob_Roller

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Re: Highway stumble/loss of power
« Reply #4 on: June 22, 2017, 07:02:01 PM »
Do you have a black and gray ignition coil ??
'81 R65
'82 R65 LS ?
'84 R65 LS
'87 Moto Guzzi V65 Lario
'02 R1150R
Riding all year long since 1993 .
I'll give up my R65, when they pry my cold dead hands from the handlebars !!!!!

Offline qwerty123

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Re: Highway stumble/loss of power
« Reply #5 on: June 22, 2017, 07:38:55 PM »
Mine looks like this

Offline Bob_Roller

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Re: Highway stumble/loss of power
« Reply #6 on: June 22, 2017, 08:05:13 PM »
Yes, that's black and gray coil, a very well known problematic part .

I would suggest taking a good look at it, most likely you will find mutliple cracks in the coil case .

Just a suggestion, replace it when you can .
« Last Edit: June 22, 2017, 08:06:50 PM by Bob_Roller »
'81 R65
'82 R65 LS ?
'84 R65 LS
'87 Moto Guzzi V65 Lario
'02 R1150R
Riding all year long since 1993 .
I'll give up my R65, when they pry my cold dead hands from the handlebars !!!!!

Offline mrclubike

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Re: Highway stumble/loss of power
« Reply #7 on: June 22, 2017, 08:31:41 PM »
When I have mine jetted to get over 50mpg
It does not  run very well until the temps are over 60
I keep mine jetted to get mid 40's mpg since we ride all year round

If you do get a new coil keep the old one for a spare on long trips
It can at least get you home 
« Last Edit: June 22, 2017, 08:35:55 PM by Mrclubike »
1982 R65 running tubeless Snowflakes
2004 R1150R

Offline mrclubike

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Re: Highway stumble/loss of power
« Reply #8 on: June 22, 2017, 08:40:25 PM »
Here is one option for a replacement coil
1982 R65 running tubeless Snowflakes
2004 R1150R

Offline qwerty123

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Re: Highway stumble/loss of power
« Reply #9 on: June 22, 2017, 11:48:51 PM »
Thanks guys!

I'm going to have to see if I can get a coil somewhere around Austin, TX (Presuming it's the coil, which seems likely. I'll have a look tomorrow morning).

Offline qwerty123

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Possible culprit?
« Reply #10 on: June 23, 2017, 09:42:42 AM »
An inspection of the black coil doesn't show any cracks, but there is a raised area that looks like a crack--however, I don't think it's a crack, I think it's a casting / moulding artifact. A close inspection reveals it's raised, smooth with no crack I can see. And the way it goes around the edge of the cylinder--I'd expect a crack to continue, this does not. The coil has been replaced, I think? This is the later BMW 12 13 1 244 426 / Bosch 0 221 500 203.

However! Close inspection of the coil led me to look at the (brand new) silicone plug leads, and...there's an area where the insulation has worn away! I'm thinking maybe under certain circumstances this could be shorting directly to the engine top cover where the plug leads lie, and so could be the culprit? I'll cover it with some silicone self-fusing tape (great stuff by the way, highly recommended) and see if the problem recurs. If it does I'll try to source a new coil.

The "crack" you can see in the photo looks for all the world like a casting / moulding artifact to me, but do let me know if I'm way off base here--I've left my magnifying glass at home.

Offline Bob_Roller

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Re: Highway stumble/loss of power
« Reply #11 on: June 23, 2017, 10:06:57 AM »
No, that doesn't look like a  crack to me either .

The OEM coil ending in 426, is red and black .
'81 R65
'82 R65 LS ?
'84 R65 LS
'87 Moto Guzzi V65 Lario
'02 R1150R
Riding all year long since 1993 .
I'll give up my R65, when they pry my cold dead hands from the handlebars !!!!!

Offline mrclubike

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  • Jungheinrich Master Tech
Re: Highway stumble/loss of power
« Reply #12 on: June 23, 2017, 05:07:04 PM »
That is why I have Vinyl tubing covering my wires
They will rub thru laying on top of the vibrating motor

It is not going to be easy to find a coil that will bolt up with out making a bracket

I would not get one with less than 1.5 ohms resistance unless you have a ignition module that can handle it

It looks to me you already have the latest coil that should have a primary resistance of about .7 ohms
Get a Volt ohm meter  and check it .
If this is the case you may be over stressing your ignition module and causing the problems you describe
« Last Edit: June 23, 2017, 05:20:44 PM by Mrclubike »
1982 R65 running tubeless Snowflakes
2004 R1150R

Offline qwerty123

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Re: Highway stumble/loss of power
« Reply #13 on: June 23, 2017, 05:25:20 PM »
Thanks for the suggestions guys.  Unfortunately repairing the spark cable did not fix the stumble :-(

Next stop new coil I guess.  I will test the resistance of the one I've got this evening.

Offline Bob_Roller

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Re: Highway stumble/loss of power
« Reply #14 on: June 23, 2017, 05:27:12 PM »
Where are you now ??
'81 R65
'82 R65 LS ?
'84 R65 LS
'87 Moto Guzzi V65 Lario
'02 R1150R
Riding all year long since 1993 .
I'll give up my R65, when they pry my cold dead hands from the handlebars !!!!!