85HP Force - Out of Ideas

[Mark9740]

I bought a 16' bowrider last year with an 85HP Force. The engine ran great this spring and summer until mid-July when my son took it out with his friends tubing on the river. Now, the motor won't idle below 1000rpm and to get it moving, I need to run it in high idle and then jam the throttle to neutral and back to forward quickly before it dies. It gets up to 32mph at 4500 rpms. I'm using a 13 1/2 x 17 prop which is much better than the 19 pitch prop that came on it. I'm not worried about going any faster I just want to be able to idle and put it in reverse if necessary without it dying. When I remove the plugs after having it out, the bottom one is shiny and the top two are covered with oil/carbon. I've replaced a coil and CD module and know I'm getting good spark. I've rebuilt the carbs and set them to 1 1/4 out from a light seat. The plugs are new. I've done a decarb twice now per the FAQ recommended procedures. There is a slick on the water at the dock when I take the boat out from the gas that apparently wasn't burned in the 3rd cylinder. I replaced the head gasket this week and the readings are 125, 125, 115 which is within range. I've got a Seloc manual that I've been using but it's pretty light in the troubleshooting dept. Any ideas out there? I'm out of them. Thanks.

 

[roscoe]

Re: 85HP Force - Out of Ideas

replace export cover gaskets.
Water likely getting in through there.

 

[Mark9740]

Re: 85HP Force - Out of Ideas

Is that the same as the exhaust cover gaskets? Also, someone mentioned to me that it could possibly be a bad reed valve. Does that sound right?

 

[eurolarva]

Re: 85HP Force - Out of Ideas

Lower plug is not firing for some reason. When you did the compression test did you do it with all the plugs out? When you replaced the head gasket did you see any damage to the cylinder? Try putting a dirty plug in cylinder 3 and the clean plug in another cylinder and run it. Does this clean the plug in #3

 

[Mark9740]

Re: 85HP Force - Out of Ideas

Lower plug is not firing for some reason. When you did the compression test did you do it with all the plugs out? When you replaced the head gasket did you see any damage to the cylinder? Try putting a dirty plug in cylinder 3 and the clean plug in another cylinder and run it. Does this clean the plug in #3

Eurolarva,
Yes all the plugs were out when I did the compression test. The cylinder looked good, no scratches but the pistons were still carbed up on all three. Would the water clean that bottom piston too if there were a leak in the exhaust cover gaskets? Maybe the fuel isn't getting to the cylinder after all?

 

[pnwboat]

Re: 85HP Force - Out of Ideas

If water was getting into the bottom cylinder, you should be able to see water droplets on the end of the plug.

To confirm that the bottom cylinder is not firing, pull each spark plug wire off with the engine idling as low as possible. You said it wouldn't idle below 1000 RPM? Motor should stall or almost stall when pulling either #1 or #2 plug wires one at a time. If you pull #3 and it makes no difference, that would confirm that the problem is with #3 cylinder. Either it's not firing (ignition), or a vacuum leak or broken reed valve. The bottom carb could also be out of whack too.

 

[Mark9740]

Re: 85HP Force - Out of Ideas

If water was getting into the bottom cylinder, you should be able to see water droplets on the end of the plug.

To confirm that the bottom cylinder is not firing, pull each spark plug wire off with the engine idling as low as possible. You said it wouldn't idle below 1000 RPM? Motor should stall or almost stall when pulling either #1 or #2 plug wires one at a time. If you pull #3 and it makes no difference, that would confirm that the problem is with #3 cylinder. Either it's not firing (ignition), or a vacuum leak or broken reed valve. The bottom carb could also be out of whack too.

