Small Engines (Lawn Mowers, etc.)/Troy-Bilt ST425SS


I have a Troy-Bilt ST425SS 4 cycle trimmer that's giving some trouble.

It has a new carb, plug,valve adjustment has been checked, and does have fire.
Gas is in the primer bulb.
Checked compression, reading at 60 PSI.
MTD talls me 60 is fine.

Still it refuses to start.

Any ideas or tips would be appreciated.


ANSWER: Bad gas?  If the gas is over 30 days old I would replace it.  Have you tried another spark plug, other than the one you replaced?  What brand of spark plug are you using?

Intake leak? Will the engine try to start if you prime it by squirting gas directly into the carburetor?


[an error occurred while processing this directive]---------- FOLLOW-UP ----------

QUESTION: Hey again Eric.

The gas is fresh and clean.
I did swap the plug out with one from a well running Cub Cadet 4 cycle that uses the very same plug.
It's an AC2.
The one from the Troy-Bilt is the MTD stock, and the Cub started with that plug.

Can't tell if there's an intake leak as yet.
I also checked the flywheel to see if the key was sheared.
It looks like new.

No, it will not even try to start with gas or ether sprayed through the carb.

I did notice that the spark when using the tester was a little on the cute pink side rather than blue.
maybe a weak coil?

Thanks again,

ANSWER: Are you using an inline spark tester to check the spark?  I have only seen 2 weak coils over the years.  One would not just the gap (.166 of an inch) in the in-line spark tester and the other coil would not allow a chainsaw to accelerate...very, very rare.

I would suspect compression or timing, mechanical or electrical (spark).  The flywheel controls the electrical spark and the valve clearance could cause the mechanical timing issue.  Compression could be a combination of valves or rings.

How many hours are on the trimmer?  Have you removed the muffler and inspected the piston, cylinder, and rings for signs of wear?

Go to this link and see if you trimmer model is listed:


---------- FOLLOW-UP ----------

QUESTION: Hi again Eric,

Thanks for spending time on this issue.

To answer questions:
I have the valves set at the recommended gap according to the manual as well as MTD tech.
MTD Tech says 60 PSI is great for this because of the compression release system.

I have no idea how many hours are on the machine, but the owner says the engine was replaced under warranty a season or 2 ago.

The muffler is off and clear.
Tried to start it without the muffler thinking along these lines as well.

I'll take a look in the cylinder to see if I can spot anything unusual.

I may also do a leak-down test and recheck the flywheel, even though the keyway is fine and magnets seem good.

Thanks again,

I have not had to perform a leak-down test on a 4 cycle trimmer.  Do you have a crankcase pressure tool and some way to block off the intake and muffler?  A piece of rubber will work.  I have 2 adapters for performing leak-down tests.  One bolts to the carb and the there others are installed in the spark plug hole.

It really sounds like compression, igntion, or timing if the engine refused to start after priming.

Let me know if the the leak-down reveals anything.

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Eric A. Jones


Lawnmower Repair . Certified Master Service Technician from B&S. Have 23 years experience on B&S, Lawn Chief, Weed Eater, Echo, Peerless, Wheel Horse, Snapper, Atlas, MTD, McCulloch, Homelite and many other numerous brands. Specialize in electrical repair.


Born and raised in the midwest. Started tinkering with engines when I was about 14 on my Suzuki RM-80. I began lawn mower repair at a small hardware store. I knew absolutely nothing. I read lots of repair manuals and met an older fellow who taught me many lessons. I continued working on small engines through high school and paid my way through college working on mowers at the same hardware store. Decided to get away from the midwest and mower repair so I joined the Air Force. I repaired air traffic control electronic equipment and ended up in Hawaii where I got a part time job at Small Engine Clinic. I gained a lot of experience from the Small Engine Clinic and had a blast repairing small engines. I then took the Briggs and Stratton Master Service Technician test and earned my MST. I then traveled to Wisconsin where I attended the factory update training seminar and received formal training. Continued working on mowers part time as I completed 20 years of military. Retired from the military on a Friday and continued in the lawn and garden industry the next Monday.

MAS Aerospace Operations BA Mathematics AAS Electronic Communications AAS Electronic Technology

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