Unresponsive idle fuel screw

[cfairweather]

I have seen a dozen or so GL1000 carbs with unresponsive idle fuel screws and usually the reason is a clogged idle circuit. Usually the slow jet or the idle discharge tubes are clogged, but not today. To make a long story short, a coil was supplying a weak spark to #4. Let me explain how I figured this problem out. My test engine stand does not have exhaust manifolds installed and I do this so I can feel the exhaust pressure. This makes it easy to adjust the carbs and discover problems. When I was testing a set of carbs, #4 had less exhaust pressure than the other three and adjusting the idle fuel screw had no effect. When I swapped #3 and #4 spark plug wires, the weak pressure moved to the #3 cylinder and #4 became normal, so I knew the coil was the issue. After I replaced the coil, pressure was normal on all 4 cylinders. I have never seen a weak coil cause this problem before. Come to think about it, I have never seen a coil fail or get weak on one side.

[gltriker]

I have seen a dozen or so GL1000 carbs with unresponsive idle fuel screws and usually the reason is a clogged idle circuit. Usually the slow jet or the idle discharge tubes are clogged, but not today. To make a long story short, a coil was supplying a weak spark to #4. Let me explain how I figured this problem out. My test engine stand does not have exhaust manifolds installed and I do this so I can feel the exhaust pressure. This makes it easy to adjust the carbs and discover problems. When I was testing a set of carbs, #4 had less exhaust pressure than the other three and adjusting the idle fuel screw had no effect. When I swapped #3 and #4 spark plug wires, the weak pressure moved to the #3 cylinder and #4 became normal, so I knew the coil was the issue. After I replaced the coil, pressure was normal on all 4 cylinders. I have never seen a weak coil cause this problem before. Come to think about it, I have never seen a coil fail or get weak on one side.

Very good information, Cedric.

Contrary to most person's belief, yes, a double outlet ignition coil WILL fail on one side and not affect the other side.

I have seen three 3 ohm Dynatech ignition coils fail on one side, only; circa 2016.
Two of them were accidentally overheated during a very brief gasoline fire, caused by a lean fuel backfire into unwisely puddled gasoline squirted in the air filter housing while attempting to get the engine running.

I slammed the air filter element back into the housing, and quickly clamped the air filter lid onto the filter element to extinguish the fire.
Apparently, a sufficient amount of hot combustion gas was expelled out of the inlet snorkel directly onto where both ignition coils are mounted and caused Both imperceptibly singed Dynatech ignition coils' lower outlet to soon afterwards go "dead" but the upper outlet on both coils still presented strong spark.

The third 3 ohm Dyna ignition coil, a new replacement "out of the box" directly from Dynatech, was "dead" on its lower outlet, too.

done now

[CYBORG]

I gave up on Dyna years ago for the same reason. To many unexplained failures at the wrong times. I believe they are to voltage sensitive for one thing.

[cfairweather]

I forgot to mention that my test stand uses genuine Honda GL1000 coils with the ballast resistor.

[redglbx]

Like Cliff, I had a Dyna coil fail (no fire) where one socket of the coil had the brass insert missing so the spark was very weak on that one, even though logic would say both cylinders using that same coil should be weak but on mine it was only the one socket.

I called Dyna and talked to someone there who claimed that they’d never seen or heard of anything like that. So they had me send both coils back and later called me to confirm that the one socket was indeed very weak, but he also said the remaining 3 sockets didn’t meet specs either.. They ended up sending me new coils at a substantially reduced cost that basically just covered the shipping. These coils were about 20yrs old at the time btw.

So,because of this I have suspected that my current chuffing out of the Left bank at idle is caused by some kind of coil issue or possibly a plug wire issue. Hoping to get back after it in the next few weeks since my back is allowing some minor work these days but just trying to get caught up on all the home & yard work that has slipped over the last 5yrs first. By the way never discount issues with the plug caps, they can cause all kinds of mysterious issues as well.

Thanks for your inputs on this guy’s , kinda confirms my suspicions on this… Cedric ,, your test bench is a great thing !! The only thing I would add to that is it might be wise to add a set of short cut off exhaust manifolds to it because in the “old” days of Hotroding it was always considered a bad thing to run an engine w/o any manifolds or headers under the belief that when you shut it off the cold outside air would migrate back up the exhaust port and possibly warp any open valves.

So I don’t know if there’s any truth to this or if it’s just an old wives tale but either way a short length of tube shouldn’t hurt so long as it’s not so long as to hide the flame. I believe that you’ve done this long enough to know how long the flame coming out is so to give you an idea of how long the tubes could be. May just be nonsense too , just a thought.

[cfairweather]

Steve- Probably a good idea to add some short pipes when I setup a permanent engine. Not sure what it would take to add some oxygen sensors/gauges, because I have never done that, but that might be helpful.

[Sidecar Bob]

The secondary windings of Dyna and some other aftermarket coils have a centre tap that is connected to ground via the mount so it is possible for one side of the coil to fail.

The secondaries of the original coils are not supposed to connect to ground, the circuit being completed through the engine block so that the spark plugs are in series with the spark jumping the gaps in opposite directions on the 2 plugs.

There was some discussion on the forum a few years ago about this and IIRC the conclusion was that if a coil is abused in just the wrong way a short between the secondary and the armature can develop, which makes the coil behave like the aftermarket ones with the possibility of one end of the winding failing and the other end still working.

This can be verified by checking with an ohmmeter. In a normal coil there should be no continuity between either lead and the armature (infinite ohms - the same as if the meter leads are not touching anything). If you can see any continuity between a lead and the armature (meter indicates any resistance value) the coil is bad.

[redglbx]

Great info Bob ! Maybe add a bit more on testing procedure to help any newb’s or electrically challenged ? Possibly ways that can damage the coils ?

[ericheath]

I gave up on Dyna years ago for the same reason. To many unexplained failures at the wrong times. I believe they are to voltage sensitive for one thing.

I agree. I wonder if a small solid state regulator could protect it with a steady 12 or 13 volts. The fuel gauge, temp gauges use a small 7volt. Probably a couple bucks from DigiKey or the like.