early 9ks0's yamaha 25 hp

[dogridgekraus]

Does anyone know what the compression is susposed to be on a early 90's 25 hp Yamaha 2 stroke?
I had right around 90 on both cylinders.Just don't want to buy a outboard with major issues.Just cant find any info.
Thanks

[rodbolt17]

its supposed to be within ten PSI on each cyl with the same gauge.

[dogridgekraus]

early 90's yamaha 25 hp 2 stroke

so does around 90 psi per cylinder sound ok ?

[zenoahphobic]

its supposed to be within ten PSI on each cyl with the same gauge.

That is a rule but somewhat limited when there are few cylinders.

Dogridgekraus, 90 might seem OK, but depends upon the gauge accuracy and how it was measured.

It needs to be cranked WOT vigorously. that is a good charged battery and starter motor. If pull start more difficult to determine.

I tend to watch how the pressure increases after every revolution. Good rings and piston would show the pressure increase rapidly to near maximum. Sluggish increase indicates a tired engine.

To test your gauge use it on a known good engine.

[99yam40]

WOT is no needed on a 2 stroke when checking compression but it will not hurt to do it. They will scavenge enough from the exhaust port to get a good reading
4 strokes are a different story though as they will only pull from the intake side

Try it both ways next time you check compression an a 2 stroke motor

[zenoahphobic]

WOT is no needed on a 2 stroke when checking compression but it will not hurt to do it. They will scavenge enough from the exhaust port to get a good reading
4 strokes are a different story though as they will only pull from the intake side

Try it both ways next time you check compression an a 2 stroke motor

Not disputing anything you say, WOT is used to limit the actual amount of fuel sucked in ( insufficient vacuum at cranking speed) diluting oil seal or adding to incompressible volume artificially raising compression. But rightfully pointed out there are differences between 4 and 2 strokes.

[rodbolt17]

ill dispute your dispute.
take any free throttle that will allow WOT. fire up the engine,warm it up. shut it down,place the throttle at WOT.
crank it and stand the heck by. it will light off at WOT.

the F100 will do it cold.
don't ask how I know that one.

most EFI motors wont due to the flood clear mode used on most EFI setups,not ALL efi setups.

[zenoahphobic]

ill dispute your dispute.
take any free throttle that will allow WOT. fire up the engine,warm it up. shut it down,place the throttle at WOT.
crank it and stand the heck by. it will light off at WOT.

the F100 will do it cold.
don't ask how I know that one.

most EFI motors wont due to the flood clear mode used on most EFI setups,not ALL efi setups.

You're talking about the presence of an ignition , and prewarmed engine to help ignition. I said fuel is limited at WOT at cranking speed.

Cranking speed is less than idle speed (the minimum speed an engine can reliably run.

An engine will rarely run after just one induction and compression cycle, so at WOT minimal fuel delivery requires a couple of cycles to have enough fuel to meet the concentration with the air to combust (fire).

A large enough explosion is required to provide enough energy stored in the "flywheel" to bring through another cycle on its own and to reach this "idle speed".

What you describe is a rapid rise in speed between successive cycles bringing almost immediately into the speed range at which the carburettor can actually suck the right ratio of fuel to air.

[rodbolt17]

it will also depend on the enrichment system.
some of the inline 3 carbed two strokes and some of the V6 2 stroke and most carbed 4 strokes use an automatic enrichment system that forces extra fuel at cranking speeds.
its why you must disable the ign or control the sparks.

even more critical when compression testing any EFI motor.
on any EFI you must disable the ign AND the fuel before compression testing.

not the first fireball I ever saw .

[zenoahphobic]

It is a very good point what disabling an ignition system should entail.

Taking out and away the spark plug and leaving the spark leads loose is not a very good idea from two aspects.
One : the system needs a load to control the possible over voltage destroying the system, and
Two :a runaway spark like a lightning strike could create a fireball as you describe, igniting the mixture pumped out of the spark plug hole.

It seems the spark plug should be connected to the lead as far away as possible, and that it is earthed, to fire as normal.

I do worry about that spark, that the plug should be in a sealed container, preventing any possible vapour being ignited.

I don't do this very often, but if I did I would have isolation containers (?) for each plug.

How is this potential danger managed in the day to day work in the workshop?

[boscoe99]

Activate the kill switch. Crank the motor till the cows come home and there will be no spark. No spark, no ignition. Let the fuel system do as it may.

[zenoahphobic]

Activate the kill switch. Crank the motor till the cows come home and there will be no spark. No spark, no ignition. Let the fuel system do as it may.

You would have to "hot wire" most engines, as the kill switch also disables operation of the starter?

[boscoe99]

Nope. In a Yamaha the kill switch is separate from the starter switch.

[99yam40]

As Boseco said,the safety lanyard is hooked to the kill switch , when pulled off it disables spark by grounding the input to ignition coils, it has nothing to do with the starter circuit

[zenoahphobic]

Yeah, forgot my remote shuts all power off to switch.

Some motorcycles also locks out stArter

Maybe differences with regard to this between countries.