225 Pro XS low compression 70psi #3, loud popping thru throttle body.

[bmac101960]

​Help needed. My 2010 225 Optimax ProXS DFI S/N 1B786424 Mirage plus 15.5 X 17 with 340 hrs.
Runs smooth at idle after warm up. surges and struggles between 1500 and 3500 rpm then runs smooth on plane after 3500 and up to 5200 rpm at wot. I believe 5800 would be expected. I have replaced all fuel filters, new ngk iridium plugs gapped at .040, new ign wires. no change after all the above. compression check reveals 90-95 psi all cylinders except #3 which is at 70psi and does not seem to be firing at idle. swapped both the coil pack and injector in that cylinder with no change. scoped that cylinder thru plug hole and looks good. Can bad reeds cause this low compression? I am an ACE certified master auto mechanic but have very little experience with 2 stroke motors. I don't know what the normal sound from the air plenum/throttle body should be on this engine but mine sounds very loud to me. Similar to what you would hear on a 4 stroke with a stuck intake valve, however in that situation you would get almost no compression in that cylinder. Any advice would be greatly appreciated prior to stripping down the engine. Also seems to be a small leak of 2 cycle oil from the bottom of the air plenum housing. Has no codes or warning from SmartCraft system.

[bpicinc]

Sounds like an intake reed valve on #3 cyl. If reed has broken it may have damaged piston.

[bmac101960]

Thank You. I scoped the cylinder and it appears to be good, at least on the upper end that can be seen with the piston still in the engine. Can the broken reed cause the compression loss? or should I assume there is damage I cant see or possibly a damaged or stuck ring?

[Savage]

Need to perform a leak down test on #3 cylinder.

[EuropeanAM]

+1

Will probably end up removing that head for inspection in the end, but agree Leakdown Test is the BEST step to follow currently.

[bmac101960]

Thank You for your advice. What is the proper way to perform a leak down test on this engine? I'm assuming piston position is very important on a 2 stroke

[EuropeanAM]

Recommend very close to TDC (on the compression stroke). Flywheel turns CLOCKWISE.

Normally done with 100psi regulated air pressure, hold that flywheel with a suitable holder and watch secondary gauge for measurement of leakdown.

Example: 100 psi supply, 94 psi on Secondary Gauge = 6% leakdown.

And of course, LISTEN for leakage to midsection, coolant/water passages, etc.

[Rip Bass]

Bmac,you can remove the throttle body from the top of the plenum and inspect the reeds with an extendable inspection mirror. Use a flashlight on the mirror and you can see all the reeds. I found a broken reed on my opti using this method.

[Savage]

Bmac,you can remove the throttle body from the top of the plenum and inspect the reeds with an extendable inspection mirror. Use a flashlight on the mirror and you can see all the reeds. I found a broken reed on my opti using this method.

But you can't really see ALL of the reed(s) by this method...…. the plenum must be removed to see the complete condition of the entire reed.

[EuropeanAM]

+1

[bmac101960]

Thanks for the advice. I have an inspection camera that is the diameter of a pencil, has LED light and a 90 deg mirror. Hopefully this would give me a good view of the reeds. I will do the inspection thru the plenum and perform the leak down test that was recommended and report back with the results. I am away on business now and will report back when I return in about a week. I'm still unsure if broken reeds can cause low compression. I guess I'm hoping for the best but will probably be disappointed. My guess is a reed broke off and damaged the cylinder wall, piston, and or rings.

[lpugh]

Thanks for the advice. I have an inspection camera that is the diameter of a pencil, has LED light and a 90 deg mirror. Hopefully this would give me a good view of the reeds. I will do the inspection thru the plenum and perform the leak down test that was recommended and report back with the results. I am away on business now and will report back when I return in about a week. I'm still unsure if broken reeds can cause low compression. I guess I'm hoping for the best but will probably be disappointed. My guess is a reed broke off and damaged the cylinder wall, piston, and or rings.

