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Rough at any speed. Old 25 HP 2 cycle

A timing light will tell you (sometimes) if your miss is due to lack of spark - if you can see the miss (I've never been very good at seeing it myself).

A spark tester, available from many discount auto places, will allow you to see if you have a strong spark (check to see if it can jump a 7/16 or 1/2" gap).

That won't tell you if you have an intermittent problem causing the hiccup but might point you in a direction to check next.

If your spark plug coils are original they would be high on my suspect list. They do develop very fine cracks and the result is what you note (but there are other possible culprits). You can wipe them down really well (usually gooky after a number of years and look for spider web style cracks in the case)

Some guys will suggest using a spray bottle and some water to try and see "power leaks" - I tend to 1) run the motor on a flusher with the hood off, listen at the back end for "snaps" or 2) do the same thing at night and look for "flashes" when the power leaks out of the coil hitting the engine block (the power just wants to go to ground and doesn't care whether it jumps a sparkplug gap to get there or not).

If you need to replace the coils I would go with the Sierra cross-ref coil instead of the OEM Merc coil. The Sierra's a less expensive and produce more power than the OEM coils (although Merc used to have an optional "high energy" coil that was equivalent to the Sierra's power which they sold for 25 horse and under models - don't know if it's still on the market or not).

If the coils are fine then you would need to start eliminating the rest of the ignition components. Ohm readings can be done on all the other parts, save the switchbox, and if any are out of spec that would lead to doing DVA tests to check for correct voltage - the ohm tests are a kinda quick "working/not working/not working well" indicator.

Other things besides the ignition can cause misses/pops/sneezes/coughs on the portable line. A blown powerhead base gasket, a blown intake transfer cover gasket, a bad carb gasket or bad upper or lower crank seal can also result in a hitch, cough or sneeze. So don't stay totally focused on the ignition if it checks out fine...
 
The "super 15" (the super didn't apply to the 20 or 25) was just built on the 20/25 horse platform (24.4 cube block) compared to the (normal) 15 which shared a platform with post-1994 9.9's (16.0 cube block).

So the "family" was:

20 horse Pro or Marathon, 25 horse Pro or Marathon and the Super 15 (which is/was kinda the idea behind the "high output" or "pro-kicker" models that followed)...
 
Thanks Graham. Plug wires are good. Cleaned up the coils. No visible short paths. Pulled plugs, grounded them and had a nice blue spark.
I don't believe this was a problem last summer although it might have run rough on a few occasions, so I am worried that it may be one of
your gasket suggestions. That is beyond my scope of brilliance. Since anything can be intermittent, I may go ahead and replace the coils.
Thanks again for all your help.
 
Good move. Coils don't last forever and I've seen many of these fail; they'll give good spark at cranking--maybe even starting and idling--but die at full throttle..

Jeff
 
I used to work on portables quite often (and own three 86, 87 and 91 models). Many have been replaced with 4 strokes now.

Having said that, the four most common issues that I used to see that resulted in a cough/sneeze/backfire/hitch were:

1) coils - as stated early, they crack and leak power

2) powerhead base gasket - more often then not would blow out in the front somewhere, (under) the carb and it would sneeze/hitch/pop

3) transfer cover gasket - I could never really figure exactly why they bothered putting a cover here (could have molded the assembly without it). Ok, it's where the fuel/air/oil passes from the crankcase into the intake ports but aside from giving you a very limited peek at the cylinders/pistons I can't see what other purpose it serves.

The cover gasket does blow out though on occasion, so some of your air/oil/gas gets blown back out through the cover (when the gasket is shot) before the piston travels far enough to cover the intake port. That causes some lean sneezes, hitching and generally running like crap conditions particularly at lower rpms (give it enough gas and it will stay running - will almost always stall when you back off the throttle).

And number 4 on the list of "most common" was the upper crank seal. It's part metal (cup) with a rubber (insert) that has a round spring molded into it (spring hugs the rubber to the top of the crankshaft, just below the flywheel). The rubber tears and some of your air/oil/gas gets "blown out" of that leaking seal when the piston is trying to push it through the transfer port (assuming that gasket is holding).

The upper crank seal, if shot, usually results in a oily mess under the flywheel/on top of the power head. The gas evaporates and leaves the oil behind leaving anything "gloopy".

