Not firing one cylinder

[dburr]

Help.... The number four cylinder on my merc 50hp 500 is not firing. How do I find the problem part? Where do I start?

 

[achris]

Re: Not firing one cylinder

Does this engine have a distributor and one coil, or a power pack and a coil for each cylinder? And are we talking about no spark at the plug or not firing on that cylinder. If you have spark but not firing, then I would suspect the lower crankshaft seals.<br /><br />If its a dissy, then the only thing it could be is the dissy cap, the plug wire or the plug.<br /><br />If it has a power pack and coil for each cylinder then it could be the power pack, coil, plug wire or plug. Swap the coil (and plug wire) on the faulty cylinder with one of the good ones and re-test. if the faulty follows the coil then it's the coil, if not then it's probably the power pack. <br /><br />Have you put a known good plug on that plug wire and checked the plug?<br /><br />Chris.........

 

[dburr]

Re: Not firing one cylinder

Its a merc 50hp 500 thunderbolt. Basically the number 4 cylinders spark plug is brand new and doesnt look like its been getting spark. I say this cause I replaced all four and the other three look like there getting used. I pulled the number 4 one and it was soaked with gas and still very clean. The engine has a single coil with a switchbox that feeds the distrubuter. The person who had it before me put a new wire on the number four thats not firing. Would it be the switchbox or the distributer. Can you give me a little insight on how this configuration operates?

 

[KCLOST]

Re: Not firing one cylinder

I need the serial# for your motor....<br /><br />Something doesn't sound right, on what you have.

 

[KCLOST]

Re: Not firing one cylinder

Dburr,<br /><br />You should have a "stator" (under the flywheel), a "trigger assy" (under the flywheel), then a "switchbox", then "four coils" one for each cylinder which has the spark plug lead leaving it going to each plug.<br /><br />The stator sends current to the switchbox, which charges it. The trigger sends a "trigger" signal to the switchbox to release an electrical charge to each coil at the proper time, "hence the term, Timing". The coil then conditions the charge and releases it to it's spark plug lead (which goes to the plug).<br /><br />IF you don't have spark coming to the #4 spark plug you need to back track through the electrical system to find the culprit. Start with the coil (like achris mentioned) switch the #4 coil with another and see if the condition moves to the other cylinder you switched coils with. If it does, you need a new coil.<br /><br />The next step would be to mess with the switchbox leads and see if you can find out if your getting no charge out of one of the leads leaving the switchbox. We'll get to that next...<br /><br />Let us know...

 

[dburr]

Re: Not firing one cylinder

Can I send you a PDF file of the exact wiring and components I have?<br /><br />It shows a single wire feeding a switchbox, a single coil assembly feeding the swithbox with a green and black wires. Then three wires go to ingition driver it calls it which has 4 output leads on it going to the cylinders. It also attaches to the flywheel gear assembly......Does this make sense? I have a official drawing from Merc, which shows all this. Pleas help I need this bugger going for the weekend......

 

[emckelvy]

Re: Not firing one cylinder

You have the older "Lightning Energizer" ign setup. Unless you have some problem in the dist cap itself (i.e. cracked around #4 or carbon track), you may have a problem with the lower crankshaft seals. If the seals are bad, it'll make #4 idle poorly and may suck water into the cylinder.<br /><br />You can check to see if you're actually getting spark to #4 by snapping an inductive-pickup timing lite around #4 spark plug wire to see if it's firing.<br /><br />If so, likely you have other problems such as the lower crank seals.<br /><br />The carb which feeds #4 also feeds #3 so I doubt the problem would be found there.<br /><br />It's possible a leaking fuel pump diaphragm could over-rich that cylinder, you'd have to check to see where the fuel pump is getting its vacuum supply. A leaky diaphragm will allow the cylinder supplying vacuum to the pump to suck excessive fuel back into the cylinder; then it won't run due to being too rich. You can easily check this by examining for presence of fuel in the vacuum line to the fuel pump.<br /><br />A cracked reed valve could also cause problems, but it would be spitting fuel back out of the carb. You can put a mirror in front of the carb's throat while the engine's running to check for this problem.<br /><br />You'd need to pull the powerhead to check the crank seals, but it's possible to pull the lower end cap and replace the seals without breaking the crankcases apart.<br /><br />HTH and Good Luck........ed

 

[dburr]

Re: Not firing one cylinder

I dont think I have a water problem. The compression in the cylinder is good and the spark plug has no evidence of water, its strickly just soaked with gas......At least I dont want a water problem. What about the switchbox you never mentioned that. could it be that...... I phoned the marina and they want two hundred and some dollars for a new one. Is there anyway of fixing a busted swichbox. What about the coil. It has just one that feeds the switchbox.....Or would you assume since it fires the other three the coil is good, likewise for the trigger.....And i checked the fuel pump diaphram and it looks good, gasket too.

 

[achris]

Re: Not firing one cylinder

dburr,<br /><br />Compression is not an indicator that you're not getting water. That very clean plug and not firing is an indicator of water, and it'll be coming in through the bottom seals. <br /><br />Powerhead off and change the seals.<br /><br />If the switchbox was faulty you would not have spark on any cylinders, same for the coil and trigger.<br /><br /><br />Chris..........

 

[dburr]

Re: Not firing one cylinder

Dam, thats not what I wanted to hear. How hard is it to change this said seal. <br /><br />Its cut and dry like that, forget about the ignition system?<br /><br />This (#()*#*(ing motor is going to be the death of me.<br /><br />I still havent checked for spark out of the cylinder, if it doesnt though what should I do, or are you saying its absulutely impossible for this to happen since I have spark on the other three. <br /><br />Thanks for helping me here Chris I really appreciate it.

