Number Three Cylinder not Firing

[gmcpheter]

1981 GS 850, with 17K. Number three cyliner is not firing...runs poorly...cool exhaust. Tried new spark plug, swaped plug wire with number two. Same results. Compression test dry with closed throttle on all cyl around 120 to 125 after about 10 cranks. I have not adjusted valves since I bought the bike 2k miles ago. Thanks

[Redman]

Well, your plug & wire swap test shows problem is not ignition.
Compression test indicates problem is not mechanical (valve, piston ectera).

By elimination that leaves the conclusion of : carburator.
Maybe simplest thing is to take out the plug on bottom of that carb's bowl, see if its getting fuel at all. But either way, you will be soon taking off carbs for inspection / cleaning.

[chef1366]

Does the cylinder ever kick on while riding? Maybe during the needle or main jet circuit?

[gmcpheter]

Unclear

The bike does kick in on all four around 4,000 rpm. Runs great. I did rebuid the carbs several months ago, mainly a cleaning, ran better than ever after that. I verified the manual sync. Have never used the sync meter which I do have. That may be helpful, but does not explain why it just started acting up. But your right fuel, air, spark. I'm hoping fuel as well. BTW. I did open the bottom of 3 and 4. Both had fuel, and both had a good stream of fuel in the prime position.

[chef1366]

Your pilot circuit has a clog in it somewhere. A fuel additive might clean it out but you more than likely will have to pull the carbs and clean out the pilot circuit.

[chef1366]

Oh yeah, vacuum sync the carbs.

[gmcpheter]

Thanks for your help

I'm getting where I can pull those carbs off in my sleep. I do think a fuel filter would help. More to follow.

[chef1366]

A fuel filter will restrict fuel flow and is a bad idea. Make sure the petcock filter is clean.

[gmcpheter]

Right now there is plenty of fuel coming out when in the prime position. I did not know there was petcock filter. I'll check it out.

[chef1366]

Right now there is plenty of fuel coming out when in the prime position. I did not know there was petcock filter. I'll check it out.

It's a filter that goes up into your tank first. It's connected to your petcock. Fuel has to flow through it first before entering the petcock.

[Freddy]

Hi I recently had the same problem on my 1979 GS1000L. It had me so confused everything checked out ok good spark etc but I did not check the coil lead with a plug in it I just had a screw driver in the end and watched a lovely blue spark jump across to the head, put it all back together and still a cool header pipe, My father came and said it's the only cylinder with a brand new plug why dont you try one out of your other bike another 1979GS1000L?, low and behold it was a faulty brand new plug it does happen. Also of note I put an inline fuel filter in as had some rust in the tank "now nice and clean" it was only put in as a precaution but I dont think I will take it out.
I hope this helps.

[gmcpheter]

Thanks for the input. I have also seen bad plugs cause huge problems, but have tried a used one and new one so far. I just own too many things with pistons and spark plugs )

[Rocketman]

One might look at the rubber diaphragm in the non-functional carb to see if its seated properly between the carb cover & body, and also inspect for pinholes. Should be easy to remove without removing entire gas tank and carb rack, as I just did it last week. I eliminated every possibility but a fuel problem, and am currently working on it too (same cylinder too...think its a conspiracy?)

[Thomas Kenny]

You sure you got that compression with closed throttle? That sounds high...