or Connect
EpicSki › The Barking Bear Forums › Off-Season Sports & The Lighter Side › General Sports › Wayyyy OT - Anyone know small engines?
New Posts  All Forums:Forum Nav:

Wayyyy OT - Anyone know small engines?

post #1 of 72
Thread Starter 
Here's my deal. I'm okay working on small engines, but have never really messed with small 4-cycle engines before. I tried looking for forums for this kind of stuff, but only found one, but it doesn't get much traffic, so I'm hoping one the folks here might be able to help. Yes, I did post my question on the other web site (Hobbytalk).

I have a 2-3 year old, straight shaft Troy-Bilt string trimmer / brush cutter, that has been running perfectly until a couple of weeks ago. It used to start on 1st or 2nd pull, but while I was using it one day, the engine started running slower, and when I opened the throttle up, it choked and cut out on me. It was very hard to start after that, and now, when it's cold, I have to pull the starter 20-30 times (at least) in order to get it to run at all. Then, if I don't let it idle for 3-5 minutes, if I pull on the throttle, it'll cut out on me. Once I do let it warm up, it's still slow to accelerate when I pull the throttle and will not run at full speed. When running it with the throttle wide open, it sounds like it's sputtering or backfiring on a very frequent basis (a few times per second).

I checked and cleaned the plug. I checked the fuel flow, tried running it w/o the air cleaner, I even completely disassembled and reassembled the carb, and everything seems to be in perfect condition, including the main fuel jet.

I've never worked on a 4-cycle before, and 2-cycles have not given me this kind of problem that wasn't easily fixed by cleaning. I honestly don't know if it's even a 4-cycle issue or not, but I'm drawing a blank on what to look for next.

If anyone has any ideas what I might be able to check or do to get it running normally, I'd really appreciate it.

post #2 of 72
How did the spark plug look when you took it out?
White? Brown? Black? Red?

Try doing a compression check...
post #3 of 72
A 4 stroke trimmer? well no matter, does it make any different noises than it used to when it ran good? like a hissing on the compression stroke? More un-powered rotations after you shut it down?

If either of those, that would lead me towards a burned exhaust valve.
post #4 of 72
Thread Starter 
Allgo - The spark plug was in perfect shape. It seems to get normal compression when I try to start it.

Skier_j - Yes, a 4-stroke trimmer . Quieter, smoother and less polluting than 2-stoke. Just not as powerful for the engine size. But it's so small that I don't care.

Yes, it is making different noises than when it ran well. It seems to backfire, knock (probably not knocking) or something a few times per second. No hissing sounds. No run-on when I stop it (that's why I don't think it's knocking).
post #5 of 72
My first guess would be something to do with the fuel-air mixture screws rattling out of postition, but the plug should have revealed that condition.
Does it have an automatic choke?

Governor malfunctioning?

Maybe your gas is off?

Timing slipped?

Magneto and associated wiring to plug good enough to run but not under load or when starting:

Good luck.
post #6 of 72
almost always the sparkplug going bad
followed by air cleaner failure
but what I hear you saying sounds like oil starvation followed by seizure
post #7 of 72
Thread Starter 

I'm no expert at spark plug analysis, I just know when they are broken or caked up with deposits, etc. If the air/fuel mixture has gotten out of whack, what should I see? the plug had (what I considered to be normal) fine black coating on it, but no buildup. I could certainly go get a new plug to see if it would make a difference.

yes, automatic choke. To start it, you do nothing but pump the primer and pull the cord.

there is no fuel shut off. I've burned a full tank of fuel with it running crappy.

Timing slipped? How could that happen on this type of engine?

I need to find the owner's manual, but I'm thinking I may try to play with the air/fuel mixture.

If it's a problem with the ignition, how can I find out before I go and buy a new one, only to find out that isn't the problem? (as a side note, I did this on my Lawn Boy mower, only to come to the harsh conclusion that it was not running poorly because of the ingition, but because it had tossed the needle bearings in the connector rod, and one of them got caught between the cylinder wall and piston, and my engine pretty much disintegrated from the inside out. $250 and about 4 hours of my time, after waiting a week for the part, I replaced the short block) I still have the new ignition sitting in my basement. I don't want to do that again. How can I test the ignition? I have a digital multi-meter.
post #8 of 72
Thread Starter 
Originally Posted by Ryel
almost always the sparkplug going bad
followed by air cleaner failure
but what I hear you saying sounds like oil starvation followed by seizure
After reading what I wrote, what makes you think plug or air cleaner?

I doubt it's oil starved. When this first started happening, I changed the oil, but the engine turns very smoothly, as if it's being oiled normally.
post #9 of 72
As a former auto shop teacher, here's the sequence:
1. Compression - means the engine is OK, valves, piston, rings.
2. Spark - do you have it? Is the timing right?
3. Fuel - is there any? Is there too much? Wet plug?

