2 cycle compression ......

[Steve B]

I have a 21cc hedge trimmer. A Craftsman. It hasn't run for a while, but the guy swore it was running when put up.

I put new gas in it, and checked the obvious. It needed a new on/off switch. It has spark. I can't get it to pop, even with shooting a little gas into the carb, or on the sponge air cleaner.

When I pull the start rope, there isn't very much "POP" at all. I mean resistance that would come from good compression, or from firing and igniting the gas/air mixture.

Just how much "POP" should be present, even if you have the switch off, and aren't trying to start it, but just basically see how much compression it has? I call "POP" that instantaneous resistance on the pull rope where it would hit TDC, or fire with a spark. Does two strokes not have the "POP" that four strokes have because of their different configuration?

Tomorrow, I'll pull the carb and see if there's anything obvious there. The primer bulb worked, no leakage, and filled up, presumably feeding gas to the carb.

Steve

[Tony Hwang]

Hi, Take a look at the spark plug. To check a compression I remove plug and plug the hole tight with chewed paper ball. When you pull the cord, it should pop out. Is choke working? Try new spark plug or heat it on the range heater and make it dry. Hope you did not mix in too much oil.

[hrhofmann]

If this is a 2-cycle engine, it make take a few pulls to get enough oil into the unit to raise the compression. you could also put a 1/2 teaspoon oil into the engine thru the spark plug opening, pull the starter a few times to spread the oil around and then put the plug back in and see if the compression seems to be higher. You could also use some starting fluid.

[Steve B]

Plug is absolutely new and dry. Even when I shoot some gas into the carb opening with a syringe. Even with shooting a small amount of gas into the plug hole, reinstalling plug, and pulling cord. I suspect I need to take it off tomorrow and see what's up.

I got an edger going today, a 3.5 hp Briggs, but it doesn't run very well, and the thing will only run with both jets at about 3/4 turn open. Can't get it to run up to very high rpm by adjusting the jets. Took off the diaphragm cover, but just the cover. There was sand. I didn't have a replacement diaphragm, and that one was on there pretty tight, so chose to just leave it alone, thinking it might run. Obviously, I need to get a carb rebuild kit and go through it. First, though, I'll take a razor knife and see if I can get the diaphragm/gasket off, and clean behind it, then reuse it.

With these little carbs, just a little bit of junk will make a big difference. They're like working on watches.

Steve

[Teddy]

Another case of ethanol damage?

[notbob]

Sounds like you want to know if there is enough compression in yer

2-stroke. When I was a motorcycle mechanic, lo these many yrs ago, the rule of thumb for both 2 and 4 strokes was: Pull the spark plug. Put yer thumb firmly over the plug hole, make sure the throttle is wide open, and cycle the engine. If there is enough compression to blow air past yer thumb, that's good enough. If not, you need a rebuild. Typically a bore job and new rings.

nb

[Steve Barker]

[Hank]

Compression may be the problem, but you have to run a compression check to verify. Plugs can be what we call "gas fouled". It may be new, look clean and may even produce a little spark. But more times than not, a person usually floods a 2 stroke to the point of "gas foulding a plug". They think that they can't put too much gas in to make if fire and usually there is so much gas laying in the crankcase, it is near impossible to get the air/fuel mixture into the flammable range.

Here's my suggestion...........

Remove the plug and pull the start cord a few times and let it sit, come back an hour or so later and pull it a few more times. This should remove any gas that is in the crankcase. Let it sit overnight with the plug out. This will let any gas in the crankcase to dry out. The next day, install a NEW plug after squirting a little juice down the plug hole. Don't choke, it should fire if there isn't a compression problem.

Hank

[Art Todesco]

Last year in August, I bought a little used but older Stihl chainsaw from the local Habitat for Humanity Thrift Store. It ran good. It was very dirty and needed a cosmetic cleaning job. Seemed to have good compression. And, it was only $50. I used it once in December and it started and ran great. This year, I tried to start it and nothing. Well, ok, there was an occasional pop, but few and far between. I checked the plug and it was wet. Dried it off and tried and tried and nothing. Emptied the gas and put in clean gas/oil. Stihl nothing :) I then put in a quick squirt of starting fluid in the plug hole, reinstalled the plug and tried again. After maybe 10 pulls, it started without the choke (only on or off) and full open throttle. It seems ok now. I used it the next day. The 1st time it took about 6 or 8 pulls before it started. After that, only one pull. BTW, I think this thing had the original chain, apparently never sharpened and still very sharp. Of course I can't tell for sure, but it everything points in that direction. So it must have been owned by someone who just used it a few times, spending most of it's life on the shelf with all the oily sawdust just left there.

