strange compression test results from a 86 moto4 250

[VikingTrad3r]

I have an 86 yfm250 (moto4 250) i picked up. 

 

It bogs and pops a bit out the tailpipe if you can even get it higher up in the rpm. It is pretty gutless. I have other 250’s (timberwolves) and its ultra gutless compared to them. 

It has an aftermarket carb on it that has a “W” on it  Its not a mikuni but it looks close  

i figured jetting was off at the high end. 

i had a mimuni that was very crusty, swapped the main jet into the W carb, and the needle, which was set on low slot 4 so left it there.  

 

it actually ran much better, and cured what i thought was a problem caused by a lean condition from the W carbs main jet being too restricted when i compared to the factory mikuni  

I had it out and was enjoying the upper rpms that i couldnt reach before without bog.  at wot it would fall on its face  low top speed.  

after getting it hot, it would actually refuse to start after dying.

 

back to the drawing board.

 

when i first got it i adjusted the valves. So i retraced my work and thought maybe id overtightened. double checked, nope all good.  .005 exh , .003 intake, check.

So i checked the compression. 

here is the trickyness. help me understand this.  I am at 3500ft asl  in calgary canada  

 

-  initiate test, it goes to 75psi right away. this is low. Even at my elevation im thinking 110psi minimum  

but....this thing starts no problem. even -20c it starts. makes no sense to me. so i press the starter again and it goes to 85. Then i press it again and no change. Then i turn over again and it jumps to 95. Then again...no change. Then again and it goes to 115.  finally finishes at 120 and i cannot get it to go any higher than this. 

 

ive repeated this multiple times. 

 

i press and hold the starter...it will max at 75. Then a series of maybe 10 more quick turnovers from the starter and compression inches up to 120psi. 

 

So, i think the low compression explains the low top end, and even the backfiring out the exhaust when hot. The exhaust is rich enough (incomplete combustion in chamber) and to ignite in the muffler. 

it gets worse when hotter. 

 

makes me think i have a crack, or something that is low down on the cylinder wall. When the piston stops just above it on the upstroke, and i then turn it over again, it adds pressure to my tester reading. 

 

thoughts?

[Mech]

A compression that climbed over a succession of compression strokes used to be attributed to weak rings. If you have a vacuum gauge you could check the readings..

If you put about a teaspoon of engine oil down the sparkplug hole then check the compression it should go up quite a lot..  Sticky or leaking valves don't get improved by oil down the bore, rings do.

[Gwbarm]

Due to the age of the machine and the reputation of abuse these engines get I agree with mech it could very well be the rings, but it could be a combination of worn rings and leaky valves. If the rings were worn bad enough to cause this problem it should be smoking a lot. Always check the compression with the throttle completely open. If this happened to me the first thing would do would be try a different compression tester, I have not had good luck with testers accuracy. You said the more you tested the higher the compression would get so the more rotation the more oil is getting pumped through the system and would also point to rings, but because of the popping and backfiring I would think leaky valves, there could be carbon deposits not letting the valves close completely, I would suspect the exhaust valve. You said that when it gets hot it won't start that does point to low compression or flooding . Maybe do a compression test when it won't start hot and see what you get. Just my thoughts.

[VikingTrad3r]

1 hour ago, Mech said:

A compression that climbed over a succession of compression strokes used to be attributed to weak rings. If you have a vacuum gauge you could check the readings..

If you put about a teaspoon of engine oil down the sparkplug hole then check the compression it should go up quite a lot..  Sticky or leaking valves don't get improved by oil down the bore, rings do.

I tried the oil down the spark plug hole trick and got no change. 

28 minutes ago, Gwbarm said:

Due to the age of the machine and the reputation of abuse these engines get I agree with mech it could very well be the rings, but it could be a combination of worn rings and leaky valves. If the rings were worn bad enough to cause this problem it should be smoking a lot. Always check the compression with the throttle completely open. If this happened to me the first thing would do would be try a different compression tester, I have not had good luck with testers accuracy. You said the more you tested the higher the compression would get so the more rotation the more oil is getting pumped through the system and would also point to rings, but because of the popping and backfiring I would think leaky valves, there could be carbon deposits not letting the valves close completely, I would suspect the exhaust valve. You said that when it gets hot it won't start that does point to low compression or flooding . Maybe do a compression test when it won't start hot and see what you get. Just my thoughts.

tried a different test with same result.

 

im trying a leakdown test to see where the air escapes. 

 

absolutely zero smoke. none. AND it starts easy as pie. 

will also run a borescope in but i wont be able to get it to turn around and face up at the valves. 

[Mech]

If the oil made no difference then it points to the valves.. Sticky in the guides, badly worn cam, play in the cam bearings, play in the timing chain, carboned up valve heads(except that doesn't happen these days), something blocking the exhaust, like a flap of rusty pipe inside,

It is sort of normal for compression readings to climb on each compression stroke, up to about four or maybe five strokes, then after that you might get an extra five pounds if you are lucky.. But you've done plenty of tests, and know that one right, and this is different in that it doesn't build up if you just keep your finger on the button. You have to let it get another breath between cranks.. Right ?  I'm just trying to get this clear..

Not enough lift on the inlet valve.. ??

[VikingTrad3r]

3 hours ago, Mech said:

If the oil made no difference then it points to the valves.. Sticky in the guides, badly worn cam, play in the cam bearings, playYou have to let it get another breath between cranks.. Right ?  I'm just trying to get this clear..

Not enough lift on the inlet valve.. ??

chain is tight, didnt see any strange behavior on the way the rockers were moving while doing compression tests with rocker covers off  

leakdown test showed minor leakdown past rings. minor. valves tight  id say the rings are slightworn yes  but down on a bit of compression/power is really different than it coughing and sputtering at high rpm  

this quad starts instantly, purrs like a seeing machine, and blows zero smoke. 

im doubting compression now. 

 

i have a second “parts” moto4 and decided to test the ignition coil of it vs the current patient. to my surprise on the patients sparkplug wire/coil, i was reading open from the coil orange wire to the spark plug connector. meanwhile the parts read 10.75 kiloohms. 

 

so i swapped them. 

 

installed a new petcock in the fuel tank and just sat it on the frame. After some minor carb tuning it ran like a sewing machine. 

 

So.....i will take it out for a rip tomorrow but im thinking....compression is a bit down yes. But thats red herring. i am thinking the sparking was jumping inside the wire from the ignition coil to the plug. somewhere there mist be an open because i could not get a resistance reading out of it. 

 

that may explain the high rpm sputtering. faster spark and the jumping of hypothesized gap crossing made the timing off. 

 

thats my theory and ill know more when i get a chance to drive it tomorrow. 

also i cant rule out the weirdness of the compression incremental rise.   yes if i keep going with the starter it maxes at 75-80 psi. if i then bump the starter several more times it randomly adds compression up to 120. strange!

[Mech]

Yeah that is strange..

If the cap comes off the end of the sparkplug lead I'd take that off and check it's resistance.. those play up.

[Gwbarm]

Ive had trouble with those caps also, when in doubt replace it, they are cheap also the carbon wires are also prone to break somewhere between coil and plug I would go ahead and replace the wire as well and check all coil connections. I agree if it cranks as easy as you say with no smoke, compression doent seem to be a problem, but strange readings, a 4 stroke engine should barely run with compression of 75 if at all, and would certainly be hard to start. So im thinking your 120 maybe closer to .correct