My own experience with a mower was a broken flywheel key which had the effect of throwing the timing off. Cost about $0.50.
post #10 of 72
Originally Posted by JohnH
Allgo - The spark plug was in perfect shape. It seems to get normal compression when I try to start it.

Skier_j - Yes, a 4-stroke trimmer . Quieter, smoother and less polluting than 2-stoke. Just not as powerful for the engine size. But it's so small that I don't care.
Yep -- I've also got a 4-stroke trimmer. I don't use it alot - but when I use it I like it.
post #11 of 72

Engines only need a few things.




And someplace to pass spent gas.

IF all these things are right, IT has to run!

So, Hard starting?

Compression or ignition.
Don't guess at compression, put a squirt of oil in the spark plug hole to fortify the rings. Listen at the exhaust port to check the valve.

Small engines should turn sharply against compression.

Ignition., Always a problem cause you often need to buy parts. I spent more on the ignition unit for the mower this spring than I paid for the mower (Lawn Boy two stroke bought second hand about 9 years ago)

Put in a new plug, If it starts right up , but then doesn't start well the next time, The ignitions going south. I like to lay the plug on the engine block to see that a good spark is jumping the gap. A good spark, Every time the engine turns over. Blue white and FAT

Fuel/air mix? Unless you did something to the carb, (Adjusting) I doubt it.

Lubrication failure that leads to poor compression is usually my first guess on a hard to start engine that runs after it get's going.

Your try with the air cleaner out is a great first check as well. Can you see fuel in the carb bore?

There must be a reason thes weed whackers are always in the dump!

If the timing is OFF due to a slipped flywheel, the thing will never run.

Don't waste your time with the manual unless it might contain Basic carb settings. I like 1.5 turns out for both high and low speed screws. IF I don't have anything better, It is someplace to start.

post #12 of 72
Thread Starter 
Thanks for the feedback. My fear, this thing being a 4-stroke, is the valves not opening and closing properly. especially the exhaust valve. If I am lacking compression, it could be due to the exhaust valve, not the rings/seals.

Yeah, I hate to think of this thing as disposable, but I'm not going to pay for labor to fix it, because 2 hours of labor would buy me a new one.

If I do buy a new one, I'll probably go high-end, with a Stihl or something like that, to avoid the disposable mentality. that's why I put a new, $250 short block in my $450 Lawn Boy, knowing full well that I could buy a new, mediocre mower for $250. But mine is 14 years old and I like it and don't want to add to the trash heap. Speaking of that mower, I need a new bag for it.... I wonder how much that'll cost me?:
post #13 of 72
Plugs are cheap and "plug analysis" is not always accurate.

Before I brought my ancient Kawasaki in for work the dealer (in a rare moment), made me buy a plug first. Insisted on it, but the old plug looked fine. For $3 .... ....

Spent $100 on "parts and labor" for my old Stihil chain saw (it had an oil leak) .... that was money pissed away. It still has a leak and won't start now.
post #14 of 72
Sure sounds like the problem I had with my mower after I re-read your posts.
This isn't rocket science. The three things I posted are in their order of importance. You say it seems to have compression so it can't be a valve. It runs so it has spark and fuel. Sounds like timing to me. Also sounds about the same as my mower acted. My two stroke weed whacker ran with a scored(melted) piston for awhile but eventually failed to start at all.
But what do I know? I've only been working on motors since 1955 (model airplanes back them)(I'm still learning though)
post #15 of 72
Have you been cleaning the cooling fins? Slatz was reminded me of my JD wacker. Three years old and started to run slower. I had hot spotted one side by not cleaning the fins.

Blasting everything with carb and choke cleaner isn't a bad thing.

