Loop frame electrical gremlins

Sorry if this is very longwinded.

Have had my 73 Eldorado for about 2 months now, but for half that time the temperature was too low for me to get out to the garage much.

Bike was running, but a bit rough. Took carbs apart and cleaned everything.
No real difference. It would idle with choke on, but not too happy otherwise.
Then I thought it was running on just the left cylinder.
Bought a Carbtune, but the reading from the right cylinder was bouncing all over the place.

Thought it might be an ignition problem, but seemed to be getting a spark at both cylinders.

Anyway, this week I changed the coil.
Kept reading that the coil should have resistance of 3 ohms, and mine was about 1.6 ohms.
No Bosch blue coils available here at the moment, so went with a Viper coil.

While I had the tank off I installed a front brake switch lead from Gregory Bender, and also a new headlight/turn signal switch that he had made up for me.

New coil on, everything back together and bike refuses to start.
Turns over fine, but no starting.
Both plugs are getting a spark.
Checked coil and the resistance between the two terminals is 1.6 ohms !
Went back to check old Bosch coil sitting on the bench and it is 3.2 ohms !

Pull tank off again and put the oil coil back on.
Bike still won’t start.
Checked coil resistance and it’s 1.6.
But, unplug the two leads and check it again - 3.2 ohms.

Is this normal ? Or have a a short somewhere.
Apart from the coil, all I have changed is the headlight switch. New wiring for the switch, but it is just a straight replacement for the old switch wiring.

Checking the primary winding resistance on the Bosch coil while unplugged is 3.2 ohms, while secondary winding is 8,300.
The Viper coil is supposed to be 3 ohms, but the numbers are 2.7 ohms, and 20,000 on the secondary winding.

Thanks for any info.

As an alternative thought - do you have a condenser on your ignition?

I do.
Could this be the culprit ?

How does one test a condenser ?

Just checking online now and neither Gutsibits or Stein Dinse have them.

Any automotive type points condenser should do the job.

Value 0.2 to 0.4 microfarads…not unduly critical.

See here:

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If you are measuring the coil impedance while it’s connected, you are also measuring any bits of the bike wiring in parallel with it. I would have to look at the diagram to see what.
Many decent multimeters can test capacitors. I have started buying NOS car ones as modern ones have proved to be rubbish. It can be mounted anywhere as long as it’s parallel to the points. On my little Ducati I have put it under the seat as the replacement is too big to fit inside the points housing.

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I’m wondering why no fire with the old coil refitted. What was disturbed when the new coils were put in?

Have you tried just swapping the HT leads over?

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Presumably you have disturbed something in the wiring. I would try hot wiring it by running a temporary power supply from the battery to the ignition coil to see if it will run on that.

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Thanks for all the replies.
New HT leads fitted today and no change.
Have ordered a new condenser, so we’ll see if that was the problem by the end of the week.

The only other thing I did was to install a new switch for the lights, indicators and horn.
Can’t imagine that has had any effect on the bike not starting, but you never know.

To expand on Don’s suggestion, I would sketch out the ignition circuit on its own and then it will be clearer to you where to measure and what to check. Or how to make a slave system to bypass the bike’s wiring as per Don’s suggestion.

Thanks for all the suggestions.
Fitted a new condenser and points, but the problem turned out to be a schoolboy error on my part.
I my defence, I haven’t done much in the way of mechanical work for about 20 years.

Anyway, although I had checked the ignition timing, the distributor had been right at the limit of it’s adjustment range, BUT, I had foolishly set it at the TDC mark, and not the mark 10 degrees before. Had to pull the distributor shaft up, and turn it a little anti-clockwise before replacing it.

Bike starting again, but idles very roughly, and I’m sure the right cylinder is not contributing much.
Putting my hand on the valve covers, the left gets warm while the right is still cold.

Check compression and today it was 160 left and 150 right.
I should have written it down, but as far as i remember it was 160 on each cylinder last time I checked, about a month ago. Maybe the engine was warmed up more.

Back to carberation issues.

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Obvious thingis to check you are getting a spark on both sides before tackling the carbs.

Just went out and double checked again.
Spark both sides.

Sorry to keep dragging up an aging thread.
Haven’t been near the bike much lately, cataract surgery, but all fine now.

Still only running on the left cylinder.
Today I pulled the carbs off and swapped them around. Left carb on right cylinder, right carb on left.
The problem still persists, left cylinder getting warm while I can put my hand on the right side exhaust.

So it has to be an ignition problem, right ?
Getting a spark to both plugs, but could the right plug not be timed correctly if that makes sense ?
Do I need a new distributor ?

Almost reaching the point where I throw my hands up and send the bike off to a professional.

Next step would be to swap the coils over to rule them out. Plus to give all the wiring to the points a good clean,
I’m not that familiar with the Eldorado model, does it have 2 sets of points or is it single points with a distributor cap that feeds the left and right HT leads? What’s the distributor cap like inside?
It would also be worth checking the timing with a strobe, you did say the right spark was bouncing about.

dont forget to make sure the valve clearances are the same on both sides. As you are getting similar compression results they should be close but I would still do it.
My money is on carburettor synchronisation.
When you swapped the carbs did you swap the tops and cables from left to right as well? If not the problem might be that one cable is slacker so that slide lifts later and less.
Have you got vacuum guages to synch the carbs?


Yes I have vacuum gauges. Carbtune with the steel rods.
Trouble is I get no real reading at all from the right cylinder.
It barely gets the rod to move.

I did swap the carb tops, but not the cables, but I have checked the slides moving in sync with a small screwdriver under each one, and making sure they move together.

Have you swapped the gauges left to right to check that the fault is not in the gauge?
They have screws that restrict the suction form the carb to allow you to damp the oscillation of each cylinder sucking.

If the slide is too open you will get a low reading which will not change on early throttle opening. the closed side will show a high reading which drops swiftly as the throttle opens.

Hope I am not telling you what you know but there are two different approaches to choke synch, if the engine, carb etc are perfect they give the same result but…
(1) for smooth fast pick up first make sure the vacuum change on opeming the throttle occurs at the same time to the same amount using throttle stops then for midrange adjust the cables to get it most even.
(2) for smoothness high to midrange first adjust throttle cables for equal suction at a steady 3-5,000 revs and then bodge the throttle stops to give best tickover

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I would try swapping the carbs over to see if the problem swaps to the other side.
Another thought if the right side vacuum gauge isn’t reacting is to check the valve operation. Pop the rocker covers off and make sure all 4 are opening and closing fully. Reset the tappets whilst you are in there.

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I have swapped the carbs and have swapped the vacuum gauges over too. No change.
Have double checked the tappets the other day too. All seem good.


I have swapped the gauges over and nothing changes.
I feel like there must be an air leak somewhere.