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.

 

My suspicion is spark/electrical inasmuch as when the bike is in “faltering mode” and rpm fall below 2200 rpm, the bike acts as if the ignition had just been switch off. Yet a bad switch would affect the bike regardless of engine speed.

I used a spare plug to test the plug wires when bike died. The plug fired fine on plug wires #3 and #4, thus verifying both coils are functioning. I may have a bad plug #3 and have ordered a replacement. I mention this inasmuch as cylinder #3 seems to be the weak link. Also, I plan to swap plug wires between #2 and #3 to see if the problem follows the plug wire.

 

I am

Resting battery voltage is 12.6v and at ~2200 rpm, voltage is 14.2v (measured at the battery). I am also getting 12.6v between battery and engine ground. Zero ohms between battery and engine ground and zero ohms between battery and starter relay.

 

I checked the main fuse and ignition fuse. Both checked out. No visible corrosion; reinstalled with dialectic grease.

 

What values (ohms? voltage?) should I be expecting when testing the leads from the TCI?

 

From the Ultimate Relay, Switch, Sensor, Diode guide:

https://www.xjbikes.com/forums/threads/the-ultimate-relay-switch-sensor-and-diodes-guide.27543/

"XJ750 air-cooled models:

Pick-up coils:
650 ohms +/- 20% = 520 ohms to 780 ohms acceptable range

Ignition Coils:
Primary side (input from main wiring harness):
2.5 ohms +/- 10% = 2.25 ohms - 2.75 ohms acceptable range

Secondary side (spark plug wires, without their end caps):
11K ohms +/- 20% = 8,800 ohms - 13,200 ohms acceptable range

Spark plug caps:
1981-83 models: 5K +/- 20% = 4,000 to 6,000 ohms per cap acceptable range
1984 RL models: 10K +/- 20% = 8,000 to 12,000 ohms per cap acceptable range

Spark plugs:
0 ohms per plug"

 

How do I go about testing the TCI? What values am I looking for (ohms?, voltage?).

 

I plead ignorance: what are pick up coils?

 

Pick up coils are under the cover on the left hand side of the crankshaft. The three wires from them black, orange and grey plug into the TCI.

 

Solved (hopefully). #Bruce W’s comment pulled my attention to fuel. I changed the little mini fuel filter and all seems good. In an effort to rationalize or understand the effects of this, it makes sense: the bike would run until it ran out of gas. If the petcock was ‘off’ the bike would run the carburetors dry pretty quickly (20 seconds), but in my case, the fuel filter was letting a bit of gas through so the bike would run for 2-3 minutes before starving out.

Thanks to everyone who contributed. As a result of these comments, I checked my valve clearances, compression, spark plugs, spark plug wires, coils, battery voltage, and alternator output as well as cleaned various chassis ground points - all worthwhile tasks.

 

Aaaahhhh!! That’s great and frustrating at the same time.

The good news is, if you didn’t know how to do all of those other things before, you are now certified!!

I’m glad you’re up and running.

 

For future reference- these are the pickup coils.

“Crankshaft position sensor” is a more logical name to me, but it turns out Yamaha didn’t consult with me when I was about 10 years old when they were naming these things.

 

Ah, yes, the crankshaft position sensor. I’m familiar with most of this because I built this bike from the oil pan up.

My advice to others is test, test, test and resist buying parts with the hope of solving the problem.

 

I had a bad gas cap that wasn't venting right. I didn't know it and it reared its ugly head at 3am when I was riding up to college on the highway. Got stranded and after 30 minutes of trying to calm down, checked the simple things because it was about all I could check. Found the fuel line was empty. Switched it to prime thinking the petcock went bad, and it started flowing. Long story short, I'm glad my crap show allowed me to give you some advice!

 

I went though the same vent-less cap with a little XT200. Simple- but so frustrating. Thanks for your help with my current problem.

 

I finally received my new gaskets today and put the Seca Turbo back together. I checked valve clearances three times and found only one exhaust valve slightly out of spec. I changed its shim out. I also enrichened the fuel:air since I had easy access. The first ride today yielded just under two miles before the sputtering, loss of power and wanting to die. Tank cap is new. I have a new larger fuel filter that I can try, after reading the prognosis above. If that doesn't change anything I'll look into the electrical suggestions.
Funny thing is the last ride before the valve clearance-check took me nearly 20mi before any bad stuff. But I sat stranded for over 30min before it would continue home....

 

After waiting 20 minutes at the side of the road, did the bike fire up and behave as it should or did you have lingering problems? Also, did the bike run for for the 20 minutes back home? Also, did the bike work fine before you installed the new gaskets? Last question: did the bike sputter to a stop or behave as if the ignition had been turned off?

