'89 Evinrude 90HP Piston #1 Keeps Blowing Out

[janker]

ARGH!!!<br />I have a 1989 Evinrude 90 HP crossflow VRO that I have rebuilt each summer for the past three years now. I can't get more than 15 hours on this thing before the #1 piston blows out on me.<br /><br />The symptom is for dentonation--the piston is melting and fusing the shards back onto itself.<br /><br />I have had the carbs rebuilt, the block honed each time by a former Mercury machinist who owns his own shop now. I have tried a variety of spark plugs that are listed as the proper ones for that engine and swapped out spark plug wires.<br /><br />I'm replacing the ignition coils this time around, but what the heck can cause the #1 cyl to constantly blow out like that? <br /><br />The only clue I have is that the last time right before I blew out, I discovered that the starter gear was not consistently dropping back into position. Would the starter gear being engaged with the flywheel generate enough load on the engine to cause the dentonation on the cylinder? If so, why is it only the #1 that shreds itself?<br /><br />Should I replace the reed block for that cyl? It seems to be in the same good condition as the other reed blocks.<br /><br />Someone please help me out here: The guy that rebuilt my carbs for me last year laughed and said I was the first person he had ever seen destroy one of these engines, and it's happened too many times with too few hours. I've gotten FAR to good at stripping this engine down!

 

[janker]

Re: '89 Evinrude 90HP Piston #1 Keeps Blowing Out

Bump, because I don't want to rebuild this again after the current rebuild.....

 

[mikeyzx2]

Re: '89 Evinrude 90HP Piston #1 Keeps Blowing Out

I'd make him eat that piston for laughing, especially as much $$$ as it costs to rebuild these motors. Were there any symptoms prior to losing that cylinder? Maybe you oughtta not let a Merc guy work on it.....j/k I understand the frustrations. Another thing: is the timing set correctly? And what sparkplugs were you using anyways?

 

[janker]

Re: '89 Evinrude 90HP Piston #1 Keeps Blowing Out

Fortunately, I've been saving a bunch of money on these rebuilds by doing it myself, but I wish I could track down what the original problem that keeps popping up. I forget what plugs I had in last year--a guy that I had double check my work and set the timing had put in gap plugs that some manuals reference as the correct plugs. My Clymer manual says to use QL77V (surface gap plugs) for sustained high RPM use, which is how I use it. Those were the type of plugs that were in it when I bought it in 1989, and I had used them for several years before this problem started popping up.<br /><br />I thought that it was the plugs at some point, too--that's why I went back to the QL77V plugs, but it happened again. And after every rebuild I have a paid professional check the timing.<br /><br />The weird thing is that there aren't really any symptoms, other than the quick loss of power once the dentonation starts. It generally idles well, hits 5500 RPM in stride and starts well--until the problem hits, which is within 10 hrs of the rebuild every time. <br /><br />The only thing I can point to as being odd is what happened this last time--that I discovered the starter gear wasn't disengaging. I don't know if that was new or how long that had been happening. It was fine when I ran it in my driveway after each rebuild, but I can't verify that it wasn't happening on the water with the cover on. I ask again--is it possible that this could cause enough load to fry the #1 piston? Where else can I be looking for the prob? I know that dentonation is usually caused by excessively lean mixtures, but I had the carbs rebuilt and even ran off an external tank to make sure that there wasn't anything funky in my main tank--another suspision that I once had. I thought I've been through all possible causes, except for the ignition coil (could this cause the issue) and now the starter gear.

 

[funpilot]

Re: '89 Evinrude 90HP Piston #1 Keeps Blowing Out

Are you sure you have good water flow to that cylinder?<br /><br />fp

 

[janker]

Re: '89 Evinrude 90HP Piston #1 Keeps Blowing Out

I can't verify that. The tell tale is strong when running. How can I verify water flow to the cylinder?

 

[R.Johnson]

Re: '89 Evinrude 90HP Piston #1 Keeps Blowing Out

An instructor at OMV service school told of an engine doing this very thing. Each time the carbs were gone over with the rebuild. They finally changed the carb to the cylinder causing the problem. That fixed it. I'm certainly not saying this is your problem, but I would have to think it is the same underlying cause each time. I would want to be certain of the cause before taking the engine to total failure each time.

 

[Dhadley]

Re: '89 Evinrude 90HP Piston #1 Keeps Blowing Out

We assume the top edge of the piston is burning -- most likely the sharp edges of the deflector on the dome. That can be caused by an ignition problem. Youll need to check the entire ingnition system starting with the flywheel. Make sure the center hub magnet is not loose.<br /><br />After you get the motor running you'll need to check #1 for shadow firing or double firing. Very easily done with your timing light. Toull have to watch it when its under a load so a test wheel would be very helpful.<br /><br />Once you get the motor running it wont hurt to richen up #1 while youre looking for the cause of the problem. You can go 3 or 4 sizes on the mid range and high speed. It may blubber but it may save the piston while youre troubleshooting.<br /><br />Also, you said you had the timing set. What did they set it at and by what method?

