91 150 evinrude compression

[cowboyup]

I fired it up for the first time since the rebuild earlier this week. Runs like a new motor. But I'm curious about what I sould expect from the compression readings. Since it hasn't been broken in (only about 15 minutes on the muffs), does the +/- 10% rule still apply?

 

[JB]

Re: 91 150 evinrude compression

Yup.

 

[cowboyup]

Re: 91 150 evinrude compression

Thanks JB.

 

[walleyehed]

Re: 91 150 evinrude compression

I would stay away from compression readings until you get to atleast 10hrs. The rings have not seated for the first few hours...<br />Actually, I'd wait until I had 5-10hrs on 50:1, so 15-20hrs.

 

[cowboyup]

Re: 91 150 evinrude compression

Thanks Walleyhead. O.K. Well, it's 1 to 1. Anyone care to break the tie?

 

[sikpnter]

Re: 91 150 evinrude compression

Dont worry about compression just double the oil and get the thing on the water! You cant break it in properly on muffs. later

 

[walleyehed]

Re: 91 150 evinrude compression

okstatefan, it really doesn't matter what the compression is at this point....everything has to be seated properly to "seal", so until the engine is broke in, it is of no value at this point.<br />JB's answer is not incorrect, as they should, technically, be within a few pounds anyway but compression readings will mean more at 20-25hrs.<br />Another fact I've been made aware of is the "Idle break-in" procedure....let the engine Idle for 3-4hrs to allow cylinders to warm to temp, T-stats open and cool back down and the cycle continues and is considered the "Best" way to properly break in an O/H as the temp goes up and down without creating over-temp or super high combustion temps.<br />Engine would need to be in a tank or boat on the water tied to dock for proper back pressure for this.

 

[cowboyup]

Re: 91 150 evinrude compression

thanks again Walleyehead. That's the answer I was looking for. I knew you had it in ya. I do have one question about the "Best" way to properly break in an O/H though. Do I follow the service manual's normal break in procecdure after this 3-4 hr break in or is there a better method?

 

[walleyehed]

Re: 91 150 evinrude compression

After the Idle break-in, follow the rest of the manual's break-in procedure, and you should be good.

 

[rodbolt]

Re: 91 150 evinrude compression

test the compression new then test it at 20 hours. wont be 2 psi different<br /> if it is there is a problem.<br /> I have leakdown tested to many to believe in the old ring seating myth on a two stroke. if it wont seal when new it aint gonna get better with age. <br /> its a habit I got into years ago after watching a co worker get bit. somehow the top ring on a 3cyl broke at installation. he messed with it a long time before we tore it back down. now once I get the short block assembled I test for leakdown. the results have been amazing. usually less leakage before running than after 10 hours.<br /> if you make a fixture to lock the flywheel you will find a leak down test is as fast or faster than a compression test and much more accurate but on the crossflow motors you can check for a broken ring with the transfer port covers off so its not nessasary.<br /> so do the compression test now and in 10-20hours and post your results. I would be interested to see the numbers.

 

[walleyehed]

Re: 91 150 evinrude compression

rodbolt, ya let the cat out of the bag....that's EXACTLY why I didn't want him to worry about it now.....If it drops in 20hrs, they want to tear it apart!<br />"The old ring-seating myth" is why the engine needs to be run at Idle for 4 hrs....FIRST.<br />Are you ok today, rodbolt???? acting like you've been messin' with a Force...???? I'm worried...

 

[cowboyup]

Re: 91 150 evinrude compression

I like the idea of having a reference point for compression. And yeah, I'm gonna think about it a time or two. But I followed the service manual to a T on reassembly and the initial startup went as I had hoped. So I feel like the outboard gods have given me a thumbs up so far. Thanks to everyone for their input and I'll post the initial results hopefully this week.

 

[rodbolt]

Re: 91 150 evinrude compression

walley<br /> while the rings will wear to the bore an infantismal amount its the piston that we want to go through several heating and cooling cycles. most owners manuals like to fast idle for the first 10 min then less than 1/2 throttle for the next 4 hours then short bursts at full throttle followed by 1/2 throttle and constantly vary the speeds. the reason to back off is to allow the pistons and walls to cool again. the rings actually wear in very little. its the piston we are trying to make happy that is why after 100K miles you can tear a motor down and stillsee the cross hatch pattern. after building many race motors we found more problems with piston scuff than rings. most 1/4 mile motors see little in the way of breakin but we are not concerned with longevity. however we found many a broken skirt and scuffed sides due to the piston not seating and conforming to the bore and instead of riding on a film of oil it rode metal to metal.<br /> I have probably built as many or more 2 stroke powerheads as any on here and most likely a lot more 4 strokes than any one. I did 3 a week for HR Floyd motors in cleburne TX for almost 3 years. plus Dans machine shop in Joshua TX and solo auto with the machinist there. old man was one of the best I ever worked with. so I do have a limited experience with tying to figgue out myth from reality. I have seen techs leave the coolant out and overheat a new motor to seat rings, I have seen them installed with only wd-40 on the claim that lubriplate or engine oil will prevent ring seal. the only one I did find that would prevent proper breakin was slick 50 in a new engine. that causes problems . but next time ya put a fresh one together do a leak down and another at the 10 hour check and log the results. my IDC 2300 dyno log has all kinds of scribbles in it but you have to remember the sealing characteristics are different on a two stroke compared to a 4 stroke. you really cant compare the two.

 

[walleyehed]

Re: 91 150 evinrude compression

No argument about who's O/H'd more engines, 2 or 4-stroke.<br />Ring to piston to cylinder still has to "Seat"..<br />I've researched this "Idle for 3-4hr." thing, and have come to the conclusion it's "The only way" to properly seat the parts involved, just on the basis of how it works...<br />I've done comp test on fresh O/H's, and followed with tests at 20-30 hrs...usually it's real close, but with assembly lube, it's gonna have a better seal, and at that point, to me anyway, compression is irrelevant.<br />No bad intended.