 

Ok...After sitting it started but was not great. I had to keep the rpm's up just to limp home. I did make it, but only because the two traffic lights were green. That was when I tore it down to check the valves. I replaced one exhaust shim.
I ran it today and it was acting up after only 1.8mi. of easy putting. I read this morning Dwmadsen solved his issue with a new fuel filter. I installed a new, bigger one and rode about 10mi with only minor glitches. Then about 1/8mi from home a car accident kept me at a standing idle for a while. She didn't like that and the behavior returned. I eeked home and I let it die. When it dies I could keep it alive, if I wanted, but I am a merciful father....
While still hot I laser-temped the headers. Cylinder 4's pipe was 20° cooler than 1-3's.
I then checked the compression. Not amazing, not even close. After checking 1,2 and 4 cylinders I quit. Cylinder 4 was the lowest, but not by much. Clueless in Seattle...

 

I don’t want to dwell too much on the fuel filter (you know just because it solved my problem), but your symptoms are somewhat similar. So staying with this thread, does the bike have the original vacuum-actuated fuel petcock? If so, you might want to ensure the valving in the petcock is letting the fuel pass. Also ensure the vacuum hose between the carburetor and petcock is good. With a properly working vacuum-actuated petcock, fuel stops flowing when the engine is off - or is there is a vacuum leak. if the vacuum hose is good, move and check if your petcock is suspect. To so this suggest you take a 3’ section of fuel hose and run it into the tank through the filler neck (just leave the gas cap ajar) and connect it to your fuel filter, thus bypassing the petcock. Ensure the fuel hose is full of fuel (so you have a good siphon). If the bike runs reliably, you know where the problem is and can chose to rebuild or replace the petcock.

 

Thanks, but I think I mistook your post as a turbo related Seca. The Seca turbo has a different petcock. The fuel is pumped out and returned back. There is an on/off position on this one.
It really does feel fuel related. When it starts running badly I can keep it going by revving higher roms, like the slow and/or mid are clogged, but why only when it's warm? It runs like a champ until it heats up. I was sure I was going to find more valves out of spec. Maybe there is something to the CDI theory?

 

Is your SECA carbureted or fuel injection? You comment sounded as if you are dealing with a FI fuel pump. If it is FI look at your fuel regulator; the system needs a minimum pressure (usually around 32 psi) and it is the fuel pump and regulator which deserve attention.

My problem was complicated by the red herring of “it started running poorly after 3 minutes or 10 minutes”. What seems to have been the problem is the fuel filter would pass fuel, but not enough to fuel the bike after a couple minutes of operation. The fact that cylinders 3 and 4 were “failing” was actually they were being fuel starved since the fuel enters the rack of carburetors between carb 1 and 2 and what little fuel making its way through the filter would be used by the closest cylinders. So while my issue certainly seemed heat related, it was actually fuel starvation after a few minutes of operation.

 

Dwmadsen,
Can you post a picture of the fuel filter you pulled off?
Do you have photos of how it was routed w/ the fuel filter on and how the fuel line was routed after you pulled it off and it was then working?

 

I use an inexpensive small engine/motorcycle fuel filter available for about $2 from Amazon: https://share.icloud.com/photos/073gLh3dRi0Q2JrGKRrw483eQ

 

Keep in mind my petcock may be different from your inasmuch as mine is an after market petcock which does NOT use vacuum to allow fuel to pass to the carburetors.

A simple test is to disconnect fuel line between petcock and carburetors and the connect a vacuum hose connected to the petcock. By sucking on the vacuum hose, fuel should flow from the petcock (use appropriate container to catch flowing fuel). If sucking on the vacuum hose does not release fuel, clean, repair, or replace petcock.

 

I

 

That is a similar filter that I had in mine but mine did not have the right angle fitting. I felt that if I had the right angle fitting , it would trap air at the top and not flood the rock.
This is ONE of the several things I changed since having fuel starvation problems a couple weeks ago.
I also was using the same petcock which I felt was sucking in air from time to time adding to the bubbles in the line.
Since I changed so many things, I can only speculate but I think the main cause was the rock filter.

 

The petcock picture was taken with the tank raised about 2”-3”. With the tank in it normal position, the filter sits slightly above the petcock but doesn’t seem to present a problem with fuel delivery - although this was an initial concern.

The 90° filter allows it to sit a wee bit lower, possibly minimizing air being trapped at the filter (high point). Since fuel delivery is gravity fed, eliminating any high spots in the fuel line might be important.

keep trying…good luck.

 

Is there a max/min gap between the rotor and the coil tip? I think my gaps are way bigger than yours

 

If you are talking about the reluctor to pickup coil gap, it's a non-wear item and non adjustable (without modification).
What issue are you having?
Please start a thread for that if you haven't yet.

 

I guess it could become a wear item if some idiot forgets to screw down the baseplate, new thread started.