Compression check question

[billy-t]

Hi, todays dumb question…

Can i do a compression check with the carburetor off the motor?

Thanks

[mr-asa]

I can’t see why not. So long as you have the reed plate bolted down properly it should work.

[billy-t]

Thanks

[wedgie]

Do you even need the reed plate ?

[mr-asa]

Now that you mention it, I don’t think so. However it might do weird stuff without it. Part of the reason that 2-strokes work is that the reed plate helps pressurize the crank case, so maybe it would need it for the full compression numbers?

[dave-bernard]

on a 4 cycle motor no, but a 2 cycle yes you can no carb no reeds no problem for compression check, only running.

[legendre]

quote Mr. Asa:

Now that you mention it, I don’t think so. However it might do weird stuff without it. Part of the reason that 2-strokes work is that the reed plate helps pressurize the crank case, so maybe it would need it for the full compression numbers?

I’ve never actually tested this, but I would imagine the reed valve is +critical+ to getting a proper compression reading. Secondary (cylinder) compression is the product of the primary & secondary compression ratios. Without the reed valve, you’re going to lose much of the primary compression, as the charge will be shoved back out of the intake before it transfers to the cylinder.

But other than that, there’s no need to have the carb installed on any engine when testing compression. 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..

If the OP has the time and inclination, it would be interesting to see figures posted for compression with and without the reed valve in place.

quote Dave Bernard:

on a 4 cycle motor no, but a 2 cycle yes you can no carb no reeds no problem for compression check, only running.

Are you saying a 4-stroke engine +needs+ to have a carburetor in place to get a proper compression reading? Sorry, that’s just not the case..

(ETA: LoL, here we go again..)

[Alan]

No carb necessary for a compression check.
The carb is not involved in compressing,

[dave-bernard]

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

[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..

[/quote]

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.

[Author]

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