'83 GS750EF, No combustion in #4 cyl.

[Macmatic]

I'm going to sit down now.

So this evening I come home and cool off a bit from my 100 degree day and then head out to the garage where I hook all the wiring back up to get some baseline readings.

Problem gone?

The bike fired right up....and ran like it did before. Fires up, runs on three cylinder. 1,2,3 all sizzlin hot after <1min at 3,000rpm. I noticed some extra smokey crap coming out of the right side muffler too...

So once I saw that #4 was still cold I switched out the plug leads between 1&4 and ran it again long enough to get #4 hot, only it wasn't. Around this time I thought it would be a good idea to do a compression test...

Three outta four had 140psi, one had barely 90psi. Want to take a guess which one was low? I'll give you a hint, when I pulled the plugs #4 was a little wet with fuel.

Did you guess number 3 had 90psi? Oh... well thats the one thats low.

Now last time I really rode the bike I took a 1000 mile road trip up to WV and back and then about 400mi up in North GA the next weekend before parking the bike for three months. No running problems what so ever. So pretty much I rode 1,500 mi in about a week then let the bike sit and now I've got 90psi in one cylinder.
I should clarify that the bike sat for three months over the winter then I went to ride and it was obviously missing on #4 so I checked for spark and figured I must have stuck a carb from letting it sit. I had bigger mains I wanted to put in anyway so I went ahead and pulled the carbs...and getting them back in dragged out until now.

Time to get that leak down tester I guess...

Thanks for all the electrical help. I hope that problem doesn't come back. Now I've got to figure out why #3 is low on compression and #4 isn't firing even though it has fuel and spark and compression.

EDIT I finally thought to put some oil in #3 and recheck the compression. Bad news, looks like a ring or cylinder issue since pressure shot up to 120psi and then 110 the next time. That sks, not looking forward to pulling the engine apart right now but if it has to be done maybe I'll put in some GS700 pistons. I guess I'm not going to learn fiberglass this summer after all!/EDIT
/\/\ac

[SPARKSS]

Howdy Mac. That part about it running again on 3 cylinders would have been a big help before we started Because one coil runs 2 cylinders, the fact that one works means the ignition primary is basically ok - move on to the secondary or HT side. Now....with the one cylinder apparently dead......low on compression.....with a wet plug.....this is how I'd proceed. Verify, using a known good plug (or spark tester) that the offended wire will or won't fire a good plug. If it will (bearing in mind that firing in atmospheric pressure isn't as severe as firing under compression), then squirt some oil into the cyl and install a new plug and restart - If you get a big bunch of smoke and things sort themselves out then you merely had a plug foul. When this happens (no spark inside the cylinder), the walls and rings get washed down with gasoline and you lose compression - not a self-healing situation, thus the addition of a little oil and a fresh plug to get it resealing and re-firing.

All this depends on the history and may require other info.......just tossing out a couple of ideas (and lots of actual experience).

[LarryA_Texas]

So here is where I stand now.

...
According to my wiring diagram the W wrie goes directly from the ignitor to the coil with no connections between. Since I can read good voltage across either O/W and the Bk/Y wire but nothing across and O/W and the W the W must have a break or high resistance, right? Only when I read the resistance it shows <.3ohm.

Any ideas? Please, please make me look like an idiot so I can start getting some miles on!

.

I had a similar problem with a GS850. It seems the coil was bad...shorted actually, so there was pretty much a 0 ohm reading across the two terminals of the coil. This in turn burned up a resistor in the igniter box so at first look I thought the box itself was bad. I replaced the burned resister and it immediately burned out again when I tried to start. It's possible that by trading igniter boxes you toasted both of them. I also noticed I had good voltage on the good coil, but voltage dropped on both when I hooked up the bad coil. If I had only tested one side then the other without going back I might not have noticed 1/2 voltage on both.

The confirming test was ohms across the coil leads themselves after it was disconnected from the wiring harness. My good coils all show about 4 ohms. Hope this helps.

[Macmatic]

Howdy Mac. That part about it running again on 3 cylinders would have been a big help before we started Because one coil runs 2 cylinders, the fact that one works means the ignition primary is basically ok - move on to the secondary or HT side. Now....with the one cylinder apparently dead......low on compression.....with a wet plug.....this is how I'd proceed. Verify, using a known good plug (or spark tester) that the offended wire will or won't fire a good plug. If it will (bearing in mind that firing in atmospheric pressure isn't as severe as firing under compression), then squirt some oil into the cyl and install a new plug and restart - If you get a big bunch of smoke and things sort themselves out then you merely had a plug foul. When this happens (no spark inside the cylinder), the walls and rings get washed down with gasoline and you lose compression - not a self-healing situation, thus the addition of a little oil and a fresh plug to get it resealing and re-firing.

All this depends on the history and may require other info.......just tossing out a couple of ideas (and lots of actual experience).

Oops! I thought I had that in the first message, that it was firing on three cylinders.

Tonight I'm going to recheck the HT side of the coils and install fresh plugs with a little oil in #3 and see what I get. I've got a bad feeling about #3 though since it's been firing all along (header pipe as hot as 1&2 by the spit test) but the compression is low. On the other hand #4 has good compression and gets fuel but isn't firing. I've been checking spark using an inline spark checker, lights up when the plug fires in the cylinder. I'll throw the battery on the charger for an hour since its down to 11.8v and then get back to testing.

I'll post back this evening.



......this'll teach me to be "too busy to ride". My commute is now OUT of Atlanta rather than into so I'm more interested in riding to work instead of just getting on the bike to get out of town. ATL drivers are not crazy, just oblivious.
-/\/\ac