Slash five tuning help

[Frog]

I am in the process of sorting out a 1972 R75/5 on which I just did a frame swap.

The bike ran well before the frame swap!

When I first fired it up, all appeared normal.

But shortly there after, a friend took it for a day ride. He had nothing but trouble getting it to idle properly and it essentially was running on the right cylinder only at idle. At open throttle, the bike ran fine.

Back at the shop, we assumed that the inline filter full of paint chips (from the tank) was a sure sign of something blocking the idle circuit in the left carburetor.

But after a thorough flush of the tank, petcocks and full disassembly of both carbs, the problem was still there.

I checked the points gap and it was good. The points are about two years old with maybe 3,000 miles on them. They have the normal wear which sometimes creates a tit on one side.

I checked the timing and it was off some, so I adjusted it. But it still ran poorly

I put my timing light on the right side ignition wire and it fired normally, but on the left wire, the light missed at the same time that the engine stumbled.

So, I swapped out the ignition wires for some spares that I had (used, but working fine the last time I used them).

The bike ran much better, but still was not right. I took it for a little ride and could tell that it struggled at idle and mid-range. At full throttle it felt strong, but maybe not as healthy as usual.

Back at the shop, I put the timing light back on the wires. Again the right side fired correctly, but the left missed in sync with the stumbles of the engine. I figured that it could be a failing coil, so I swapped out the left coil for a good used spare.

I put the timing light back on it and it still stumbled on the left side.

So, where do I go next?

My thinking is that:

-Fuel and carburetion is okay
-If the points were causing the miss, then both sides would show it on the timing light

Is my replacement coil bad too? Should I swap coils and ignition wires? Is there anything else that can effect the ignition on just one side?

Thanks in advance for your help.

[robtg]

Electrical grounding.

[Frog]

Electrical grounding.

Aha...

The frame was powder coated...so most of the connections are not coated.

What connections should I check? Coils to frame? Grounding wire at left coil? I can't think of any others that would be affected by the paint.

Thanks.

[Deleted User 62]

Just a thought, since the ignition fires both cylinders at the same time, I would think you have a loose or faulty connection somewhere between the coils and the plug. Possibly that short jumper that connects the two coils? Also, try swapping the wires to the plugs without removing them from the coils and see if the problem swaps sides. Are the coils wired correctly? (+-+-) Did you use new gaskets and o-rings when you rebuilt the carbs? Was the idle jet visibly clear of debris? Are the valves adjusted? Does the timing advance fully?

[Frog]

Just a thought, since the ignition fires both cylinders at the same time, I would think you have a loose or faulty connection somewhere between the coils and the plug. Possibly that short jumper that connects the two coils? Also, try swapping the wires to the plugs without removing them from the coils and see if the problem swaps sides. Are the coils wired correctly? (+-+-) Did you use new gaskets and o-rings when you rebuilt the carbs? Was the idle jet visibly clear of debris? Are the valves adjusted? Does the timing advance fully?

I will check the connections at the coils. They are wired correctly....I checked a few times.

The carbs were rebuilt (by me) recently, so I just did a cleaning.

Valves were adjusted 600 miles ago.

Timing advance works well.

I think robtg is onto it with the grounding issue. I will check tomorrow.

[Deleted User 62]

Frog, http://www.eubmw.com sells carb kits cheap. The fact that it runs ok at higher rpms still makes me wonder about the idle jet. I use carb cleaner with the red plastic tube inserted from the top of the jet. The jet opening is about the diameter of one bristle of a toothbrush. It doesn't take much to plug it, and it only takes a minute to check.

[Garnet]

I had an isue last fall with a bad harnnes ground at the coil. Turned out the connetor was bad, but it only showed when the headlight was turned on. The head lamp would glow dimmly along with all four signal lights. It had no affect on how it ran.

The coils do not need to be grounded to the frame to work. They ground internaly through the pioints.



I would be suspect the coil or the connectors. Ususaly bad connectors will affect both coils.

I asume you swapped plugs as well.

[Ken in Oklahoma]

The coils do not need to be grounded to the frame to work. They ground internaly through the pioints.

I may be looking at this wrong, but . . . . . .


I agree that the coil case doesn’t need to be grounded for the coil to work. And I have read many times that the spark is grounded through the points. I have a hard time with that last statement. Hopefully someone will point out my error, but until then:

Looking at the diagram that Garnet supplied, we can easily see how current flows through the primary windings of the coil, thus establishing a magnetic field caused by the primary windings and shared by the secondary windings. The coil windings are resistive and inductive, mostly inductive, and we know that an inductor resists changes to current flow. In fact the inductor will generate a voltage trying to keep the flow going increasing the magnetic flux in the process. And whatever happens with the coil primary is happening with the coil secondary because the two are connected by the same magnetic flux.

We also know that the spark is initiated by the points opening. But with the points open, how can the circuit be completed through the points? There is a capacitor across the points, but the role of the capacitor is to absorb some of the inductive “kick” voltage, thus preventing the points from arcing and burning. The capacitor also serves to reflect its stored energy to the coil secondary which can enhance the spark energy, but that’s not all that germane to this discussion.

OK, we have the points open and no arcing across the points. Current, that is the flow of electrons, cannot now be flowing through the points.

So how does the coil cause the spark plugs to spark, which are connected to ground? The answer can’t be through the points as already discussed.

The answer, it appears to me, is that the coil positive or negative going voltage sparks relative to the +12V potential (voltage), not the 0 volt ground potential. Think about it. The wire going to the spark plug is charged with thousands of volts of electric potential. If the spark plug gap is small enough there is going to be a spark happening. It matters not whether the coil voltage is thousands of volts compared to the +12V side of the battery or the ground side of the battery. And as for the micro amps of current that actually have to flow through the battery, that’s nothing considering that the battery is capable of flowing hundreds of amps.

Now for the caveat. I might be overlooking something. What I wrote about is not something I remember hearing. But looking at the circuit, until I’m better informed, I can’t see any other way it can happen.


Ken, out on a limb in Oklahoma

[Garnet]

I agree with all your theory Ken. I also agree with your other secert thought about 2 12V coils in parallel should work like 2 6V coils in series. But they don't, and I can't tell you why.
I do know that my 2 6V rubber mounted ungrounded coils work on my conversion. The only thing I can think of is that they are internaly grounded.

Hopefully someone who actually knows something will splain it to us.

Garnet, ungrounded in Victoria.

[Ken in Oklahoma]

. . . I also agree with your other secert thought about 2 12V coils in parallel should work like 2 6V coils in series. But they don't, and I can't tell you why.

Wow, you do have a good memory. I had almost forgotten about my two-12V-coils-in-parallel-ought-to-work idea. Still seems to me like they should.

I do know that my 2 6V rubber mounted ungrounded coils work on my conversion. The only thing I can think of is that they are internaly grounded.

Well, in accordance with my theory, both coils are "connected" to the positive side. So there still is that +12 volt potential that the those high tension kilo volts can "relate" to and transfer electrons to.

Hopefully someone who actually knows something will splain it to us.

Yeah! Where is that Rob Frankham dude when we really need him?



Ken