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Merc 4hp missing

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bcripps

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Trying to diagnose a Mercury 4hp-2stoke that misses during acceleration and always has since I owned it. Compression pegs out at 105 psi. Replaced plug, filter, carb, fuel pump, tank, hose, and ran three tanks of premium gas through it. I think the fuel system is good. Coil: Primary Winding Resistance- 00.8 milli ohms. Secondary Winding Resistance- 6.29 milli ohms. Used a pencil in the spark plug hole to bring the piston up to TDC. The mark on the fly wheel is about ¾ of an inch off the mark on the casing. Manual suggests that the fly wheel has been installed wrong (is that possible?), or that the key has been sheered off. The engine is only about 5 years old, starts on third pull, runs well at speed, and has never been run aground. Is sheering off the key common? That seems unlikely to me but I'm not a mechanic and don't have a fly wheel puller to check. Just looking for comments or ideas on how I should proceed. The closest Merc dealer is about 800 miles away so need to fix this bugger myself. Much thanks, in advance and all best for the holiday season. Bry
 
Thank you for responding. Serial Number: 18680388 Model Designation: 1004201EK. Engine was purchased in Santo Domingo, Dominican Republic.
 
Everyone has trouble with that Serial Number including Mercury's Caribbean Rep, Miami Office. I order parts by the 'year' (2016), and that seems to work. Their Rep figured out which service manual to use and I bought that along with spare parts. I've tried to upload a couple of photos but the wifi signal is too weak presently. I'll try later in the day and send one at a time. If they go through, you'll see I have converted the fuel system to run off an external tank. Reducing the 3/8 fuel line down to the ¼ inch inlet on the fuel pump and including a fuel filter meant for a lot of connections and reducers. I thought my problem may be an air leak so I ran a length of fuel line from a bottle of gas directly to the fuel pump and the engine still ran rough.
All best, Bry
 
No response from fitz73222. It's the Holiday Season so hopefully he is with family and friends. Anyone else care to respond, you're input would be much appreciated. Bry
 
Okay... that's interesting. Really interesting. The engine was bought from a Mercury dealer in Santo Domingo. And the replacement Mercury carb I bought was a perfect match. The Mercury Service Manual I bought for the 4hp-2stroke seems to correspond to my engine. You've got me a bit confused and not sure how to proceed. But please, more of your thoughts... Bry
 
Look on the Nissan / Tohatsu parts books to see if there is a similar motor.-----Boat.net----Is one place to look.-----Some ( not all ) of these small motors all came off the same assembly line !!-----Painted white on Monday for Johnson , blue for Evinrude on Tuesday. black on Wednesday for Mercury.----Simple marketing concept.
 
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Thanks racerone, Yes, the 4hp-2stroke Tohatsu listed on Boat.Net certainly looks like my Mercury. Could easily be one and the same engine. Must make reviewing outboards for boating magazines difficult :) I couldn't, though, confirm my SN# (1004201EK), is a Tohatsu number. I have sent the photos connected with this thread to Mercury in Miami to see if they can shed some light on the matter. But referring back to my original post, the engine misses in the mid power range, not just during acceleration. And it makes little difference if the engine is warmed up. I think my fuel system is good, unless there is a problem with the Low Speed Jet or the Main Nozzle. If you can confirm my coil is good, I can only think it must be a timing issue. Is sheering off the fly wheel key common? Should I buy a timing light? Thanks again, racerone... Bry
 
Trying to diagnose a Mercury 4hp-2stoke that misses during acceleration and always has since I owned it. Compression pegs out at 105 psi. Replaced plug, filter, carb, fuel pump, tank, hose, and ran three tanks of premium gas through it. I think the fuel system is good.
Coil: Primary Winding Resistance- 00.8 milli ohms. Secondary Winding Resistance- 6.29 milli ohms.

Download the Tohatsu service manual and fault diagnose electrics as per Tohatsu 5hp 2 stroke

https://www.tohatsu.dk/forhandler/Service/Service Manual_1-2 Cylinder 2-Stroke.pdf

If you struggle with the download let me know and I’ll post the info you need up but your coil figures look very suspect to me at 00.8m Ohms and 6.29 m Ohms be sure you are checking it properly and begin by starting at the exciter coil and working your way out to the spark.
 
Thanks Tony Outboard for joining in. And thank you for the Tohatsu Service Manual. I had no trouble downloading it and am very happy to have it. I went directly to the Ignition Coil Specs to compare them with what is outlined in my Mercury Service Manual: Tohatsu 5hp: Primary Winding .26 - .38 Ω Secondary Winding 3.0 – 4.4 KΩ Mercury 4hp: Primary Winding .02 - .38 Ω Secondary Winding 3000 – 4400 Ω I'm not sure I understand the discrepancy between the Primary Windings but in any case, my results are different across the board. I will retest the coil on my engine tomorrow using the test procedure outlined in the Tohatsu Manual and verify my results. Again, thank you for your help... Bry
 
No probs Bry, you will most certainly need a dva adapter for your multimeter to get to the bottom of it so if you don’t have one, may be worth ordering one, I would most certainly suggest starting at the exciter coil
and while you are testing, remove the ignition coil and check it on the bench.
 
