Problems with 1981 Seca xj750

Positive lead on the black wire on the TCI connector, neg lead on the battery negative post. If there is a poor ground there will be a voltage drop. I would expect this to be less than a couple hundred millivolts. There are two eyelet terminals on the Seca main harness that are secured via the coil mounting hardware - these two eyelets account for all the grounds and attach them to the frame. Engine to frame mounting provides the connection to battery return.

That is correct. Current path is from the battery, through a single plug connector, main fuse, ignition switch, ignition fuse, kill switch, and then to the coils and TCI. Any of those points can cause a voltage drop with poor connections. Typical voltage 12.20V battery, and approximately 11.5V at the TCI R/W wire.

I'm not a TCI expert, but I can tell you on my 82 Seca that only the 2 and 3 cylinders spark after two seconds. You can verify this by placing a meter on the gray wire on the TCI. At power up it will read 1.5V for 2 seconds, and then switch to 12V, which collapses the field in the coil producing a spark. For some reason, the orange wire at the TCI always seems to power up at 11.5 volts, so the 1 and 4 coil is not going to fire.

Either the orange or gray wire needs to be low when checking the above voltages, otherwise the current is so small poor connections are not going to be obvious. When the orange or gray wire is low for that two seconds, the TCI is sinking approximately 2.5 amps. That is the point to Polocks suggestion to use a brake light bulb, sometimes you need the extra current to show the fault. He suggested it on the coils, you could also disconnect the TCI and attach the bulb from the engine harness R/W to the black wire to simulate approximate current demand on the ignition circuit. While the bulb is connected voltages could be verified.

 

One other note on the spark after two seconds is make sure the other plugs are installed and the plug wires connected. The wasted spark system requires the corresponding plug that you are testing to be connected to ground and electrically connected.

If 2 and 3 circuit failed the 2 second spark, I would try the 1 and 4 for good measure, or vice versa. There might be a good reason that the design of the electronics within the TCI results in an either or. Theory on that is electrically it makes no sense to fire on power up after two seconds (other than to scare the crap out of you if you have overflowing carbs and all the Yammie engineers are still laughing their A$$ off). The reality is it actually produces a bit of a fire hazard.

 

Your electronics acting up the way they did indicates a grounding problem. Yes to #2; you should see the same voltage as going across your battery posts or close to it. no to #1. You can have continuity with a meter, (its rare that a meter has a Ohms x 1 scale anymore), and still not have a good connection. Best to for the TCI to have a ground of its own. This has fixed a lot of TCI problems in the past.

 

Swing test the coils and wires with your running '81 Seca. I had similar symptoms from bad coils on my '81 Seca. They also ohmed out good but didn't perform under load. Can you High Pot the coils?

 

What does High Pot mean?

 

It is a testing method that applies 100% or more of the normal working voltage to the windings in order to test the insulation on the windings. It is normally used during manufacturing, and I have not heard of it being used as a diagnostic tool otherwise. I humbly suggest that the coils are not the cause of the problem. The poor ground needs to be addressed regardless, and prior to investigating any other possibilities.

 

I thought I had read every post from the OP twice. Somehow I missed the flaky electronics issue on page 2.

I withdraw my adamant coil issue suggestion until further testing is completed.

BTW, if you have the ability, and I realize most won't, High Potential testing can show weakened insulation in a coil winding that may be problematic when the coil heats up under load.

 

OK. So I reinspected my fuse box and they all "looked" good but when I hooked the voltage test up on each side of the fuse it was apparent I had a problem. The fuse labelled "signal" had a hole in the end and that was causing my dashboard problems. I replaced the fuse box with inline fuses last night and my dash lights all work as they should.

However the main problem still persists. Now I can't get the engine to run at all.

So here's what I did this morning. I took the carb off both bikes - my 82 Seca that works and my 81 that does not. I sprayed 1 squirt of starter fluid in each intake port on both bikes. Then I hit the start switch. The 82 started up immediately and revved to 4000 rpm and then died because the fuel was gone. The 81 sputtered and didn't really start - in fact it backfired through the number one intake port - a nice yellow flame...

Now here comes the part you won't believe. I hooked up a timing strobe light to the 81 with the induction lead on the number one Cylinder. It seems the light is strobing at about 15 degrees AFTER tdc. How is that possible? I have already verified that the cams are timed correctly to the crank. As a sanity check I hooked the strobe up to the 82 and hit the starter. The strobe showed on tdc for Cylinder number one. Keep in mind the the bikes are not running during this test. I'm just cranking the starter - the strobe light has a built in tach which told me the engines were cranking at about 525 rpm.

