260 Merc Instant Misfire!!

[Sea Raider I]

Hello everyone!!

I am glad I found this site as it is filled with valuable information...

However I am afraid after hours of searching I cannot find a related issue to which I am facing.

I am working on a Mercury 260 5.7l in a 78 Sea Ray Weekender. I just bought this beauty not to long ago and have been performing routine maintenance and engine condition checks.

I decided to change out the plugs and perform a compression check just for information as the motor ran just fine BEFORE I started.

I will admit I performed the compression check in the wrong manner by just checking one at a time as I changed the plugs - which all plugs removed were consistantly covered with oil deposits. I also removed the coil wire as opposed to grounding it.
#1 120
#2 150
#3 120
#4 150
#5 95
#6 150
#7 120
#8 0

No compression reading in #8 freaked me out as I have never seen absolutely no pressure...

Anyway, After replacing all plugs I went to fire her back up and she ran like @$%&... I have lost spark to cylndrs 3,5,4,6,8 running on three. Ran motor and pulled plug wires to find this determination. Also removed plug and turned over motor to see if any spark - none.

I guess my question is HOW the heck does this happen just by changing the plugs? I thought I fried the coil because I didnt ground it, bought a new one to no avail...

Plugs gapped to .035
Point Gap @ .018

Could it have jumped time? Fuel?

Its ok to slam me as its my first post of many to come!!! THanks for any advice you all may have, and no - none of the wires are crossed

 

[mthieme]

Re: 260 Merc Instant Misfire!!

If all you did was change the plugs, it has to be the firing order.
Recheck you wires.
1-8-4-3-6-5-7-2

Sounds like you had the distrributor cap off - did you replace that too?

 

[Sea Raider I]

Re: 260 Merc Instant Misfire!!

New dist cap, but I didnt change the rotor while I was in there.

Checked wires, all are correct.

My inclination leads me to either, condenser or voltage reg- I just dont understand how I could lose spark in so many cylinders immediately.

Thanks!!

 

[JustJason]

Re: 260 Merc Instant Misfire!!

the question is.... how are you checking for spark in the first place?
Are you using new plug wires or old???? have you ohmed the plug wires out?

Spark is an easy fix... so I wouldn't waste to much time on it yet. You need to figure out why your down on #5 and Kaput on #8.
Bringing a small compressor on board and doing a leak down test would be best. But realistically.... you probably have an exhaust valve that isn't closing for whatever reason. Pull the valve covers and adjust the valve lash, check the rocker mounting studs, and check the valve height at the stem from the outside. Recheck your compression... if nothing changed or if #8 doesn't come up to over 100psi the motor is gonna have to come out. (#5 needs to come up a bit too.... but it's probably not going to be a dealbreaker.)

 

[newport dave]

Re: 260 Merc Instant Misfire!!

If you did the compression test with the coil wire removed and not grounded, you may have burned the points, which could explain the ignition problem.

However, if you really have zero compression in one cylinder and 90 PSI in another, you need to find out why and make repairs. Running with one or more cylinders with compression issues can cause the engine to ingest water and then you will have even bigger problems.

 

[chiefalen]

Re: 260 Merc Instant Misfire!!

Simple question one bank of cylinders is not firing? Or low on compression?

 

[Sea Raider I]

Re: 260 Merc Instant Misfire!!

Well Guys, Thanks for the advice. I ended up taking her down to my mechanic and we ran a camera down #8 hole.

Found out what looks to be the valve seat/seal lose and riding with the valve causing zero compression.

I will be pulling the entire top end and giving a full valve job.

At this point, forget about the spark as I intend to convert to the electronic igintion while Im at it. Maybe that will help, but right now I need to get the heads done before I blow the block!!!

Sure am glad I decided to change the spark plugs!!! Lesson learned at minimal expense.

If I need any help on the tear down/rebuild I will surely post here.

Thanks again to all! :redface:

 

[Sea Raider I]

Re: 260 Merc Instant Misfire!!

Hello All:

Just thought I would update everyone on my experience with this job....

I finished tearing down the motor and pulling heads, #8 exhaust valve seat came loose and was riding with the valve!

Had heads rebuilt and put her all back together. Just out of curiosity I decided to perform a comp check with all spark plugs in their wires on top of the exhaust manifold just to see if I had spark.

Compression fully restored, and wouldnt you know it - I had spark to every freakin plug.

Skeptically I plugged em all back in and went to fire her up - BANG started right up and ran smooth at propert temp for 50 minutes.

Only glitch - Idle surge of roughly 300 rpm. She doesnt die, but I wonder if a fuel pump may need some love. I replaced the fuel/water filter, but not the filter in the carb.

I find it interesting that loss of compression in one hole would cause a multitude of issues elsewhere - out of the BLUE!!!

Thanks for the help guys!!

 

[Bondo]

Re: 260 Merc Instant Misfire!!

Only glitch - Idle surge of roughly 300 rpm. She doesnt die, but I wonder if a fuel pump may need some love. I replaced the fuel/water filter, but not the filter in the carb.

Ayuh,...

If you got the Points, Timing,+ Dwell set properly,...
I'd think the Idle circuit of the Carb is where I'd be lookin'....

 

[Sea Raider I]

Re: 260 Merc Instant Misfire!!

Thanks Bond-o

I eyed the distributer back in, so I pulled out the timing light set at 8 deg while adjusting idle.

Perfect!! Thanks again for the help

Goin
FISHIN!!