Now that you mention it, I have never seen water droplets on the plug. I did confirm after taking it out of the water that I have a strong spark on all 3 cylinders. After reading another similar thread, I'm wondering if it might be the bypass valve/cover gasket? I've never heard of it before and not sure where to find it on mine. Might be the next step before looking at the reeds. How can I determine if this is the problem? Thanks for your help troubleshooting this pnwboat. Mark 9740

 

[pnwboat]

Re: 85HP Force - Out of Ideas

By-pass covers are on the side of the block. Your motor has 3 of them. One for each cylinder. Your fuel pump is actually mounted on one. 4 bolts hold each cover in place. They're bolted to the block. I would check the bolts on the bottom one for #3 cylinder to see if they're loose first. You may have to remove the coil/CD Module mounting plate to get to them. Pain in the you know what.

See item 47 in the diagram below. Zoom in for a clearer view. Item 48 is the gasket.

http://www.boats.net/parts/search/Merc2/Force/85%20H.P.%20%281987%29/856X7A/POWERHEAD/parts.html#

Double check your electrical connections on the 12 position terminal block where the CD Modules, stator and trigger are connected. Sometimes the spade lug connections get corrosion. Doesn't take much to cause problems. Also there is a ground wire from each coil that goes to the mounting plate. Check those. Then there is a very important ground wire that goes from the mounting plate itself to the block. Check both ends of that wire. I added an additional ground on the opposite side of the mounting plate to be sure. Later model Mercury Thunderbolt ignition systems had a Service Bulletin to add another ground to the mounting plate.

 

[Mark9740]

Re: 85HP Force - Out of Ideas

Thanks for the help. I'm traveling all week and won't be able to check the bypass cover until the weekend. If the bolts are tight, I assume there no reason to remove the plate is there? If that's not it, my next stop will be back to the carb and then the reed valve I guess. I'll let you know what I find after looking at it this weekend. Thanks.

 

[pnwboat]

Re: 85HP Force - Out of Ideas

Usually if the by-pass cover is leaking, you'll see puffs of smoke coming out of the side of the motor. If the bolts are tight, and you don't see any smoke from the side, especially when the motor "sneezes" when starting, you should be OK.

Typically, when you have a plug that still looks brand new when removed from a running motor, it's because it's not firing. Successful ignition of the fuel/air/oil mixture causes the black carbon deposits to form on the plug. Weak, intermittent, or lack of spark is usually the cause. Only exception would be if there was no fuel/oil mixture getting to that cylinder/plug at all. The carburetor would have to be really out of whack for that to happen. Possible but not that common.

 

[Mark9740]

Re: 85HP Force - Out of Ideas

Would a bad reed valve keep the fuel from getting into the cylinder or otherwise be the culprit in a case like this?

 

[pnwboat]

Re: 85HP Force - Out of Ideas

Yes a bad reed valve will affect fuel flow into the cylinder.

The reed valve is necessary to create positive crankcase pressure to push the fuel mixture into the combustion chamber when the by-pass port is uncovered during the piston's downward movement. Depends on how bad the reed valve is leaking as to how much it affects fuel mixture flow into the combustion chamber. Reed valve petals should be laying perfectly flat against the reed block. Maximum gap is 0.01 in.

They're relatively easy to check, however, there's a couple of gaskets that may be damaged when removing the carburetor adapter plate, reed block assy. You may have to do a "link and sync" on the carbs after you put it all back together. "Link and Sync" and gaskets are easy to do, not a show stopper.

 

[Mark9740]

Re: 85HP Force - Out of Ideas

Thanks for the explanation. I'll look it over this weekend. If the bypass covers are intact and not leaking, I'll move on to the carburetor. I'm thinking about spraying some gas (50:1 mix) into the carb while cranking the engine with just the bottom plug in to see if it fires (I'll ground the other 2 plugs). If not, I can only think that the reed valve is the next step. I'll let you know what happens. Thanks again for your help in talking me through this.

 

[speedwrench]

Re: 85HP Force - Out of Ideas

DING DING DING! We have a winner: reeds. Been there done that, used T-shirt to clean ny hands.

 

[john from md]

Re: 85HP Force - Out of Ideas

Before pulling the carbs and reeds out, pull the fuel pump and check the diaphram. If your kids backfired it a couple of times when starting, it may have torn the thin diaphram. An engine with a torn diaphram will idle poorly, leave an oil slick on the water and give poor top end performance.

John