Compression test

[uncledj]

I've an old Polaris Trail Boss ATV.   I keep it around for yardwork and to ride around at a friends farm.   

Anyhoo

  It started running poorly last year so I put it in storage with ethanol free gas with seafoam as a stabilizer.  Pulled it into the garage and did a compression test and it was really low.   Around 50psi.   I put 5 or 6 drops of oil in and checked again with the same result.   Been a while since I've done this, but I seem to remember that if there's no change after adding a bit of oil, it's most likely the valves.

I'm hoping I can just pull the head off and give it a valve job.   Don't want to put too much money into it.   

So...As the compression reading didn't change after adding oil....can I be confident the problem's with the valves?   I'm guessing burnt.

[vzuden]

5-6 drops sounds a little light.  I usually use an oil can with a trigger pump and give it two to three squirts.   The idea is that the oil will make up for worn rings so if the compression is low and then increases with the oil, the rings are worn.   If there is no change then there can be leakage at the valves

[uncledj]

Found a manual that says normal compression should be 50 to 90.   Compression ratio is 9.2 : 1, but there's an automatic compression release that I was not aware of.

Either way, ...popping thru carb and backfiring, combined with a black (carbon) spark plug tells me there's something going on.   I'll see if I can figure out this compression release and see if it's working before I tear the head off.   I'm still betting on valve issues.

[BlueSky]

Lapping the valves may be all that is needed if the valves are a problem.  I would put a lot of seafoam or gumout in the gas and see what that does.  

[saddlebum]

I would move one step further and do a cylinder leak down test. This I have found to be the most accurate way of determining were the engine is loosing its compression. You may have to disable the decompression system in your case to do this, which may or may not be possible depending on the type of system used.

I will often forgo a compression test and go directly to a leak down test, specially when I am working a multi-cylinder engine were compression has to be taken with the engine running such as a two stroke Detroit diesel, were you have to take down the top end replace the injector with a compression adapter put the engine back together, run it on the remaining cylinders and repeat for each cylinder. Bad enough to do on a 4 cylinder but worse when you get into 8 or even 16 valve cylinders.

[uncledj]

Pulled the head off and found it really carboned up, but saw no evidence of damage, or anything leaking through the valves.   I cleaned it up and really don't think there's an issue with the top end.  Cylinder walls look good,...no scoring, no ridge at the top.

SO...The 50 psi compression reading is within specs...(50 to 90) The carb was put on just last year. (Chinese), but I'm wondering about the carb....non adjustable, except for idle...but running rich wouldn't explain popping back through the carb.   Maybe a bad ignition control module...weak coil???... dunno...SO

  For right around $100 I was able to order a top end gasket kit, another Chinese carburetor, an ignition coil with wire and plug, and an ignition module.   This stuff is crazy cheap.

  If I clean up the carbon and throw all these parts at it, it's GOTTA run right.....right???

[saddlebum]

Sure ! what could possibly go wrong

[SpencerPJ]

Always fun tinkering.  You also might check the exhaust side for air leaks, I mean since it's all apart.  

[Marcarl]

Put the valves and springs back in, turn head upsidedown, fill the cavities with water and see if there is a leak in an hour or so. I know it sound strange but it seems to work. Water will leak down if there is a gap.

[saddlebum]

48 minutes ago, Marcarl said:

Put the valves and springs back in, turn head upsidedown, fill the cavities with water and see if there is a leak in an hour or so. I know it sound strange but it seems to work. Water will leak down if there is a gap.

This is a good suggestion,  however if you can get your hands on a bit of diesel fuel, this would be a choice over water.

[uncledj]

Finally had time to get it back together.    I ended up cleaning the carbon off of everything, didn't see a need to grind / seat the valves...Put a new carb on and I did notice that the old carb (1 year old) had a tiny bit of corrosion (rust) on the spring that keeps the choke shut.   The choke is simply a needle that pulls away from an orifice when the choke knob is pulled..   They call it an "enrichment valve".   I think this all came down to that valve not closing properly.

I adjusted the valves, and the exhaust valve was a few thousandths tight, but other than that I put it all back together and it seems to run pretty well, albeit throttle response seems just a tad slow, but not a problem.

Further found a bent rod in one of the control arms.   Straightened it as best I could (actually straightened out pretty good_)   Looked like someone nailed a tree or rock with that wheel.   Changed control arm bushings, ran it around the property for a bit, then put it away.

Now lets get that old TT600 freshened up.