Problems with 1981 Seca xj750

Timing perhaps. Is it possible the cam tensioner has failed and the chain has jumped a tooth.... Ignition timing is another possibility..

 

Ok... So theoretically everything seems in order because it works fine on the other bike.

So, lets look at some things that didnt get swapped around ( unless I missed it)

Fill your tank, Pull the fuel line off of the petcock ( put a fuel-catch underneath first). Now turn it to PRI and see how the gas runs out.... It should gush out.

Try swapping tanks

Swap your spark plugs

Swap your pick-ups under the left engine cover

Check all your ground wires

Swap your batteries

Do only one thing at a time, and swap back before going to the next thing, so you don't "contaminate" the cause

Before doing all that...

Pull the seat and air filter ---- can you get the bike to run with either starter fluid carefully sprayed into the carb throats first ?

Or, if you pull the plugs and squirt some "extra" gas right into the cyls first?

I'm trying to narrow down whether its not getting enough electric signal, or if its just not getting enough gas yet.

Let me know these things, and if answers still seem normal, then we start all over again.

Hang in there........

 

You have spark, but do you have spark on the correct cylinders at the correct time? Check your plug wire locations.

 

He said he had 1/4 so at least the pairs are correct, and I don't think he would have been able to ride that far with them switched ----- so I didn't go there----

 

Ya. I though about that after, but then thought that they might have gotten switched while he was checking things.

 

Thanks again for all your support and suggestions:

bstig60 - Love your bike!!!
k-moe - The wires are hooked up to the correct cylinders and the camshafts are timed correctly to the crankshft.

I'm focusing on the electical - especially the TCI. I read today about a TCI test - pull a spark plug and ground it (still attached to the plug wire) and turn on the key. After 2 second there will be a spark across the plug. I tested that on both bikes (the 82 that works and the 81 that I'm having troubles with). The TCI on the 82 passed the test (spark after 2 seconds). The TCI on the 81 did not pass the test (no spark after 2 seconds). So I swapped the TCI from bike to bike and re-ran the test. With the TCIs swapped, neither bike passed the test (no spark on either bike). Then I put the TCIs back on the correct bikes and the 82 bike passed the test (spark). The 81 did not pass (no spark).

So it seems to me that the TCI on the 81 might be bad since it did not pass the spark test on the 82. However the known good TCI from the 82 did not work on the 81, so there must be something else wrong with the 81...

 

I've never heard of that spark test before ----

 

Thanks, Steve. Not familiar with that spark test either. I don't have my book with me to verify this, but I believe there is a black ground wire on the TCI. It should be a ground wire. If so, you might want to try cutting it where you can splice it back together if need be and running a wire from the TCI side to the negative ground post on the battery. Also, If you haven't already done so, use a good brand of contact cleaner and clean the connectors on the TCI and the wiring plug that attaches to the TCI. Also check the RW wire to make sure you have a good power supply. Did you put a new battery in the bike before you started the trip home? If not, swap the battery with the 82.

 

RICH, LEAN, or possibly even ritually UNCLEAN or OBSCENE: THE CARB REBUILDING FAQ:

Why does my engine sometimes backfire when I first turn on the key (without attempting to start the engine)?:

- When you kill the engine, the intakes will still have some remaining (un-burned) air-fuel mixture remaining in them. Upon powering up the bike (turning the key on), the ignition system will apply 12 volts to the coils, thus charging them. Of course, that constant current is not good for the coils, so few seconds later (if the engine is not started) the TCI shuts down the coils by grounding them (to protect the coils from overheating) which cuts the current to the coils and thus triggers a spark to the plugs. If there is enough un-burned fuel in the intake manifolds or the combustion chambers, and if the valves are held open by the camshafts, a backfire thru the exhaust header and/or thru the intake manifolds/carbs/airbox can result.

 

That's true. I have seen it on other Yamaha models. It would appear then that the TCI is not putting a pulse out to the coils. But why would the test not work when the TCI's were swapped?

 

That is the $ 64,000 question............

 

Someone said it before - air filter?

 

Oh.... I didn't realize that was a way to test though....

 

When you test the voltage at the coils, use a brake light bulb, not a digital meter

 

While I was looking for clues I found a reference on this site (XJBikes.com) that linked to here:
http://www.jetav8r.com/Vision/IgnitionFAQ.html#a12p0

The above link is a TCI FAQ for the Yamaha Vision motorcycle, but I learned a lot about TCIs on here, and that's where they mentioned the "TCI Power-on test".

Last night I conducted a leak-down test - I have between 15% and 18% leakage on all 4 cylinders - from what I have been reading, those are good numbers. So I'm double sure the engine is fine (compression for all cylinders was between 145 and 150). I know the carbs are fine (since they run on my other bike.) So it MUST be something electrical.

I have another symptom that I neglected to mention, but it might provide more insight now that I am convinced it's an electrical problem. A few hundred miles before the bike refused to run I started having these problems: The headlight and dashboard stopped working. All the fuses were good and the headlight bulb looked fine. I spent an hour removing the front fairing and checking all the wires and everything looked fine. And the headlight started working again. But the dash was acting really weird - when I turn the key on and started the bike, all the insturments seemed to work. But as I shifted gears, the whole dash would lose power and regain power and cycle through the diagnostics as if it were starting up. That would happen each time I shifted. Keep in mind that the bike was running crappy at this point and I could not go faster than 50 MPH. Also the tach would read about half what the engine was actually doing (I know that each 1000 RPM equates to about 13 MPH - so at 50 MPH the tach was reading about 2200 RPM where as I would expect to see about 4000 at 50 MPH. The day before it was showing the correct RPM for the bike speed.

The current state of the bike is that it takes over a minute to stat - if it starts at all, and it idles very rough and any throttle will stall the engine. Battery is good and strong. So after everything I've learned so far, I suspect the TCI might be bad and/or I have a loose or corroded connectin somewhere that is eating up a lot of current and that's causing the spark or other electronics to be weak.

If I get deperate enough, where can I but a good TCI?

Thanks for all your help and suggestions thus far. I'm learning a lot...

-steve

 

Jeez... you left THAT out?! Start looking very carefully for the intermitant electrical break that you have. Does it have the original fusebox? The fuses can look fine but the fuse clips likely are not fine at all. At this point I seriously doubt that the TCI is bad, and there really won't be a way to test that until the rest of the electrical system is working as it should be.

 

Beginning to sound like a bad ground somewhere...........

 

It could also be grounding. If I recall, there is a ground multiple that runs throughout the wiring harness and provides ground to all systems. I have seen the connections inside the wiring harness fail before. That is one reason I suggested providing the TCI with its own ground directly to the Negative battery post.

 

You all have been a wealth of information.

Is there a write-up on how to exhaustively test the wire harness and electtical connections?

bstig60 - I re-read your previous suggestions just now. I have some (stupid??) questions:
1) Is there a way to test the black ground wire that is connected to the TCI? Can I hook up my digital multimeter to the wire blade that connects to the TCI and also to the negative battery post and meaure something to validate the wire is good? I know you suggested cutting the wire and hooking the TCI side directly to the negative post - I'm just trying to understand if there are other methods to test.
2) When you said "check the RW wire to make sure you have a good power supply" - I assume I would use a multimeter and connect the probes to teh RW (red/white) wire and the other to ground and that I would check for 12 volts and that would be a good value?