120HP (circa 1986) running slow

[jnpcook]

I have a 1986, 22ft Sylvan Offshore (basically a Starcraft Islander copy) powered by a 2 stroke, 4 cylinder 120HP Force outboard (I believe around the same year as the boat).

The first time out this year (just bought the boat this year), it started right up and I took it on the lake and ran it up to full throttle which according to GPS ran at around 20mph. I then drove around several minutes at top speed and then idled down to troll for fishing. After about 15 minutes of trolling (probably around 1-2 mph) the motor just turned off and I looked back and saw some steam and it sounded like sizzling sounds. I left it off and had to flag someone down to tow me back in.

I then took my boat to a mechanic and he installed a new impeller, cleaned the carbs, checked compression (which he said was adequate), drained and filled lower unit with new lube oil, put in new spark plugs. He originally said that the 2 lower cylinders plugs looked dry and mentioned that it seemed like maybe it was only running on two cylinders and that maybe no gas was getting to those cylinders. The mechanic took quite a while working on the motor and I finally got it back about a month later. He said it was running good and started it for me at his place and everything seemed OK. Started up, ran at idle, revved up OK.

I took the boat out to the lake for it's 2nd trip and had a problem getting it started. It probably took me 5 minutes to finally get it started. As soon as I put it into reverse to leave the dock (which was very busy that day) it stalled and I had to keep starting it. I would let it idle a while in case it needed to warm up but every time I put in reverse to leave the dock it stalled out. I then started it up and rammed it in reverse at a high speed and was successful in leaving the dock. I went in forward and went up to full throttle and made sure I had water coming out the 2 holes at the rear of the engine (which I did) and according to GPS I was at about 20mph maximum. It looked like the tach was reading between 2000-2500 RPM (which seems too low). After maybe 10 minutes the boat started slowing down to maybe 10mph and I still had the throttle lever maxed out. I checked the 2 holes in the back of the engine and I didn't see any water coming out. I shut off the engine and waited for a friend to tow me in as I didn't want to risk overheating the engine.

I have just purchased the manufacturers factory service manual for this outboard and plan to look into this problem myself as the motor seems to run even worse after I had the mechanic look at it and "repair" it.

I have a few questions:

1. Is water always supposed to come out of the 2 holes at the back of the engine at all speeds or only at idle or max speed, etc?

2. Is my RPM too low? I would say so as the Force manual says 120HP @ 5250 RPM.

3. Where should I start to look to diagnose this problem?

I lost all summer to this problem and have not been able use my boat at all and would like to get it sorted out before it is too cold to work on (I live in Michigan so only have a couple months at best to work on it).

Thanks,

John

 

[Frank Acampora]

Re: 120HP (circa 1986) running slow

Ahhh! I just hate to see people having trouble after having a mechanic work on their engine. Most of the times you can screw it up equally well yourself without spending the money. LOL

That engine should run in the range of 4500-5500 RPM at full throttle with rated horsepower developed at 5250. Idle speed should be 700-750 RPM in gear in the water. Too low and the engine will always stall when put in either gear but will catch when "slammed."

However, before adjusting idle speed, you need to verify that the engine is running on all four cylinders.

20 MPH on a 22 foot hull is too low. You should be up around 28-32.

Now, first, with the bottom two plugs dry, did the mechanic service the carbs? Did he verify spark on those two cylinders? Did he do anything? Did he care?

Before running the engine, with the help of a friend, crank it and verify spark is present on all four sparkplugs. You can do this: Remove and clean the carbs. Replace the fuel pump diaphragm and gasket. Be certain there are no restrictions in the fuel line. Buy, borrow, or rent a compression tester and verify that all four cylinders are adequate--say over 120 and equal--within 10 psi or so. Check the setting on the carb(s) low speed needle--be certain it is screwed out about 1 turn from lightly seated otherwise the carb will not deliver adequate fuel.

Water needs to exit the two idle relief holes in the back at ALL speeds. If it is not, then there is a problem somewhere. You need to recheck the impeller and remove the thermostat cover and check the thermostat.

 

[jnpcook]

Re: 120HP (circa 1986) running slow

Thanks a bunch. I will try each of those things you mentioned.

I kind of thought that speed was too slow. I forgot to mention my boat is all aluminum so I felt definately it should go faster due to the lighter weight than a glass boat of the same size.

I do know the mechanic cleaned the carbs (well he said he did). I thought he said he checked spark. I don't know for sure. I plan to do the work myself from now on with the help of the manual and these forums.