How about the bottom bearings (or something stuck), down at the wacker head? That will rob lots of power.
post #16 of 72
Dumb question but is your fuel fresh?
post #17 of 72
post #18 of 72
By the numbers 1. 2. 3. If you don't do it in that order you're wasting your time.
Didn't the "hot spot" score the rings and cause low compression?
John H
Wasn't it running OK then something went wrong while it was running and it hasn't been right since? That would mean something went wrong right then so it couldn't be bad fuel or wrong plug gap.
I make my living by how fast and accuratly I can find the problem. There have been many times I've tried to start in the middle because it sounded familiar to something else only tro find it was something simple in a step I'd skipped.
1. An engine is an air pump. You have to move air in and out and compress it. If valves, pistons, rings or head gasket are bad the compression will be low. Problems with bearings or crank and rods will either make it hard to turn over or be loose and noisy.
2. There has to be spark to light the fire at the right time. Small engines usually have a magnet that rotates past the coil built into the flywheel. This "magneto" provides spark and, by it's placement, timing. (that's why the broken key makes it hard to start and runs bad. The timing is off)
3. When the first two are OK there needs to be something to burn. Small engine carburetors are not much more than a tube that air flows over and sucks gas out. When they fail the motor usually won't run. If you squirt gas into the venturi and it fires, burns the "prime" and stops it's not getting gas. If the spark plug is dripping with liquid gas the needle isn't controlling the fuel.
I re read the symptoms again. It's obvious something failed. If it runs enough to "run a whole tank through it" I'd say it's either bad compression, bad timing, since the spark sounds like it's consistant, or the last thing might be dirt in the carb.
I guess if I were doing it I'd make sure the compression is OK, then I'd turn the fuel mixture screw (counting the turns) in until it lightly seats(if there's two screws it's the one farthest from the motor or closest to the inlet, the lower one is the idle). Next take the screw out and blow out the carb with compressed air, or possibly a straw attachment to a can of choke cleaner if there's no air. Next screw the screw all the way to lightly seated and back it out the same number of turns plus one (that should be a good starting point) If that works and it runs OK run it at wide open throttle and turn the screw in slowly until it starts to stumble when it does back it out about 1/4 turn.(you can play with it a little but always set it just a little richer(backed out) than peak RPM.
If that other stuff doesn't work, take the starter off and expose the flywheel nut. Remove the nut and unseat the flywheel from it's tapered shaft (a puller is best) Here's how I did it without a puller. Put the nut on flush or slightly above the top of the threads, put two large screwdrivers at opposite sides so you can pry it off (make sure you don't damage the surfaces you're prying against, they should be solid) With pressure on the screwdrivers give the nut a SHARP rap with a LARGE hammer (don't beat on it, just a sharp rap should unseat the taper)(I managed to do it alone but a helper would be better) Once it's off if the key is broken replace it and that should do it. If the key isn't broken inspect the magnet and coil plus any othe "trigger" device you might find.
I'm betting, if it's got compression, (CHECK it FIRST) on the key or the dirty carb.
post #19 of 72
I'm with Slatz on this one. Something happened at the time you noticed a decline in performance. I'm still leaning towards compression issues. I had to replace a head gasket last season due to a mouse nesting under the shroud of my mower.

With small engines we sometimes get lazy and try this or that or don't do things in an order. I'd replaced the short block 2 seasons earlier and I don't think I torqued the head bolts (hey its just a lawn mower right?/??)at all, just snugged them up "properly". The mouse's nest compromised the cooling airflow and I was lucky to have to replace only a 3 dollar head gasket to recover from that.

Look at various compression issues first. You will probably have to remove shrouds to get a good look at the head and flywheel.

Once you have it all unbuttoned, check everything for tightness. Head gasket, carb mounting, exhaust (to a much lesser degree)---even the cylinder, if it is bolted on to the crankcase, although thats not very likley.

The top of some flywheel keys can be seen if you just remove the nut. By removing the nut---you may be able to see if the key is damaged right away.

I used to replace a lot of keys in lawn mowers that had tried to cut a sprinkler head off: (of course they usually needed a crankshaft as well) and in chain saws too. The chain saw usually was accompanyed by a pulled out starter cord and almost always brought in by a guy with 25 inch biceps. Gee I wonder how that happened?? I was JUST trying to start the thang!!:

It has been quite a while since I did this for a living (college days to be precise, 25 years ago), as I remember tho, a bad flywheel key ususally ended up with a no start -- not a hard start--but it certainly is possibly to have a hard start. The reason was that in order to the key to shear, the rotating mass needed to have a pretty severe jolt (like hitting a rock or sprinkler head) If it deformed enough to just be hard starting, something else was happening, like a loose flywheel nut from a backfire, or too much starting force or overheating---and it would soon be sheared.
post #20 of 72
Thread Starter 

Thanks for the info.

As I mentioned the carb is clean. When I slowly pull the starter, there seems to be a LOT of compression (very firm to pull through the compression stroke, and a nice "putt" sound as it goes past, and seems smooth through the rest of the stroke). So I guess I need to pull it apart and check the flywheel key. I've replaced them on lawnmowers and outboard marine motors, so I know what to look for.

I found the manual, and was noticing the rocker gap specs (remember it's a 4-stroke). I'm wondering how easy it is for those to get thrown out of whack, and how it would affect the engine performance if the gaps on the rockers were off. I don't have a gap tool for checking gaps.

I'll definitely replace the plug, just in case.

