No. 1 piston - Bad!!!!

[fourchows]

First time poster!! Hi guys, I have a '93 90 hp Johnson VRO. Boat had been running fine; however, I had the compression checked and to my disappointment found out that the number one cylinder had no compression WHAT-SO-EVER. Since then I removed the head and found that one piston ring had disintegrated and melted the piston at the side that exits into exhaust. All 4 spark plugs looked the same (i.e., oily and wet with fuel). I removed block, cleaned and honed cylinder, and replaced piston and rings. It runs fine in driveway and compression has been restored. I have always had good pee hole indication and water pressure at gauge, but I replaced T-stats anyway. I am left wondering, however, what other possible problems might have caused it. How does water travel through this motor? Any suggestions would be greatly appreciated?

 

[Dave Abrahamson]

Re: No. 1 piston - Bad!!!!

Let me be 1st to say welcome aboard iboats! You're gonna get some good advice on this board, some real pro's lurk here.(present company excluded). Do you decarb the motor? I do know rings can coke up from carbon deposits and let go.<br />Someone with more experience will chime in here soon. <br />Good Luck!<br /><br />Dave

 

[Dhadley]

Re: No. 1 piston - Bad!!!!

Welcome!!<br /><br />We need to figure out what caused the piston to have damage. If the rings broke it could be as simple as the motor was being lugged (top rpm in the low 5000 range). If it burned off the corner by the exhaust ports, it could be another problem.<br /><br />Lets start at the begining. What plugs were in it when this happened? What plugs are you using now? What was the top rpm? What fuel are you using?

 

[OBJ]

Re: No. 1 piston - Bad!!!!

Yes...welcome fourchows! (the name come from having four Chow dogs?.....just curious)<br /><br />It would sound like pre-ignition in the one cylinder maybe caused by heavy carbon build up. Was there much carbon in the cylinder head? Did you notice if the head was pitted from a ring failure? Pits can collect carbon which can actually glow and cause pre-ignition as the fuel enters the cylinger.<br /><br />We would hope that you had all the cylinders honed out and installed new rings. Also, rebuilding the carbs is a great idea to do along with the rebuild.<br /><br />Cooling water usually enters the exhaust cover first, travels through the head from the t-stat and exits out the exhaust.<br /><br />Replacing the t-stats is a good idea but better yet is replacing the water pump impeller also if it hasn't been changed out in a while.<br /><br />Did you have an experienced machinist do the bore and honing? Cylinders were miked and still in specs? Intake and exhaust port edges broke over to remove the sharp edges?

 

[OBJ]

Re: No. 1 piston - Bad!!!!

You type to fast Mr. D......

 

[Dhadley]

Re: No. 1 piston - Bad!!!!

Now thats the first time I been called a fast hunt-n-pecker...Lol!!

 

[MCM]

Re: No. 1 piston - Bad!!!!

Just curious as to the condition of the deflectors and if they were replaced as well.

 

[fourchows]

Re: No. 1 piston - Bad!!!!

Guys, thanks for the help!! I appreciate the fast responses. Certainly, lots to think about. To provide some clarity (hopefully), The piston looked more like heavily pitted. The head had very little pitting and not much visible carbon buildup. Only the first ring dissolved. The one that looks cupped stayed intact. The edges on the ports were smoothed out during the honing process.<br /><br />The fuel is the lower octane of the 3 at the gas station and, since I don't trust the VRO, I premix the oil (I use a measuring cup ritually and use "Pennzoil Premium Plus"). I have always used the same spark plugs which are champion QL77JC4 gapped at .030". The water jackets did have the deflectors in place and in what appeared to be in good condition. As to the work done, I typically do most of my own work (I am not an experienced machinist) on the motors I have had, but considering the problem is not in my work but rather in an existing cause, I did a pretty good job. The compression is back up to 125. The propeller pitch allows for a top end RPM of 5200 to 5500; however I seldom go past 5300.<br /><br />As a follow up question to see if I got it right: As the water is pumped up through the pick up tube, it enters through the T-stats, through the hoses, then does its cooling through the heads and water jackets, and exists back down through the exhaust area?

 

[fourchows]

Re: No. 1 piston - Bad!!!!

OBJ: Yep, originally two chows, but wouldn't you know it they had kids.

 

[Dhadley]

Re: No. 1 piston - Bad!!!!

Good choice on plugs, fuel and oil. The motor was definately being lugged. Once you get it broke in we'll help get the r's up. Whether you use them all or not it has to be capable of 5800 for longevity.<br /><br />As for the water flow, you got it but the other way around. It comes up thru the block & heads first and then to the t-stats.

 

[Dhadley]

Re: No. 1 piston - Bad!!!!

Oh yeah -- MCM has a very good point about the deflectors. Its worth responding about that.

 

[OBJ]

Re: No. 1 piston - Bad!!!!

Chow.....what Mr. D. said about the rpm's is really important. I have an 86' 90hp Johnny that will turn 5800 to 6000rpm's depending on how many I have in the boat. It's been running like this for about three years now and I havn't had bug one get into it. I know what the OEM specs say...but really, these blocks breath easier and last longer at higher rpms'.

 

[fourchows]

Re: No. 1 piston - Bad!!!!