Do I have to purchase new gaskets to inspect the thermostat and fuel pump diaphram? Can I reuse the ones that are on there or will they be destroyed when I check these items?

Where is a good place to find and purchase parts for these motors?

I do have access to a compression tester so I will do that. How do you perform the compression test?
If the thermostat is stuck will water not flow out the 2 rear holes?

 

[Desertsky]

Re: 120HP (circa 1986) running slow

I think I would go back to the "mechanic" and either get my money back or have him explain why the engine runs no better after he "fixed" it. If he says it is fine, demand that he go out on the water with you and prove it. I would NOT let these "mechanics" get away with charging you a bunch of money and not fix the problem. One BIG lever you have with them is if you pay by credit card, you can have your card company charge back the shop until you are satisfied with the work.

I hope you are able to get in at least some boating this year!

 

[Frank Acampora]

Re: 120HP (circa 1986) running slow

Gaskets will most likely need to be replaced if they have not been removed before. If they come off clean, then no, you will not need to replace them.

To perform a compression test, remove all four sparkplugs. Screw in the compression test gauge one cylinder at a time and crank over the engine 5-10 times. Read and bleed the gauge and go on to the next cylinder.

You can try going back to the mechanic, and yes, you may have some leverage with a credit card, but the aggravation, fuss, arguing, etc. sometimes just isn't worth it--but that's your decision.

Personally, I never let a "pro" work on my engines. Why should I, when I know more about these engines than most of them? And, Force engines are so simple that all but the most mechanically inept owners can repair them.

Yes, there are good ones out there and in speaking with them I can quickly determine that they know their onions, but then, there are the bulls**ters, and a relatively inexperienced owner will have trouble seeing through the crap. Some of it sounds logical when said with authority.

 

[jnpcook]

Re: 120HP (circa 1986) running slow

When I perform the compression test, after I remove the 4 spark plugs, do I leave them all connected to the spark plug wires? Do I pull them out of the spark plug wires? Do I need to ground the unused wires?

I found out that I actually have a 1986 125HP motor (model# 1258F6A) if that makes any difference from my original post. It looks like I have 4 ignition coils and then 2 modules that sit next to the ignition coils. I just want to make sure what to do with the spark plug wires. I don't want to risk blowing up a module as these look to be pricey.

 

[jnpcook]

Re: 120HP (circa 1986) running slow

Got a chance to get back over to where my boat is kept and did a few checks on it today.

Compression test:

Top Cylinder: 130psi
Next Cylinder: 125psi
Next Cylinder: 40psi
Next Cylinder: 125psi

So it looks like I have some serious problem in the 3rd cylinder down from the top.

I used a spark tester and set to 7/16" gap and got good spark on all 4 cylinders. Looking inside two of the spark plug wires the metal connector was pretty rusy so I plan to either clean them with a wire brush or if possible replace the terminal that connects to the top of the spark plug without having to replace the entire ingnition coil.

I pulled the thermostat out and took home to test. Still need to do that.

My next idea is to pull off the head and inspect the pistons tops and cylinder walls.

When we checked compression when we bought the boat we measured around 120-130psi all 4 cylinders if I recall.

Am I on the right path?

 

[emoney]

Re: 120HP (circa 1986) running slow

Doesn't seem like there should be such a disparity between your compression
test and your mechanic's? Just thought I'd bring that up.

Hopefully Frank or Jerry will jump in here, but from everything I've read here
in the forum you're definitely on the right path. Head gasket could be a
problem, which you should be able to see when you pull the head. But
at this point you've got to look inside there. Good luck.

 

[jnpcook]

Re: 120HP (circa 1986) running slow

All compression testing was done by myself and/or my brother-in-law. The mechanic never told me any compression numbers.

What does a blown head gasket look like?

I will take the head off next and inspect but probably won't be until next week.

 

[Frank Acampora]

Re: 120HP (circa 1986) running slow

Like I said before: Did the mechanic do anything? Did he care?

With compression on #3 cylinder at 40 PSI, it is possible that a head gasket has blown BUT with the added note that the bottom two plugs looked dry (did the mechanic do anything to correct?) it is more likely that the piston is damaged. Dry plugs should be a strong warning that there are carb problems that need to be corrected before running the engine. (Did he care)

So now, you must remove the head to see what damage has been done.

A head gasket is either a steel gasket covered on both sides with black or orange fiber or a straight fiber gasket. Around each cylinder hole there will be a steel ring. If the gasket is damaged, the steel ring will be severely dented or broken at one spot. That spot may only be 1/8 of an inch is size bit that will be enough. Additionally, the fiber portion mat appear burned , charred, or perforated at that spot.

