Engine only firing on one cylinder... Help!

[kryptek49]

Hi, I own a 1992 GS500e and am based in the UK.

The engine is only firing on one cylinder and I can't work out why.

I swapped out the ignition coil and this appeared to solve the problem, however when you actually ride the bike for a few minutes and it gets hot, it cuts out again.

I have given the carbs a quick clean and also swapped out for some spare carbs i had, and the exact same problem persisted.

So far things I have checked and ruled out are: Carbs, Ignition Coils & Spark Plugs

Engine compression is at 130 psi on the left (the dodgy side) and 125 psi on the right, a bit under spec but most likely not the problem.

Any thoughts on where i go from here would be really helpful.

Thanks

[mr72]

When you say "cuts out", what exactly is the behavior? In detail?

Also how have you worked out which cylinder is going out? How do you know it's only running on one?

Depending on the cause of the low compression, I would first guess that when it gets hot the valves don't seal and that's the problem.

Most likely statistical cause is a bad coil. This is precisely the behavior of a bad coil that open-circuits when it gets hot. Swapping side to side wouldn't rule it out unless you had a definitive way to determine exactly which cylinder wasn't firing.

[kryptek49]

Thanks for getting back to me.  I'll try be more specific.

The bike starts from cold on both cylinders.  It idles fine, so I drive it down the road and after about 1 minute of riding the power drops completely.  4th gear is the only gear that can hit 60mph and there is almost no torque.

I have worked out which cylinder is not firing by pulling the spark plug cap on the left side, which makes no difference to the running.

I am sure the coil is fine as I have swapped the left and right coils, and exactly the same problem happens on exactly the same side.  I have also tried 2 different coils on the side that is not firing when hot and the same problem persists.

Would you agree that this rules out a bad coil? If so what else could be a problem but only when the engine is hot?

Thanks again, I really appreciate any help as I'm desperate to get out on the road

[mr72]

Given what you said, I'd have to guess it's a bent or burnt valve on the bad cylinder side. Try doing a compression test with the engine hot, while it's failing.

You can also try this... if you can get it to start its one-cylinder thing, if you get it stopped in neutral can you make it run ok at >7K rpm? Like, does it run real bad at 1500-6K but if you can get the revs up all the sudden it starts running right? That'd help verify a valve sealing problem in my mind.

Of course it could also be a gross leak at the intake boot on that cylinder. Crack in the plastic part, warping, etc. Worth checking. You might not be able to find the problem unless you heat the part. When they went to rebuild the top of my motor, one intake boot separated the rubber part from the hard plastic part, it had obviously been leaking. Plus there's an o-ring between the boot and the cylinder head that is almost guaranteed to be bad unless you just replaced it. I'd say it's worth it to check/replace those o-rings and inspect the boots for any signs of cracking or separating of the rubber and plastic parts, but I would have to guess a top end rebuild is in your future soon especially considering those compression numbers. They are low by any measure.

[kryptek49]

The compression test I ran yesterday showed 130psi on the failing side, and 125 on the good side.  This was a hot test.

I understand these are below spec, and am going to check the valve clearances in case this is the reason why.  Could the valve clearances be the only reason that the compression is low?

When the bike is idling at 1.2k rpm, it runs perfectly on both cylinders, it is only when the bike is taken out and heats up that the cylinder fails.

Does this rule out a valve sealing problem then seeing as it is perfect at lower RPM's?

I will check for any cracking etc when I am home in half an hour.

Seeing as the coils are fine, does this rule out the spark being the problem? The bike has done less than 700 miles over the last 10 years, I have cleaned the carbs quickly but is there anything else worth checking? 

Thanks again

[mr72]

Could the valve clearances be the only reason that the compression is low?

No, could be a lot of things. Most likely NOT valve clearances. If the valves are not sealing then it's things like receding valve seat, bent valve, carbon build-up on the valves (which leads eventually to bent valve), burnt valve, etc. Basically lots of things requiring a top-end rebuild to fix.

When the bike is idling at 1.2k rpm, it runs perfectly on both cylinders, it is only when the bike is taken out and heats up that the cylinder fails.

Does it still idle correctly when hot? Only fails when trying to run above idle when hot? This is an important distinction. Poor valve sealing would result in it probably stalling at idle when hot.

I will check for any cracking etc when I am home in half an hour.

Good plan. Think about what happens when it's hot to help you find the problems. Things expand. Some things expand more than others. So that plastic flange on the intake expands more than the cylinder head that it's attached to, so it can be made to form an arc that allows a leak at the head. This air leak would cause it to run so lean it'd not run that cylinder. A hardened o-ring in there would expose this potential leak, while a good o-ring would flex enough to keep the seal. A crack may seal when cold because the material contracts and then open up when it's hot allowing air in. In the same way a crack in the cylinder head would cause bad compression when hot. Could be something as simple as a plug wire. Did you swap the wires side to side?

But I'd say now the most likely cause is sticking float needle on that carb so once the carb is drained of fuel in the bowl that cylinder dies. But you said you tried other carbs, right? It's entirely possible that there are multiple causes making it hard to pinpoint. Double check the float needles if you get a chance. While you're in there check the float height, heck do the entire "fix up the carbs" bit like I posted in my blog.