bcripps said:
Thanks Tony Outboard for joining in. And thank you for the Tohatsu Service Manual. I had no trouble downloading it and am very happy to have it. I went directly to the Ignition Coil Specs to compare them with what is outlined in my Mercury Service Manual: Tohatsu 5hp: Primary Winding .26 - .38 Ω Secondary Winding 3.0 – 4.4 KΩ Mercury 4hp: Primary Winding .02 - .38 Ω Secondary Winding 3000 – 4400 Ω I'm not sure I understand the discrepancy between the Primary Windings but in any case, my results are different across the board. I will retest the coil on my engine tomorrow using the test procedure outlined in the Tohatsu Manual and verify my results. Again, thank you for your help... Bry
Click to expand...
I have a few coils here new and used which I know all work, if you’re wondering at all let me know your resistance figures on the bench and I’ll cross check on my spare coils. It will certainly help the dva tests if you have an electric drill rigged up driving a socket on the flywheel nut so that with the starter and plug removed, you can drive the engine at a consistent speed to check the outputs.
 
Oh boy. That's thrown a wrench into the works. No I don't have a DVA Tester; never heard of one. And I won't be able to locate one locally. It will take a month to six weeks to get one shipped in from the States. Anyway just ran my test with the multi meter again and confirmed the results. Primary Winding– .6 milli volts. Secondary Winding– 6.29 milli amps. The coil has a Part Number: 1G 3930 160211 Much thanks for the help sorting all this out... Bry
 
Hi Bry, are all those figures with the coil removed and the terminals scrubbed up with a bit of scotchbrite or something or are they taken with the coil in situ, confirm you read primary 0.6 milli ohms and secondary 6.29 milli ohms ?
 
Just to note, when checking the coil on the bench use the tag with the blue arrow to it as a ground, the tag is slightly sprung out and does not contavt the core unless it has a bolt through it snugging it up to the engine, I've also had problems with an accumulation of crud between the tag and the core giving high resistance readings. You really will need a DVA meter to get to a conclusion
https://www.ebay.co.uk/itm/362782990898
coil tag.jpg
 
Just an aside, when you say the engine is "missing" do you mean it's hesitating on acceleration and in the mid range and it's OK at higher revs? If so, what colour is your spark plug afetr a bit of idling, it's possible the fuel air mix needs a tweak as the plug is getting a bit too much carbon build up on idle, or the carb needs a good clean out. Try starting and running the engine for 5 minutes at idle and then pulling the plug and checking the colour. In particular see 7. Below
2021-12-10.png
 
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Yes, failed to mention that I pulled the coil from the engine before taking the readings with my multi meter. And, again yes, I cleaned the contacts with a good scrubbing with emery cloth and I jammed the probe between the "tag" and the contact. Readings: Primary Winding– 0.6 milli ohms; and Secondary Winding– 6.29 milli ohms, confirmed. The engine runs rough in the mid power range, not just during acceleration. And it makes little difference if the engine is warmed up. It starts fine, idles fine, and smooths out at maybe ¾ throttle and above. The carb is new, less than a year old, and installing it didn't alleviate the problem. And I've backed the mixture screw out so far I was afraid it would fall out! To my eye, the plug color looks good but I will do as you recommend and let the engine idle for five minutes, pull the plug, photograph it, and include the pix in my next post. Hey Tony, you're a friggin' brick. Thank you... Bry
 
Hi Bry, let's see how it goes with the plug but 6.29 m Ohms is very odd, try the mixture screw/plug colour first and let's go from there, as a guide it should be sensibly seated fully in and backed out about 2 turns as a start point.
 
Here's a photo of the plug, pulled after running the engine for five minutes. The plug is wetter than I expected but no sign of water. Engine idles fine. Bry
 

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I’ve just finished a similar engine which had similar symptoms because it was running rich and the plug was fouling, slow to accelerate had to modulate to get any revs up. It’s tough to see with the plug having some moisture on there but it does appear to be running rich. I would be very tempted to run through the steps from 4 onwards on the idle setting above, I would wind the screw in fully, back out 2 turns and run it for a little bit, compare the plug then to what it is now and then go through step 4 onwards. No matter what, you will need the carb set up properly before diagnosing any other running problems.
 
Fuel Mixture: 3 oz oil to 1 US gallon of regular gasoline. When adjusting the mixture screw, I typically wind the screw out until the engine starts choking and then turn the screw back in a half turn. Things usually smooth out at that point and the engine is easy to start. I have only witnessed the wet plug this morning, after letting the engine idle for five minutes. In any case, running rich or lean has never made any difference to the mid-range lack of performance for this engine. Bry
 
Excuse the daft Brit but is that 50/1 or 100/1. I’ve just rebuilt the same engine model and it runs sweet as a nut. I’ll run it at idle tomorrow and the take a piccy of the plug and post it up. My thoughts are around the carb setting first as you posted earlier that you had the screw pretty much out without it making too much difference.
 
50:1 Good morning. Adjusted the mixture screw. It was 1-3/4 turns out. Turned it out 2 complete turns from lightly seated position. When you say run it for a little bit, do you mean 5-minutes again, or a day or two? Bry
 
I would say just 5 minutes you are looking for a comparison to your first picture so replicate whatever you did for the first run. This is some piccies of mine which I’ve just run for 5 mins at idle.08675E55-C29C-4DA5-98C2-B9AD77A9AD5F.jpg
 
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