One oddity about the idle screw. All it does is stop the throttle cable at a specific point. It doesn't enter the carb. I could find nothing in the manual or by looking, that would adjust the idle speed other than that. FYI, the little carb on this motor is the most complicated carb I've ever seen on a small engine. But when I took the carb apart, I didn't see any of the normal adjustment screws.
post #21 of 72
Originally Posted by JohnH

One oddity about the idle screw. All it does is stop the throttle cable at a specific point. It doesn't enter the carb. I could find nothing in the manual or by looking, that would adjust the idle speed other than that. FYI, the little carb on this motor is the most complicated carb I've ever seen on a small engine. But when I took the carb apart, I didn't see any of the normal adjustment screws.
That is an idle speed adjustment screw NOT an idle mixture adjustment screw. Look for another screw somewhere near the throat of the carb.
post #22 of 72
How many cylinders is it?

Have you checked you're getting a spark?
(any health and safety people, or those with pacemakers please look away now)
Once the engine is running, pull the lead off one of the spark plugs. Touch the metal contact at the end of it (the bit that would normally be attached to a spark plug).
Do you get a shock? (don't panic, itwon't be as much as 15kV, and a couple of microamps, so it won't do any serious damage, just make you jump a bit)
Now, while that lead is off, does the engine sound different?

Repeat for each spark plug.

1. If you don't feel the shock, then it may be the spark plug lead, or dirt on the rotor arm.
2. If you DO feel the shock, and notice a change in sound on one of the cylinders, then it's most likely the spark plug on that cylinder.
3. If you do feel the shock, but don't notice a change of sound, then it's not the electrics.

If the problem appears to be 1, try swapping two of the leads around (before you do this, mark each lead for the cylinder it belongs to, so you can put them back the right way). Also, take the distributor cap off (where all the spark plug leads meet), and you'll see the rotor arm. Pull it off, then wipe it on a cloth to remove any dirt. Give the contact on it a gentle rub with sandpaper. Check the contacts on the distributor cap. Are they clean? If not, give them a clean too.

Just a couple of thoughts.
post #23 of 72
Good one Fox
post #24 of 72
Slatz, it was how we used to do it back in school - you want to check the spark plug leads, well, grab them!

And it's fairly safe. Voltage rarely kills - it's amps that do the damage.
So, 10kV at 0.0001A will probably make you jump, but 1V at 100A will probably kill you.
post #25 of 72
Valve adjustment can effect the running without causing loss of compression. It would probably be a good idea to get a cheap feeler guage and check it before you pull it apart. (valve adjustment is part of step #1, engine)
As for the carb if there aren't any screws for mixture adjustment then it may use a diaphragm or something to control fuel. Sounds like you've covered it.
post #26 of 72
I know it works but I'd rather try to hold the wire and watch the spark jump than touch it. These days there can be 20K or more volts there. It won't kill you but it's no fun.
Actually on today's cars, where everything works off the computor, pulling a plug wire like that can cause all kinds of "glitchs".
Now I know why, in all those old pictures of mechanics, they wear those little beannie hats. It's so their hair doesn't stand on end when they do that.
When I went to school they had just taken delivery of one of those new fangled oscilliscopes so I was spared your experience.
post #27 of 72
Thread Starter 
Originally Posted by Wear The Fox Hat
How many cylinders is it?
It's a 4.7L V8 weed whacker! 253 hp and 275 lbs of torque. Kinda heavy and hard to hold onto, but you culd trim all the weeds in a rain forest in under 20 minutes.

Ummmm, to check the plug, I'll go for a slightly less painful method. Do you also check your 9v batteries by putting your tongue across the two tabs?

SLATZ, Thanks. Yes, the carb has a diaphram in it. When I took it apart, it seems to function properly and is very clean (pristine). I'll get some feeler gauges/gap tool to check the gaps. While I'm at the auto parts store, I'll get a new plug and check the spark to make sure it's strong. I'll go through those before I pull the flywheel off to check the key.

And here, I thought I was doing the planet a favor when I went with a 4-stroke! Am I saving the planet by emitting less gunk, but end up tossing the thing in a land fill?
post #28 of 72
Don't forget to set the proper gap on the spark plug.
post #29 of 72

I have never worked on a 4 stroke before, but I do work on 2 strokes, (daily)

We have 6.

The problem you describe will probably be the carb., however, before ordering a rebuild kit, remove the spark arrester from your exhaust muffler, & see if it's caked with carbon deposits. If not , throw it away anyway, since eventually it will cake.
When we buy new trimmers for the business, I remove the arrester before putting it into service.
If you have any other questions, you can pm me.

post #30 of 72
Since this is "Off Season Sports", did you see the guy at Bonneville who set a land speed record for lawn mowers? 80+ MPH I guess.
New Posts  All Forums:Forum Nav:
  Return Home
  Back to Forum: General Sports
EpicSki › The Barking Bear Forums › Off-Season Sports & The Lighter Side › General Sports › Wayyyy OT - Anyone know small engines?