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Old 07-10-2011, 12:09 PM
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Varmit Varmit is offline
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Default Small Engine Diagnosis



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Question for the gear-heads on the forum.

I've got a 17 year old lawn mower that finally refused to start yesterday.

OK, 17 years of faithful service with nothing more than an occasional oil
change, cleaning the foam air filter, and sharpening the blade -- That's
a winner. No complaints. I'll buy a new mower and count myself lucky
for all the years of service I got out of the old one.

However, in the process of tinkering around with it, I find myself intrigued
as to why it won't run. Ran fine the time before. Now, not even a hint of
a sputter.

5.5 HP Briggs and Straton pull-start engine.

- Pulled the air filter and cleaned it.
- Also tried to start it with the filter entirely removed. No go.
- Checked the priming bulb while I had the air filter out. Gives a good squirt of gas.
- Removed the spark plug and cleaned it up. Gapped at 0.033.
- Plug was wet when removed. (Tells me I'm getting gas in the cylinder.)
- Also tried a replacement spark plug I had on the shelf. No effect.
- Pulled the plug and jumpered it to the block and pulled the cord. Seems to be getting a good spark.
- Cycled the engine repeatedly with the plug out, trying to clear any flooding. No effect.
- Engine cycles freely with the plug removed. Not seized up.

Absolutely zero hint of any sort of combustion happening.


- Its getting gas.
- Its getting a spark.
- Its got air through the filter.

Have the rules of physics, chemistry, and combusion changed?
Did I miss the memo?

Only thing I noticed was that I had to put a pretty good tug on the pull
cord to get a spark. If it cycled too slow, no spark. Don't know if that's
normal or not. Maybe with the plug installed, the compression slows the
engine down enough that the plug isn't actually firing when installed in the
cylinder? Maybe I'm not getting a good enough spark after all. Something
to do with the points, or electronic ignition, or wherever the heck the spark
is coming from on this engine. (Over my head and beyond my skill level to repair.)

This is just a post-mortum and learning process for me at this point.
Everything else on the mower is absolutely on its last legs also.
Got it -- Its time for the old girl to retire. But its just nagging at
me that I can't figure out what finally killed it.

Any suggestions?
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Old 07-10-2011, 12:44 PM
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Quote:
Originally Posted by Varmit View Post
...

Only thing I noticed was that I had to put a pretty good tug on the pull
cord to get a spark. If it cycled too slow, no spark. Don't know if that's
normal or not. Maybe with the plug installed, the compression slows the
engine down enough that the plug isn't actually firing when installed in the
cylinder? ...
Can't say for sure but if your engine bearings are going out so that it is difficult to turn over, that might be the problem. In order to start you need to get the thing spinning fairly good. With the spark plug removed there is no compression that you are working against so it should spin easily.

One other thought is the brake (if it has one). If it is rubbing due to built up crud this could be slowing down the cranking effort. Also look for things wrapped around the shaft.
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Old 07-10-2011, 01:49 PM
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Quote:
Originally Posted by MattB4 View Post
Can't say for sure but if your engine bearings are going out so that it is difficult to turn over, that might be the problem. In order to start you need to get the thing spinning fairly good. With the spark plug removed there is no compression that you are working against so it should spin easily.

One other thought is the brake (if it has one). If it is rubbing due to built up crud this could be slowing down the cranking effort. Also look for things wrapped around the shaft.
No brake on the thing (that I know of) and I'll check to see if anything is
wrapped around the blade. Stray piece of twine or such, never know.

It does seem like it was turning very sluggish when I tried to pull-start it.
Barring something wrapped about the shaft, I'm liking your theory on the
bearings.

Thanks.
Old 07-10-2011, 01:57 PM
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Engines require..
Air, at the right time
Fuel, at the right time
Spark... at the proper time... small engines usually have a 'shear' pin or key on the flywheel.. if that breaks... you may have a spark, but at the wrong time.

Points... they wear and are replaceable... is the spark strong (bright blue) or weak (yellowish color)

If need be, check the compression... pull the head if needed (head gaskets are cheap).
Good luck!
Small engines are very simple as a rule, .. reliable for a long long time, but need a little maintaining.
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Old 07-10-2011, 02:12 PM
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The thought I am having on this suject is very common. The new gasoline contains some alcohol which in turn eats away the little needle valve seat. Thus causing the carburater to receive only very small amounts and sometimes none at all, gasoline. Hope this helps.
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Old 07-10-2011, 02:46 PM
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The thought I am having on this suject is very common. The new gasoline contains some alcohol which in turn eats away the little needle valve seat. Thus causing the carburater to receive only very small amounts and sometimes none at all, gasoline. Hope this helps.
I expect that is what happened to the engine on my lawn mower this year. The darn thing would start but would not keep running. Unfortunately the BS engine had been discontinued and Sears had no carburetor parts (I love Sears parts website it has helped me get stuff for real old machines). I went and bought a real inexpensive engine from HF and re-powered my mower. Since the old mower (it was about 17 years as old as well) was a high back wheel with powered drive, it would have been very expensive to replace. Not to mention they simply do not make as good as design for the new models.

