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Troubleshooting 2 Stroke Ignition Problems
Author: Stuart Buckingham Date Posted:30 March 2017
The most common thing I get asked over the phone to assist with is diagnosing outboard ignition problems. Whilst I am always happy to try and help in diagnosis, it can be very hard to do so over the phone. Apart from having the right tools to assist in diagnosis, a basic understanding of how a two stroke ignition system works is fundamental. The following is a list of what makes up an ignition system from start to finish.
Your boat battery is where it all starts. Outboards require higher cold cranking amps than many other engines and therefore a dedicated marine battery with the correct cranking capacity for your outboard is a must. Without the correct voltage, your outboards starter motor may not be able to crank the flywheel fast enough to activate your ignition system.
2. Ignition Switch
To activate your engine and crank it over you need a switch. Every electric start outboard has an ignition switch and it will open and shut the starter solenoid. It also turns the engine off by shorting the CDI unit (power pack or switchbox) to ground. A faulty ignition switch can cause all sorts of problems with your ignition system and is more often than not overlooked.
3. Starter Solenoid
This is effectively a gate. The ignition switch sends a signal to the solenoid when you turn your key allowing it to open; this allows voltage from your battery to pass through it to the starter motor. If you're turning your key and the starter motor isn't cranking then you may have a faulty solenoid.
When your starter motor is turning over it is spinning your flywheel to create voltage for your ignition system and battery charging system. On the inside of the flywheel are magnets that spin around a stator at high speed to create AC voltage. The flywheel also spins around the trigger (or timer base) to determine your engines firing sequence or timing. Broken or loose magnets on your flywheel will cause ignition issues so they are definitely worth checking.
Your stator creates voltage for two reasons. To send voltage to your CDI unit (power pack or switchbox) to run your engine and to send voltage to your rectifier/regulator that will charge your battery. Engines with no spark or intermittent spark on one bank can be caused by a faulty stator.
Nestled in with the stator under the flywheel is the trigger (or timer base). Probably the most overlooked part of the ignition system the trigger sends a signal to the CDI unit (power pack or switchbox) and tells it when to release voltage to the coil. A faulty trigger will result in no or intermittent spark and is often misdiagnosed as a fault with the power pack. Additionally, a faulty trigger shorting to ground will damage a power-pack so be careful if fitting a new power pack.
7. CDI Unit
Depending on your outboard brand these units will go by different names but, they will all have the same function and that is to store voltage from the stator and release it to your ignition coil when the trigger tells it to. Often a faulty CDI unit will see intermittent or no spark on cylinders and may only breakdown when your engine warms up.
8. Ignition Coil
Receiving voltage from the CDI unit the coil will amplify voltage and shoot it through to the spark plug. Faulty or damaged coils will have weak or no output. We see a lot of coil failures where customers are using a spark plug that is not recommended by the manufacturer.
9. Spark Plug
This is the last component in your ignition system. The spark plug receives voltage from your ignition coil and creates a small electrical spark within your engines combustion chamber igniting your engine's fuel and creating the explosion that drives the piston to spin the crankshaft and the flywheel where the ignition process begins over and over again. Spark plugs can foul up with oil and carbon on two stroke engines resulting in poor spark. They are by far the easiest part to check first.
Fitting in somewhere between all this is your engines rectifier/regulator. It is always working and its job is to replace the voltage in your battery that you've lost due to engine cranking, using electric tilt and trim, running fish finders, lights or other accessories on your boat. It converts the AC voltage to DC voltage and regulates how much voltage it is putting into your battery. In a lot of cases, a rectifier/regulator also sends the signal to your RPM gauge (tacho) telling you what revs your engine is doing. Sometimes the first sign of a faulty rectifier/regulator is when your RPM gauge stops working and this is very common with older Evinrude/Johnson outboards.
There are variances here and there, but I’ve gone for the most common and basic approach on this occasion. I hope the above has given you some insight into how your outboard ignition system works. Keep these points in mind if you ever have a potential ignition problem and hopefully, it will make a diagnosis that bit easier. But remember, in order to diagnose ignition problems properly a Digital Multimeter with a Direct Voltage Adaptor (DVA) is a must have tool for accurate diagnosis. Another very helpful piece of material is the CDI Electronics Troubleshooting Guide, click here for pages of helpful information on almost every two stroke engine.
