2 cylinders not firing

I've got a 1981 Seca 750 that was running OK last week. Suddenly it runs like crap. I have verified that the two center exhaust pipes are ice cold while the 2 outer pipes are hot while the bike is running. So I assumed that I had a bad coil for cylinders 2 & 3. I read the specs in the Haynes manual and both coils test OK - (2.8 ohms on the low tension lead wires and about 21K ohms going through the plug wires.)

I thought it might be a bad TCI, so I swapped the TCI with a know good TCI from my other bike - a working Seca. No difference. With the TCI's swapped, the problem bike still is not firing on cylinders 2 & 3, and the known good bike works like a charm. So I swapped the TCI's back to their original locations.

I have not checked the pickup coils because the manual says that it is unlikely that they go bad.

The wiring diagram in the manual shows that the right coil (the one that drives cylinders 2 & 3) also drives the tach - and my tach works, so I suppose the coils are getting power...?

Looking for suggestions and what I should test next.

thanks,
-Rustee Gold

1981 Seca XJ750 times 3

 

did you ohm out the spark plug caps?

sounds like your pickup coil failed but

tach reads the firing of the tci to the primary side of the coil

if tach is working I would suspect the spark plug caps or spark plugs.

 

Swap coil 1/4 with 2/3 and check again, even though it ohms out, could be bad. Other than that, wiring or pickup.
Also did you check the plug caps? 5K ohm +- 10%

 

I did not check the plug caps independently - however I did check the ohms from cap to cap and I got 22K ohms. The manual says the coil is supposed to ohm at 11K so 22K - 11K = 11K which means each cap is contributing about 5.5K... (It that a good assumption?)

I'll swap the coils tonight to see if that makes a difference.

One more thing - I'm bringing this bike back from the dead - PO left it outside for 2 years - it had a lot of rust (I've removed most of the rust). Do the coils need a good ground to the frame? (might sound like a dumb question for this group... but I don't know the answer...) So I wonder if it requires a good ground to the frame - if there is rust on the bolts where the coil mounts to the frame - could that cause a problem? Or does all the electricity run through the wires and the frame connection doesn't matter..?

Rustee Gold

 

the bolts serve as a ground for 1 or 2 black wires that the bolt holds down. I would replace the bolts. the bolts are just for holding the coils in place.

it only takes a minute to check the pickup coils. unplug the connector at the tci and check the orange wire to black wire and the gray wire to the black wire . easier than swapping coils unless your spark plug wires are long enough.
also check the orange and gray wire to battery ground should not get a reading for battery ground

 

I would say yes.

They do not need a ground - however you might check either the 2 or 3 cap to ground just to rule out an internal short in the coil to the iron core that is grounded to the frame. The coil would ohm out fine even if a short was present but would not function correctly.

And +1 on xj550H about the pick-up coils, it just too easy to do to not go there.

 

One other thing to look at is the plug wire insulation. If there ia s pinhole or crack the electric faries will spill out all over the valve cover.
You can see them at night. A spray bottle of water will make the fairies visible if they have gotten out.

 

So I think I figured this out. I had the bike running with cylinders 1 & 4 only... exhaust pipes 2 & 3 were ice cold, so I decided to pull the wire off of spark plug #2. That's when the fairies got me... The ones that kmoe talked about. 20,000 volts... That woke me up. Basically the coil woke up and started working again. Now the bike runs on all 4 cylinders. So I must have a loose wire somewhere. I'll just have to wiggle everything to find what's loose.

 

the caps unscrew off of the plug wire remove them trim back plug wire and reinstall

 

Spray them down at night to see the arc. You may have cracks in the insulation and move the plug wire just enough that it's not able to bridge the air gap. If the wire moves again (and it will) you'll end up with the same fault.

Also trim the ends of the wires as suggested above. You want the best path possible for the fairies to dance down. If the insulation has begun to break down you can wrap the wires with self-amalgamating tape as a temporary measure until you can get coils with replaceable wires. We used to reccomend other methods for wire replacement, but since the coils are all getting quite old I think it's best to just replace them too given how much time it takes to replace the moulded in wires on the OEM coils.

