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Merc 4hp missing

A little “clean” but it is a new plug, it would be worth you scrubbing the plug with a small wire brush and some petrol before installing and running it just to make sure you are starting with a clean plug and will have a very good idea of what is going on when you look post running.
 
[FONT=Times New Roman, serif]Gosh! That almost looks too clean. Okay, run mine for 5 minutes and pull the plug for a look. And, out of curiosity, what's the part number on your coil?[/FONT]
[FONT=Times New Roman, serif]Thanks, Tony... Bry[/FONT]
 
Okay... cleaned the plug and turned in the mixture screw another full turn. This is where Mercury says the starting point should be: 1-1/2 turns out. Engine started and idled well. I took the boat for a quick run up the harbour but the problem still persists. Missing at mid range, during acceleration as well as running at mid speed, even with the engine nicely warmed up. Starts to smooth out as I approach wide open throttle. Pulled the plug when I got back: Dry and ash gray in colour. Sure appreciate your thoughts... Bry
 
So a recap, this engine has always had exactly the same problem, you have replaced the carb and that made no difference at all and it also had exactly the same problem before and after you modified the fuel system. When you say the engine is “missing” does this mean that when you add some throttle, the engine hesitates to accelerate and almost feels like it wants to cut out and it does the same thing when sat in the mid throttle range, if all that is correct I would still be looking at fuel initially, purely because you don’t have access to a DVA yet to allow you to fully diagnose the electrics. When you are sat in the mid range i.e. there is no change in demand to the carb, what you are asking it to do is a constant. Will the problem persist or will the engine sort itself out ? For the fuel system I would strip and thoroughly ultrasonically clean the carb looking for water in there when I disassemble it, make sure the float height is good make sure of course there is no crud in there, at the same time what is the resistance on your exciter and trigger coils ?
 
If the problem occurs at mid to high speed operation I would look at the high speed fuel circuit and check the hi-speed jet (if it has one) and the float level.
 
Thank you, Quicksilver, for joining in. Been raining here which is a double whammy... curtails my outdoor activities and usually interferes with my wifi signal. I choke, start the engine, immediately push in the choke lever and let the engine warm up a minute while I untie. When I apply throttle, the revs pick up for a second or two then drop off, then pick up again and drop off. What my father used to call "Kangaroo Petrol." Things don't improve until I'm close to wide open throttle. I don't get the feeling it is going to die. And with no demand on the carb, running at a constant half-speed, it runs rough. The problem does not eventually sort itself out. If the engine is warmed-up, I usually just gun through the mid range and run at speed. In the beginning, I cleaned and rebuilt the carb using a rebuild kit with new jets included. I switched over to an external fuel tank, so new tank and replaced all the fuel lines between the tank and the fuel pump. Bought and installed a new carb and fuel pump. I'll be the first to admit that "new" doesn't always mean "set-up and in good working order." But I can take a look inside and check the float. No ultrasonic cleaning capabilities here but I have carb cleaner and compressed air. Tony, do I have to pull the fly wheel to check the resistance on the exciter and trigger coils? Thanks, guys... Bry
 
They are all good against the figures in the manual you have downloaded Bry. You mentioned earlier about replacing fuel lines and fitting a smaller diameter, was the problem present before you did that ?
 
I didn't reduce fuel line size. I increased the size to accommodate an inline fuel filter. I was concerned about an air leak with the additional fittings so by-passed everything and ran the engine from a bottle of fuel with a short length of hose attached directly to the pump. That made no difference in performance. Still raining here... day four. I dung out my old carb and saw that I replaced it in early November 1920. I stored it away clean with new jets, etc. Took it apart and cleaned it again, checked the float and the mixture screw. Everything seems to be in order. I'll install it tomorrow if I get a break in the weather and let you know how the engine runs. Thanks for bearing with me... Bry.
 
Run with a timing light and see if there is steady spark to full throttle.-----Checked crankcase compression ?
 
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Hi Bry, bloody hell don’t move to Devon, it rains for 4 moths at a time sometimes ��. Sound move with the carb, let us know how it goes Bud.
 
Been working on the Merc all morning and wish I hadn't touch it. It starts runs for about two minutes and then dies. Switched carbs back and forth, makes no difference. Fuel filter is full. I don't see water or dirt in the fuel I removed from either of the carb bowls. Ran it with the fuel tank top removed. Does this thing have a power cut off if it over heats? Bry
 
Four months of rain? Devon reminds me of why I left Canada. Snow drifts vs sand and palms. It wasn't a hard decision. About ready to start in on the Merc this morning. As stated, it starts, runs a bit ragged for a couple of minutes then the revs increase just before it croaks. Classic "surge and die" syndrome I have experienced with diesel boat engines and usually caused by an air leak in the fuel system. Maybe when swapping out carbs, I disturbed something. I'll start be dismantling everything between the fuel pump and the tank, check and tighten connections. Any suggestions, as always, much appreciated... Bry
 
Checked all fuel system connections and don't seem to have air leaks. But to be sure, I bypassed the fuel system by running a different fuel line directly from the tank to the fuel pump with an inline filter. Engine started and ran a bit ragged for almost ten minutes. When the revs started dropping off, I was able to compensate by squeezing the bulb. I didn't notice the "surge before dying" syndrome this morning. Engine finally wound down and stopped. I found the carb to be quite wet under the baffle cover (air cleaner?), like blow-back through the carb. Reed valve? If that's a possibility, how do I check? ...Thanks guys ...Bry
 
Sorry for the delay, been a bit busy fishing ��. A surge followed by cutting out is pretty much always running out of fuel so it leans itself out before it stalls. You have stripped, cleaned and sorted out your fuel pump haven’t you?
 
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