So based on your symptoms, those are the first four things I would check.
 
How have you eliminated the carb? A bad primer will cause that to happen. Need a serial # because the marathon is a later motor,
 
Got brand new fuel. Emptied the filter and back flushed it. Rebuilt the carb. Not a pro so that might be the problem. I just checked the compression
with my antique gauge and it showed 80 PSI on both cylinders. The gauge is over 30 years old I would guess.
 
Serial is 0D088766. Rebuilt the carb which included fuel pump and primer. It starts right up but pops and spits every couple of seconds at any speed.
I mentioned to another contributor that I just checked the compression with a very old gauge so I don't trust it, but it shows 80 PSI on both cylinders.
Maybe it only goes up that far and gets caught.:D I forgot to tell you. I just replaced the coils with no change.
 
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80 is way to low get a different gauge. Did you look at the spring and ball under the primer? What do you have the low speed needle set to?
 
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Under the primer assembly is a brass screw in that, is a spring loaded ball that acts as a check valve. Be careful taking it out as parts like to fly. It is not uncommon for that ball to get stuck in the spring and not seal. I am curious how you rebuilt the carb as the new kits would have had a replacement spring for that in it.
 
flyingscott said:
Under the primer assembly is a brass screw in that, is a spring loaded ball that acts as a check valve. Be careful taking it out as parts like to fly. It is not uncommon for that ball to get stuck in the spring and not seal. I am curious how you rebuilt the carb as the new kits would have had a replacement spring for that in it.
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This has been a long project and I don't remember much about the rebuild. I'm pretty it had a new spring but I'll need to check it tomorrow. THanks.
 
Everett1939 said:
This has been a long project and I don't remember much about the rebuild. I'm pretty it had a new spring but I'll need to check it tomorrow. THanks.
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Checked out the kit I got and it didn't have the ball. Turned out it is called a gasket and needle kit. So I did not removed the bottom screw. It did contain and new spring and diaphragm for the primer. Oh, the primer does squirt when pressed. Again, thanks to everyone for your suggestions.
 
Everett1939 said:
This has been a long project and I don't remember much about the rebuild. I'm pretty it had a new spring but I'll need to check it tomorrow. THanks.
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Primer check ball good. Compression is 120 in both cylinders. Is there any way short of a new carb, to rule out same? It starts and runs well except for the popping and coughing.
 
Thanks Graham. Compression good, new coils and plugs, wires are good. Ready to think gaskets. Can the gasket blowout be seen? Can the crankcase be pressurized to check for leaks?
 
I decided I'd better get a better understanding of 2 cycle engine operation so of course I have a question. Most mockups on line show a valve between the crankcase intake and the carb to prevent pressure in the case blowing back through the carb. I don't see such a thing on my engine. How is this prevented on my engine?
 
The valves you are talking about are behind the carb and are called reed valves. Did you put the correct gasket on behind the carb. The kits usually come with more than one and it is critical you used the right one. That fuel pump gets it's pulse through the carb body from the crankcase if you used the wrong gasket or put it on wrong it will cause problems. When you have the carb off you can look through the hole and see the valves to make sure none are broken.
 
flyingscott said:
The valves you are talking about are behind the carb and are called reed valves. Did you put the correct gasket on behind the carb. The kits usually come with more than one and it is critical you used the right one. That fuel pump gets it's pulse through the carb body from the crankcase if you used the wrong gasket or put it on wrong it will cause problems. When you have the carb off you can look through the hole and see the valves to make sure none are broken.
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Darn it Scott, you got me so excited I went up immediately to check the valves. I think I saw them before but didn't know what they were. They are perfect. Not broken and seated well. Also verified the correct gasket. I've had the carb off so many times, I could do it in a flash and could probably do it with my eyes closed, given the right tool. I'm closer to paying through the nose to have it fixed.
 
You are probably going to have to start looking at gaskets and seals. One more thing to try is get it running at night and see if there is any electrical arcing. Also go to www.cdielectronics.com look up the troubleshooting guide to test your ignition.
 
Another clue for you that probably indicates gaskets. With the throttle plate fully closed, it idles way too high
and the mixture screw doesn't seem to change it much except when far one direction or the other
 
the idle screw for that is on the throttle linkage under the flywheel. It is a funny looking slot head bolt with no head on it. Your motor could need a link and sync.
 
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