Problems with 1981 Seca xj750

The front end is strapped down for transport. Does make it look nice though.

 

Duhhh... I guess the BRIGHT ORANGE RATCHET STRAPS should have been a clue. How'd I miss that?

 

I'm glad everyone likes the Widow. I'll post some pictures of the front and yes it does have a round headlight.

However I'm glad to report that I found a problem with the 81. Up till now it seems that everything I tested was working so I couldn't figure out what to fix.

Today I bought a variable spark tester. Basically it's a screw that you twist in or out to adjust the gap the spark has to jump. I hooked it up and the spark can jump at least a quarter of an inch and it looks blue. However I don't get a spark each time. When the starter is cranking it might spark many times then skip once or twice then hit then skip. Keep in mind that I have swapped the ignition coils and the TCI and battery and I have new plugs. The only thing not swapped that has to do with electricity are the pickup coils... Do pickup coils fail slowly? Could that be my problem? How difficult is it to swap the pickup coils with the Widow? It looks like I have to cut wires. Can I even get new pickup coils if that turns out to be the problem?

 

You do no thave to cut wires. The plug for the pickup coils is the small one at the TCI. You do have to mess with routing the wires though. There is a test for the pickups, but I don't have it handy to look at.

 

XJ750 air-cooled models:

Pick-up coils:
650 ohms +/- 20% = 520 ohms to 780 ohms acceptable range


I wouldn't cut the wires, although it is a pain to get their harness out from all their routing clips, etc. on their way to the TCI.....

 

I'd check the timing prior to removing the pickup plate (see if the dots on the cams align with the marks on the caps with the mill at TDC). If the cam chain has jumped a tooth or wasn't set correctly that might be the problem.

Gary H.

 

I double checked the camshafts timing last night. Everything lines up at TDC.

My pickup coils measure at 657 ohms and 659 ohms.

But do they sometimes work and sometimes not? Or do they either work or not work?

Is there a post that explains how to take the pickup coils off the bike?

 

They either work, or they don't work. There could be an intermittent break somewhere in the wiring harness tough.

 

They're in range but like k-moe said could be a break in the wiring. Got a impact screwdriver? The plate is located with two screws. Try not to disturb the pointer. Get a camera and take pics of how the wiring routes to the TCI. Idk about the 750 but there are two clips assembled with two of the shifter cover bolts the wires route through on my 650.
Note: The neutral switch and oil level sender feed are also part of this harness.
Hope this helps.

Gary H.

 

do you have a link to one of those "variable spark tester" things? It seems like if there's spark sometimes and then it quits, the bike should run sometimes and then just quit.
no word yet on wet or dry or new plugs yet

 

A spark gap tester, or high resistance tester. looks like this.


It is placed inline between the coil and the sparkplug, and the screw is adjusted to increase the air gap between it and the coil to see just how strong the coil is.

 

For members that may not understand completely...the spark will be seen in the window. As the screw is opened the strength of the spark will continue to jump across the window.

Gary H.

 

That's exactly what my spark tester looks like except I don't have a glass window.

I decided to buy a used pickup coils from eBay last night. It's already been shipped this morning so I should have it in a few days. Even though I decided to get another pickup coil it doesn' make sense to me that this should fix it but the whole friggin' bike doesn't make sense to me...

 

well that's just dandy but if the plug is fouled and is arching at the base of the insulator that little gadget might lead you astray.
if you just pull the boot up off the plug a bit you can hear it fire, that's a impressive tool though

 

I have some new information that I don't really understand. I have a video that shows what I did. Here's the link:Timing light:

I suspected a problem with spark or ignition since everything else seems to be in spec. I hooked up 2 timing lights. The red one in the video is hooked up to plug wire #2. The silver light is hooked up to the #1 wire. In the video you can see the reluctor in the bottom right. The video is in slow motion. In the video the engine is cranking but it did not start.

From what I understand each wire should either have spark or no spark. However in the video you can see that plug wire #2 sparks every time the reluctor passes its pickup coil. Plug wire #1 fires EVERY OTHER time it passes its pickup coil... What The Heck is going on????

 

Nice job of capturing that and slowing it down.

I don't know what it means but eagerly await an answer.

 

Nice camera work! How did you do that? The Seca 750 uses a waster spark system so from what I understand both cylinders show spark on every revolution of the crankshaft. My first thought would be the #1/4 PU coil. Second would be the connection from the PU coil to the TCI or the wiring from the PU coil to the TCI. Since you have changed the TCI, I doubt its the problem.

 

The camera work was done by my son with his iPhone.

 

Check the pickups from color to color, tci unplugged. black is common so color to color should be pu1 + pu2 and with the tci unplugged the black should be OL to ground.
don't know the colors, I'm on the phone.

 

The wire colors from the p/u coils to the TCI will match the color of the wires from the TCI to the ignition coils:

- the ignition coil for the #1/#4 spark plugs is mounted on the left side of the bike, and has the solid orange (ground) wire and the red-with-white-tracer-stripe (hot) wire input leading to it.

- the ignition coil for the #2/#3 spark plugs is mounted on the right side of the bike, and has the solid grey (ground) wire and the red-with-white-tracer-stripe (hot) wire input leading to it.