 

[Dhadley]

Re: '89 Evinrude 90HP Piston #1 Keeps Blowing Out

If youre paying someone to work on that motor who has never seen this before -- you need a new guy with experience.

 

[Blueclaw]

Re: '89 Evinrude 90HP Piston #1 Keeps Blowing Out

doesn't that have two carbs? If it has two, I doubt that the carb is at fault. It would show up in more then one cyclinder. I'd think more of an air leak or spark/timing problem. The latter being more suspect.

 

[Basscat 1]

Re: '89 Evinrude 90HP Piston #1 Keeps Blowing Out

Yes it has two carbs. The top carb controls fuel air to cylinder 1 & 2, while the bottom carb controls fuel air to 3 & 4.<br />I would listen to Dhadley and richen up the fuel metoring circuit until the problem is identified.

 

[Dhadley]

Re: '89 Evinrude 90HP Piston #1 Keeps Blowing Out

It has 2, 2 bbl carbs. Each barrel or venturi feeds one cylinder.

 

[janker]

Re: '89 Evinrude 90HP Piston #1 Keeps Blowing Out

Yes, Dhadley, it is the top of the piston that is burning up. They have the flywheel marked at 28 degrees TDC. I don't know exactly what method they used to set the timing. But seeing how far into this I have gotten methinks that a test wheel is a worthy investment at this point--much cheaper than another rebuild!<br /><br />How do I richen up the carb for #1? I was under the impression that the carbs for this model weren't really adjustable other then bending the float? Also, as a side note, can anyone tell me exactly where the idle speed screw is? I keep hearing reference to it, but even my Clymer manual doesn't say where it is exactly. <br /><br />If I do find that this thing is double firing, is that the ignition coil or power pack's fault? I've ordered a new ignition coil already for it, but won't balk at picking up a new power pack.<br /><br />PS Dhadley, it's not that he'd never seen it before, he'd just never seen it before on this particular model--it'd been his experience that these were dependable, work horse engines with little troubles--which was my experience with it until this recurring issue! He actually specializes in racing boats and did the carbs for free for me more as a favor to the machinist up in Osh Kosh that referred me to him. I question the work of the two different guys that checked the timing the past two times, but I have the utmost confidence in the guys who did my block work and carbs, esp since I watched them as they did it.

 

[Dhadley]

Re: '89 Evinrude 90HP Piston #1 Keeps Blowing Out

Assuming you have the original carbs for a 1989 90 the high speed jet should be a 56C. You can go to a 59C or even a 60C. The mid range jet is in the bowl and should be a 26C. That can go to a 29C. <br /><br />To check for a double fire mark TDC on all 4 cylinders and number the mark. As youre watching any one number cylinder, if another number comes up also -- thats a double fire.

 

[janker]

Re: '89 Evinrude 90HP Piston #1 Keeps Blowing Out

So if I understand you right, you're talking about replacing the jet in the carb?<br /><br />How about the idle speed screw--I know it's not related to this problem, but where exactly is it located?

 

[Dhadley]

Re: '89 Evinrude 90HP Piston #1 Keeps Blowing Out

Yes, to change the rich/lean run characteristics you have to replace the fixed jets.<br /><br />The idle stop screw is on the throttle arm. On that motor the idle is set by the position of the rod going back to the timer base with that thumbwheel.

 

[janker]

Re: '89 Evinrude 90HP Piston #1 Keeps Blowing Out

okay, now I'm a bit confused--there is an adjustable screw in the block on the starboard side. What is this screw for?

 

[Dhadley]

Re: '89 Evinrude 90HP Piston #1 Keeps Blowing Out

Where in the block? I know you say the starboard side but where in the block? Are you looking at the one in the throttle arm?

 

[janker]

Re: '89 Evinrude 90HP Piston #1 Keeps Blowing Out

No--it's actually screwed into the block, near the bypass cover for the #3 cyl. It's a screw that goes into the block, does not hold anything on, but has a lock nut on it so that it stays in place once you adjust it for whatever it does.

 

[buddyholly]

Re: '89 Evinrude 90HP Piston #1 Keeps Blowing Out

the stop you are referring to is the WOT stop. it is at the bottom of the power head and when you open the throttle up all the way it hits it right? the slow idle stop is at the topand the adjusting screw is on the lincage and hits the land on the block at full throttle closed.