#4 Cylinder Dead - Can't find Cause

It wouldn't be the first time for that either. Have you checked the condition of the wires?

Yes, the spark plug wires were surprisingly supple and void of any visible cracking from age.

The bike was put up in 1999 as I mentioned, but it was stored inside the whole time. Most rubber on it survived the storage period really well.

Thanks.

Right 1 & 4! Dont even know what the hell I was thinking of

[tkent02]

It wouldn't be the first time for that either. Have you checked the condition of the wires?

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...

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.

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.

***

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.

[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.

[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.

.

[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.

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]

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.

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.

***

[tkent02]

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

[Nessism]

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