Help - motor went to WOT and couln't get it stopped!

[Chaparralbob]

I have a 1990 Evinrude 70HP 3-cylinder. I refurbished the pontoon boat that this came on and have been trying to get the motor running right. It has caused me nothing but grief so far! I probably have the ignition and fuel system sections of the manual memorized!

I just finished rebuilding the carbs. I followed every instruction in the manual and have reviewed a bunch of posts on the subject. I think(???) I got them right. The engine will not fire on the #3 cylinder. I pulled the drains on all 3 carbs, just to make sure, and they all had gas in them. The jets all looked clear when I had the carbs apart, but I removed and cleaned them anyway. All cylinders appear to have a good spark, but just to make sure, I swapped the #2 and #3 spark coil. The problem is still with the #3 cylinder.

Today, I was running the motor at idle on muffs trying to figure out why the #3 cylinder wasn't firing. Suddenly, the motor took off at full speed and I couldn't shut it off! The kill switch has been working, but it seems to have had no effect! I finally disconnected the gas tank and it stopped when it ran out of fuel. What could have happened? This was very scary!

The throttle was closed the whole time! I suppose it is possible that the butterfly's could have been sucked open(??) against the springs, but the throttle linkage was at the idle position. I checked, and the carb linkage moves freely, and the springs return the butterfly's fully closed when I let go of the linkage.

I checked the compression after this, and it is about 100 in all 3 cylinders, very closely matched. It didn't seem like it seized, but the telltale had steam coming out! Prior to this, the compression was 110 in all 3 cylinders, but the motor was cold when I originally measured it.

I am scared to try running the engine again! Is this " run away" a clear symptom of something that has failed? I've never done much with 2-strokes, and I'm starting to feel pretty dumb!

Thanks for any help with this.

Chaparralbob

 

[oldcatamount]

Re: Help - motor went to WOT and couln't get it stopped!

You got me, Chap! That's a heck of a question. I am going to guess that it's a ignition timing problem, but, both me and you are going to need some expert advice here!

 

[Chaparralbob]

Re: Help - motor went to WOT and couln't get it stopped!

I did a bit more research on the web, and found that this is apparently a problem that sometimes happens after extended periods of idling.

Apparently, excessive idle time builds up carbon in the combustion chamber. The hot carbon can act like a "glow plug" in a diesel engine and cause the engine to run without any spark at all!

Several posts I found suggest that one way to stop the runaway condition is by applying the choke. Apparently, this will add enough fuel to cool the carbon and stop the dieseling.

The kill switch has no effect when the engine is running as a diesel! Since it is being ignited by the hot carbon, killing the spark does not stop the engine.

Hopefully, running WOT for a few minutes will not have damaged the engine - we'll see. Most of the posts I found said that once the engine has cooled off, it should run normally again.

Chaparralbob

 

[Joe Reeves]

Re: Help - motor went to WOT and couln't get it stopped!

For the engine to hit a runaway engine scenario, the throttle would have to been open at one time to allow those high rpms to take place. Never attempt to open the throttle when the engine is in the neutral mode.

The spark should be checked with the spark plugs removed and using a spark tester with the gap set at 7/16". The gap is important..... checking spark using the spark plugs is a waste of time. The spark should jump that gap with a strong blue lightning like flame.... a real SNAP!

Check that you have spark of some sorts when the key is ON and no spark with the key OFF.

Carefully and closely check all rubber connecting plugs associated with the ignition system. The pins and sockets have been know to back out somewhat resulting in a poor of non existing connection. Also the wires connecting to those pins and sockets have been known to break away from the wire but are still firmly held in the rubber plug... giving the illusion of being connected when they are actually not.

Check the position of the timer base under the flywheel. Standing over the flywheel, looking directly down at it, the timer base moves in a "counter clockwise" movement to advance.... "clockwise" to retard to the idle position.

That is the timer base to vertical throttle arm connector should move towards the rear of the engine when the throttle is moved to the neutral position. (Not trying to insult your intelligence.... just avoiding any lack of communication).

Head bolts should be torqued to 18 to 20 foot pounds in the following sequence. If the engine has overheated bad in the past, they may be somewhat loose. Check them.

9....10
5.....6
1.....2
4.....3
8.....7
Continue on in a circular pattern.

