1996 mercury 9.9 outboard, stalls when decelerating

[Mc Tool]

I picked up a 6 HP Johnson for next to 0 beer tokens.----Tag says ---" low compression " ----as reason it was set aside.------I tested it at 90 PSI.-----Doubt there is an issue.-----These motors have a hole above exhaust ports.-----Bleeds off compression at low speed.-----Makes for smoother running.----This has fooled more than one person.

I thought the hole above the exhaust port was to act as a decompressor at cranking speeds , if anything it would make the idle worse .
2 stroke compression figures ( like 9.5:1) are static figures ie ,if you do the maths , in motion the effective comp ratio depends on the porting to some degree. At cranking speeds you have to wait untill all the port windows are covered by the piston before any compressing happens , at faster speeds the fuel mix is delivered via the transfer/bypass ports to the combustion chamber at a partially compressed state and the effective comp ratio goes up and thusly some engines will tolerate a lower static comp reading and still run fine . Typically a 2 stroke with to lower comp will be a pig to get to idle /off idle properly and will probly be a poor starter.....particularly when hot .
At the end of the day you are the judge on that , If your happy with the way it starts and runs then it has enough compression

 

[racerone]

Compression in 2 strokes is a confusing subject I think.

 

[Mc Tool]

Compression in 2 strokes is a confusing subject I think.

Yep , both above and below the piston

 

[flyingscott]

I thought the hole above the exhaust port was to act as a decompressor at cranking speeds , if anything it would make the idle worse .
2 stroke compression figures ( like 9.5:1) are static figures ie ,if you do the maths , in motion the effective comp ratio depends on the porting to some degree. At cranking speeds you have to wait untill all the port windows are covered by the piston before any compressing happens , at faster speeds the fuel mix is delivered via the transfer/bypass ports to the combustion chamber at a partially compressed state and the effective comp ratio goes up and thusly some engines will tolerate a lower static comp reading and still run fine . Typically a 2 stroke with to lower comp will be a pig to get to idle /off idle properly and will probly be a poor starter.....particularly when hot .
At the end of the day you are the judge on that , If your happy with the way it starts and runs then it has enough compression

Lower compression engines tend to idle smother

I thought the hole above the exhaust port was to act as a decompressor at cranking speeds , if anything it would make the idle worse .
2 stroke compression figures ( like 9.5:1) are static figures ie ,if you do the maths , in motion the effective comp ratio depends on the porting to some degree. At cranking speeds you have to wait untill all the port windows are covered by the piston before any compressing happens , at faster speeds the fuel mix is delivered via the transfer/bypass ports to the combustion chamber at a partially compressed state and the effective comp ratio goes up and thusly some engines will tolerate a lower static comp reading and still run fine . Typically a 2 stroke with to lower comp will be a pig to get to idle /off idle properly and will probly be a poor starter.....particularly when hot .
At the end of the day you are the judge on that , If your happy with the way it starts and runs then it has enough compression

Decompression holes make the idle smoother.

 

[Texasmark]

A---" crossflow "-- has a deflector on the piston crown to direct incoming charge to scavenge the cylinder.-----A looper has a flat / nearly flat crown and intake ports positioned to loop the incoming charge.

Plus the Looper has a tuned, reflective exhaust function where the exhaust from one cylinder helps to keep the raw fuel in the combustion chamber of a cylinder as the piston is closing off the exhaust port before the big bang. Initially that came to being with the OMC, Evinrude, Trump 3 cyl. 55 HP engine as at the time 3 cylinders were required to get the correct exhaust reflections to make it work. Over time designers figured out how to get 2 and 4 cylinder engines do function properly....have no idea what that took.

My experience with both types of engines is that the cross flow was bullet proof in a lot of situations except for gas consumption. Loopers run circles around them hands down in that accord. But loopers have their differences that have to be dealt with....in my experiences with half a dozen crosses, and later half a dozen loopers. If the cross had the efficiency of loopers I would prefer the cross.s.......my opinion.....and everybody has one........

 

[flyingscott]

Plus the Looper has a tuned, reflective exhaust function where the exhaust from one cylinder helps to keep the raw fuel in the combustion chamber of a cylinder as the piston is closing off the exhaust port before the big bang. Initially that came to being with the OMC, Evinrude, Trump 3 cyl. 55 HP engine as at the time 3 cylinders were required to get the correct exhaust reflections to make it work. Over time designers figured out how to get 2 and 4 cylinder engines do function properly....have no idea what that took.

My experience with both types of engines is that the cross flow was bullet proof in a lot of situations except for gas consumption. Loopers run circles around them hands down in that accord. But loopers have their differences that have to be dealt with....in my experiences with half a dozen crosses, and later half a dozen loopers. If the cross had the efficiency of loopers I would prefer the cross.s.......my opinion.....and everybody has one........

Plus the Looper has a tuned, reflective exhaust function where the exhaust from one cylinder helps to keep the raw fuel in the combustion chamber of a cylinder as the piston is closing off the exhaust port before the big bang. Initially that came to being with the OMC, Evinrude, Trump 3 cyl. 55 HP engine as at the time 3 cylinders were required to get the correct exhaust reflections to make it work. Over time designers figured out how to get 2 and 4 cylinder engines do function properly....have no idea what that took.

