Compression Issues.-

[Sea Rider]

If a 2 strokes OB should have 120 PSI in both cylinders and only has 95 on upper one and 90 on lower one, will this be the direct cause for OB to be difficult to start and once started doesn’t want to maintain idle or even fast idle, just dies as it were a fuel starvation issue. The OB was tested geared forward on muffs at different rpm ranges.

The fuel pump and diaphragms are doing their homework right, carb and filters are clean, has perfect time advance and carb synch, new plugs, fresh fuel and oil. Thermostat is missing, could it be the cause for OB to behave in that strange way ?

Happy Boating

 

[pvanv]

You need a thermostat, or the plugs will foul.
Compression should be within 10% of specs... 5% is better. But it would still run, just not make as much power, and could well be hard to start.

 

[Sea Rider]

Paul,

What's the factory specs if any regarding which is the least PSI to consider before going for new standard ring set or standard new piston and ring set, worse case escenario a 0.5 piston/ring kit ?

If say 120 PSI is ideal from new, right now has 90/93 on both cylinders. Only starts or re starts with choke, dies at idle, fast idle rpm. Stays on passing fast idle...All parts including electrical ones are OK.

This horse is full of issues, was used with : no thermo, incorrect spark plug set (Champion L78C) cold plugs, bad timing, badly taken care among other issues. Has no more than 150/200 worked hours. A real OB misfortune.

Happy Boating

 

[pvanv]

Anything under 100 will cause idle and starting issues.

 

[Sea Rider]

Anything under 100 will cause idle and starting issues.

Thanks, good to know and not guess nor waste precious time troubleshooting with no avail what's happening to that OB. Don't have much experience dealing with cylinder compression issues as didn't count with a tester. Now counting with one it's a different story.

Will install a new thermo, do you think that spraying say half can of power tuner through carb's throat while OB runs geared on muffs at 2 K rpm will increase cylinder compression ?

Happy Boating

 

[pvanv]

It might. Possibly carboned-up rings.

 

[Sea Rider]

It might. Possibly carboned-up rings.

Hope so, this OB has been extensively used for long time periods as a tender on very short distances while working with high oil ratio excess. Has never run full grip on longs distances outings. Will post results once it's figured out how to remove both salted stuck thermo bolts.

Happy Boating

 

[Sea Rider]

Update :

Broke both thermo bolts heads when trying to remove thermo housing, were extremely salted and stuck, didn’t want to cede a bit after a long W-40 soak application. Heating both bolts with a propane torch did the trick, Internal thermo housing was found a mess, rusted, crusted, was polished to perfection with a Dremel Drill, small wire brushes and holy patience.

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Prior installing new thermo and gasket, all water passages were conveniently high water flushed with a Karcher washing machine set at min pressure. Now OB pees as a teenager than before. This is no indication that heavy salt layers already built inside powerhead will have been removed to perfection, for this task will need to tear head, side lids apart to have all water paths crusts mechanically removed.

Installed a new thermo and gasket. Installed muffs while OB sat on a stand, opened the water faucet, throttled the OB till reached its proper operating temp. While thermo opened and closed properly could feel the water temp difference on hand palm at rear middle leg.

OB tended to die at idle speed, so idle screw was adjusted a bit CW to increase rpm for OB not to die.when geared forward. Throttled to 2-2.5 K constant revs while squirting small doses of CRC EngineTune Up for 15 minutes. OB needs to be geared forward to have a load. Just be aware that prop is spinning high and will chop anything standing next to it.

As the product was removing carbon build ups on piston rings while being burned on combustion chambers, OB started to smooth out. Squirted a final overdose till OB died, let it soak for one hour, started OB and let product clear itself till min smoke was visible exiting through prop.

Mounted OB on a boat and with 2 up went for a 10 mile wot run as if stealing the combo. Now OB remains ON at idle, purrs as a kitten and revs very fast throughout the whole throttle range than before. Can't precise how much compression the OB has gained as didn't have my compression tester handy to check that out again. Saint CRC Engine Tune Up has done a superb long decarbonize job. LOL!!

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This product is highly recommended, just don’t expect to remove severe carbon build ups with few product squirts, need at least to squirt 1/4 to 1/3 can contents through the carb. If applied as previously described will clean much better, deeper and faster.

Happy Boating

 

[ahicks]

When doing compression checks, let me toss an idea. Something I've seen cause a lot of unnecessary grief over the years.

A friend of a friend had a 2 stroke engine that read low on a compression check. As the cylinder bores looked good when the head was removed, he replaced the pistons and rings after giving the cylinders a light scuffing. After a short break in, he decided to check the compression again. There was no change from the original check that led to all this work.

Why? After a lot of head scratching, his compression gauge was found to be reading incorrectly (low), which wasn't discovered until AFTER the rebuild when they compared it to a quality compression gauge.

Lesson? If doing a compression check, look for a big difference from cylinder to cylinder. That's a guaranteed issue. When close (as in the example of 90/95), ALWAYS confirm uniform low readings with a second compression tester unless using one that's used regularly that you know and trust (even if it's new!).

 

[Sea Rider]

Compression tester is brand new, have tested my 2 strokes horse with it and reads 118 and 116 PSI respectively and that's having 850 metered worked hours. When turning flywheel by hand on mine, you can feel medium high compression restriction.

On this OB matter of the present post you can turn flywheel same with near least compression restrictions compared to mine, a clear call sign that rings and pistons or just rings are bad along worn cylinder walls. That needs to be correctly diagnosed at a machine shop.

According to the answers received on this forum, less than 100 PSI of compression will make OB to be hard to start, needs lots of choke operation and tends to die at idle rpm.

Lesson learned : If Ob is in good running shape with 0 issues throughout the whole throttle range, simply ignore what pressure gauge indicated..

Happy Boating