What caused my motor to go?

[Stretch.]

Hello, I have a 1985 Johnson 90hp VRO on a 16' Blue fin. After buyng it last year, I noticed it would not idle very well and also started cutting out at WOT momentarily. Also on one occasion I couldnt even get the boat to plane out which it usually does without hesitation. I brought the boat to my mechanic and found the compression to read 120-125 on the bottom 2 cylinders, and only 80-90 on the top 2. I ran the boat for the rest of the summer (which I use mostly for fishing) and on my last trip out, noticed a lot of blue smoke hanging around as I was trolling. I limped it back to shore and the next morning it wouldnt even start, but puffed a lot of blue smoke as it tried to fire. I have since had the motor rebuilt ($$$) but am VERY hesitant to run it as I am not sure what the cause of the uneven compression was in the first place. There was scoring on some pistons and cylinder walls. The mechanic thought maybe wasnt oiling propperly due to the scoring. If it was the VRO pump it should have affected all cylinders? Fuel/air too lean? Any input that might have caused above problems would be helpful. Thanks

 

[emdsapmgr]

Re: What caused my motor to go?

It is possible that the top carb on the engine was the fault. If that carb had some debris or dried fuel in it, the fuel jets could have been restricted, causing the upper two cylinders to run lean on both fuel and oil. Over time, this could have resulted in the failed condition of your powerhead. Running, idling and performance problems are common symptoms of dirty carbs. Typically during a complete powerhead overhaul a mechanic will also replace the thermostats, new fuel filter, overhaul both carbs, replace the lower unit impeller and install a new oil/ful pump. This will insure the engine rebuild will give you many years of good service. Normally that carb overhaul service will have corrected any fuel restriction issues on that top carb. Check your bill or call that mechanic to check to see specifically was done during the overhaul. If the boat is laid up for long periods of time, make sure you have it prepped for long term storage properly (winterized.)

 

[Stretch.]

Re: What caused my motor to go?

Thanks for your input, a lean condition was what I was leaning towards but I am no mechanic. I'll give mine a shout like you suggested.

 

[wilde1j]

Re: What caused my motor to go?

VRO failure would damage most, if not all, cylinders. Lack of oil doesn't discriminate! Likely cause is lean condition. Mechanics, when the don't know, will often blame the oiling system.

The bad news: if you don't find the cause of failure, it will surely happen again.

 

[AEROCOOK]

Re: What caused my motor to go?

Clean the carbs (soak method) as a precaution and as a matter of routine maintenance.
Lean condition can also be caused by bad crank seals in 2 cycle motors but if yours was completely rebuilt, they would have been replaced as part of the repair. For piece of mind I would monitor the engine very closely by checking the colour of your spark plugs frequently...Brown/tan color is good, white is too lean (bad) and black or oily is too rich.
Do you have a workorder from the repair facility and if so is there a parts list? It would be nice to know the actual level of repair, i.e. complete rebuild as opposed to partial rebuild and also any notes that would allude to the cause of failure.
Also, were you told to run with extra oil till it's broken in?

 

[Faztbullet]

Re: What caused my motor to go?

I would look the ignition system over throughly as the miss you described could have been the problem starter(preignition).Index the flywheel and check timing on all 4 cylinders.

 

[Stretch.]

Re: What caused my motor to go?

I dont have a parts list but I know the pistons and rings were done, slight over bore, rods, bearings, gaskets etc. He ordered a kit for all the bearings and there was another one which didnt come in the kit that was replaced as well. He also cleaned the carbs. I spoke to my mechanic this morning and he thought it was possible that the low compression in the top 2 cylinders could be due to lack of winterizarion over the years (after all the boat is 25 years old). He is also going to come along with me for the maiden voyage after the rebuild so hopefully he can catch anything that isnt quite right. I appreciate all the input. This is a great site and the store here has good products, service, and pricing. Just received my new prop the other day.

 

[Stretch.]

Re: What caused my motor to go?

P.S. yes my mechanic told me to mix an additional 50:1 in the tank during break in. He felt quite sure that the engine should be good now. But without having pinpointed the problem, leaves a guy feelin a little uneasy.
Will look at all things suggested by your inputs. Thanks again