HHMMM....Please correct me if I am wrong. I always understood it that the RPM range for a motor of this size was between 4500 to 5500 rpm, and as long as it ran in between you were o.k. Am I wrong? If so, a higher pitch prop would do it, ...wouldn't it?<br /><br />In between postings I've been doing some testing on the motor to better answer some of you're questions. I connected the water hose to the cups, and cranked the puppy up. After a few minutes I could tell the T-stats opened up because the water outlets at the bottom increased in water flow and temperature (moderate temp.). I disconnected the hoses between heads and T-stat housing and noticed considerable flow. I then turned off motor and noticed water flow stopped even though faucet remained on. This gives me a preliminary indication that the pump is at very least working to some degree. Once I disconnected hoses I placed my hand on the water coming out of both heads and noticed a gradual drop in temperature, which to me would indicate good water cooling effect.<br /><br />Next, I turned my attention to ignition. I turned her back on and disconnected one spark plug cable-at-a-time and on each I noticed a drop in RPM. Once I connected each back, the RPMs went up again. I believe this helps with the powerpack and adequate spark theory(Or am I missing a step). I do notice a fuel smelling GOOO out of the exhaust as if not all fuel is burning. Gas is old by the way.<br /><br />I hope this helps you help me.

 

[jy118lfd]

Re: No. 1 piston - Bad!!!!

Do yourself a favor and drain that old gas! These motors do not like old gas at all. <br /><br />You need to get it on the water and break it in correctly (search it here and read on and on).Then you need to work on the setup to get the top rpm in the 5800 rpm range. When a motor is set up to run at 5000 you lug the motor kinda like starting out in third gear in a car. An outboard works very hard and it needs the ability to get to that rpm so when you are running at say 4000 the load on the motor is managable.<br />If a motor is setup to run at a lower wot rpm when you run it at any other rpm it will be lugged and can be damaged just as easy.

 

[Rabbitdawghunter]

Re: No. 1 piston - Bad!!!!

How long had it been since the motor ran fine, with the "old gas" sounds like gummed up carbs may have been the culprit here. I'd be sure to clean them all before returning to service. A piece of crud may have made its way to the high speed jet causing a lean condition and this is the result.

 

[Dhadley]

Re: No. 1 piston - Bad!!!!

A higher pitch prop will lessen the rpm even more. More bite. Dont worry about changing props right now. Changing props on an outboard to gain rpm is a last resort. Get the motor broke in then we'll go into set up.

 

[Dhadley]

Re: No. 1 piston - Bad!!!!

I have no idea how that happened. Excuse the double dribble....

 

[andy6374]

Re: No. 1 piston - Bad!!!!

First off I have a 88' 200hp Merc. Black Max.<br /><br />I've heard SO much about under propping or not reaching the proper RPM's at WOT in this forum. My guess is the same must be true for Merc's.<br /><br />What's the problem? Is it that the spark is too advanced for the low RPM, thus causing detonation or something else. I thought that the max spark advance was reached far before WOT.<br /><br />-Thanks<br />Andy

 

[Dhadley]

Re: No. 1 piston - Bad!!!!

Max rpm has more to do with set up than anything. Many folks take a boat to a dealer complaining of poor performance, lack of speed, poor fuel economy and sometimes a damaged motor. The dealer puts the motor on a dyno and declares the motor healthy. Which is true.<br /><br />However, a healthy motor doesnt mean the rig is set up properly. To be set up properly the motor must reach its full potiental. If the dyno and compression / leakdown tests tells us there is no problem with the motor, we then look at set up. The dealer who doesnt run the boat in the water is only doing half the job.<br /><br />Overpropping and under propping are common but horrible discriptions of the problem when it comes to outboards. If a motor cannot reach its full potiental, as in this case, the common reaction is to reduce pitch first. The most likely result is a better holeshot and more top rpm but at the cost of speed and economy.<br /><br />We would rather work with the set up to reduce drag using the same pitch prop. Raising the motor can have dramatic results. As you lose gearcase drag (by raising the motor vertically -- we're not talking about trim angle) you gain holeshot, top rpm, speed, economy and extend the life of the outboard.<br /><br />Throwing money at a prop is a last resort. And most likely the owner is just as dis-satisfied and frustrated. Once the set up is optimized some folks cant believe its the same rig.<br /><br />Why dont dealers and boat compinies do this to start with? Some do. Some dont. It may be a lack of knowledge or experience or the ol "time is money" thing. "It floats, starts and runs. Next!"<br /><br />The good thing about working with your existing set up yourself is that if you try something and you dont like it, put it back the way it was. In most cases all you've spent was a little time and whatever beverage you enjoyed while you worked. <br /><br />And face it -- you were probably going to have that beverage anyway.

 

[fourchows]

Re: No. 1 piston - Bad!!!!

Mr. Dhadley: I like your thought process. I should give you some insight as to fishing and boating in South Texas. I fish in the South Padre Island bay area. Water depth for the popular type of fishing down here is approximately 1 to 2 feet of water--better known as shallow water fishing. To get in and around these areas you need a rig set up for shallow water fishing. My boat is a popular brand and is designed with a tunnel which moves water efficiently as a plane. My motor is rigged up with a CNC brand powerlift and tilt/trim. I can traverse the bay in waters of 9 to 10 inches while on plane and take off in approximately 15 inches. To take off without cavitation I have a anti-cavitation plate installe above the prop. If raising the motor is part of the proper set up for higher R's then I don't see a problem there.