Troy Built Trimmer Wont Rev Up

[StarTech]

It has 120 psi compression, if the crankcase seal was bad would it still have good compression? Are you referring to the crankcase cover gasket?

Several parts places sell this coil (753-04336) for around $100 vs the eBay/Amazon $20 versions.. Sure wish I knew if that was it but it does kind of lead in that direction I guess.

I wish I hadn't thrown out the original coil, I can't believe I did that, I usually keep stuff forever.

Yes it would still have compression. There several seals and gasket involved. A two cycle uses the crankcase as part of the intake system/

And the coil should only cost $81.81 at a dealer. Below is my cost and resell price.



Now with the compression at 120 and the fact that the starter fluid caused the engine to run rich I would say that eBay coil is defective. Though it is a pain to get get to it would hurt to check the flywheel key. On this unit it made as part of the flywheel so if the damage a new flywheel would be needed. Be careful as it can look bad when it is not.

 

[BillCT]

Thanks for all the good info and sources, I’ll report back on the results. If I get this thing running I’m going to paint it gold!!

 

[BillCT]

Yes it would still have compression. There several seals and gasket involved. A two cycle uses the crankcase as part of the intake system/

And the coil should only cost $81.81 at a dealer. Below is my cost and resell price.

View attachment 61957

Now with the compression at 120 and the fact that the starter fluid caused the engine to run rich I would say that eBay coil is defective. Though it is a pain to get get to it would hurt to check the flywheel key. On this unit it made as part of the flywheel so if the damage a new flywheel would be needed. Be careful as it can look bad when it is not.

Thanks. Yes I did pull the flywheel and checked the key, everything was fine, the key was not damaged and was aligned in the keyway. Flywheel keyway was not damaged

 

[BillCT]

Inside the coil is a timing chip that basically replaces the points .
You can have 10 identical looking coils with 10 different chips that will cause the ignition to throw the spark at different times.
If the factory makes the entire magneto then it would be relatively safe to assume that two units with the same number on them were the same units.
However if that factory just stamped the metal laminations then assembled the unit from bought in parts then the 173 could be nothing more than a type 173 coil which could be fitted with a dozen different chips . we don't know.
Next a lot of trigger chips are fitted with a rev limiter so your "new" coil could be one of those where the limiting chip is cutting off the spark at 5000 rpm in place of 15,000 rpm
We don't know and this is the problem with unbranded parts from an annonomus source, way too many unknowns .
Now if it ran fine on the old carb and you did not fiddle with it then it should run fine with the new coil unless you did not hook up the throttle wire properly.
And yes this easily happens so the cable is not anchored properly thus does not open the throttle butterfly fully , which will of course limit the revs .

You don't have to buy everything from a dealer but you do have to buy them from some one who actually knows what they are selling you
SO these are places like,
e-replacement parts
Small engine warehouse
Barretts
K & C Mowers
Parts Tree
JacksSmall Engines
etc etc etc
GO back to the place you ordered it from and have a look at what else that vendor sells
If they are selling everything from maternity bras through to Condoms including trimmer parts then they are a residual / surplus seller and usually have less than no idea what they are actually selling
And if that includes both condoms & maternity wear then better than average chance what they are selling is defective .

Thank you! I'll get one on order from one of these guys.

 

[Jeff P51]

I put it back together and tested the spark, visually. The spark is fat and purple. I used a rod in the plug wire grounded on the muffler. I didn't measure the gap but it was substantial. . I also tested the plug which is a new NGK grounded against the muffler and saw lots of spark, same color and not a weak thin yellow. Gap is set to .020.