I got the new 6.5 hp engine for $65 dollars and bolted it in place after adding some spacers and other modifications to make it workable. It works great! Have used since Spring. Hopefully it will last a few years and I will get my money out of it.
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Old 07-10-2011, 07:58 PM
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Thanks guys, for all the great responses.

>>is the spark strong (bright blue) or weak (yellowish color)
Great info. Exactly the sort of education I was hoping for when I posted.
Yeah, now that you point that out, the spark was definitely yellow. And
that was with the plug removed and grounded. Who knows how little
(if any) spark I'm getting with the plug screwed in and the pull-start
fighting against the engine compression.

>>...but if your engine bearings are going out so that it is difficult to turn over, that might be the problem.
Might be going hand in hand with that weak spark problem. It's tough
to turn over, so I'm getting a weak (or no) spark.

>>I got the new 6.5 hp engine for $65 dollars and bolted it in place...
Tempting, but the rest of the mower is about done anyway. Wheels wobble,
the deck is rusting through. It's just done. Cost me about $120 17 years
ago. I think I got my $$ out of it.

>>The new gasoline contains some alcohol which in turn eats away the little needle valve seat.
I've heard of that with regard to it eating plastic fuel lines.
Yeah, we run about 15% etheynol here as a standard.
Thanks, government intervention. I will try some carb cleaner before
I declare the thing to be a gonner. Guess that won't fix a worn needle valve,
but maybe there's some other crud in the carb.
Just didn't have any spray cleaner on hand at the time.

From what I'm hearing, and given my limited small-engine repair skills,
I think a new mower is in order so I can keep up with the lawn. However,
I'm going to not junk the old one yet. Keep it to tinker with, and maybe
get some hands-on wrench time.

Thanks for the education.

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Old 07-10-2011, 09:26 PM
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interesting post
Old 07-10-2011, 09:38 PM
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Quote:
Originally Posted by Varmit View Post
...
>>I got the new 6.5 hp engine for $65 dollars and bolted it in place...
Tempting, but the rest of the mower is about done anyway. Wheels wobble,
the deck is rusting through. It's just done. Cost me about $120 17 years
ago. I think I got my $$ out of it.

...
Sounds like you did. Do not be too shocked at the cost of the new mower. I know I was, especially for anything of quality when I looked around this year. I think the Craftsman Highwheel I have probably cost over $250, 17 years ago. For a similar quality one looked to be over $350 today.
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Old 07-10-2011, 09:46 PM
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Sounds like the pickups on the flywheel and coil are rusty. A little emery paper and you should get lots-o-spark.
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Old 07-10-2011, 10:04 PM
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Most of those small engines arnt ment to be fixed. Lots of plastic parts INSIDE the block, head, crappy mags... They dont make them worth a damn these days sadly.
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Old 07-11-2011, 12:15 PM
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Check compression, if its low maybe valves. I bought a generator, same symptoms you have. Good spark, good fuel, etc. I pulled the valve cover off the rocker arms...(this was a OHV motor), and adjusted the valves. You have to look up online to see what they need to be set at... And walla, she fired right up... I still had to do some carb work to get it to run "perfect", but the main cause what the valves out of adjustment. She had VERY low compression, like 30 psi or something. that was the tip off for me to check the valves. Good luck.
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Old 07-11-2011, 12:19 PM
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what about timing? did you mess around with it?
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Old 07-11-2011, 01:27 PM
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Small gasoline engines are simple, incredibly reliable, and are too often retired too early in favor of a shiny new model. Case in point...




