Xj750 SECA missing after 10 minutes

1982 750 SECA
Issue: bike begins missing after 2-3 miles (5 minutes). Missing seems to get worse and within 20-30 seconds the bike will not have enough power to continue. As the rpm’s drop below 2000, the bike just dies (as if the kill switch was hit).

Fuel vent: I have ridden with the tank cap open and the behavior is the same, thus ruling out a gas cap venting issue.
Bike is stock with the exception of the fuel petcock which was replaced with a non-vented petcock (yes, the vacuum line has been plugged).
The plugs appear pretty clean (see picture of plugs).
The exhaust header temperature (after 2-3 minutes) was 105°, 120°, 109° and 115° which may indicate all cylinder were firing (as does a visual inspection of the spark plugs)
After five minutes, the bike starts and goes fine, but the missing symptom reappears within 45 seconds.
A brass mesh fuel filter is in-line and allows fuel to flow unimpeded.
There are no visible cracks in either coil or coil wires or caps. The primary and secondary coil circuits test within spec (obviously tested at room temperature, not at engine operating temperature).

A change which may or may not have anything to do with the missing problem is recently the enrichment (choke) function does not seem as effective as in the past. The cable is properly adjusted and is lifting the tabs on the carburetors. This being the case, cold starting is not as instant as it was. Again, this may be a separate issue.

 

Have you checked the valve clearance, tight valves will cause loss of compression and result in zero power or not start when warm
How about coils check values when warm

 

The valves are a great suggestion, but also consider the ignition fuse if you are still running the original fuse block as corroded / broken terminals can heat lowering ignition voltage to the point of not running

 

If you need to change valve shims balance your carburettors afterwards.

 

Okay, three good suggestions. I’ll tear into things this afternoon. Also, is there a way to test the ignition system (coil+plug wire+spark plug cap+spark plug) other than by the color of the spark?

 

You can check what voltages and ohm readings you are getting on the ignition system components.

 

Sometimes with bad coils/ ignition components, when they are cold they will have contact and will ohm out correctly, but when they heat up and things shift around, will lose contact.

 

boy this sounds familiar to me

 

The valve clearances are within spec.
I may have identified a potential issue with the spark plug caps on #2 and #3. Their design/construction was very sub-par and I have ordered new spark plug caps and will continue troubleshooting once new compliant spark plug caps get installed.

 

Also classic TCI symptoms

 

Guess that it is a electrical component that is giving up once it heats up. Either the TCI or coil or coils are causing you the issues. Anyway of riding the bike close to the tools, when it happens pull a plug wire and have a extra plug in hand. Slap the extra plug on that plug wire and see if you have spark?

Good luck with the trouble shooting.

 

I thought of doing this…but then didn’t. I will put a plug and plug wrench in my tank bag when I venture out today. I agree this will be a good way to see if an electrical component is failing due to heat.

 

As I begin to button things up, I came across a discussion on resistance at the spark plug and spark plug cap. Some bikes of this era call for a total of 5k ohms of resistance and others call for 10k ohms. The manual actually calls for 5k ohms on cylinders #1 and #4, and 10k ohms at #2 and #3.
Comments?

 

This is the note in the xj4ever catalog (https://www.xjbikes.com/forums/threads/xj-parts-catalog-section-d-electrical-system.44641/):

NOTE: some sources specify that XJ650 models, XJ750 air-cooled models, and XJ900, XJ1100 and XS1100 models used 5K caps on the outer plug (#1 and #4), and 10K caps on the inner plugs (cylinders #2 and #3). Additionally, it is noted that non-North American models would indeed use 5K caps on all four cylinders. This conflicting information from a variety of sources can lead to quite a bit of confusion. Our recommendation is to use 5K caps on all 4 cylinders

 

Not much of an update, rather more information which may help in the troubleshooting process:
I replaced the sub-par spark plug caps to NGK’s (inexpensive swap and eliminates a variable)
The bike starts missing after 2 or 3 minutes. Spark seems strong (visually viewed spark on cylinders #3 and #4 thus testing spark from each coil). Disconnecting plug wire to cylinder #3 makes some (minimal) difference in idle; disconnecting cylinder #1, #2, or #4 causes the engine to die. This indicates to me cylinder #3 is in some way may be part of my problem. Value clearances and compression are within spec or consistent with other cyclinders.

One characteristic is in limping back home with the engine running on no more the three cylinders (maybe less?), if rpm’s fall below ~2500 rpm, there is an immediate shut down - not a “faults ring” (would seem to be electrical). Yet, as the bike comes to a stop, restarting attempts cause the engine to fire (but - alas - not restart), causing my previous conclusion that the problem is electrical to be questioned. And just to cover as many bases as possible, as long as engine speed is 3000 or higher this “electrical problem” keeps its head hidden - but the missing is still present.

I believe the coils can be ruled out because the primary and secondary circuits test to specification when tested at 65° F. I have not tested the coils at normal operating temperature, but spark at operating temperature is strong. Ditto with TDI since it provides the signal to the coils.

Is there an electrical component (or ground) that would function at higher engine speed but cause a fault when engine speed is less than ~2500 rpm?

 

The alternator does not work until 2000 rpm. If you can keep the tacho at 3000rpm put a voltmeter across the battery terminals to see what voltage the alternator is producing. Alternator brushes?

 

The alternator is a good idea, but if it is cranking strongly after it dies, the battery is likely not dead. I think you really ought to take the bike out, get it in its no start status, then take the plug wires off one by one and see if they are still sparking (bring a spare plug so you dont have to mess with a very hot spark plug!). You need compression, fuel, air, and spark. Compression is good because you said it was. Nothing should cause a disruption to air, so we can assume that is also good. The next time the bike stalls, do us a favor and do two additional things. First, check the fuel line and make sure gas is in there, I know you ruled out the gas cap, but visual confirmation of gas in the line would be a good step. Second, bring a can of starting fluid or carb cleaner with. Just for giggles to truly rule out fuel issues, give the bike a whiff to make sure that even if there is gas in the line, it isnt somehow getting obstructed. The plugs dont look lean, and dont look rich, so I doubt it is fuel related, but it would really fricken suck for you to go through all this work diagnosing spark, just to find that it was fuel, especially when the check for fuel is bringing a can in your backpack.....

With that, the only thing left would be spark. The alternator is a good idea, but if it is cranking strongly after it dies, the battery is not dead. I think you really ought to take the bike out, get it in its no start status, then take the plug wires off one by one and see if they are still sparking (bring a spare plug so you don't have to mess with a very hot spark plug!). Given the above things, 1 or 2,3,4 of the cylinders will hopefully not have spark, and we can go from there. Also, if you have a heat sensing infrared gun, you could check the four exhaust pipes to see if they are warming up at the same rate. Could confirm your suspicion on cyl 3.

 

The TCI could still be suspect as suggested earlier. They have a circuit inside that monitors the pickup coils. If no signal is present it will shut down. I could imagine a failure where low RPM might create a shutdown that occurs after it has heated up a bit. Unfortunately, there is no good test for a TCI other than to replace it or have it tested n another bike.
Checking the pickup coils in the failed condition might also be a good idea. They can be checked quickly and easily by pulling the connector from the TCI to connect a DVM. Note they will change hot (increase a bit) but you would be looking for something drastic like an open. If you haven't already they should be checked with the bike cold as a marginal pickup coil might show up there too

 

The resting battery voltage (bike off) is 12.6v. Voltage increases to 14.2v above ~2200 rpm.

 

Yes that's fine.