Johnson HD-25 and TD-20 Troubleshooter's Index
Starting a TD-20 or HD-25 is easy. The process is similar and the motors are close enough in design to not confuse anything between the two...
First check
Make sure that there isn't any old fishing line or weeds or anything else behind your prop.
Remove the kotter pin on the prop nut
Remove the 3/4" nut from the prop shaft.
Remove the prop and check for fishline, etc.
Now you can inspect for lower unit leaks down there also.
Clean junk out and replace the prop, nut and kotterpin.
NEXT!
is to be sure that you have NO OLD GAS in the tank!
This will make your motor not run and need to be cleaned out.
Removing and draining the old gas (HD and TD models are the same for removal of tank) by removing the tank is a lot easier than cleaning the carb and everything else. CARB CLEANING by spraying a generous amount of carb cleaner spray into the carb will help if the motor has NOT been sitting a long time, in which case you should remove the carb 1/2" nut and flathead screw and clean a little more thoroughly.
I usually remove the carb, take off the bowl cover (three screws) and flathead screws on the bottom of the carb and spray a lot of carb cleaner through each screw hole. This a majority of the time, solves any carb problems.
If the carb floods/pours gas out of it and you're sure all the carb screws are in place, the float may be flooded. I haven't made many, but hand cutting to shape to match the original and coating with gasproof epoxy works well.
Make sure the gastank is clean as well as the gas valve!
I remove the mesh screen on the valve if it's really rusty or corroded and discard it. If the tank is clean, you won't have to worry too much abnout any foreign material getting from the gas to the engine block. Ideally it should be replaced, but the motors will work without the filter inside.
Make sure you thoroughly flush the tank of anything old, rust, dirt, gas, etc. If there is a lot of buildup in the tank, you can use a cup of aquarium gravel and shake it vigorously, flush with water and repeat until the sediment is completely gone from the tank. that has worked best for me in the past.
I've also found using a 20 ounce bottle of LAs Totally Awesome from the dollar store works wonders. Pour the entire bottle in the gastank with the valve closed. Place the cap back on and shake the solution around in the tank and let it soak for anywhere from a few hours to a few days. Flushing with plenty of water and thoroughly drying the tank will be of great value when trying to get the motor running again. Traces of water or dirt or old gas, especially, will be trouble and end up with a motor that won't run.
after you have flushed and completely dried the tank (you could air dry or use a hair blowdryer blowing hot air into the gas cap opening to speed things us a little)...
Being certain there is fresh gas in the tank (mixed approximately 5.5 ounces per gallon of gas for these motors or 24:1 ratio)
continue onward by checking your mixture screws, the top on the left

, and on the back of the engine on the right side,

Turn these in until they are flush, don't force them in too far or you will damage the needles.
Next, back them out approximately 3/4 turn only. Anything more than 1 turn out is too far and won't make your motor start any easier.
Open the gas valve on the left side of your motor.
Next, wait a few seconds for your carb bowl to fill with fuel.
Next, press the primer plunger on the top of your motor on your lefthand side.
Press this about 5 times.
You will hear an audible swish as the gas is pumped into the engine.
NOTE: GAS MAY SPRITZ WITH A BIT OF FORCE SO BE SURE TO HAVE "GRUBBY CLOTHES" ON as well as eye protection.
WHEN GAS PUMPS OUT THE BACK HOLE ON THE ENGINE, IT'S OKAY, it's showing that gas is getting inside the engine. If gas is pouring or leaking past the carb on the side or around the bowl or float, then the float may be flooded as mentioned above.
IF NO GAS COMES OUT, there is a block in the line or carb.
After priming...
Pull recoil/starter rope.
Motor should fire on third or fourth pull on a cold engine if everything is set right and there are no other problems.
rough running can be corrected a little with the mixture screws, the one on the back of the motor should be adjusted to correct rough idling. If it doesn't help, there may be an air leak/vacuum leak on the engine block somewhere.(read on)
I will add information as it becomes available or is encountered by myself.
The most common problem is that the motor won't start at all.
My experience has been dirty points as the first place to check.
This page is for TD-20 and TN series motors, but shows in general how to check for spark.
Next step to checking/cleaning the points is removal of the flywheel.
Though not necessary to disassemble, you may find it easier to remove the magneto plate from the engine.
But this page will show you the location of the points and it's pieces and locations of pieces.

Rough running motor checklist.
If your motor is running, but not right, below is a list of some things to check.
Spark Plug with water on it
This problem occurs when a gasket or head is bad.
Symptoms: motor won't start easily or at all.
If gas is flowing, check for spark.
If no spark is observed, clean and gap points
If gas is not flowing, check for plug/dirt in line, carb, or tank including valve.
If starts, runs rougher than it should, lots of grey/blue smoke that simply doesn't smell right.
This typically occurs if there is a leak on the block or crankcase as seen on this same page.
KEEP IN MIND, if your motor hasn't been disassembled and cleaned all the way inside and out, a certain amount to a large amount of blueish/grey smoke is normal. consider that 'back in the day' 30w oil was mixed with gas very heavily, so a lot of old buildup can created 'burn off' of the old carbon, etc on the pistons and rings.
off smells can usually be caused by water getting past one of the gaskets.
Keep in mind that a rough running motor can often times be a simple tiny piece of dirt in the carb.
I've found that running the motor for a while clears this up in many cases.
If running fine then revving up or quitting, check that the impeller is functioning properly and you are getting proper engine cooling.
also, make sure point gap and spark plug gap are correct.
rough running can be caused by frayed or broken spark plug wires also.
low speed idling should be able to run so slow that the motor is barely firing once a second, acquired by turning the "L" shaped slow speed idle on the engine block to less than 1/2 turn from seated.
a properly cleaned and tuned up engine will idle to a slow trolling speed, the 2.5, 5 and 10's can all idle down and troll nicley.
HD-25 INDEX