My experience with both types of engines is that the cross flow was bullet proof in a lot of situations except for gas consumption. Loopers run circles around them hands down in that accord. But loopers have their differences that have to be dealt with....in my experiences with half a dozen crosses, and later half a dozen loopers. If the cross had the efficiency of loopers I would prefer the cross.s.......my opinion.....and everybody has one........

How does a crossflow not have a tuned exhaust? I dont think you know the difference between a looper and a crossflow.

 

[racerone]

Some of the V-4's and V-6 models ( bubble back ) from 1973 to 1998 certainly had the tuned exhaust.

 

[Mc Tool]

Some of the V-4's and V-6 models ( bubble back ) from 1973 to 1998 certainly had the tuned exhaust.

I think that 2 stroke outboards are pretty crude compared to a 2 stroke motorbike engine .
Im sure they did what they could exhaust wise but to get an effective exhaust scavenge it really needs one exhaust per cylinder........as evidenced by ALL 2 stroke bikes since the 60's. There simply isnt room for this on an outboard so they make the best of it .
And .......its a decompression port ....not got nuffin to do with smoothing idle . Thats why you only see them on pull start ( or kick on motorbikes ....my RD 400 has them )motors . It has little or no effect at idle speeds . I have seen plenty of small motors that have become hard to pull over because this port is blocked with carbon . Being as one of the symptoms of low compression is a poor idle it simply doesn't make sense that this will improve idle . 2 stroke outboards are appallingly inefficient ......but inherently reliable so they survived .

 

[flyingscott]

I think that 2 stroke outboards are pretty crude compared to a 2 stroke motorbike engine .
Im sure they did what they could exhaust wise but to get an effective exhaust scavenge it really needs one exhaust per cylinder........as evidenced by ALL 2 stroke bikes since the 60's. There simply isnt room for this on an outboard so they make the best of it .
And .......its a decompression port ....not got nuffin to do with smoothing idle . Thats why you only see them on pull start ( or kick on motorbikes ....my RD 400 has them )motors . It has little or no effect at idle speeds . I have seen plenty of small motors that have become hard to pull over because this port is blocked with carbon . Being as one of the symptoms of low compression is a poor idle it simply doesn't make sense that this will improve idle . 2 stroke outboards are appallingly inefficient ......but inherently reliable so they survived .

Piston ports which is what most dirt bikes are, is a terrible design, No amount of exhaust tuning can fix the Holes they have in their powerband. The reed valve design is significantly better. You also have to look at the ports timing which is different than outboards.

 

[flyingscott]

Some of the V-4's and V-6 models ( bubble back ) from 1973 to 1998 certainly had the tuned exhaust.

The 15 hp Evinrude from around 1984 up has a tuned exhaust and later 9.9s also.

 

[Mc Tool]

Piston ports which is what most dirt bikes are,

Yeah? Not since the 80's . Im trying to think of even one 2 stroke that doesnt have reeds ,or discs as some Suzuki and Kwaka's did .....in the 70's . Chainsaws are piston ported.
I guess one thing we should remember is that when these motors were designed or made no one gave a 5hit about emissions etc ,and efficiency took 2nd place to reliability and simplicity .
Things have changed

 

[Mc Tool]

The 15 hp Evinrude from around 1984 up has a tuned exhaust and later 9.9s also.

Yes , it was an improvement over an exhaust port dumping into free space ,but your playing fast and loose with the term "tuned "

 

[flyingscott]

Yes , it was an improvement over an exhaust port dumping into free space ,but your playing fast and loose with the term "tuned "

No I am not, because if it didnt do anything why make a round and square one. Just because you dont understand it doesn't not make it tuned.

 

[flyingscott]

Yeah? Not since the 80's . Im trying to think of even one 2 stroke that doesnt have reeds ,or discs as some Suzuki and Kwaka's did .....in the 70's . Chainsaws are piston ported.
I guess one thing we should remember is that when these motors were designed or made no one gave a 5hit about emissions etc ,and efficiency took 2nd place to reliability and simplicity .
Things have changed

Any snowmobile built into the 90s. Any dirtbikes built into the 90s. Any chainsaw built today. How are a piston reed or case reed any less reliable than a piston port?

 

[Mc Tool]

Any snowmobile built into the 90s. Any dirtbikes built into the 90s. Any chainsaw built today. How are a piston reed or case reed any less reliable than a piston port?

Well ,obviously ,I thought , the piston ported engine doesn't have reeds to go wrong.

 

[Mc Tool]

No I am not, because if it didnt do anything why make a round and square one. Just because you dont understand it doesn't not make it tuned.

Wow , thats powerful logic there ,I .....ah......yeah

 

[flyingscott]

Wow , thats powerful logic there ,I .....ah......yeah

Just trying to explain it in words even you can understand. Should I make it simpler?

 

[Mc Tool]

Just trying to explain it in words even you can understand. Should I make it simpler?

Nah , Ive moved on