OTC compression gauge off?

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The OTC plug hole fitting is longer than most and fills more chamber volume than other models. At least that's what I've found.

Getting too caught up in static compression reading can be a slippery slope and can make you nuts.

I've had a ported 262 that would only pull 165 psi in 90* 100% humidity weather at 250' asl. The same saw pulled 238 psi on a cool dry day a month later.

It's inconsistent and just a comparative gauge at best. If you've got a clean piston, a good feel when you tug the rope, and a saw that runs fine and has power-the test really isn't necessary.
 
crotchclimber said:
OTC makes the tester for Snap-On from what I read. I suppose you would have to make sure the volume of the test fitting exactly makes the volume of the spark plug it is replacing to be totally accurate. Does Stihl or Husqvarna sell compression testers that match the common plugs?
Click to expand...

Some things I found out when constructing a homemade back-up compression tester so as to confirm a low compression reading is not a compression tester fault when I see a low compression reading when testing a small cc engine.

First: The special low pressure Schrader valve (not a standard tire valve) needs to be located at the spark plug adapter that screws into the spark plug hole. If it's not you are already SOL.

Second: make sure the gauge itself is accurate. Connect it too a air compressor and compare to another gauge, but just because a gauge reads accurate from a air compressor does not mean that it will read the true compression of a low CC engine. ( just confirms that the gauge itself is accurate and is capable of reading correctly when properly connected to the proper adapter for a small cc engine)

3rd: Crank the small cc engine (chainsaw) until the comp gauge reading quits climbing. Might take 6 pulls or might take 15 pulls. This will vary by the size, length and even hardness of the hose from the spark plug adapter to the gauge. The more cc volume of the hose the more cranks it will take to reach the max reading.
AND on a chainsaw you do not have to hold the throttle wide open when cranking to get a accurate compression reading. (like is required on a 4 cycle engine) It will get air thru the muffler port.:surprised3:
 
I've got a MS461 in the shop that won't start. First thing I noticed is that compression is very low (decomp off). Hooked up my gauge and it's saying 60psi. So 60+28=88 at sea level, sounds bad right? It won't start, pulled a bunch choked and the spark plug never got wet. Getting gas to the carb, filters are good. Could the compression be so low that the impluse line isn't delivering enough pulse to the carb to pump the fuel?
 
crotchclimber said:
I've got a MS461 in the shop that won't start. First thing I noticed is that compression is very low (decomp off). Hooked up my gauge and it's saying 60psi. So 60+28=88 at sea level, sounds bad right? It won't start, pulled a bunch choked and the spark plug never got wet. Getting gas to the carb, filters are good. Could the compression be so low that the impluse line isn't delivering enough pulse to the carb to pump the fuel?
Click to expand...
Pull the muffler and look at the piston I bet its scored all up. also spray soapy water on the decomp and spark plug base to look for leaks.
 
I'm not real familiar with that model, but 88psi is really too low, especially when a chainsaw compression gets below 100psi is a bad thing. Do like lone wolf says and while you have the spark plug out or the muffler off give it a squirt of mixed gas into the crankcase then try for a pop with a new spark plug. (just a little bit mixed gas primer, don't flood the crankcase and you might have to hold the throttle wide open when cranking and no choke applied)

If the piston is scored real bad you are wasting your time, the saw will be weak even if you get a start/run.

If the piston looks ok squirt some oil into the spark plug hole pull the saw thru couple times with the spark plug out and then check the compression. (warning: this oil is going to inject into your compression gauge Schrader valve and might cause it too leak back slow if it carries little pieces of carbon with it, if so just remove and gently clean the schrader valve and it's seat)This is also why you need spare Schrader valves handy for your gauge.

If the compression comes up momentarily quite a bit after oil is injected into the spark plug hole you have a bad piston/cylinder/rings area.

You might get more replies to this if you would start a new thread about that model saw issues!
 
No leaks from the decomp or spark plug so I pulled the muffler. Piston is scored badly. Guys are destroying all our saws. Going to try premixing the gas for them. Main problem is guys rarely tell me when the saw isn't running 100% right.
 
crotchclimber said:
No leaks from the decomp or spark plug so I pulled the muffler. Piston is scored badly. Guys are destroying all our saws. Going to try premixing the gas for them. Main problem is guys rarely tell me when the saw isn't running 100% right.
Click to expand...

Just some thoughts to ponder: (few things I've seen)
Premix yourself is a good thing, but if a saw starts leaning out in a heavy full throttle cut it can ruin the piston also even though you have the correct pre-mix. You might search around this site about using a IR thermometer and a tach to check the block temp of a chainsaw for overheating lean conditions. (too make sure a saw not being operated lean which causes overheating and piston scraping) If into a full bar cut and a saw is running lean at wide open throttle (rpms are below spec, not excessive rpms causing overheating due to heavy load, too much air is getting into the crankcase causing the lean and heat) it's not readily apparent sometimes to the operator that the saw is self destructing due to overheating) the rpms will slow and kinda act like chain is binding or being pinched and the operator will remove the saw throttle it a few times and it starts acting ok and back into the cut and same thing again within few seconds.
Piston is expanding into the sides of the cylinder and scraping slowing the rpm,s. Saw may not show any lean symptoms on light cuts where throttle is only wide open for few seconds.
I've started using a IR and it will detect almost immediately (once you get familiar with what to expect) if a saw's block is headed towards overheating when testing under full load.

Ask the guys to get a heads up when a saw is not first running correct so you can give it a check. (and also tell them to not do any carb tuning adjustments themselves) Hide the screwdrivers.
 
Good info. Saw' s carb was probably set slightly off. I had replaced the carb about a month ago but this happened after a day of hard use bucking logs all day. This saw has a rev limited coil which I found hard to get the tune spot on. Trial and error in the cut. Used a tachometer too but not as useful as on an unlimited coil saw.
 
crotchclimber said:
Good info. Saw' s carb was probably set slightly off. I had replaced the carb about a month ago but this happened after a day of hard use bucking logs all day. This saw has a rev limited coil which I found hard to get the tune spot on. Trial and error in the cut. Used a tachometer too but not as useful as on an unlimited coil saw.[/QUOTE

Eventually get yourself a good IR thermometer and you will find many other uses for such.
I use one quite often for example to name a few, checking the cylinder balance temps on engines, (a weak misfiring automobile cylinder will be cooler than the others) block coolant temp's on auto's so as to see if dash gauge is accurate, can check temp right at the block thermostat to see if thermostat is regulating properly, heating and A/C, check for hot spots in electrical panels due to bad connections and do this from a safe distance, check running temp of bearings on equipment.
Click to expand...
 
crotchclimber said:
Good info. Saw' s carb was probably set slightly off. I had replaced the carb about a month ago but this happened after a day of hard use bucking logs all day. This saw has a rev limited coil which I found hard to get the tune spot on. Trial and error in the cut. Used a tachometer too but not as useful as on an unlimited coil saw.
Click to expand...
Better to err on the side of being too rich when you set them up. Your guys may have straight gassed that saw pull it apart and if the scoring is on both sides of the piston heavy then yes, one side, the exhaust side, indicates too lean either from a wrong carb setting or an air leak. Going to be an expensive repair if the cyl is destroyed.
 

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