Low Compression, rebuild questions

zachzez · Oct 24, 2010

Hi, I have a late 70's 140hp Johnson V4 outboard. My father gave me the boat since he was not using it. I took it out the other day for the first time. It fired right up and seemed to be running good. I used the boat for about 30 min and on my way back to the dock I was running it at full throttle when all of a sudden the motor stopped. I raised it out of teh water and took the cover off as a small amount of smoke was coming off the block and was very hot. I took it to a marine mechanic who did a compression test and told me I needed to do a rebuild. Today I took off the cylinder heads and went ahead and took the powerhead off. I did a compression test and found 2 cylinders in the 20's, one at 30 and one at 60. One piston head was white and really rough. The other three looked to be in good condition. All the cylinder walls looked to be good. The pistons had some play in the cylinders. My questions are; what would have caused this to happen, what is the white on the one piston, can I get away with just replacing piston rings? Any thoughts and feedback will help out greatly.

Zach

emdsapmgr · Oct 25, 2010

Re: Low Compression, rebuild questions

What year is the engine-the 78 and newer V4's have the telltale indicator for the water pump-was it pumping water out the nozzle? The powerhead may have overheated due to lack of cooling water. If you continue to run an overheated engine, the pistons can swell and eventually expand enough to actually lock up the powerhead. It's not unusual for the rings to be affected by that type of overheat. A pitted/white piston crown can be from: overheat on that piston hole, too much ignition timing, lean condition on the carb throat that feeds that cyl. You can reuse pistons if they are not damaged-new rings only. You will need to replace that damaged piston. Would be good to know why it failed. Some things to do for an overhaul: replace the impeller in the lower unit and see what the old one looked like. Also, you should plan on overhauling the carbs and probably replace both thermostats. Check the rubber water diverters in the block to make sure they are whole and properly in place-new ones are inexpensive.

zachzez · Oct 25, 2010

Re: Low Compression, rebuild questions

It is a 1978 engine. When I took the boat out I watched to make sure it was pumping water out. I checked it before I started to head back to the dock and it was still pumping. After the motor shut down and I took the engine cover off and let it cool down, I tried to restart it. It turned over, just did not start.

How would I go about checking to see what made this happen? I would hate to rebuild this powerhead and have this happen again. I will take some pictures tomorrow and post them.

wilde1j · Oct 25, 2010

Re: Low Compression, rebuild questions

Unless the head gaskets were blown, those compression numbers suggest boring all four will be needed, along with new pistons and rings (oversize) all around. You're right about finding the cause of the failure. If you don't, it will happen again.

zachzez · Oct 25, 2010

Re: Low Compression, rebuild questions

Can I take the powerhead to any engine machine shop to have them bore out the cylinders or are there engine shops just for marine motors?

Any thoughts on how to diagnose the problem?

emdsapmgr · Oct 25, 2010

Re: Low Compression, rebuild questions

A single cylinder can still overheat-even if the impeller is good and the horn does not sound. Check the location and placement of those water diverters when you pull the heads. I'd also pull that carb apart and specifically look at that barrel which feeds the damaged cylinder. Check the warning horn system to make sure the horn is functional and that the cylinder head sensors are also functioning. An original factory manual is really helpful if you are not familiar with the engine. You need to have a machine shop familiar with outboards do the boring. They can tell you if/which cylinders need boring and which can simply be honed.

zachzez · Oct 26, 2010

Re: Low Compression, rebuild questions

Here are some pictures of the cylinders and heads.

wilde1j · Oct 26, 2010

Re: Low Compression, rebuild questions

Can't tell much from the pictures. A good marine machine shop (some listed below) will tell you what oversize pistons to get.

Mar Fab 931-738-2815 (Tennessee)

Flagship

Tribble

Blackburn

Roehrs

Riverside

emdsapmgr · Oct 27, 2010

Re: Low Compression, rebuild questions

It is interesting that in all the stress on that dry piston, that the ring did not let go. The head and cylinder walls look ok. You have some rubber water diverters that are slightly out of position. I'd replace them all. The center (mid) water diverters have swollen and are encroaching on the water flow passages. Picture #2, shows the middle diverter problem the best. Also, the bottom ones show they are in different positions between the two sides of the block. These minor differences could be enough to account for cooling flow differences within the powerhead-causing hot spots internally which a head temp switch might not pickup to signal an overheat. Check out the factory service manual. You need 4 pcs of 322411.