A melted or otherwise damaged piston will be obvious as will a scored cylinder wall. Just compare it to the others. Sometimes if the damage is not too severe, a simple honing and piston replacement can be done without dismantling the engine.

SO: When you remove the head, take a couple of photos and post them here.

These two photos clearly show the damage to a piston due to broken rings. At the top of the photos you can see where the top of the piston has been dented by the broken ring catching in the exhaust port and flipping out. You may also be able to see the light cylinder scoring which was corrected by honing. There was some port damage which could not be corrected but should not affect engine performance.

 

[jnpcook]

Re: 120HP (circa 1986) running slow

Well I found out why I have low compression on cylinder #3. I pulled the cylinder head off and took several photos and inspected things.

cylinder head with plugs still installed

cylinder 1 head

cylinder 2 head

cylinder 3 head

cylinder 4 head

piston #1

 

[jnpcook]

Re: 120HP (circa 1986) running slow

piston #2

piston #3

piston #4

head gasket

cylinder #3 head gasket

close-up of cylinder #3 head gasket

As you can see the metal ring in the cylinder #3 portion of the head gasket is broken. I noticed that when I removed the head, the bolts were easy to remove. I was expecting to have to use a lot of force to remove them. I used a 12 point socket as I expected to have to really torque on the bolts to remove them. I didn't have a torque wrench but like I said, there was very little force required to remove the head bolts. Is this normal?

I plan to purchase a new head gasket and install. I inspected the pistons and the rings and they appear to be in good condition. I did see some light scoring on one part of cylinder #3 wall but it didn't look bad. There was a light coating of oil/gas on each of the 4 cylinder walls. I will replace the thermostat and the thermostat gasket as well. I still need to figure out why the engine overheated. I am wondering if the overheating caused the metal ring to break or if it may have been damaged or broken the whole time I had the boat. I am hoping my RPM will now be in the correct range. I probably will drop the lower unit and try to spray some water up the cooling tube to make sure water is getting to the powerhead and while I have the lower unit off I will replace the impeller (even though the mechanic replaced it already).

Thanks for the help and please let me know if you have any other suggestions, comments, etc.

John

 

[jerryjerry05]

Re: 120HP (circa 1986) running slow

The tourqe is only 20# on the head bolts.Head gasket on #3 is definatly bad.
Any Mercury dealer or this site has this gasket.
The original overheat could have blown the gasket.The impeller is probably good but check it anyway.It should have a 6 vane impeller.They don't usually break apart and clog the tube.
It's possible the mechanic did a comp test before the gasket went(not likely but possibe)bad.
Make sure the air screws are set right.Ethanol!!! J

 

[jnpcook]

Re: 120HP (circa 1986) running slow

Where are the air screws located and what is the correct setting for them?

 

[emoney]

Re: 120HP (circa 1986) running slow

Almost like the little 'cricket' chirping in your ear, but;

Timing can be an issue, as well, can't it? You've definitely got a head gasket
issue on #3, but his saying that "possibly firing on 2 cylinders" could be an
issue with timing being off.

By all means, don't take my word to the bank, but I've had similar issues
lately on a different scale.

 

[bman1bpm]

Re: 120HP (circa 1986) running slow

You'll need a torque wrench to properly set the torque on the head bolts to make sure it gets sealed evenly.

 

[jerryjerry05]

Re: 120HP (circa 1986) running slow

Setting the air screws.The top front of each carb there is a screw head.Turn it in untill it just bottoms.Then out 1 and 1/8th turn.This should be done when the motor is warm and in the water in gear at an idle.Idle is about 750-800 in gear.
Timing shouldn't be much of an issue,unless you have messed with the trigger or stator.Check it, but it shouldn't have changed.
Read everything you can about Ethanol.It's probably your next problem.J

 

[jnpcook]

Re: 120HP (circa 1986) running slow

Thanks for clarifying that Jerry.

I am about to order some replacement parts. For replacing the head gasket, do I need to use some type of RTV or gasket sealant on the head and on the block before I put the head gasket on or do I just clean off both metal surfaces real well and use the head gasket without any sealants, etc? Same question for the water pump impeller gaskets and thermostat gaskets and carb gaskets?

 

[jerryjerry05]

Re: 120HP (circa 1986) running slow

No sealer needed if you clean the mating surfaces.
If there is a lot of corrosion then some sealer on anything but the head gasket. You might need to get it resurfaced.J

 

[tasmania105]

Re: 120HP (circa 1986) running slow

I would check your ignition coils...(after you fix your head gasket)