Seeing as the coils are fine, does this rule out the spark being the problem?

Not 100%. Could be the plug, plug wire, bad ground at the coil, etc. Lots of things. But it sounds unlikely. That's the best I can do. Likely/unlikely, not ruled out.

[HPP8140]

Think I saw this earlier...electronic...crank trigger on right side sensitive to heat.

[mr72]

Yeah but the crank trigger/pickup could be it. Try taking the cover off, get some freeze spray. Get the bike to do it's failing thing, then immediately shut it off with the cover off and while on the side stand quickly check the resistance of both pickups (you can measure right on the solder joints on the each pickup coil under the cover).

If one is failing it'll read infinity ohms. If it's like under 1K ohms then it's fine. Careful because once it starts to cool it may restore continuity and fool you into thinking it's good when it's really bad. That's why you have to be quick. For that matter if you are seriously precise and dexterous you can measure it while the bike is running. Just don't touch the rotor

[J_Walker]

or he could just clip the multi meter leads to the part in question and ride around with the multimeter double sided taped to the gastank. just keep the wires away from the spinny spin spin thing..

[Big Rich]

Being that it is a 92 GS, check the valve clearances. When your valves are out of spec, it causes all kinds of problems, including low compression and problems running under temp.

Seriously, check the valves.

[kryptek49]

Thanks for all the replies everyone, after further investigation my problem has got more confusing.

I just started the bike from cold and the left cylinder did not fire at all.  So it is not just when the engine is hot unfortunately.

Previously this has been fixed by swapping the ignition coils round, although both ignition coils are working correctly so I am not sure why swapping them has any affect?

What is a crank trigger and how can i test it? Thanks for all these ideas.

I've ordered a shim tool and feeler gauge so will check the clearances tonight.

So as a recap, the left cylinder works intermittently from cold, swapping the ignition coil seems to solve the cold start problem, but as soon as i take it on the road and the engine gets hot the left cylinder cuts out.  Any more thoughts on things to check??

Thanks again.

[rocketgirl]

You said you've ruled out the sparkplugs.  Did you replace them? 

[kryptek49]

Yes, I replaced with brand new NGK spark plugs yesterday

[kryptek49]

I have noticed that the crank trigger is often mentioned on threads related to one cylinder not firing - in my case would this be worth replacing?

[J_Walker]

test the crank trigger first, I thought that was what was wrong with my bike too. after testing it, determined it was not what was wrong.

did you make sure the wires leading up into the coil packs [black things connected to the spark plugs, mounted to the bike] that the connector, the orange and white wire, and black and yellow wires are actually ON there? take em out and give the ends of them a LIGHT squeeze with some pliers if they are easy to come off, or go on, and by easy I mean they slide right off with almost no pressure.

[kryptek49]

On the left hand side one of the connectors to the coil is quite loose however it has never been disconnected after running so I didnt think this would be the problem.  I will tighten it up and check when I'm home in an hour though.  Even when the left side is not firing though i still have a visible spark so don't think this would be the problem.

How do you test the crank trigger?  And is it the same thing as a pick up coil? I've searched all over the forum but just can't get my head round it.

Thanks

[J_Walker]

On the left hand side one of the connectors to the coil is quite loose however it has never been disconnected after running so I didnt think this would be the problem.  I will tighten it up and check when I'm home in an hour though.  Even when the left side is not firing though i still have a visible spark so don't think this would be the problem.

How do you test the crank trigger?  And is it the same thing as a pick up coil? I've searched all over the forum but just can't get my head round it.

Thanks

Start on a cool bike.
take off the side cover on the very right hand side of the engine, 3 little bolts no gasket or anything.
grab your multimeter. put it on 2k Ohms.
The black electrical device has two pins on it that are where the wires connect to it that are soldered on, just test at this point, doesn't matter what multimeter lead touches what side.
measure it, write this number down.
Now heat up the part, either by warming the bike. or what I did was boil a pot of water, unbolted just the black electrical bit, and unplugged it from the bike. dangled the part [dont let it touch the bottom of the pot] by its wire, and heat it up to 100C [212F] pull it and test it as fast as you can. write down the read number.

of course if you can heat it up with a heat gun, or just running the bike if that's an option for you. what ever gets it around 100C.

Post the readings back here.

[kryptek49]

Okay, so I've tested it cold as you said, and the reading was at 390 ohms. Heated it up using a heat gun for a minute or two and the reading decreased too 369 ohms.

Does this mean anything to you?

I don't know what temperature I heated it up to as I don't have any tools to measure that but the heat gun is quick normally.

I've already bought a used one on eBay as it was going for £15 so it's not the end of the world if I don't need it.

There was a lot of rust on one of the screws holding it in, is this a problem? Here's a photo: https://i.imgur.com/gOiYM5g.jpg

Thanks for the help I really appreciate it!

[J_Walker]

weird. the resistance should increase with heat, not decrease...

but your part seems to be within spec.

[kryptek49]

Oh, not what I was hoping to hear! Any thoughts on where to go from here?