[notbob]

Not familiar, it being decades since I was a motor wrench.

One quickie trick is to squirt some heavy motor oil (50-60wt) in the sparky hole to give the rings some temp sealing power. It will go away, but often gives jes enough seal to get the engine running.

nb

[Stormin Mormon]

I've heard of using a squart of two stroke oil, which burns more cleanly. Same idea.

Christopher A. Young Learn more about Jesus

.

One quickie trick is to squirt some heavy motor oil (50-60wt) in the sparky hole to give the rings some temp sealing power. It will go away, but often gives jes enough seal to get the engine running.

nb

[Harry K]

Could be it was straight gased, pull the exhaust and check for scoring on hthe pison/jug.

Harry K

[Mr. Austerity]

I've messed with a lot of the trimmers. Most of them now days are not for this world long. Keep messing with it and you will get it running but don't expect to much. The carbs are set to run as lean as possible nowdays with very limited adj to richen them, which is why half of them have to be run with the choke partially on. Any chance someone ran it without the oil mixed with the gas?

[Bob-tx]

Here is a trick that seems to be little known, but one that has worked for me at times over the many years I owned a small engine shop.

For a vertical cylinder engine, remove the spark plug - rotate the piston to TDC (Top dead center) and then pour in paint thinner. It dissolves carbon on piston rings and valve seats. Let stand overnight. Then drain oil and refill with new oil. This will often bring compression up from virtually nothing to a workable engine. Not a permanent fix, but gets you by. For a horizontal piston (vertical crankshaft), tip the engine on its side with the spark plug up.

Bob-tx

[Steve B]

Went to the local small engine repair place today to get a couple of other parts for another machine. He checked the compression, and it was an astounding 45 psi. So, apparently there are major problems in the engine, probably rings. Not sure whether to dump more $$$ into this, or just junk it and get the electric used ones at yard sales for $15.

Oh, wait, I think I know the answer to this one.........

Steve

[Jules Richardson]

Pull the head on it and check the bore for damage - typically on these smaller engines the head gaskets are quite meaty and will come off clean (and can be reused). Unless, of course, it's a head gasket problem in the first place...

I don't know, I don't like electric stuff - too much of a pain to run cords everywhere. On the other hand, gas prices... :-)

cheers

Jules

[clare]

Which WILL wash the oil off the cyl, killing the compression if it doesn't start right away.

[clare]

Those are still carbs - not injectors. They are "regulator carbs" with a diaphragm instead of a float, and a pulse powered fuel pump (which has another diaphragm)

[clare]

The head and cylinder are one piece and bolt to the crankcase. Nearly all small

2-stroke engines have one-piece "jugs." Stange, NONE of the numerous 2 stroke engines I have floating around have integrated jugs. They ALL have removeable cyl heads.

[Steve B]

I have a vintage Honda FourTrax, the first year they were made. It wasn't ridden much, then restored by Grandpa for his kid at a cost of $900. Tires, and "stuff". Kid got busted for drugs, so Grandpa sold FourTrax. I picked it up.

That early model has an electric fuel pump, as the carb is higher than the gas tank. It is a damn nightmare. I finally went to a friend who works on them, and asked what to do, as it was always acting up.

He sold me a Mikuni pulse operated fuel pump. All you need is a vacant vacuum port on the carb, and the pulsation works a small diaphragm. Marvelous concept, except for the small detail of no port on that Honda.

So, anyway, I got the electric one working. Hope it doesn't act up out in the bush.

Sold the Mikuni to a friend who has a race bike, and he installed it on his, and it fixed his fuel starvation caused fluttering.

The thing is elegantly simple. Three little nipples... vacuum, gas in, gas out. Bolt it on anywhere.

Steve