Force 120hp L-Drive Overheats At High Rpms

[pmkohout]

Long story short... had a 1990 Bayliner w/ Force 120hp L-Drive given to me with every piston ring broke. Bored block 0.030" over, new pistons/rings, new gaskets, new water pump, new t-stat. Fired it up w/ muffs to tune it, runs great. Dumped it in the lake only to find overheat alarm goes off while cruising above 3500 rpm, never goes off at idle. Removed t-stat and tried again... same thing. Head temps are at about 140 at idle, but climb to 190-200 at higher rpm(overheat alarm trips at about 195) with or without t-stat. Water flow seems good, even flushed and back-flushed powerhead and all hoses... no debris or old impeller chucks. Nothing should be restricting water flow, considering every power head component was hot-tanked and cleaned via power washer. Any ideas?? I hate to melt a freshly rebuilt powerhead!

 

[pnwboat]

Re: Force 120hp L-Drive Overheats At High Rpms

What is your timing set at? Too far advanced can overheat the motor.

Makes sure the inlet cooling hose from the water pump to the block (#12 in diag. below) and the outlet hose from the block to the exhaust (#11 in diag. below) are not kinked or blocked.

Make sure there are no blocked water passages in the exhaust housing itself.

http://www.crowleymarine.com/mercury-outboard/parts/1342_140.cfm

 

[pmkohout]

Re: Force 120hp L-Drive Overheats At High Rpms

I will double check those hoses. Timing is at 30 BTDC, which was a pain setting with no timing marks or reference point. I had to physically find TDC 1, mark the flywheel, measure the circumference of the flywheel, divide by 360 and measure 30 BTDC (obviously no degree wheel was used!). Since it is a freshly rebuild powerhead, I am running a 25:1 fuel/oil mixture. (My Force manual said to run double oil during breakin) Would that possbily lean it out too much, causing overheating? I haven't tried 50:1 yet, but i will just to see. Thanx!

 

[pmkohout]

Re: Force 120hp L-Drive Overheats At High Rpms

I forgot to mention, I am using the updated one piece exhaust gasket #819729A1, which takes place of the exhaust gaskets and plate. Since it was revised by Force to supress water entry into the cylinders, I would assume that would not effect water flow or circulation. Has anyone else used this one piece gasket? Also, the one thing i didn't do when building the powerhead was seperate the cover from the cylinder head itself... has anyone seen a restriction in the cylinder head cover? (it would have to be tiny stuff to get past the t-stat) Thanx again!

 

[pnwboat]

Re: Force 120hp L-Drive Overheats At High Rpms

That gasket looks like the gaskets that I've seen and used many times without any problems.

You mentioned that the block was hot tanked and power washed. Some blocks have 3 small rubber tubes inserted along the length of the cylinder in the water jacket to divert the flow of water. Do you know if your block has these? If so, were they in place when the motor was put back together? See picture below.



Here is another diagram that shows water flow. Disregard the pressure relief valve. Your motor does not have one. Note how the "blocked passage" in the water jacket between the cylinders diverts the flow of water around the outer perimeter of the cylinders.



I've never seen any obstruction in the water passage under the head cover. Not that it couldn't happen but it usually occurs in the water passage in the exhaust housing. Probably because the water passage there has some openings that are pretty small.

Also the inlet hose actually attaches to the adapter plate which is bolted to the bottom of the block. The adapter plate has a water passage that runs to the bottom of the block, which then is fed to the cavity that surrounds the exhaust cavity and the exhaust housing.

 

[pmkohout]

Re: Force 120hp L-Drive Overheats At High Rpms

Thanks pnwboat for the good info! I think it had all the diverter hoses in place upon re-assembly, but I will probably pull the head and double check (rather pull the head off in boat vs. pulling the entire powerhead again to check exhaust manifold!) Also, with the water outlet hose disconnected at the base of the powerhead and a length of heater hose connected to the base, I am getting water out both the heater hose and the bellows... is this normal? I have checked various pics of exhaust manifolds and I believe I can see water/exhaust cooling holes in the collector portion on the manifold. I hope it should be normal and not a cracked manifold. Thanx!

 

[pnwboat]

Re: Force 120hp L-Drive Overheats At High Rpms

I'm not sure that you should be getting any water coming out of the bellows with the outlet hose disconnected. I'm not really that familiar with the L-Drive set up as I much as the regular outboards. I noticed that the diagram has a couple holes for fittings on the downward "tubular" portion of the exhaust housing that look like may be water fittings. Are there any hoses hooked up to them? Is this what you are referring to when you say "collector portion on the manifold"? Maybe the water is coming from there and dropping into the bellows?

Bottom line is that (and this is just my guess) there should not be any cooling water used to cool the block also coming out of the exhaust bellows. Any water going to the exhaust bellows should be supplied via the outlet hose. Not sure I'm clear on my point, but you get the general idea. All of the incoming cold water should be fed to the block and only the block. If you have a "T" connection somewhere on the inlet hose that is feeding the exhaust cover, it should be moved to the outlet hose.

 

[pmkohout]

Re: Force 120hp L-Drive Overheats At High Rpms

There are two pipe fittings on the tubular portion of the manifold (thats what i was referring to as collector... im new to boat repair, an auto mechanic attempting to speak marine) but they have plugs installed. The manual and no other posts found say anything about water entering the exhaust stream within the exhaust manifold... but all the l-drive exhaust manifold pics i found show holes in the tubular wall. Any ideas?