Someone suggested that I swap the pickup coils on the 2 bikes. I have not done that yet. It looks complicated but I suppose I need to do it unless someone has another idea?

 

Pull the left side crank cover (4 JIS machene screws, has the YICS emblem on it) and take a picture. I want to see what the pickup plate looks like and see if someone monkeyed with it, or if there are just loose screws. I think we're right back to not getting spark at the right time being the problem, and I hope that the solution will be apparent once that cover is off.

 

I already had the cover off to do the timing test. In fact I have set the reluctor to the same orientation that the strobe lights up when I did the timing test. All the screws feel tight. Here's the picture.

 

it might be a good time to take that rotor off and make sure it's on the crank correctly

 

+1

 

OK. I'm an idiot. I didn't believe my last post even as I was writing it. How could the timing be firing 15 degrees AFTER tdc? I decided to test again tonight. There might have been interference from plug wire number 2 on my previous test so I removed plug number two and grounded it far away from plug number 1 this time. The strobe lighted exactly where it should - between the two little lines before tdc. So now I believe the timing is correct. Sorry for the confusion. I feel like a fool....

It still doesn't run.

 

Does my bike have a ballast resistor?

I was reading an article on the XJ4ever.com site about ignition. This is what it said: "Using a voltmeter set on 12VDC, connect the positive meter lead to the Orange or Grey coil lead at the TCI. Connect the negative meter lead to the black (negative) lead at the TCI. Turn on the ignition. Voltage should come right up to about 10 - 11VDC. Crank the ignition and observe the meter. Look for a wide voltage swing during cranking. A strong swing indicates that the pickups and TCI are working OK and your trouble is between the TCI and the plugs. Possibly a bad ballast resistor, bad coil, bad plug cap or just corroded connections. Repeat this test for both Orange and Grey coil leads."​

 

OK, makes sense. So you have good compression, fuel with the petcock on PRIME and spark at some time. But not enough to get the engine to start. If you are certain the compression is good and the fuel supply is good, then the only thing left is the spark isn't there sufficiently to start the engine. Either its the ballast resistor or the spark isn't there at the right time. The purpose of the ballast resistor is to increase the intensity of the spark during start-up, then reduce the spark to prolong the life of the ignition coils. Perhaps you should try swapping the ballast resistors between bikes and see if that helps. If it doesn't I would suggest you start tracing out ignition wires. Since the fuse blocks was history, I would suspect some of the wiring connected to it, particularly the wiring for the Ignition fuse and the wires going from the TCI to the coils. Are these wires connected correctly? I would also suspect the wires coming from the PU coils to the TCI as this controls the spark timing. Might think of swapping the PU coils too. But one thing at a time. Wish I was closer, I would come over and give a hand.

 

The article that this information came from dealt with XS1100 models (if I recall correctly).. I don't think that the XJ-series bikes not use such an animal.

 

what heat range are the plugs and how are the leads?

 

The plugs are brand new bp7es - stock plugs. The leads are in good shape. I measure 11K ohms from lead end to lead end on both coils and the plug caps measure 5K okms on 1&4 and about 8K ohms on 2&3.

 

Did you ever get any voltage readings at the TCI and coils to see if you have a voltage drop occurring in the ignition circuit?

The symptoms of barely runs, can't give it any throttle without it dying are so typical of a weak spark. If the supply voltage is OK at the TCI, I would do as other members have suggested and swap the coils with the 82. Also, as others have suggested, coils can ohm out OK, but breakdown under the stresses of high voltage. Seems like an easy try compared to swapping carbs and such.

Any continuity checks done on the pick-up coils? Not just resistance but checked to ground also. It's also another very easy check.

The other flaky issues related to the tach and instrument cluster have already been explained by the defective signal fuse you found and corrected with the fuse box replacement.

 

I searched thru the wiring diagram and I can't find a ballast resistor. I would question the 8K plug cap resistance on the 2-3 plugs. It shouldn't be any different than 1-4. Have you tried removing the plug cap on those two wires and cutting off a 1/4 inch of the wire, then screw the cap aback on? Another possibility is removing the resistor in the plug caps and going with a resistor plug.