Have fun!
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Old 07-11-2011, 01:42 PM
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Quote:
Originally Posted by Pangea View Post
Sounds like the pickups on the flywheel and coil are rusty. A little emery paper and you should get lots-o-spark.
Quote:
Originally Posted by splat View Post
Check compression, if its low maybe valves. I bought a generator, same symptoms you have. Good spark, good fuel, etc. I pulled the valve cover off the rocker arms...(this was a OHV motor), and adjusted the valves. You have to look up online to see what they need to be set at... And walla, she fired right up... I still had to do some carb work to get it to run "perfect", but the main cause what the valves out of adjustment. She had VERY low compression, like 30 psi or something. that was the tip off for me to check the valves. Good luck.
Quote:
Originally Posted by Inventor View Post
what about timing? did you mess around with it?
Pulling flywheels, coils, valves, and timing are sadly a bit beyond my
present skill level. (I can build you a pretty fair room addition, but as
soon as it goes from lumber to mechanical and electronics, I get stupid
pretty fast. Just didn't grow up working on engines, so I'm kind of starting
from scratch here.) But I do appreciate all the suggestions. The mower
deck is rusted through in a couple of locations, so it's time to retire it
anyway. Which means that the old mower has now become a learning
project. So I guess I'm going to try to learn how to pull flywheels and
adjust timing. We'll see how it goes. Goal is to to get it to sputter
back to life, even if only for a short duration.
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Old 07-12-2011, 04:55 AM
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Quote:
Originally Posted by Varmit View Post

- Its getting gas.
- Its getting a spark.
- Its got air through the filter.

Have the rules of physics, chemistry, and combusion changed?
Four things you need for an IC engine to run.

Air.
Fuel.
Compression.
Ignition at the right time.

Have you got all four?
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Old 07-12-2011, 05:10 PM
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Quote:
Originally Posted by SamboRoberts View Post
Four things you need for an IC engine to run.

Air.
Fuel.
Compression.
Ignition at the right time.

Have you got all four?
Thank you for adding Compression... Like I said in my earlier post, I would start there before pulling fly wheel and stuff like that... That would tell you alot. Valves, rings, hole in the piston
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Old 07-12-2011, 05:27 PM
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Quote:
Originally Posted by SamboRoberts View Post
Four things you need for an IC engine to run.

Air.
Fuel.
Compression.
Ignition at the right time.

Have you got all four?
and here's how I test for these things.

Air: take off the air filter, make sure the carb isn't blocked. That's about all you need in this category.

Fuel: Take off the gas cap. If the tank isn't venting, it won't allow fuel to the carb.
Second - take the plug out and lay it in the hole (not threaded), while pulling.
It should pop when it ignites the gas. Fuel is tricky so I'll include another hint at the end.

Compression: This is a rough test - put your thumb over the spark plug hole while someone pulls the cord. It should literally blow your thumb off. Doing a real test with a compression gauge wouldn't hurt either.

Spark: Should really be #2 in sequence. Take out the plug, ground it on the block while pulling. Should see a spark.

Timing: Really shouldn't be too far off with a lawnmower. Even if it is a bit off, it should run albeit feebly. Worry about this after you get it to turn over.


Fuel again: If all of the above pass muster, spray some starter fluid into the carb. If it turns over, you have a fuel / carb problem that needs to be addressed.
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Old 07-12-2011, 08:12 PM
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Ill vote for the contacts being rusty.
It's a lot simpler than you think.
Once you get all the covers off, you will expose the flywheel. It's steel with magnets embedded on the outside edge. There should be 2 sets of magnets. Lightly polish both sets.
The coil should look something like a couple C-cell batteries wrapped in plastic with contacts nearly touching the flywheel. A couple small bolts hold it in place. remove those, and polish the contacts.
Replace the coil, with a standard piece of printer paper between the contacts and the flywheel. Once tight, remove the paper and replace all the covers.
The whole thing shouldn't take 15-20 minutes
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Old 07-17-2011, 12:54 AM
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You could check your magneto clearance, just loosen it, it will have two small bolts holding it in position, place a good thick piece of paper in there, like a cereal box. Turn it to where the magnet is pulling it tight and reset. I see where it is said to use a piece of typing paper, great if it doesnt ever touch, most pieces of equiptment run for 17 years have a slight amount of slop and you do not want it to touch, everything I use is ancient and I always use something a bit thicker and it always works for me, a matchbook paper will also work well.

You might also check the spark cable and make sure it isn't cracked or covered with oil and grease. Make sure the contact isn't rusty etc.

My bet though is that your needle is not seating and you are flooding. Most usually a little tiny piece of junk in there keeping it from closing all the way. Often times just loosening and removing the can and letting a little fuel flow will get rid of that.

Some of the older engines will have a rubber needle tip, these can eventually wear a groove in the tip and stop them from sealing. A simple way of solving that is to take a small drill bit and softly place it up on the seat and slowly turn it by hand, even a tiny tiny amount will change the seat position and solve the ridge problem in the rubber needle, you may have to do it a few times but does work well. Even the metal needles with enough time can wear a slight groove, this is a good fix for them as well.

The rubber tipped needles are having more trouble these day with the alcohol additive in our fuel, they still work but they wear the groove much faster now.

Never give up on a good peice of equiptment, they are usually better than anything you can buy now.
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