No Spark 1981 XJ750

I have a 1981 Seca 750 and believe it may not be firing... Can someone tell me a simple way of checking spark and proper grounding with the aluminum block? I have heard of Stator and other issues with this bike. I just bought it with apparently only 19,900 km/s on it, but who knows... Any help would be appreciated! Thanks!

Tom

 

Re: Checking Spark 1981 XJ750

Easiest way is get a spark tester and stick on each plug one at a time since they have a neon lamp that flashes.

 

Re: Checking Spark 1981 XJ750

I've used an inductive timing light to check for firing. Another option is a small inductive firing checker. Their cheap but work just fine. If given the option, I'd get the timing light since it can be used on more than just the bike.

 

Re: Checking Spark 1981 XJ750

Thanks guys, keep you posted...

 

Re: Checking Spark 1981 XJ750

I screw out the plug(s) and set against the head. Turn it over and look for spark. Don't need to buy anything that way.

 

Re: Checking Spark 1981 XJ750

Still no spark... I checked all cylinders and nothing. Lubed the clutch switch, kickstand switch, etc. Thinking about trying a voltage regulator a/o TCI; however wiring looks fine to Voltage regulator connector - thought it might have got hot becuase of too many boosts. Are coils a problem with these bikes? I want to try and troubleshoot this myself before taking to a garage for sure if anyone has any other suggestions would be much appreciated, thanx.

 

Coils tend to be problematic with these bikes. Age catches up with them and they just drop (typically from corrosion in the secondary high tension leads). Ohm them out to ensure you have fuctioning units before dismissing them. 2.3 Ohms for primary reistance, 11K inboard 5K outboard for the High Tension leads. These numbers should be good but I'm not 100% about the High Tension leads. The point is you should see something close to the resistance readings I've mentioned.

 

Thanks Robert. They look original and the wires are really weaknear the plug end... Probably a weak question to ask but would a faulty kill switch cause this also?


Tom

 

have you tried ohming the connections out with a multimeter?

 

Not yet philbewer, more work tonight...

 

If you pinched the kill switch wire to ground sure you could "kill" the bike. Got my Maxim cheap after the P.O. did that and couldn't figure it out.

 

Finally I received my Yamaha "Fours" manual so I can take some readings and narrow this down. Keep you posted!

TPD

 

Still nothing, and now getting a bit anxious. As far as I can tell I am getting 2.3 - 2.5 ohms off the primary ignition coils, infinite off the secondary(s) with my meter set properly. I am also getting an infinite reading off the pickup coils on both connectors off the TCI. I haven't checked the regulator/rectifier yet and wondering if this may all be caused by overcharging or a really bad short. A connector with a small red wire that leads off the solenoid also looks like it got hot at one time. I think I'm in over my head, any ideas other than selling the bike would be most helpful! Thanks...

 

Ok TD, we're not done yet.
You say you are reading infinitive ohms on the coils. Pull the spark plug caps off and try again. Make sure you are on the right pin of the primary coil. Caps can and do go bad.
Open TCI pickups? Again, check the color code and ensure you are on the right wires. They do go bad too.
It is possible that someone spiked the system trying to jump it off of a running vehicle (hopefully you are sharper than that).
One foot at a time, we'll get to the bottom of it.

 

...I am going to try another meter tomorrow. I did follow the wires back to the source as you suggested. I believe the PO must have used a vehicle to boost if in fact the ignition and pick up coils are gone bad.

I checked resistance with the 4 pin connector off the TCI across the blue and orange wires and then orange and grey on the other connector both infinite..

Also need to test the regulator to but need 2 meter's to do so. All the connections and ground(s) seem fine throughout the harness and can't see any other evidence of hot wires. Fuses are good, kill switch seems to be working fine also. Have not checked side stand switch however...

Thanks again, back soon with an update...

TD

 

So I now have 2.5-3 ohms on the primary side using a different meter.. I trimmed the HT secondary(s) back a bit and presto! got 11kohms! Can't seem to get a reading with the caps back on though... Have replaced the plugs and will try again tomorrow... Also need to re-measure off the TCI back to the pick-up coils with the new meter... Keeping my fingers crossed!! Bring on the spark!

TD

 

hi Tom,

I recently bought a Seca Frankenstein bike with an xj750 engine. I had the same issues you have mentioned. I used the Haynes manual and checked the coils and pickup coil (stator). All were fine. I then assumed that it must be the cdi box. When I spoke to 3 different salvage yards to by a ne cdi box they all told me they didn't think it was the cdi. I checked the wiring and fixed any visible problems. I finally read the section in the Haynes manual about carburation and realized that the carbs needed to be synchronized. If one or more is out of whack it may seem like it isn't firing in those cylinders. I synchronized the carbs and sure enough all cylinders are firing. This may or may not be a solution to your issue, but it just so happens to be that on my bike 1 and 4 were also the suspect cylinders. Before synchronizing you need a yics tool. This will disable yics so that you can synch the carbs. I did a search online and found a site that showed a diagram and parts list to create your own. It cost me less than $10 and worked great.

Good Luck!

Tim