 

[dburr]

Re: Not firing one cylinder

Oh ya another thing, can I run it like this for awhile or will it cause more damage to the cylinder?

 

[achris]

Re: Not firing one cylinder

dburr,<br /><br />Pull all the plugs out, reconnect them to their leads and lay them on the engine block. Crank the engine and check that all four are sparking. Then put them back in the engine and run it. One by one pull the lead off each spark plug, you'll notice a drop in rpm. If you don't get the same drop on the bottom cylinder then that cylinder isn't running. Now swap the plug with one that you know is working. If #4 still isn't running you are now absolutely postive that you have not got a spark issue. Pull the powerhead off and replace the seals.<br /><br />Running the engine with the leaking seal will damage the engine. That water is getting in passed the seal, around the crankshaft, including the bearing surfaces, and into the combustion chamber. Rust on the bearing surface will kill the crank very quickly.<br /><br />Chris.............

 

[KCLOST]

Re: Not firing one cylinder

You need to check the darn spark on that cylinder!!!!!!!<br /><br />I've been trying to tell you on this post and your previous... It's an easy check...<br /><br />I recommend a spark gap tester, that you can buy at any auto parts store. It runs about $10.... Save you from getting shocked, and possibly damaging other electrical components if the spark can't hit ground.<br /><br />By the way achris, he doesn't have a trigger assembly, so we can at least rule that out.

 

[emckelvy]

Re: Not firing one cylinder

Concur, you MUST determine whether or not that cylinder is sparking under load. You can have a decent looking spark with the plug laid out on the block, but under compression you may find the spark is blown out.<br /><br />Not so likely on the Lightning Energizer ign, but you are trying to determine, via a progressive troubleshooting process, what your problem is.<br /><br />If you don't know whether it sparks or not under compression, you don't have enough information to determine whether to proceed to powerhead removal or other courses of action.<br /><br />The idea of the 'sparky' tester is a good idea, they're hard to see in the sunlite but show up good in the evening or in the shade. My suggestion of a timing lite will work any time and will definitely show you if you have fire at the spark plug, under running conditions.<br /><br />We're not saying it's impossible for #4 to not have spark if the other 3 do, but it's highly unlikely. Still you need to check it out to be sure.<br /><br />Another good point made is, if the plug is being washed clean of carbon etc, water does a very good job of doing that! <br /><br />Many of us have Been There, Done That and are just trying to get to the root of the problem in a logical manner.<br /><br />HTH and Good Luck...........ed

 

[achris]

Re: Not firing one cylinder

KCLOST,<br /><br />Sorry to disagree with you, but he does have a trigger. It's a magnetic (hall effect) sensor in the dissy and fires the powerpack as the current direction changes. It fires the powerpack 4 times per crankshaft revolution and it's part number is 393-2918A1<br /><br />Chris..............

 

[dburr]

Re: Not firing one cylinder

Sorry guys for make this hard. Im on business right now away from home, thats why Ive been asking so many questions so I can return home and quickly find the problem. I understand exactly what you guys are talking about, hopefully narrowing down the possibilities and going after the most probable cause. I dont trust the wires that much to touch them while the engine is running, so maybe I will throw my oscilloscope on them to see if they are bleeding voltage first... I will probably try to get a timing light from a budding as one of you have suggested. So correct me if Im wrong, I will pull plugs one by one and verify that they have spark out of the cylinder. Then put them all back in and verify they have spark in the cylinder with a timig light. If the number four fails under load then its the crank seals. How involved is this changing of the crank seal. Im an electronics engineer and not a mechanic.....<br /><br />Is this something i should consider having done at the shop?<br /><br />I leave for vacation on Saturday with the boat no matter what. I will be camping for a week probably using the boat every day, do you guys think this may ruin the engine? I just redid the carbs last weekend and put all new fuel line on it, it starts fairly easily, idles fine, and does have enough power now the carbs are clean to put the boat on a plane with me in it. Does this change anything?<br /><br />Thanks again guys

 

[dburr]

Re: Not firing one cylinder

Oh ya, another thing... I checked the gear oil and its fine, no water leaks in there. It was pressure tested as well and was fine. So I assume that you guys are saying the water is coming from the cooling system. Whats the exact name of this seal or seals. Part numbers would be great but I guess Im not much help cause Im not sure the exact year (late 70's) and dont know the serial number.......Dont hate me guys...hahaha<br /><br />If you have any electronics questions I would be happy to help....

 

[achris]

Re: Not firing one cylinder

The name of the lower crankshaft seals is 'Lower crankshaft seals'. The part number is 26-41953 and there are 2 of them.<br /><br />Chris.........<br /><br />Do I need to use a diac to control a triac in a motor speed controller?

 

[dburr]

Re: Not firing one cylinder

thanks Chris.... How long of a job is this. I just read that it could be a exhaust plate as well... This thing has a thru hub exhaust does that mean theres no exhaus plate on this hemmy.<br /><br />You need a diac to fire a triac, a diac is a bidirectional diode with unique properties to fire Triacs or SRC's. They have a reverse on resistance........

 

[achris]

Re: Not firing one cylinder

dburr,<br /><br />I would allow about 3 hours for the job. BUT that's in a workshop with all the required tools at hand and assuming I didn't hit a snag, like a broken bolt or 3 or another problem found in the process. To do it at home I allow at least double that. To do the exhaust plate gaskets at the same time is a very good and sensible idea, provided the bolts all come out ok. The 'thru-prop exhaust' is nothing to do with the powerhead.<br /><br />Chris.............