Spark, then no spark....on rear cylinders - HELP!

[hartleybiker]

New to the forum. I have looked over a lot of threads on rear cylinder no-spark conditions and it has been very informative, but here is what I am experiencing. Recently got this bike (83 VR) in trade. P.O. said " rear cylinders aren't getting spark", and that he tried installing new plugs (I removed and confirmed that they are new plugs and gaps are all the same) and believed it was a bad TCI. So I had a tech open up TCI and replace all those problematic diodes and one capacitor that appeared to have "blown". Ok, now with 4 new plug wires here is what I get: Front cylinders fire just fine, rear cylinders only spark once, as the starter is beginning to turn over the motor, and then no spark after that. My testing method is a spare spark plug grounded to the engine case. I have checked the pickup connector along the frame, and the connections to the TCI. None were corroded or dirty, and the inside of TCI showed very little corrosion as well. All TCI harness connections seem to be in spec with what I've read in most of these threads and I got continuity through the troublesome pickup connector on the frame.. I have not yet switched the connections to the coils from front to rear to see if that changes, but the P.O. claimed to have done so and that the rear cylinders worked. Also worth mentioning, on a test ride, I experienced a sudden surge of power at 1/4 throttle that felt as though the rear cylinders kicked in for about a second. Any ideas?????

[dacheedah]

Have you checked the coils and the resistance in the caps? Often clipping a little off the end wire and screwing the cap back on the wire works. Coils are not expensive and known to go bad.

 

http://venturerider.org/forum/showthread.php?t=69917&highlight=coil+test

[MiCarl]

If you've got two cylinders not firing, or misbehaving the same way, it makes sense to start out by tackling the items they share.

 

- They each have their own coil, unlikely both failed the same way at the same time.

 

- They share a TCI, but my understanding in the box is it's essentially 4 TCIs in one package. In other words they all fail or just one goes out.

 

While the motorcycle has 4 pick up coils, they're actually 2 pairs. I suspect within a pair they share some common wiring. What I don't know is if 1 & 3 (the rear cylinders) are on the same pair. Some work with the electrical diagram could help sort it out. On the other hand, DINGY probably has the answer memorized.

[Squidley]

You mention that you replaced plugs and wires, did you also replace the spark plug caps? There is a small resistor in them that gets corroded and can cause a lot of grief. Also too, the pick up coils are in the stator case and could also be a culprit (there are 2 of them in the '84) There is a 6 wire connector on the left side of the bike that hooks the coils into the wiring harness. Check that for issues, my '86 had a bad set of pick up coils and it would cause 1 of the coils to not fire due to bad wire connections.

[dingy]

Also check the pickup coil connector. It is located near the shock damper knob on left side of bike. 6 pin connector with one blank space.

 

I have seen this connector very dirty and/or oily.

 

Use an ohm meter and check from the black wire to each of the other 4 wires. Reading should be from 93~126 ohms. My opinion is that they should all be close to the same reading. Not two at max and other two at minimum, just an opinion though. Then check all 5 wires to ground. There should be infinity ohms to ground.

 

I am not sure which wires run which coil.

 

Wiring diagrams are linked below.

 

http://www.venturerider.org/forum/showthread.php?t=42358

 

Gary

[hartleybiker]

Squidley....When you say spark plug caps, am I assuming correctly that you are referring to what I call the spark plug "boot" aka the black rubberized connection point to the actual top of the plug? If so, then yes, they were already part of the wire set I installed.

My pickup coil connection point near the damper knob was very clean and I got continuity through it, but i don't remember my ohm readings. Thanks for the tips everyone, I will post my findings with the ohm meter soon.

[hartleybiker]

Dingy....Thanks for the wiring diagram link.. it turns out I was testing the wrong plug near the damper knob! I was testing the connector coming from the rectifier (also 6 way plug with one left blank) After comparing the wiring colors, I found that the P.O. deleted the pickup coil connector as I know many people do, HOWEVER, the connections weren't soldered or crimped or anything resembling decent electrical work, they pulled right apart with no effort... Just some electrical tape and a prayer I think! Pickup coils all measured 120 ohms so I'm going to solder all connections and hopefully that will help. I will post results.

[Yammer Dan]

Sounds like you found it. Let us know. I need to replace that thing myself as mine has over 76k on it but it don't miss a beat.

[hartleybiker]

Well, I soldered all connections where the pickup coil connector used to be, but no change in symptoms. I will now check the coils and plug wires, etc.

[hartleybiker]

Ok, so I switched the ignition coil connections coming from the tci front to rear and rear to front, just to see what would happen. I got multiple, reliable spark on all four cylinders (using the extra spark plug method) however, the motor would not crank. I assume it wouldn't crank because the timing was mixed up since the coils were switched. When testing the ignition coil resistance at the tci connector plug, I get 3.2ohms on all 4. That's the primary resistance right? How do I test the secondary resistance, or do I have those two terms mixed up?

[Flyinfool]

Since after rearranging the connectors you did have spark on all 4, did you try putting it back the way it was to see if you would still have spark on all 4? Since you now have spark on all 4 that means that all circuits of the TCI are firing and all of the coils are working. Just by the plugging and unplugging of connectors you may have cleaned a dirty connection and fixed the problem.

[hartleybiker]

Yes, I returned it to original connections and rode it up and down the road and only the front two are firing. I should point out that I do get spark on the rear cylinders when the starter is turning over the motor, but not once the motor has begun to fire. Does the TCI have a different behavior when starting the motor, and then switch to a "normal operating setting" or something like that? Just a thought. This has got me stumped. Any ideas or help is welcome! Anyone near Atlanta with a spare TCI I could try?

[easternrider]

Ok, so I switched the ignition coil connections coming from the tci front to rear and rear to front, just to see what would happen. I got multiple, reliable spark on all four cylinders (using the extra spark plug method) however, the motor would not crank. I assume it wouldn't crank because the timing was mixed up since the coils were switched. When testing the ignition coil resistance at the tci connector plug, I get 3.2ohms on all 4. That's the primary resistance right? How do I test the secondary resistance, or do I have those two terms mixed up?

 

From what I am reading, I think you have the terms mixed up. Maybe this is what you are looking for? Testing coil resistance

[ATTACH]79007[/ATTACH]

Edited August 6, 2013 by easternrider

[hartleybiker]

thanks easternrider i will chec secondary resistance as shown. primary resistance was good. as for the "electro-tester" in the schematic, I don't have one of those,