#4 Cylinder Dead - Can't find Cause

[Guest]

All,

I'm still in progress on my '83 GS650GL restoration project. (Just a reminder that the bike it sat in storage since 1999.)

Also, I spent about 20 hours total removing, cleaning, rebuilding, and re-installing the carbs. I followed the "CV Carburetor Rebuild Guide" on BassCliff's web site to the letter. Didn't skip any steps. These carbs were like new when I was done.

I finally got the tank cleaned and lined with a POR15 kit so I was excited to get some gas in her last night and see if she would actually fire up.

She did! But only on three cylinders. #1, #2, and #3 are working, but not #4. (Even running on only three cylinders she ran pretty good.)

After several minutes of running, I noticed that the exhaust pipe on #4 cylinder was not getting hot like the other three. So, I pulled the plug boot off of #4 and NO difference! #4 was dead.

Next I pulled the #4 spark plug out, hooked it back up to the boot and laid it against the side of the engine. She HAS spark. So, not that's not the problem.

This morning after the engine cooled down I loosened the drain screw on the #4 carb fuel bowl. There WAS fuel in the bowl! I was not expecting to see fuel in there, knowing that the spark was good on #4.

I didn't notice whether the brand new spark plug that I removed from #4 for the spark test was wet from unburned fuel. It did not seem wet to me.

So, where do I look next?

Could it still be the #4 carb, even though it has fuel in the bowl and has all new/cleaned guts? If so, where's the blockage?

I have not checked compression yet, but I do have an automotive compression gauge. (Just don't know if I have the proper fitting to connect to the spark plug hole on this bike.)

I'm not hearing any suspicious engine noises at all, no valve clatter, no noises of any kind. Engine revs up high on 3 cylinders just fine with no strange noises.

I have not checked valve clearance yet, but can't imagine that the valves could be SO far off that the cylinder would be completely dead.

HELP! WHAT DO I CHECK NEXT?

???

[sinkankan]

Valves ok?
Are they opening and closing completing the compression cycle, I suggest cranking it over with the plugs out and see if there's difference in the "noises" coming out of the holes. After sitting for a while it may be a stuck valve guide, but the proper way would be to take the tappet covers off.
If you have spark and fuel, the only other thing missing is air to intiate the compression cycle.
Another remote possibilty is the coil is breaking down and only delivering a partial spark, a broken or damaged spark lead, and maybe a stuffed plug that works occasionally? (Usually they work or don't)
Failing all that, a compression test may shed some light.

[lrgguy]

Just for a quick test id take #4 plug and switch with say #1 ..
I have heard of a plug firing out of the engine and not in before ..

[Nessism]

Newbie Mistakes thread linked in my signature. You may want to brush up. Several red flags going up...

[tkent02]

Adjust the valves before one of them burns up.
It my have already. That would explain the not running on that cylinder.

[WingMan71]

Newbie Mistakes thread linked in my signature. You may want to brush up. Several red flags going up...

Only item in that ten item newbie mistakes list that applies is #3. Valves haven't been checked yet.

Not really a newbie. Been wrenching on bikes for 49 years, plus an ME degree.

Will take a compression reading next and then get after the valves.

***

[Nessism]

Only item in that ten item newbie mistakes list that applies is #3. Valves haven't been checked yet.

Not really a newbie. Been wrenching on bikes for 49 years, plus an ME degree.

Will take a compression reading next and then get after the valves.

***

1A) (NEW) Trying to diagnose running problems on a bike with an unknown maintenance history is an exercise in futility until a baseline is established through proper maintenance.

It's best to fix the valves and put a couple hundred hard miles on the bike before trying to judge compression.

[WingMan71]

It's best to fix the valves and put a couple hundred hard miles on the bike before trying to judge compression.

Ed,

In your opinion, do you think that valves can be off far enough to lower the compression significantly to cause a dead cylinder?

Also, what do you think about the "valve stuck in a guide" possibility?

Thanks!

[Nessism]

Ed,

In your opinion, do you think that valves can be off far enough to lower the compression significantly to cause a dead cylinder?

Also, what do you think about the "valve stuck in a guide" possibility?

Thanks!

Absolutely!

Maintenance neglect (no valve adjustments) = all clearance goes away as the valve hammers into the seat = valve hanging open = burning valve and seat = no compression.

Thankfully, GS engines are tough and can often survive valve maintenance neglect.

"valve stuck in a guide" is extremely rare. If that happened either the engine won't turn over or the piston will crash into the open valve, bending it. Since your engine runs you can cross off this possibility.

[Steve]

Have you done a carb sync yet?

It's possible that #4 simply isn't open enough to admit any mixture to the cylinder.

.

[tkent02]

Ed,

In your opinion, do you think that valves can be off far enough to lower the compression significantly to cause a dead cylinder?

There can be no compression if the valves are not closing, or if they are burnt from running hard while they are not closing. An ME should know this.

[WingMan71]

Have you done a carb sync yet? It's possible that #4 simply isn't open enough to admit any mixture to the cylinder. .

Steve,

I did a bench sync when rebuilding the carbs. I would expect that should be close enough for that cylinder to at least run.

Will do a sync with a manometer when I get all 4 cylinders running.

Thanks.

[WingMan71]

There can be no compression if the valves are not closing, or if they are burnt from running hard while they are not closing. An ME should know this.

Thanks for your help.

Yes, I do know that if the valves are not closing there will be no compression. That's why the "valve stuck in a guide" idea from another member caught my attention. (Particularly since this bike has been sitting since 1999.)

I've personally never seen a valve burned significantly enough to cause such a drastic drop in compression that the cylinder would not operate at all. But, I guess there's always a first time.

I do have a little bit of history from the son of the original owner of the bike, who I bought the bike from. When it was put up in 1999 it was running fine. Bike has 25,000 miles on it and the outside edges of the valve cover gasket looks to me like it's fairly new. That lead me to think that someone was in there not long before the bike was put up.

(My humble apologies for mentioning my degree, it was not meant as a dig, just wanted folks to know I'm not completely clueless.)

Thanks again for the help.

I'm still gonna try that spark plug swap idea before I take the valve cover off and check the valves.

I never heard of a plug firing out of the engine, but not in the engine. But, maybe there's a first time for that as well.

***

[SocialAnomaly99]

So do you know for sure that you are getting fuel to the cylinder? How old are your coils? Coils may check out OK when cold, but they can act a little funny once they warm up. I had a coil that checked out fine at room temp, but once it was on for a couple minutes it would cease fire on one of the terminals. You might wanna try switching #2 and #4 and seeing if #2 stops working as the engine runs.

[WingMan71]

So do you know for sure that you are getting fuel to the cylinder? How old are your coils? Coils may check out OK when cold, but they can act a little funny once they warm up. I had a coil that checked out fine at room temp, but once it was on for a couple minutes it would cease fire on one of the terminals. You might wanna try switching #2 and #4 and seeing if #2 stops working as the engine runs.

Charlie,

Thanks for the idea!

My service manual shows that one coil fires #1 and #4 plugs, and the other coil fires #2 and #3 plugs.

So... I'm thinking that if a bad coil is causing #4 not to fire, then #1 cylinder should be dead as well.

But, there's always a first time...