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4hp woes

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caleb2003

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Got hold of a 4hp Mercury possibly 1978, cannot find the exact year on the serial number sites listed (s/n 8072749).

Won't start. Has spark, compression is 90 psi on both, seems to backfire when trying to crank and some smoke comes back out of carb (Carb has been cleaned and float seems ok etc).
Found a manual which states the rich/lean needs changing to be richer but this doesn't help.
The original model had a fuel tank which isn't there now so using a remote tank with primer bulb. After lots of reading on here, I checked the crankshaft key which is intact and not worn.

Anyone have any other ideas and also maybe any info on this motor serial.

Many thanks
 
That looks to be a australia serial number just a guess puts it as a 1979 gnat motor. They only produced the 2 cyl 4hp for a few years and they are real problematic when it comes to troubleshooting and parts. Did you check spark with a spark gap tester? It must jump a gap of at least 3/8 inch open air gap looking at the plugs tells you nothing.
 
kimcrwbr1 said:
That looks to be a australia serial number just a guess puts it as a 1979 gnat motor. They only produced the 2 cyl 4hp for a few years and they are real problematic when it comes to troubleshooting and parts. Did you check spark with a spark gap tester? It must jump a gap of at least 3/8 inch open air gap looking at the plugs tells you nothing.
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Hi, thanks for the reply, yes I'm in Australia.
I don't have a spark plug gap tester although it does emit a good spark, I tested by increasing the gap and visually checking the spark (after grounding).


Thanks
 
That is all a gap tester does can you get a brite blue spark holding the wire 3/8 inch from a good ground?
http://www.ebay.com/itm/CDI-Electro...ash=item5428771ce3:g:OD4AAOSw~otWc0C-&vxp=mtr
Check the resistance of both stator coils. The idle air preadjust is 1 1/4 turns out from lightly seated. These motors are real finicky with sync and link adjustments and it must have a good spark and compression before anything can be done with the fuel adjustment. You will need a good manual and take your time. Turn the idle air needle in until lightly seated and back out 1 1/4 turns. Make sure all the linkage is tight and the trigger is working smooth. Check the ignition coils primary and secondary windings are good. Another test is to put a little fuel mix directly in the spark plug holes one at a time and see if she pops. Crankshaft seals will cause starting issues if there is a bad seal (crankcase precharge) is real important and difficult to nail down.
 
kimcrwbr1 said:
http://www.ebay.com/itm/69498-reed-...ash=item23481cca1f:g:A8UAAOSwkNZUobjD&vxp=mtr
Another possibility is the reeds are dirty bent or broken. It is easy to make a new gasket, pull the reed block and see what you find?
Are the spark plugs wet with fuel mix when it coughs? A leaky diaphram in the fuel pump will flood the motor see if it has a hole in it?
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Hi, the gasket seems to have had some work done previously and reattached with some kind of orange sealant
 
I never put silicone anywhere on a outboard motor. Pull the reed block and clean and inspect it good. Get a jar of permatex aviation gasket maker and cut out a new gasket or buy a new one. then lightly coat the gasket on both sides and snug the reed block down. Let it cure for about 24 hrs before you try to start the motor. Fuel and silicone is never a good match that could very well be where your losing crankcase precharge?
 
Some pictures for clarity

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Are the plugs wet with fuel or dry? Does it fire if you put a couple tablespoons of fuel directly in the spark plug holes. I use a length of vacuum tubing and dip it into the fuel tank and plug the end with your finger stick the tube in the plug hole and release your finger. Quickly put the plug in and see if she starts briefly. That narrows it down to crankcase and carb.
 
The lower plug is fouled but the upper one is fairly dry, tried the test but no start at all, is it worth initially just getting new plugs, I did notice the spark was more of a white than a blue if that helps
 
New plugs yes. Does the spark jump a 3/8 inch open air gap? You can make a tester with a wire and a board clip the one end to engine ground staple a 3/8 gap in the middle of the wire and stick the other end in the spark plug boot. Its easier to see the color in the dark but even in the day you should hear a nice crisp snap. That being a CD type ignition it will knock you on your ass if you get bit. Once a plug fouls it needs replaced and you need to replace the plugs at least yearly. More often if you use the motor less. Are you using fresh fuel mix in the tank? Just a small amount of water in the carb will give you all kinds of problems. Drain the float bowl in a small dish and see if water is in the carb?
 
Just been out to get plugs but none of the books in the shop had the model and the correct serial number/spark information so I will have to find a marine shop when they open. Changed the fuel and recently cleaned out the carb. I actually took this to a Marine mechanic but after waiting 3 weeks he seemed to do not much with it, although he claimed to have got it started but not easily, I think he didn't want the job as he was only focusing on the big jobs but he did clean the carb.
 
I've had a look at that key before, it seems ok, you can just about see it in the last picture above, it has a straight edge with a curved back (to fit in its slot). Thanks
 
All it takes is a little crease in the key to throw the timing way off. Turn it upside down and see if it helps? Be sure and torque the flywheel nut to spec.
 
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