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Old 02-24-2012, 11:14 AM   #1
Highwood OP
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Coil Diagnosis 77 R100/7

I have a suspicion that I have a weak spark problem on my 1977 R100/7.
Bike has 44k kms (27.5K miles) and its sole modification is a two-into-one exhaust.
Plugs, points & condenser recently replaced. (1000 kms ago)
Coils are original and outwardly seem to be in good shape. No visible cracks.
Plug wires appear to be original and outwardly in good condition, but because they are cheap, I'm replacing them.
However, I put my cheap ohm meter on the coils and got the following results:

Haynes Specs
Primary 1.15 - 1.32
Secondary 7.5 - 9.15
Left Coil
Primary 1.4
Secondary 6.3
Right Coil
Primary 1.5
Secondary 6.4

I take these results to mean that my coils are under performing or not making as big a spark as they should. Am I correct?
Never had the misfortune of riding in the rain on this bike so I don't know if wet affects the coils. Primary wiring seems to be in good shape.

So, the questions:
1) Do I need new coils?
2) If I need new coils, where can I get them without darkening the doors of my local stealership?
3) Will VW coils fit?
4) Do I need coils with internal resistors?
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Old 02-24-2012, 01:10 PM   #2
supershaft
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Even high dollar good multimeters vary greatly when reading such low ohms. The specs have to be read with that in mind. Your test results point to good coils but that means little. Other than testing bad and not working, the only coil test is to replace them and see if it makes a difference. I don't suspect new ones will make any difference but they might. They very often last a long, long time but then again sometimes they don't. I had a bad coil on a /2 30 years ago. I had a crack-o-matic coil go bad. Other than that, I am running the same coils I have had for decades. They're Dyna's but still.
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Old 02-24-2012, 01:29 PM   #3
Garbln
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Not a bad idea to do the plug wires but I'd really check out the plug caps if they are the metal covered ones, they are iffy at this age.
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Old 02-24-2012, 02:27 PM   #4
DoktorT
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Easy to see spark energy. Just hook up a plug and hold against the head for good ground, then crank the motor. If an orange pfffffft or less is seen, low spark energy. If a blue/white audible snap, good spark energy.

Accurate very low ohms measures require certifiable to standard meters designed specifically for such. If you can find a pro who has one because he cannot afford to act on bad data and must prove the right in every case, do try comparisons with your lower cost item.

Throwing money at coils is money wasted 90% of the time. I always replace them if I see corrosion in the tower. It cannot be fully cleaned out and will come back to haunt you. Otherwise cracks and purty colors on the terms due to excess heat stress can be seen. Those I replace.

Just poor batt cable connections can give you weak spark as can a worn starter motor using up more amps.

Do confirm some many basics prior to spending money on coils.
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Old 02-24-2012, 02:41 PM   #5
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Just to be clear here - what is it that makes you think you have a weak spark? Is there some running condition that makes you think that? Are you just looking at the spark outside the cylinder and thinking "hmmm, that looks weak?" What makes you think you have a weak spark?
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Old 02-24-2012, 03:16 PM   #6
Highwood OP
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Why my suspicions

Why do I think I have a bad coil?
Well, like you, I'm not really certain I do.
However, I have read that many folks have had improved cold starts and smoother running with fresh coils.
In my case, I have rebuilt, tuned and synced the carbs and the bike is running very nicely, much improved mid range and smoother idle. (Yes, all gasket and o-rings were changed. Enricher circuit is clean. Chokes are full off in off and full on in on positions.)
But, cold starts continue to be less than perfect. One cylinder picks up first and then, with just the right amount of gas, the other gets going but the engine doesn't smooth and settle out until a brief, fast run up to 3000 rpm. Once it has gone through those conniptions, it idles perfectly at about 1000 rpm, vacuum balance between cylinders is spot on and it pulls smoothly through the entire rev range.
When I speak of a cold start, I am speaking of the first start of a day, worse when it is colder outside.
I guess what I am wondering is could a weak spark on one side be the culprit?
While my OHM measuring gear cost a whopping $13, I thought it may be telling that it indicated resistance was "lowish" on the secondary, indicating, perhaps, maybe, an inefficient spark.
Of course, the other possibility is it is the nature of the beast.
Simply, I hoping for some guidance. I'd rather not buy the coils. Perhaps fresh plug wires will help.
Thanks to all for their replies.
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Old 02-24-2012, 05:33 PM   #7
TimTowtdi
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A guy I know has the same problems. We just put new coils on his bike last weekend. It did not fix it.
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Old 02-24-2012, 06:18 PM   #8
Wirespokes
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I'd suspect something's up with the choke on the non-running side. But you can test the plug leads by swapping them left to right. If the other side now doesn't run, you'll know it was them.

