81 XJ750

Hey Noobie here. Bought a 1981 XJ750 that sat for 8 years. The bike cranks over, but I have no spark. Great conpression. Removed the safety relay and still nothing, I have power to the coils, but im not sure if its enough and im not sure how its supposed to change when I crank the bike. Any advice would be amazing. Im thinking of replacing the coils wires and caps. But im worried I missed something in my diagnosis.

 

Is your battery new? Check the fuses old Yamaha glass fuses clips get weak, common electrical problems because of this. Replace with a blade type fuse box. Make sure all terminals are clean. Don't replace parts, test them then replace might be a little corrosion on the end of the coil HT wires. Put a meter across the battery terminals and press the starter button, make sure the voltage does not drop below 10V under load. Carburettors will need attention too. You should have battery voltage to the coils no voltage drop at the low tension wires feeding the coils.

 

I put a brand new battery in the bike. And replaced the main and ignition fuses with inline fuses temporarily until I can find a proper blade style box to wire up. Testing the wires feeding the coils, there is a very minimal drop when I crank the bike over. Ill try cleaning the spark plug caps and double check the resitance through the coils, and through the wires. I know the resistance between the caps are about 5-6k ohms.

 

Clean all the connections on the starter circuit you can and the grounds to the frame. Check also your switch housing on the handlebar has not corrosion between the case and the handlebar, it earths to the handlebar. The spark is weaker on the TCI machines than on the CDI, and is harder to see depending on the light conditions. If you have no spark on all four it is a waste spark system so I think the coils are not the problem. Faulty kill switch, side stand switch needing cleaned, connections can get corroded. Have you tried for a spark with the bike on the centre stand and the side stand up?

 

https://xjbikes.com/forums/threads/no-spark-1982-xj750.119449/

 

Yea ive got no spark across all cylinders, Ive cleaned the grounds and most connections from the tci, but ill start cleaning around the start circuit. Alongside checking the connections around the handlebars. Ive been trying for spark with the side stand up, clutched pulled and in neutral. Thank youu

 

@SQLGuy read his write up here.

https://www.xjbikes.com/forums/threads/how-to-ignition-troubleshooting.21932/

 

Will do thanks

 

Hey, just wondering. I have power to the pigtails that connect to coils, 20V stagnant and then 15V while cranking. Are they supposed to pulse while cranking or are they in proper spec? If not would that mean the tci is bad?

Also Ive cleaned and checked all the wiring in the starter circuit, and anything leading to the tci. Tci Recieves proper power. Ive check and tested all safety switches, fuse box is good and all the wiring around the handlebars are good including the killswitch. By process of elimination im down to the tci or the coils/wires. I think im going to go ahead and replace the coils/wires bc its cheap and easy to do and probably should be done for proper matenance after the bike sat for so long.

 

Usually if there is a problem with the TCI one transistor affects a pair of cylinders. Only way to test it is to put it into another XJ to see if the bike runs. I think the TCI is not the problem. How can you have 20 volts from a 12 volt battery? As for the pulse do you mean the power being cut same as it would with points in the old days so it generates high voltage in the coil secondary windings?

 

Sorry no clue where I got those results or why I didnt question them. I retested with a different ameter and got;
Primary coil: 9.7V drops to 7.5V cranking.
Secondary coil: 11.5V Drops to 9.5V cranking.
And yes sorry did some more research and realized I was completely off with that. Again still new to this but trying to learn.
Anyway I just ordered new coils + wires + caps and hopefully this fixes my issue. Checking the spark plug wires, wire to wire they were not reading within spec. The ones I ordered were meant for a 83 cb750, another thread mentions that they are compatable but I will double check the ohms before hand.
Just wanna say thank you for all the help and advise also.

 

That voltage is too low to create a spark. You need ten volts minimum when pressing the starter button for the transistors in the TCI to work.

 

I keep a bigger test battery around the shop just to check for spark. I hook it to the bike with thick jumper cables and crank the bike. Turning the lights off in the area also helps to see if you have spark. Looking for a good blue spark not a yellow color. Any spark is better than no spark thought.

 

Ah okay, so im probably losing power somewhere. Ill try a different battery just in case. And go over all connections.

Edit; Tried a larger battery, no change. Losing power somewhere along the way.

Replaced the ingnition fused, brought me up to 11V on either connection to the coils. But both are dropping to about 9V whilst cranking

 

Take the time to look at some of the connectors. I know it sounds like busy work, but it is not. Finding that one connection that is maybe green or worse and is not making good contact will really help out your troubleshooting. Do you have a manual yet? Do you have a multi meter? Those will be your next steps in tesing the coils and other parts of the bike to make sure it has good connections and the specs are correct for that parts that are making voltage and in the starting ckt.

 

I have both, checked all connections and cleaned. I get decent power to the coils, especially when boosting the battery. 14V and then 11V cranking on both. Checked the primary wiring on the coils is only slightly high in ohms. Second I cannot even get a reading on. Also realized I realized the mistake of running BPR7ES instead of BP7ES. I replaced thoses with some autolight equivelents for the time being. New coils/wires arrive tomorrow so hopefully that fixes the problem.

 

Think I might have missed this one, connects to the rectifyer and tci. Gonna replace the whole connector bc I dont think I can save that.

 

Good chance that is the issue.

 

Turns out it wasnt. I am reallyyyyy struggling with this bike.

I have new coils/wires/plugs. Proper spark plugs, all connections I can find have been cleaned/replaced. Tci gets enough power, coils get enough power, coils become warm when trying to starting bike?? All relay switches click, cleaned and checked clutch safety switch, and same for the side stand switch. Ive cleaned the starter button, ignition switch and kill switch, checked all wiring behind the head light (which is burned out). And I have a good battery.
I am at a total loss, last thing left would be the tci correct? Or am I forgetting something stupid?

 

That really shouldn't happen unless the TCI has shorted transistors. If you put a DMM on the Orange or Grey wire and turn the ignition key on one of them should go low for about 3 seconds. Knowing which one (orange or gray) is dependent on the position of the reluctor, so just as long as one of them does. Then after that 3 seconds or so both should remain high (near 12V) until the bike is cranked.

Note also when you see this happening a spark should be produced on the plugs for whichever coil went low and then high.