 

hmmm. I don't quite understand the details of what is happening with the bike. I hooked up a timing light to each plug wire and all 4 wires are getting spark, but only the 2 outside cylinders are firing. If I let it run for a few minutes on just the outside cylinders, then the #2 and #3 cylinders start to fire (the exhaust pipes warm up).

I took the bike into my dark garage and started it (like K-moe suggested) and sprayed it down looking for the electric fairies, but I did not see any. However I do believe that spark is traveling through wires 2 and 3 because: a) I got shocked last week when I took #2 wire off while the bike was running, b) the timing light shows there is spark in wires #2 and #3, and c) after the bike warms up, #2 and #3 cylinders get warm on the exhaust.

Even though I don't understand the details, it sounds like I can do a quickie fix by wrapping the wires and plug caps in electrical tape. Is that the quick fix?

 

electrical tape will break down quickly. self amalgamating tape is what you want. it is plumbers tape that is used to stop leaks and shrinks up real tight by it self.

 

The timing light is showing you what you have already felt: the coils are delivering voltage to the plug wires.
Because you felt that the voltage is not reaching the plugs reliably.
Whether or not you see a lightshow is somewhat dependant on the wires being close enough to the head or frame when you are spraying them with water.

Now....it may very well be (and is liekly so) that you have more than one problem.
If after wrapping the plug wires you find that #2 and #3 still only fire once the get warm enough, then you very likely have a fueling issue. Probably the enrichment jets in those float bowls are plugged, preventing them from receiving enough fuel for a cold start.

 

Funny you should mention that K-moe. I also have a carb problem which I did not mention because I am trying fix one thing at a time. I often get a run-away idle. I've cleaned the carbs pretty good - not quite the Church of Clean - but I got all the way down to breaking the carb bodies apart and used my newly-purchased ultra-sonic cleaner to soak each carb body and all parts. The floats are set and the butterflies are sync'ed. The run-away idle is not predicable. After the engine warms up and all 4 cylinders are firing, the bike will sometimes idle at about 1100 rpm. After a minute of two, the idle will start to climb on its own and settle at about 4000 rpm. I find that odd because I'm not twisting the throttle at al when this happens. It just does it on its own. If I take it for a ride, I can sometimes force the idle back down by forcing the engine to slow by applying the brakes. And then the bike will idle at 1100 RPM for a little bit before it starts to climb again on its own.

I have not found any vacuum leaks.

There might be a simple solution that I'm not aware of - but I'll probably pull the carbs again this weekend and re-clean everything. Maybe I forgot something.

 

rpm climbing will open the main jets even with out twisting the throttle. when any vacuum leak areas start to heat up more air comes in burning hotter

when you ultronic cleaned the carb bodies what else did you do to clean out the little passages?

did you confirm the enrichment wells in the bowls were clear?

ultrasonic cleaning breaks up the gunk but does not mean it clears the passages

 

Whenever the rack is broken it's time to replace the throttle shaft seals.
Was that done?
They are inxpensive and the cause of many hanging idle issues.

BTW, thank you for disclosing that here instead of making a seperate thread. Ignition and fuel are related systems, and both have to be dealt with while knowing about the status of the other.

 

Xj550h - yes on the enrichment wells. They were clear. I poked toothpicks and guitar strings through passages to make sure they were clear and also compressed air.

K-moe: I actually purchased the throttle shaft seals from Chacal but did not use them. The screws on the butterflies were stripping out so I did not remove them. Maybe it's time to drill out some butterfly screws...

 

you can use an easy out on those screws I did it the other day

 

A little direct heat works on those screws as well. Putting each carb in a padded vice to get a good angle and pry on those screws really helps. Make sure your tip of the screw driver is correct or you will end up drilling. I too have had to drill one out, made sure you line it up correctly and to go small enough to leave those threads in the shaft.