89retta · Oct 27, 2010

Re: Low Compression, rebuild questions

In pic 3 it looks like the insulation is burned off one of the wires for your coils.

zachzez · Oct 31, 2010

Re: Low Compression, rebuild questions

I was able to remove the pistons and it was interesting that none of the rings were broken. The pistons did have some scoring on the sides. I took the block to a machine shop and they said I need to bore it .020 over and buy all new pistons. They said everything looked good and most likely the problem was from the one carb not getting enough fuel to that one cylinder. I am going to have the carbs rebuilt. Any estimates on cost for having the carbs rebuilt? Also, I am not familiar with the water pump. Should I rebuild the entire water pump of just replace the impeller?

emdsapmgr · Oct 31, 2010

Re: Low Compression, rebuild questions

Normally, replacing the impeller is all that is required if the plastic housing is not warped/deformed by overheating. Check it when you have it apart and then determine if you want to go with just the impeller or the whole kit. Since you are doing the whole powerhead, might be the prudent thing to do and get the whole kit (including a new housing.)

Solittle · Nov 1, 2010

Re: Low Compression, rebuild questions

Replace the thermostats while you are at it.

zachzez · Nov 4, 2010

Re: Low Compression, rebuild questions

I just got my carbs back and it cost me $205 to have them rebuilt. My pistons and other parts should be in tomorrow, so hopefully I can get the motor back together this weekend!

zachzez · Nov 6, 2010

Re: Low Compression, rebuild questions

I need your help once again. I am in the process of putting the motor back together. I just pulled my lower unit to take a look at the water pump. When I was removing the bottom screw to drain the gear oil, a good amount of clear liquid came out, assuming water. The motor has always shifted fine and I have never believed there to be a problem with the lower unit. This was a pretty big suprise to me. The gear oil also has a terrible burnt smell to it. What are your thoughts on this issue? Should I just buy a used lower unit or should I mess with taking it apart? Is there an easy fix? Thanks again for the help!!

emdsapmgr · Nov 6, 2010

Re: Low Compression, rebuild questions

Water in the lower unit can be a problem. The oil used in marine lowers is a hypoid oil, which will mix with water to maintain some amount of lubrication, despite the water. You will have to get into your lower to determine just how bad the gears, bearings are. You should be able to see something when you pull the carrier out. If you are lucky, you may get by with a gearcase reseal only. Black gear oil is a sign that it was severely stressed, either from overheating, or was run with low oil in the housing. Best to keep track of the gear oil by changing it once a season-typically in the fall during winterizing.

James R · Nov 6, 2010

Re: Low Compression, rebuild questions

Was there any metal particles in the oil you removed. A little amount is not terrible. What color was the oil you removed. If it was black no biggie but it shows that the lower was not run with water in it. If it was milky then water was mixed in during running. What I am getting at is that water got in with the motor at rest in the water and it could just be leaking fill or level plug seals that are the problem.

zachzez · Nov 6, 2010

Re: Low Compression, rebuild questions

When I removed the plug it was only water then followed by black oil. There was no metal in the oil and it was not milky. I pulled the water pump and the plastic housing has a crack in it and looked like it had started to melt. The impeller was not in good condition. I took off the bushing housing and the shims were completly shot. I am thinking of replacing the shims and o-rings and just servicing the lower unit with fresh oil and giving it a shot. It still is shifting and spinning free. Does that sound ok?

Solittle · Nov 6, 2010

Re: Low Compression, rebuild questions

On the lower unit - one of the most common problems is a leak from the seals between the plug and housing. Do the water pump then refill the gearcase with fresh oil and NEW seals. Run it and check the gear case for water again. If you still have water you will either need to replace the lower unit or rebuild yours.

James R · Nov 7, 2010

Re: Low Compression, rebuild questions

Do what you are going to do and as Solittle says don't forget to replace those plug seals.

Low Compression, rebuild questions

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