 

[pnwboat]

Re: Force 120hp L-Drive Overheats At High Rpms

Looks like they may feed some of the incoming cooling water to the exhaust, or there's something wrong with your exhaust assy. I wouldn't focus too much on the water in the bellows just yet. Check the simple things like the external hoses, the little rubber tubes inside the water jacket etc.

 

[pmkohout]

Re: Force 120hp L-Drive Overheats At High Rpms

I replaced all the hoses and flushed thru all the external water passages and fittings, ie L-Drive and transom. I gave it one more test, still overheated. One thing I found was no matter how I tested (muffs, dunk tank, or lake) it cools just fine in neutral. I can wrap it up to 4500-5000 rpm and head temp stays normal. Only under load will it overheat above 3500 rpm (only tested in gear in lake). Does this still indicate a restriction of water flow, or something else, like a lower unit issue? I haven't tore down the powerhead yet, but I'm running out of options. Just say the word, and parts will be ordered and powerhead gone thru again.

 

[pnwboat]

Re: Force 120hp L-Drive Overheats At High Rpms

I don't know but I'd take another look at the timing. Can you post a picture of the flywheel? I've got several and maybe I can measure the distance from TDC for 30? advance. Also need to know how far out the fuel mixture needle valves are turned from lightly seated.

 

[pmkohout]

Re: Force 120hp L-Drive Overheats At High Rpms

I pulled the powerhead and tore down just to be on the safe side... but found no restrictions. All my water diverter tubes are in place as well. 30* BTDC is aproximately 6cm or 8 teeth from TDC. My top carb was turned out 1 1/4 turns, bottom was at 1 1/8 turns. My exhaust manifold does have water jets in the collector/snout area. Only thing I can think of is that with that one piece exhaust gasket water can flow between the block and manifold without any restictions. Maybe I need that plate in there to restrict my source water from just being pushed into the exhaust stream, thus creating more flow to the cylinder water jackets and head. Sounds like a long shot...

 

[pnwboat]

Re: Force 120hp L-Drive Overheats At High Rpms

I measured the flywheels I have and you're pretty much spot on with the timing. 8 teeth to the left of TDC is 30? BTDC. I noticed something odd about you block. Maybe it's just a shadow or something. It appears that there is an opening in the water jacket that leads right to the water exit for the block. Note the red circle. Water enters the water jacket by the red arrow.



It looks like there is a cast in depression in your block just to the right of my red circle. This is normally open and is the water exit passage from the head. Here are pictures of one of my blocks.



This slotted passage should mate up to a slotted opening in the head by the red arrow.



If what I'm seeing is correct, then the water is coming into the water jacket by the red arrow in the first picture, flowing down the left side water jacket, then right out of the block. It looks like the water exit for the head is blocked (see arrow in picture #3). Again it may be just the pictures but it looks like you are not getting any water flow around the right side water jacket and through the head. That would certainly overheat a motor.

Here is a picture of the bottom of the block and that rectangular water exit. You'll see a small 1/8" hole drilled through to the water jacket, but not an opening like what I'm seeing in your block.





The last picture is the bottom of the block where it is bolted to the adapter plate. The adapter plate has the inlet and outlet hose connections.

 

[pmkohout]

Re: Force 120hp L-Drive Overheats At High Rpms

Once again, thank you for all your help. I really appreciate a pro like you taking the time to help me out! My garage has terrible lighting, I have some better photos for you. I noticed that the rubber hose diverter where the water enters the cylinder jackets isnt fully seated, maybe a 1/4" gap at the bottom. Im not an engineer, but since water can enter the jacket on both sides of that tube, I see no use for it. I'm not gonna remove it (unless I replace it), but I would think that without it, flow would be increased, however the circulation would be altered.

View attachment 149159View attachment 149161

Another scenario I was pondering... since the block is bored 0.030" over, I have gained approx 30cc displacement. Would it be possible that my standard 0.094" main jets be too small, causing it to lean out. Only downside side is that if I drill out my jets larger, there is no going back!

 

[pnwboat]

Re: Force 120hp L-Drive Overheats At High Rpms

For some reason, when I click on your attachments, I get a message saying that they are "invalid". Maybe the file size is too big? Anyways, I'm familiar with the rubber diverter hose that you are talking about. I'm pretty sure that the water jacket area behind it is a dead end.

I guess the question is, and I can't seem to open your attachments, is there a passage between the water jacket and the exit passage that I circled in red on the top picture in post #13? Or is that just a shadow? My guess is that it is just a shadow.

Forgot to mention that I doubt that boring 0.030" over would cause it to run lean with 0.094 main jets. I have the same set-up on one of my motors and I rarely see temperatures above 150? F.

 

[pmkohout]

Re: Force 120hp L-Drive Overheats At High Rpms

Heres another attempt at uploading the photos...

 

[baylner_boy]

Re: Force 120hp L-Drive Overheats At High Rpms

First off - thanks for this great thread. I, too, am struggling with heat issues around the exhaust area. The engine stays cool, however the exhaust manifold gets extremely hot - not sure exactly how hot, but if bubbles are coming off of the hose clamps that hold the elbow to the through-hull connector - um, not good. I've already had this beast burn through one exhaust coupler, so I'm cautiously watching this issue.

My question is more of where you got your Gaskets (the exhaust one).

For what it's worth, I *think*/*hope* my problem stemmed from several semi-blocked passages. I found bits of an old impeller in both the bottom of the exhaust manifold and in one tiny hole in the water passage on the manifold, as well as in the water jacket surrounding the top cylinders. I also had near 1/4 cup of sand/dirt in the bottom of the exhuast manifold.

This is the first time I had the engine removed from the boat, finally got tired of "guessing" and bit the bullet to look everything over.