XJ650RJ Cranking but not starting

Hi everyone,
I recently bought a 1982 Yamaha XJ650RJ. I have been starting to do some modifications on the bike and now I have this starting problem. The bike cranks but doesn't start.

I have gone through a troubleshooting for the TCI and I am stuck at step 2. The bike reads 12volts (at red/white wire on the TCI) with ignition on, then while cranking goes down to about 7-8 volts. There is a weak spark at all 4 spark plugs. The fuse box is the original but in top shape. The battery, I recently changed for lithium-ion battery (I had previously tried running it with this battery and it did so on the first try). I did the same test while plugged on a car battery and had the same result (drop of voltage to 7-8V). My Ignition switch works fine, I changed my run/stop switch but it works fine (I had previously tried running it with this switch with no issues). The starter worked fine when tested initially. Still works when plugged on car battery. Ignition coils have the required resistance at their terminals. All the electrical contacts visually look fine, no corrosion, no melting, all clean. This bike was in great condition!

The only modification I made from my initial testing to now is the installation of a digital speedometer. Nothing too fancy in terms of wiring. For the moment though, I do not have a RPM scale because I didn't know where to connect that wire. I tried connecting it to the Orange wire going to the ignition coils, but I thought that it may be what was causing the issue so I disconnected it, but still no change.

Can anyone help?

Thanks!

 

Sorry I can't comment on your voltage drop, but what can you tell us about the carbs? Any recently acquired 82 should have the carbs thoroughly inspected.
Are the plugs getting fuel? Was it running great just prior to your wiring mods? Just curious, do you have the silver RJ or the red RJC?

 

"top shape" or not, the fuse box is the likely culprit.

You can verify by sticking sharp probe tips, or pins, through the insulation and into the R/W wires on either side of the Ignition fuse and metering DC voltage there. If that shows any significant voltage during cranking, your fuse box is bad.

You should probably also double-check that the voltage at the battery isn't dropping that far during cranking.

 

This has a lot of great information.

http://www.jetav8r.com/Vision/IgnitionFAQ.html#aapp

 

The carbs seemed fine. At first I thought it was a fuel issue. So I tried putting fuel directly into the carbs myself and trying to start it. Once I was certain that the spark plugs were getting the fuel I looked at the spark itself and it looked very weak. That is why I went to check the voltage. It was runnging perfectly before i installed the speedometer... Though that wiring seemingly has nothing to do with the ignition system. I have the silver RJ

@Franz I went throught that guide and things are a little for the XJ650RJ, so it didnt apply perfectly.

@SQLGuy I'd have to try and see how that goes. Though the connections are really clean and the bike was running fine with that fuse box before

 

I would think that if it's cranking, and you tried putting some gas directly into the cylinders (not just the carbs) and you didn't get any kind of fire (even a light chug), even with a weak spark, that it would be compression related. If the carbs were bad, you could still get a quick fire by putting fuel into the cylinders (or a gas soaked rag over the intakes as I have heard some people doing). Or the inverse could be true. It could be constantly flooded (but that would be pretty obvious by the sparks plugs).

The speedometer should have nothing to do with the bike firing. I have completely required an XJ650J with pretty much all the wires not attached and it still fired. Obviously the ignition circuit still needs to be good, but other than that, lights, speedo, horn can all be unplugged and still have the bike fire.

 

you need 9.8 volts for the tci to fire. do a voltage drop test on battery. you should clean and check all the connectors in the ignition system.
glass fuse are a known problem . you could replace the glass fuse for ignition with an inline fuse holder as a fix.
charge your battery then let it sit over night to see what voltage it has after sitting. looking for 12.8 volts.
what kind of battery do you have? if lead acid make sure it is full to the lines use distilled water and then charge the battery.
make sure battery connections are clean. you can find the connector from the right control to main harness. red white wires there are 2 of them measure voltage there too. you are looking for voltage drop points along the red/white wire. .
measure at battery main fuse then ignition fuse, then wires in and out of kill/run switch, at tci .
clean all the connectors with electrical contact cleaner apply a dab of dielectric grease reassemble and make sure they are tight. look to see if any of the pins push out when reassembling them.
run kill switch may need to be cleaned, ignition switch may need to be cleaned, tci and ignition connections will need to be cleaned.

you should also ohm out the ignition system to check it is all in spec, ignition coils , pickup coils. best time to do this is as your cleaning the connectors.

you will find your specs in this thread
The Ultimate Relay, Switch, Sensor, and Diodes Guide