Puzzling ignition problem

[nabeel.alsalam]

My 1990 Johnson 40 hp has developed an ignition problem. It starts easily and idles after a warm up period. However, after I drop it into gear and increase the throttle it will not develop any power or go above 2000 rpm. <br /><br />What I’ve done so far:<br /><br />1) I hooked up a timing light to spark plug wire #1 and the light strobed and the timing seemed to be correct. I hooked up the light to spark plug wire #2 and the light strobed about 180 degrees out of phase.<br /><br />2) Looked at plugs: Top one is dry and tan colored. The bottom one is wet and gray. <br /><br />3) Took the spark plugs out and connected the plug wires to a home made spark plug tester. Both are jumping the required 9/16” of an inch while the engine is cranking.<br /><br />4) With the plugs removed and grounded and the wires connected, I verified that they are sparking. They are but I was not impressed with the quality of the spark.<br /><br />5) Replaced plugs, started engine with fast idle up a bit, and pulled off the primary wire to the upper coil for #1 cylinder. Engine dies. Repeat and pull off primary wire to the lower coil for #2 cylinder. Engine continues running with no change in rpm! <br /><br />6) I reverse the primary and second leads of the coils. I pull off the primary lead to the upper coil for #2 cylinder and the engine continues running with no change in rpm. I pull off the primary lead to the lower coil for #1 cylinder and the engine dies.<br /><br />From #2, #5, and #6, I would conclude that I’m running on 1 cylinder and something upstream of the coils is the problem. <br /><br />From #1, #3 and #4, I would conclude that both cylinders are getting spark. <br /><br />I’m worried about the contradictory information. Does this happen? Why? (I’m not experienced with Johnson engines, outboard motors, or 2-cycle engines).<br /><br />What would you suggest as my next steps? I certainly will make sure the primary wire for cylinder #2 that leads from under the flywheel to the lower coil is in good shape (the part that I can see without the flywheel removed looks good). Assuming it is, is there anything else I can do to isolate the problem? I’m guessing that a bad charge coil would affect both cylinders. I’m also guessing the ignition module is expensive, so I hate to replace it without being fairly sure that is where the problem is. <br /><br />I'm trying to develop Zen with this engine but it is lonely (time consuming and expensive) work. I really appreciate the help you provide.

 

[WillyBWright]

Re: Puzzling ignition problem

The clean/gray sparkplug tells me that something failed in that cylinder. But not necessarily spark. Clean means it's not firing, but gray means there's a compression problem and the piston isn't pulling anything to burn. The gray is from metal such as aluminum and/or steel from the sleeve. When the RPMs didn't change, that could also indicate that nothing is getting in there to burn. You need to do a compression test.<br /><br />Such damage could occur from a major overheat, broken ring from carbon build-up or overrevving, the carb may not be delivering any fuel to that cylinder depriving it of lubrication. Another possibility is water in the oil supply tank. It'll settle to the bottom and the pump will be pumping water instead of oil.

 

[nabeel.alsalam]

Re: Puzzling ignition problem

A part of my Zen building process with this engine was to completely rebuild it. It has new pistons & rings, rebored, new rod bearings, etc. <br /><br />I haven't formally checked the compression yet since the rebuild, but with the old thumb over the spark plug hole method it feels great. <br /><br />I'm using 25:1 and was being careful about rpm when this new problem occurred.

 

[WillyBWright]

Re: Puzzling ignition problem

Then I'm betting on the carb theory. That cylinder isn't getting gas. Why the rebuild? Always find out why the cylinder failed when rebuilding or replacing a shortblock to avoid a repeat failure.

 

[nabeel.alsalam]

Re: Puzzling ignition problem

The fact that cylinder #2's plug was wet made me think it was getting at least some gas. I did rebuild the carbs, check fuel (VRO) pump pressure, and replace the in-line filter during the rebuild. <br /><br />I rebuilt the engine because it lost some compression in one cylinder (110 vs 130 psi) and starting making clacking sounds. That turned was due to a bad rod bearing and damaged crank rod journal. I'm assuming that was caused by insufficient lubrication, i.e. a fuel supply problem. But maybe is was caused by being run on one cylinder -- I let a friend use it. Or by being run cold in cold water due to a bad thermostat -- it was corroded by salt water. Anyway, I'm trying to make sure everything has been checked and is working properly. This "discovery" from reversing the primary and second leads on the coil is the first time I've received any "clear" indication of some engine system not working the way it should. All other checks pre loss of compression and now post-rebuild have all passed.

 

[jim_in_day]

Re: Puzzling ignition problem

We see that problem occasionally, it appears to fire with a spark test, but doesn't fire under compression. Week spark cause by a bad coil, the fact that when you switched wires, the #2 plug appeared to fire fine would indicate that your coil was bad. If it was the power pack/ cd, you will usually not get a trigger voltage to the coil. A quick test to make sure that you are is to look at the CD output, i believe it should be about 130 Volts on both lines, if it is not, its the power pack, if it is, it is the coil

 

[nabeel.alsalam]

Re: Puzzling ignition problem

Thanks. <br /><br />Actually, #2 cylinder is not firing before and after the switch of wires. #1 is firing through either coil. So that does point to the primary for #2 cylinder not working. <br /><br />So last night I ordered an ESI 640 DVA adapter that says it will allow me to read peak voltage with my digital multimeter. I assume I can't do that reading without the peak voltage feature, right?