S/plugs should be Champion QL77JC4 plugs, gapped at .030 . You may also try the older recommended gap of .040 after the engine is running properly to see if either setting gives improved performance.

With the throttle roller loosened so that it is not in contact with the cam, check the linkages between the carburetors to make sure that the throttle butterflies open and close at the same time..... and that they actually do close.

Then, adjust the throttle roller so that the scribe mark on the cam makes first contact when it is dead center with the roller, not before or after. Hopefully you have not disturbed the adjustment between the vertical throttle arm and the cam. If you have, you'll need to set the idle timing.... that is a certain degree setting when the cam scribe mark is dead center with that roller (see your service manual if this is needed).

Let us know what you find.

Thousands of parts in my remaining stock. Not able to list them all. Let me know what you need and I'll look it up for you. Visit my eBay auction at:

http://shop.ebay.com/Joe_OMC32/m.html?_dmd=1&_ipg=50&_sop=12&_rdc=1

 

[Chaparralbob]

Re: Help - motor went to WOT and couln't get it stopped!

I did advance the throttle a bit to check to see if the engine would come off idle and run correctly. This was after I had swapped the ignition coils on #2 and #3. This is when the problem happened. Once I recognized what was happening, I backed the throttle down to idle, but with no effect. I wasn't running the engine fast, as it would not go to a very high RPM. I'll be a lot more careful from now on.

I don't know much about the history of the boat. It came with a piece of property I bought at a local lake. It had set for at least a couple of years on some bricks, and pretty clearly had not been run for some time.

What is the likelihood of stripping a head bolt by re-torquing it?

Wouldn't the compression value of 100 indicate that the head gasket was probably OK? (Just asking, I don't know!) Based on what I have read, this would indicate the rings are getting tired, but the engine is still usable. The fact that the cylinders are well balanced seems to indicate that one cylinder isn't scored or one of the pistons damaged, but I would appreciate any comments about this.

I think I have been through the basics pretty well. I have pulled the plug leads with the engine running, and all 3 cylinders jump a pretty good gap from the plug to the connector. I have found a spark tester at Auto Zone, and I will get it tomorrow just to be sure. All of the carbs have gas in the bowls. The jets were cleaned during the rebuild. I even pulled the core plugs, and checked in the cavities. The VRO pump is new - I can see a little smoke, but it doesn't seem excessive.

The timer base rotates with the linkage and moves in the direction you specified. I am not offended in the least by your explanation - the more basic the better.

The only remaining mystery to me are the reed valves. I have seen some on an old Tecumseh lawn mower engine, but have no real experience with them.
I have read somewhere that if the valve is stuck open, the fuel is not pumped into the cylinder. Looks like kind of a pain to get to. I have already stripped several bolts on the lower unit while replacing the water pump, and I am leery of pulling any bolts I don't need to!

This boat was purchased at the lake where I got it from, and had no trailer. Most likely, it has only been run in fresh water.

Thanks so much for the responses. I appreciate the help more than I can say. This is a project that my entire family has helped with, and I hope to have it at the lake next summer for their use.

Chaparralbob

 

[Chaparralbob]

Re: Help - motor went to WOT and couln't get it stopped!

Just one additional comment --

When I pulled the plug wires from #1 and #2, there was a noticeable drop in engine RPM. The engine came very close to stalling in both cases. When I pulled the plug lead from #3, however, there was no noticeable change in the way the engine ran. This is why I think there is a problem with #3.

Chaparralbob

 

[Joe Reeves]

Re: Help - motor went to WOT and couln't get it stopped!

The fear of stripping bolt threads is understandable.

I'd recommend torquing the head bolts to the minimum 18 foot pounds as a test. That should be sufficent.... and should that minimum torque strip threads (unlikely), the threads were damaged to start with.

The drop in compression psi from 110 to 100 may simply be a gauge thing considering that the reading are equal. If damage existed, it would be extremely unusual to affect all three cylinders.

 

[Chaparralbob]

Re: Help - motor went to WOT and couln't get it stopped!

That sounds pretty reasonable. I'll give it a try. If they are already tight beyond 18 ft-lbs, then they won't move, anyway.

I will do some more research about the reed valves, but would appreciate your comments.

Thanks very much!

Chaparralbob