Engine will not start in the water but starts and runs fine with muffs out of water

[ahunter4063]

Having problems getting boat to start in the lake. I've had it hooked up to muffs in the yard and motor will start and run great, no doubt that it would run till it ran out of gas as long as it's on muffs and in the yard. I backed it into the lake today and it would crank over but wouldn't hit a lick. I then pulled it just out of the water and tried to start it out of curiosity.... Dang thing started up no problem. Left it running and backed it back into the water and it died and would not start back while in water. Pull it out and it will start no problem. Had someone mention that it had BACKPRESSURE on the exhaust. What is this and what do I need to do to fix it? FYI... Boat is a 1990 4.3 mercruiser with 2 barrel mercarb that I just rebuilt. Any ideas or help greatly appreciated.

 

[thumpar]

There are idle relief ports at the transom so backpressure would not be an issue. The only thing I can think of as far as testing is to get a tub big enough to have the drive in it with water up to as high as you can. It could be you sucked up junk from the tank when it was low and clogged up the a filter or a passage in the carb.

 

[alldodge]

I would check compression of the motor. Then check electrical connections if compression is good. Something happens when the motor and boat are tilted.

If you get it running I think your going to need to replace the impeller

Left it running and backed it back into the water

 

[Bt Doctur]

Throw 2 asprins in the gas tank or it`s allergic to the water.
On muffs or a body of water should not make a difference, theres got to be something else going on.

 

[Scott Danforth]

Staring and running the motor out of water toasted the impeller.

That being said, your motor should run in the water. Check compression, you should have 150psi or more in all cylinders.

 

[ahunter4063]

Just finished checking compression on motor. No compression at all on 4 out the 6 cylinders. Puzzled as to why this thing would even start. It has a new (rebuilt) short block on it so I wouldn't think the short block itself is the problem? Where to next? Pull heads off and check them, or any other suggestions? Thanks

 

[alldodge]

How new was the rebuild and when did this problem start?

 

[Scott Danforth]

Just finished checking compression on motor. No compression at all on 4 out the 6 cylinders. Puzzled as to why this thing would even start. It has a new (rebuilt) short block on it so I wouldn't think the short block itself is the problem? Where to next? Pull heads off and check them, or any other suggestions? Thanks

I would start by doing a leak down test, this will help determine what went awry

 

[Grub54891]

Yeah, a leakdown will tell ya if its the valves or rings. I purchased a vehicle years back that was overhauled, but would not run very well, got it cheap, I had checked the compression beforehand, and yeah, I had cyl's that were good and bad. Pulled the heads and found the valves were shot, they did not bother to do them. Ended up with a strong running 318.

 

[Fishermark]

.. It has a new (rebuilt) short block on it so I wouldn't think the short block itself is the problem? ...

Who did the install? Has it ever run since the install? First thought would be simply that the rockers are adjusted too tight. Try readjusting the rockers and see if that helps.

 

[ahunter4063]

I will check/re-adjust rockers within next few days(seems like first thing to check). After I readjust rockers i will re-check compression and advise of outcome. AllDodge- the short block was professionally re-built in Texas and I did the install.

 

[ahunter4063]

FisherMark- I did the install and motor has only ran for a total max of 30 mins out of water on muffs. That's why I assume the short block itself should be fine.

 

[ahunter4063]

ScottDanforth and Grub54891- As to leak down test if adjusting rockers doesn't solve my issue... Is this in the manual? If not what is this test and what are the steps to do it?

 

[alldodge]

. AllDodge- the short block was professionally re-built in Texas and I did the install.

With only 30 minutes on it, I to will wonder if the valves were not adjusted correctly. Since its a new motor I would suggest contacting the builder before you touch the rockers.

Now if they just did the block and you did the heads, then yes go for adjusting rockers

 

[ahunter4063]

AllDodge- I did install the heads and they were not re-built. When the OLD motor locked up I took the heads off and sat them in the garage and that's where they stayed till I was able to get new short block ordered and installed(roughly 3-4 months). I definitely feel like I may not have the rockers adjusted correctly. Also the LIFTERS were supplied by the place I ordered the short block from and I have a question about them..... Should they have a spring like movement to them or should they feel solid or tight(hopefully I've worded that to make sense)?

 

[alldodge]

Proper method to adjust valves on pre 1993 motors.

Need to make sure the valve your adjusting is fully closed. You can rotate and do it according to the manual, or the way many do it. Rotate the motor until you see one of the two valves per cylinder going down (opening valve). Now back off the nut on the other valve until you get some up and down play on the push rod. You can also turn the push rod in you fingers, but the up and down is what your feeling for.

Now start turning the nut back in until all the up and down movement is gone. It doesn't matter if you can still turn the push rod, just no up and down. Now turn nut one full turn. That valve is adjusted, mark with grease pencil or other method and go to next valve stud. Repeat until all are done

Oh and here is the section in the manual
http://boatinfo.no/lib/mercruiser/manuals/mercruiser7.html#/152

 

[Fishermark]

....Now start turning the nut back in until all the up and down movement is gone. It doesn't matter if you can still turn the push rod, just no up and down.

^ Yep. Cannot emphasize enough that you simply want to remove the slack. Very easy to overtighten. Then you do the extra turn as mentioned which puts the push rod basically in the center of the movement for the hydraulic lifter.

 

[Bondo]

....Now start turning the nut back in until all the up and down movement is gone. It doesn't matter if you can still turn the push rod, just no up and down.

^ Yep. Cannot emphasize enough that you simply want to remove the slack. Very easy to overtighten. Then you do the extra turn as mentioned which puts the push rod basically in the center of the movement for the hydraulic lifter.

Ayuh,..... Agreed,.... I hate it that so many write-ups, 'n manuals even mention twistin' motion at all,.....

It's the up/ down slack, Nothin' more,.....

The twistin' motion, as yer checkin' for slack, just breaks the oil film resistance, makin' it easier to feel the up/ down slack,.....

 

[ahunter4063]

UPDATE- So I couldn't resist re-adjusting rockers this afternoon after work. My rockers were extremely over tightened due to trying to get rid of all the slack in the rocker arm like Bando mentioned. I used AllDodges method and now I have compression in all cylinders!!! Numbers are: #1= 135. #2= 135. #3= 150. #4= 135.
#5= 145. #6= 130.
Are these numbers good enough to put boat on water and see if she will run or do you all recommend any other testing before doing so?

 

[Fishermark]

You're good to go. Your numbers will raise after the rings set and the engine warms.