395XP Cylinder Compression

[TexasTimbers]

My big Swedish gal just laid down on me. She was running fine when I shut her off. Refilled her with mix, bar oil, but she wouldn't restart. I had not ran her dry either, I hardly ever do that. So it isn't like she had no fuel, was refilled with bad gas, and just wouldn't start. I checked the gas anyway (was two days old) for water.

Getting a blue spark at the plug, and getting fuel in the cylinder. I hate to use starting fluid, in fact I didn't even own a can but I didn't know what else to try so I went to town and bought a can. It barely hit. Certainly not like it should with a blue spark and ether in the cylinder. Just more of a whimper barely can tell it hit at all. It only popped the valve out once in fact.

So I checked the compression and it read 125. I couldn't tell for sure if the valve was all the way back out so I pulled it with some needle nose. I don't know of the valve works like that though or if it only closes fully from a good combustion.

My gauge is old so I bled my shop compressor off until it came on and pumped up to the 150psi it cuts off at. Stuck a air nozzle to the compressor gauge and it only read 130 when the shop compressor was reading 150 (really was reading like 148) and I know the gauge on the shop compressor is correct. In fact it has a gauge at the tank and another at the other end of the shop and they both read within 1psi all the time. So maybe the 395 has more like 145 psi.

I know this makes no sense at all. Good ignition, good fuel in the cylinder. Isn't 125 - 145 psi enough to bust it off or is that too low? It won't even bark. Scratching my head on this one.

[Hilltop366]

Some times a saw will be in between hot an cold after refuleing it wont start with out choke but when you try the choke it floods.
so you either have to wait and try later or remove plug and pull eng over (shut off the kill switch so there is no spark) with the throttle in the ful position to clean it out, you can also blow some compressed air in the plug hole while turning saw over, do this outside        (BE CAREFULL gas fumes and vapour are very explosive).

When checking compression do so with throttle full open to get correct reading.

I would avoid using starter fluid try using mixed gas from a container with a small spout so you can control amount.

[TexasTimbers]

I did exactly what you said - blowing out the fuel from cylinder - but she won't start even now 2 days later. The plug looked fine but I put a new one in anyway.

I will check the compression with the throttle wide open I didn't know that's how to do it. 

[logwalker]

Did you shear the woodruff key on the flywheel? Could be a timing issue. Not likely but just throwing out ideas.

[logwalker]

I read down the line a little and see that Bandmiller had the same problem. Strange...

[Hilltop366]

The spark should be a fine blue snap it is hard to see in the sunshine on a bright day when checking on a plug, bend the wire on a plug  out straight or cut it right off (the side electode of the plug) to make the spark jump farther if it can jump that far with a blue snap in the open air it should be good enough to jump the proper gap under compression.  A flashie orange spark is not as good.

Causes for weak spark can be excess coil gap, chafed plug wire or kill switch wire, bad kill switch, weak fly wheel magnets, bad plugwire end. or shorted coil/ or electronics.

my teacher would say you need the following FACTS to make a engine run: F= fuel  ,  A= air  , C= compression   ,  T= timing  ,  S= Spark
also with a two cycle engine it is important to have good crank seals to have enough draw to suck in the fuel/air mixture.

[TexasTimbers]

i talked to my small engine buddy today ~ he said like you hilltop. said the spark might look good to me but i needed to just remove the plug altogether and see if the spark would jump to a grounded screwdriver or something at least a 1/4" but preferably more.

said if it didn't remove the coil and make sure the screws where it mounts are clean etc. I will do it tomorrow maybe tonight but still have a pile of wood to plane and seal. thanks for the ideas

logwalker how far down this page or where i would like to thread.

[Dave Shepard]

Kev, here is the link to the sheared key thread: DanG key

Sorry to hear your saw went down on you. It's a terrible thing to loose a loved one.  : ;)

[timber tramp]

  Texas- Why not put a bit of oil in the cylinder (through the plug hole) put the spark plug back in, and try starting your saw? If you truly have a compression issue the saw should fire maybe once or twice might even run for a moment then dead again. If it's not a compression issue you have'nt hurt anything, and you've entirely ruled out 1 possible problem.        TT

[Ironmower]

Sounds like a timin' issue, but I would try another new plug first ;) Just because it's new don't mean it's any good ;) ;D

[TexasTimbers]

I tinkered with it again yesterday. It's not throwing spark any distance at all. Just the spark in the gap, and I guess I was dreaming about it being blue initially because it's a weak white spark.

I haven't ohm'd the coil because I don't know what kind of resistance I ought to see. Plus, I know coils can read good but break down under a load.

I'll read your thread again Dave, and I'll look at the other things y'all mentioned.

TT I no longer think the compression is the issue, but good idea on the oil I'll sock that away.

[nmurph]

i'm not sure if the 394 and 395 are the same in this aspect, but a couple of weeks ago my 394 had a similar issue. gas, fuel, spark were all ok and the saw would not crank. for some reason i had removed the shroud and tried it again. it started and run perfectly. ok, i thought it was a glitch. i went to re-install the shroud with the saw running (for fear of it cutting off) and the saw bogged as i put pressure on the plugwire. i repeated this action and the same thing happened. ok, i'm on to something. it turns out that at the point where the plugwire comes out of the shroud and makes the 90* bend towards the plug, it had a small crack and was shorting. when i checked for spark with plug removed, the wire was straighter and i was getting a spark when i checked. when i put it in the shroud, the sharp angle was opening the wire inside of the insulation and i was not getting enough voltage to run the saw. i soldered a splice in the wire and covered it with shrink wrap and the problem was fixed.