I tried the suggestion of spraying a half second of starting fluid in the carb and starting. It did not rev up any more than usual and when it started it was pretty rich for a few seconds.( I know you did say to remove the carb but I thought this would be OK)
Since this engine ran exactly the same with the original carb as with the new carb I'm thinking the carb isn't the problem. And other than replacing the fuel lines and filter the only other new part is the coil.
It has 120 psi compression, if the crankcase seal was bad would it still have good compression? Are you referring to the crankcase cover gasket?
I've checked the wiring with a meter and neither the switch nor the wiring to the coil appears to be faulty.
Man it makes me grit my teeth to buy another coil but I hate to give up and set this thing out for target practice after all this. Also, both these coils I bought were made in China by the same company. I'm sure MTD could have had them made in China also.
Several parts places sell this coil (753-04336) for around $100 vs the eBay/Amazon $20 versions.. Sure wish I knew if that was it but it does kind of lead in that direction I guess.

I wish I hadn't thrown out the original coil, I can't believe I did that, I usually keep stuff forever.

I put it back together and tested the spark, visually. The spark is fat and purple. I used a rod in the plug wire grounded on the muffler. I didn't measure the gap but it was substantial. . I also tested the plug which is a new NGK grounded against the muffler and saw lots of spark, same color and not a weak thin yellow. Gap is set to .020.

I tried the suggestion of spraying a half second of starting fluid in the carb and starting. It did not rev up any more than usual and when it started it was pretty rich for a few seconds.( I know you did say to remove the carb but I thought this would be OK)
Since this engine ran exactly the same with the original carb as with the new carb I'm thinking the carb isn't the problem. And other than replacing the fuel lines and filter the only other new part is the coil.
It has 120 psi compression, if the crankcase seal was bad would it still have good compression? Are you referring to the crankcase cover gasket?
I've checked the wiring with a meter and neither the switch nor the wiring to the coil appears to be faulty.
Man it makes me grit my teeth to buy another coil but I hate to give up and set this thing out for target practice after all this. Also, both these coils I bought were made in China by the same company. I'm sure MTD could have had them made in China also.
Several parts places sell this coil (753-04336) for around $100 vs the eBay/Amazon $20 versions.. Sure wish I knew if that was it but it does kind of lead in that direction I guess.

I wish I hadn't thrown out the original coil, I can't believe I did that, I usually keep stuff forever.

I could possibly be either the crank seals, or a crankcase gasket.. Not really compression so to speak, but it will affect gas fuel mixtures and intake cycling. Depends on how bad the leak is

 

[bertsmobile1]

Not trying to be arguementative Don, but bad seals generally make the tool hard to start because at cranking speeds there is a lot of time for the fuel to leak out and air to leak in.
The faster the engine spins the less effect the leak has part from making the mix too lean and causing overheating then seizure if the piston does not hole first .
Lean mixes tend to cause over reving with little to no power not limiting the top speed.
If you repair these thing regularly you should be able to hear the difference between a lean burn ( hard sharp & high pitch ) exhaust note and a rich burn ( deeper & muffled ) exhaust note .

From the get go, the engine just stopped then was found to have no spark
The coil was replaced then it developed the new low reving problems
Motor mechanics 101 is a fresh problem will most likely be cause by the last part replaced .
And a fat blur spark at cranking speed tells me the timing chip is firing way too late unless it is a short stroke big bore engine as used on concrete & demolition saws or it is a professional grade engine and has a capacitor discharge ignition which it is neither
Remember the spark has to be at the right time and when you are talking 7000 to 9000 rpm that most of these units run at that timing is down to pico second accuracy
A retarded spark will very effectively limit the peak revs and if the coil is producing a fat blue spark at 300 rpm cranking speeds I doubt it will be producing a good spark at the right time for 7000 rpm .
Just about all of the hand held engines I have worked on have a faint spark that is very hard to see at cranking speeds that gets bigger , better & stronger the faster the engine revs

 

[BillCT]

Sorry for the late reply on this, I've been away on vacation.

Well you guys were correct on the coil, I ordered a factory replacement and it revs right up now. Carb needs a little adjustment for the high speed but not bad, it'll work fine the way it is.
Hope this thread helps out somebody else with a similar problem. Thanks to everyone who replied with all the helpful input!!

Bill

 

[Jeff P51]

Glad u got it fixed..way to stay with it