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No spark when warmBy: David on 5 October 2018Help please. I have a johnson v4 140 1985. It runs sweet but after about 15 mins and I go to neutral the motor cuts straight out. It wont restart unless I leave it alone for about 3 to 4 nins then it fires up and again runs fine. I can go in and out of neutral with no problems but after a few minutes when warm again it cuts out again. Any suggestions please. Cheers David
Outboard Spares Response
Hi David, from what you've said it sounds to me like a power pack issue. Power packs that are on their last legs will typically fail when they get warm (due to engine/block getting to temperature). I may be wrong, but that's where I would be looking. Cheers
1981 Johnson 50 hpBy: Tom on 2 October 2018Runs great for about 1/2 hour then out of nowhere it cuts out. Let it sit 5 10 15 minutes and it will run for a short Time again. I don’t believe it’s fuel and I can’t check spark because I’m always breaking down in a bad spot,
Outboard Spares Response
Thanks for sending in your problem. It's a tough one to diagnose. To me I instantly think that it sounds like a power-pack, but it could also be stator. If it's not ignition, it could be something as simple as the breather on your fuel tank being restricted and the engine running for a period but then starving for fuel. To check if it is a spark issue, you really need to get the engine to fault in a controlled environment and have a spark tester ready. A boat ramp on a quiet day is a good spot if there is no boat traffic. From here you'll be able to safely check if it is an ignition problem or something else altogether. Good Luck. Cheers
1989 Evinrude 70 hp cutting off at high rpmBy: jeremy crager on 25 September 2018Recently my motor started cutting on and off at higher rpm. Motor has ran fine up to this point, I took it out and it was real sluggish coming up in rpms, when it finally got up to 3000rpm range it started cutting on and off like I was doing it with the switch. I have changed the power pack being the easiest but it had no effect. Sometimes it will cut on and off and sometimes it don't.
Outboard Spares Response
Thanks for getting in touch. The problem could be your trigger. A bad trigger will destroy power-packs, so even though you've put a new power-pack on, if the old trigger is shorting to ground then it will have ruined the new pack you've just fitted. Ok, so a couple of other things you could try. 1) Disconnect the yellow wires from the stator to the rectifier and re-test. If the miss goes away then you've got a faulty rectifier. 2) If the engine does not have a miss below 2500 RPM then it could be a faulty temperature probe in the cylinder head (assuming your model has the S.L.O.W feature). Disconnect the Tan wire from the temperature switch and re-test. If the miss goes away then your temp switch in the head is faulty....HOWEVER, check that your engine does not have an over-heating issue and that your control box alarm/warning horn is working correctly. Normally when the engine overheats the alarm sounds and the S.L.O.W function kicks in and reduces the engine to 2500 RPM. Anything over 2500 RPM and the engine carries on a bit. 3) If none of the above work replace spark plugs and check coil leads for shorts to ground. 4) The problem could be a bad stator. Higher RPM problems generally indicate bad stator. There are a bunch of tests to diagnose this but you'll need a workshop manual, or pay a technician to test it for you. Good Luck! Cheers
Evinrude 70 1995By: Cuan Muyllaert on 15 September 2018Hi there, I have an evinrude 70HP with a tiny spark on the bottom cylinder. Not enough to fire but there is something going through. I've tested the stop circuit, blocking diode, changed the leads, checked the plugs put in new timer base, stator, power pack, coils. It's possible that I blew something in the new power pack because I replaced it before putting in a new timer base, but the resistance is what it should be on all the wires so maybe not? Only components left are the rectifier, wiring loom and ignition switch. Could it be any of these??? Thanks
Outboard Spares Response
Thanks for getting in touch. I think you've probably answered your own question. Replacing the powerpack before the timer base is typically not the best option as a bad timer base will destroy powerpacks. I had one trade customer fit 3 new power-packs (bought from a competitor) until I mentioned to him the trigger was probably the underlying issue. This was on the same model as your outboard. I couldn't be 100% certain but unless you've got a weak trigger magnet in the flywheel I'd say your powerpack is the cause. Good luck!
1973 20 hp Mercury 3669833 wont get up on plainBy: Darryl Ryder on 11 September 2018It runs great after i went threw the carb but still wont plain out, its on a 15ft 1987 dura craft semi V so it should push it pretty good. But it doesnt. what do i need to check next?
Outboard Spares Response
Thanks for your question. I may be wrong, but I think lack of Horsepower is most likely your problem. I'm not familiar with the boat model you have, however most 15ft boats require about 40hp as a minimum to plane. If it's not a lack of horsepower then it could be an engine height issue. One most boats the cavitation plate should be about level with the bottom of the hull. For example: If your cavitation plate is 4 inches below the bottom of the hull then you'll find the engine will struggle to plane the vessel. Especially a 20hp on a 15ft boat. I hope this helps. Safe boating. Cheers