Compression check question

[legendre]

quote Dave Bernard:

My mistake but if the carb is on a 4cycle it must be open to get a true compression .Correct?

Correct.

In some cases, it’s absolutely mandatory. In all others, it’s just a Very Good Idea as it reduces the number of cycles the motor must make to max-out the reading.

Easier on your starting system, be it electro-mechanical or manual (your arm, leg, etc).

[legendre]

quote Chris_P:

quote legendre:

The carb should always be held at WOT for a compression test, which is not all that different from not having a carb at all..

Actually for our 2 stroke outboards throttle setting is totally irrelevant for a compression test. Hold it at no throttle, half throttle, full throttle, doesn’t matter.

I can’t see any reason a 2-stroke OBM would be any different, but I’ll defer to your experience for now.

But let me ask this – what kind of compression reading does your OBM give if you +plug+ the carb inlet(s)? How does that plug differ from a fully closed throttle plate?

[frankr]

But it does matter on a 4-stroke.

[retiredoz]

On a 4-stroke, the only air inlet to the cylinder is thru the carb. On a two-stroke, there’s also the exhaust port, if the reed plate is removed…

[frankr]

This subject has been discussed to death many times in the past. There is no mystery if one considers the basic principle of operation of the two types in engines.

On a 4-stroke engine, the cylinder draws air through the intake valve on the piston’s intake (down) stroke. That air passes through the carburetor on it’s way to the intake valve. A closed throttle plate creates a restriction and a partial vacuum in that incoming air flow. That partial vacuum prevents the cylinder from filling completely. An incompletely filled cylinder has less air to compress on the compression (up) stroke, thus lower compression pressure when it reaches the top of the stroke. Blocking the throttle plate open removes the restriction to incoming airflow, there is no partial vacuum, and the cylinder completely fills with air to compress on the up stroke (both valves closed).

A 2-stroke is a completely different animal. On the piston’s up stroke, it draws air through the carburetor, into the crankcase. On the down stroke, that air in the crankcase is compressed (within the crankcase). When the piston nears the end of the down stroke, the exhaust ports are uncovered first, allowing the previously admitted (now on fire) air to escape as exhaust and the cylinder is now mostly empty. Only once all this has happened, does the piston uncover the intake (bypass) ports. When the bypass ports are opened, that air which is compressed within the crankcase rushes into the cylinder, and some goes on out those open exhaust ports. The result is a near-atmospheric pressure situation within the cylinder as the piston begins its upstroke. There is no partial vacuum because the air is compressed (a positive pressure, not a partial vacuum). Very important: Note that a 2-stroke simply cannot run without this crankcase compression to fill the cylinder. Therefor, the throttle plate never completely blocks the airflow path. There must be enough air enter to accomplish a compressed-air crankcase situation, or the cylinder will not fill and the motor cannot run.

So, understanding all this, it is evident that a 4-stroke PULLS air into the cylinder (partial vacuum), and a 2-stroke PUMPS air into the cylinder under pressure. That pulled-in air of a 4-stroke can be restricted and not fill the cylinder. The pumped-in air of a 2-stroke must be adequate to accomplish a near-atmospheric pressure situation at the bottom of the stroke when all ports are open. So, the piston begins its upstroke at near-atmosphere pressure regardless of throttle position.

All this is speaking about cranking speeds to test compression, not actual running conditions. When running, stuff like exhaust back pressure and goodies like tuning and supercharging come into play. But that’s a whole ‘nother topic.

Everybody got it???

Final word: Go ahead and block the throttle open on your two-stroke if it makes you feel any better, I don’t care. Neither does the motor.

[legendre]

@FrankR

Regarding my views on the matter, here’s some background – one of those godforsaken anecdotes – for what it’s worth.

A number of years ago on a popular moped enthusiast’s site, this same topic was hotly debated. I was on the "carb WOT" side of the debate, which should come as no surprise. But there were a number of dissenting opinions, so several members agreed to test their compression with the throttle both at WOT and fully closed. I also tested one of my ‘peds, which I believe was a Motobecane 50V.

Now bear in mind that virtually all mopeds are 2-stroke engines, and +most+ of them are simple 3-port (aka loop scavenge) designs lacking reed or rotary valving. The results of the test were interesting. In this case, all engines tested were in fact 2-stroke.

One member reported no significant difference between WOT and closed, while a couple other members (including myself) reported a non-trivial difference in the two figures. It was agreed that while it’s not always a necessity, holding the throttle WOT is simply good, workmanlike practice when testing compression.

It certainly cannot harm anything to do so, and it eliminates any questions about the measured figures. And again, that’s just good shop practice.

[frankr]

That was my "Final Word". After I practically wrote a text book on the subject. I stand by my word in both cases. Now, I’m going to lay awake tonight pondering the loop charged factor. Also, are you doing those moped tests with the muffler installed? Just as having an outboard in the water, there would be a small amount of back pressure. I’m not concerned enough or interested in pursuing that topic.

[legendre]

quote FrankR:

That was my “Final Word”. After I practically wrote a text book on the subject. I stand by my word in both cases.

That’s perfectly fine, FrankR – and you may well be entirely correct – at least in terms of the OBM hardware discussed on this site, if not more generally.

What I offered was an anecdote, nothing else, and should not be taken as some sort of unequivocal declaration of a universal truth. To be clear, I don’t place very much faith in anecdotes, and neither should you or anyone else for that matter.. they are subject to innumerable biases, errors and outright manipulations.

quote :

Also, are you doing those moped tests with the muffler installed? Just as having an outboard in the water, there would be a small amount of back pressure. I’m not concerned enough or interested in pursuing that topic.

I can’t recall whether or not any of the mufflers were removed for these tests, other than in the case of my machine, which did have a muffler. Either way, it doesn’t matter as IMO the resultant course of action should be the same in any case. That is, since we know some engines are susceptible to this issue, we hold the throttle at WOT in all cases simply because it’s one more variable that can easily be eliminated.

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