You have been given some very good advice from some top people in the marine business, Don't ignore what they have told you, Don and Savage make their living in this business. You can not check reeds properly with a camera, pull the intake. If you know how and are capable of interrupting the results you can quite often tell if you have reed problem by performing a running compression test. Very doubtful that a reed would cause cranking compression to be low, would cause running compression on a snap test to be low though
A DSO set up properly to scope test the MAP sensor wave form will also show a bad reed, just trigger it off a the plug wire to sync the pattern

[bmac101960]

Finally was able to perform leakdown test. Results on warm engine piston slightly before TDC.
#1 8% #3 4% #5 5% #2 2% #4 2% #6 2%
Leakage was heard out of prop and plenum equally on cylinders 1,3,and 5 and none detected on 2,4,and 6
I was able to see all of the reeds (10 reeds on 6 blocks) with the scope, but not the entire reed. 2 Pictures attached but all look the same.

reed pic.JPGreed pic2.JPG
I also used Power Tune as directed on the can prior to the test.
What would cause # 3 cylinder to have lower compression then the rest with only 4% leak down. Should I pull off the starboard head and inspect the cylinders?
what should my next step be?

Thanks for your help.

[lpugh]

A bent rod or a serious intake air restriction on that cylinder though very unlikely
I would recheck compression and leak down just to be sure of accurate tests

[bmac101960]

leakdown was done twice with a new quality gauge. I will recheck compression with a new gauge to confirm. A bad gauge would certainly be better news then a bad engine

Thanks

[lpugh]

Probably not a gauge if you get the same readings every time on the same cylinder, some guys only crank the motor thru 3 compression cycles which I feel is not accurate. I crank the motor till peak pressure is recorded paying close attentions that all cylinder rise to peak pressures in very similar cranking time. I do expect peak pressure by about 5 pulses. Usually slow rise times is a problem with the gauge not the motor. Such as debris in the shrader valve
Every now in a gauge can be damaged causing the needle to hang-up resulting in erroneous low readings, That's why I recommended retesting to verify
Bent rods rarely happen to the degree of a 30 psi loss and a restricted intake even more rare on a two stroke

[ChrisCarsonMarine]

My opinions on leakdown testing procedure are a little different than most,and render the most accurate results,again my opinion.Most folks follow the directions and put the piston at top dead...where there's no wear,and usually get low leakdown results accordingly.A poor mans leakdown test is really better,and is faster,cheaper,and easier...do a comp check,just write down the compression numbers,squirt a couple shots of 30 wt oil in each cylinder,spin the motor over a few seconds and recheck the compression...the amount the compression goes up will be close to overall leakdown percents.
Now my way.Follow the normal procedure but instead of locking the flywheel near tdc,pull the motor from ports open to tdc on every cylinder,stopping at several spots along the way,and look at the resulting leakdown along the working stroke of the cylinder.
We do this with a ratchet on the flywheel nut,with a long bolted in place tube extention on the ratchet handle...(3 foot long,specificly for this task).
DO NOT USE A BREAKER BAR for this job...when you pull the piston over tdc the 40-50 pounds of pull required will turn to push and may DE-BRAIN you...the ratchet will allow the flywheel to spin to to open the ports.
This procedure will locate bulged cylinders,scoring above the ports etc.,that a locked flywheel test won't know about.
Also,for the most accurate results check the port side of a "v" motor by pulling the flywheel clockwise,and the starboard counterclockwise to place the piston in its thrust position.I test
at 75 lbs.
I figure if you're going to test you might as well get the best results you can,I should'nt have to tell you to keep fingers clear,and be careful,as the flywheel is going to spin as it goes over tdc,but i will anyway,BE CAREFUL
That being said...
A twenty or thirty pound difference in compression with a good leakdown report makes no sense...something dosen't add up,unless there's major damage just above the ports,my method checks the whole cylinder,not just the top that rarely gets damaged.
A broken reed will make little to no difference in cylinder compression.
You may have an air injector sticking open,that might explaine the low compression and not running on that cylinder.

[bmac101960]

Thanks Chris.
I did the test as you described at 75 psi. This was scary at 75 so I did not attempt any higher. I also only checked #3 cylinder as that is the only one with low compression. The results on #3 was 72-73 psi the entire compression stroke. I did retest the compression on all cylinders with a brand new gauge. All except #3 were at 90psi warm engine wot. #3 again was at 70psi. Is there a way to clean or test the air injector? Even if it needs to be replaced with a new one, that is much better then an engine tear down. Also #3 cylinder is now firing at idle after the power tune treatment but doesn't seem to have the same effect on rpm as the rest of the cylinders.

Thanks so much for your advice.

[ChrisCarsonMarine]

Switch the no 3 air injector to no 1 and vice-versa,see if the problem moves with it,let me know,Chris

[bmac101960]

I Will do and report back.
Thanks again