But I highly doubt it's the plugs, leads or coils.

Double check the choke. Make sure the little holes on the disc are open and not clogged. Make sure it's positioned the correct way. Is the pick-up tube clear? Is the (float bowl) gasket sealing around the pickup tube? Is the choke jet open and free?
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Old 02-24-2012, 07:00 PM   #9
supershaft
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I know a lot of pros including myself that say the cheap ones sometimes work better than the high dollar ones. I have them both and I have used quite a few of each type. Cheap ones are often better at some functions in a lot of "pro's" opinions that I know of. At least the ones that I listen to.

In my experience with BMW motorcycles, the caps are the most failure prone component in the system but is could still be a coil. Besides the points and condensers themselves, of course. Swap plug leads and find out. Two minutes and one start up!

Never re-use the enricher gaskets. Always replace them.

Float level?

Needle jet/jet needle issues can cause that very symptom. I have only seen that in later carbs but . . . .

Has the atomizers been cleaned lately? They get gunked up regularly.

All good advise IMO except for the meters. You do not need a $500 (or more!) meter to get good readings. I probably would just have a cheapy if I didn't need to sometimes make amp draw readings (only the pricier ones have that feature).

Good luck!

supershaft screwed with this post 02-24-2012 at 07:28 PM
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Old 02-25-2012, 04:52 AM   #10
bikerfish
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I didn't see it mentioned yet, but maybe I missed it, are your valves properly set? if you have a valve starting to tighten up, this can lead to the condition your experiencing. I'm also with wirespokes on double checking the choke/enrichers.
been riding these old gals for 25 years and have yet to have an issue with coils on the older bikes. now the post 81 bikes are a different story!
I'd check everything else before spending the money on coils.
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Old 02-25-2012, 06:57 AM   #11
Highwood OP
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I'll check the valves

I rebuilt the carbs myself and I am certain everything is correct -fresh gaskets, ultra clean (toothbrush and varsol for hours) and all passages are clean.
I confess I'm doubtful about the valves. They were set about 1000kms ago by a factory-trained BMW tech, a great guy with some grey hair. He worked for BMW in the early 80s so my bike was nothing unusual for him.
That being said, the costs of pulling the valve covers and having a look is extremely budget friendly.
I'll report back in a few days when I have some time to get the wrenches out and have the fresh plug wires.
Thanks again for all the help and suggestions.
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Old 02-25-2012, 11:27 AM   #12
supershaft
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On airheads I have never seen a leaking valve from being too tight or otherwise not make the mixture screw mostly to completely ineffective.
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Old 02-25-2012, 12:55 PM   #13
Wirespokes
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I don't recall ever replacing one of those early style coils, though I have had the newer dual style go bad. Even there, it was due to epoxy cracks allowing moisture to short them out, not a weak spark.

Like shaft says, the only other thing is tight valves.

I don't see that you took an ohm reading of the plug wires/caps. What do you get there?

If none of that pans out - do re-consider taking another look at the offending carb. The best of us have somehow managed to get things in backwards, despite knowing better. You wouldn't be among bad company.
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Old 02-25-2012, 08:19 PM   #14
Highwood OP
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Plug WIres

At $15 a side, I didn't bother to check the resistance on the plug wires.
Visually, they look fine and they seat nicely on the plugs.
I have an abundance of evidence that I am prone to mistakes and errors and there is every reasonable possibility that I farked something up putting the carbs back together. But, I was unusually patient and methodical and I remain confident that I put them back together right.
I'm loathe to pull the carb back off after getting the throttle cable sync just right.
I'll start with the valves as that is the least invasive option.
I was ripping up boot-top powder today so exercising the R100/7 is still some way away, notwithstanding I did sneak it out for a ride last week.
Remember, I'm in Canada and riding year round in these parts is simply impossible. I damn near dumped the thing on the ice in the alley after getting stuck on the very gentle grade into my garage.
Again, my sincere thanks for the suggestions, help and encouragement.
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Old 02-26-2012, 07:11 AM   #15
Bill Harris
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Quote:
Plugs, points & condenser recently replaced. (1000 kms ago)...

...the valves. They were set about 1000kms ago by a factory-trained BMW tech...
Double check the point gap and timing. Sometimes there is a lot of initial wear in the rubbing block, and the point gap decreases and the ignition timing retards. Neither of which help a cold start.
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