Can anyone advise me what the compression should be on a 50CC yamaha qt50 engine. I bought for my kid the other day and it ran but the plug was all carboned up and now i cant start it up. How many cranks do you do to calculate the compression?


Do you have a compression gauge?

On most Mopeds, compression should be at least 110-120 PSI.

Re: compression

Leon Swarmer /

No choke, wide open throttle, at least three revs.

Re: compression

You do not need to turn the throttle, actually you do not want any fuild in the cylinder when you are checking as it will damage the compression tester. You should do at least 3 different test on the cylinder and then average them together to get a more accurate picture of the cylinder compression. On a two stroke the most resistance on the start and/or pedal in usually the compression stroke. Also to not be slow and baby it also, it will screw up the reading.

Re: compression

_You do not need to turn the throttle_

In my experience, this is untrue. I've gotten substantially different readings on various bikes, when testing with throttle open vs. closed. All my compression tests are done wide-open throttle.

_you do not want any fuild in the cylinder when you are checking as it will damage the compression tester_

Compression testers use brass bourdon-tube gauges and have (generally) fuel-resistant seals & o-rings. They should not be damaged by gasoline vapor.

And I think you've got this backwards.. do you think that cranking an engine with the throttle closed introduces weaker mixture to the cylinder, than if the throttle were open? It's the opposite.

If your statement were true, then the time-honored process of clearing a flooded engine (by cranking with the throttle wide open) would not work; it would in fact make matters worse.

Re: compression

Steamboat Aka J. R. Stevens /

Right on Legendre. Jim.

Re: compression

What would the throttle have to do with displacement in the cylinder? NOTHING. THUS THROTTLE NOT USED.. If this was true then how would you check a blown motor with no fuel pump attached. This is what they do to many cars and dragsters.

The seals might be fuel proof, but the diaphram will only take so much fluid (which can only be compressed MUCH LESS than air).

Re: compression

Steamboat Aka J. R. Stevens /

Insert the compression tester into one cylinder spark plug hole at a time.

Hold the throttle to full open position to ensure the engine gets adequate air intake. FYI Jim.

Re: compression

You are correct; the postion of the throttle has nothing to do with engine displacement - nor does nominal engine displacement have anything to do with compression figures - so let's just discard that entire notion as a red herring.

Try this: open your mouth all the way, and breathe in for one second - you get a good lungfull of air. Now, purse your lips and do the same - you'd suffocate trying to breathe that way. In this way, the throttle controls the maximum volume of air that can be taken in - and later compressed. Remember: the basic job of a throttle is to control airflow.. not fuel flow. In many cases, the fuel metering components are also under throttle control, but this is a secondary issue to the basic funtion of the throttle.

Whether you are taking in pure air, or an air/fuel mixture has no relevance either; a compression figure is simply an indication of the maximim static gas pressure that the cylinder can develop when not running. It doesn't tell us all that much about the realtime pressure (BMEP etc) that the engine will produce while running, but gives us a good baseline indication of the condition of the engine (rings, valve sealing, etc).

About the construction of the gauge - again, compression testers use brass bourdon-tube movements, not diapgragms. Even so, the gauge could be filled up to the brim with fuel, and it would not make a lick of difference. If the gauge were filled with fuel, the only effect is that the needle would rise to the maximum figure sooner, than if it were filled with air. All you have done, is reduce the total compressible volume of the cylinder + gauge, nothing else.

Think for a moment about a pressure gauge on a steam boiler. It's pretty safe to say that those gauges take in a mixture of air, water vapor and liquid water. Does that make them any less accurate? Absolutely not.

For more information on the behaviour of liquids and gasses under pressure, look at the work of Robert Boyle and Blaise Pascal, two noted scientists of the mid-17th century.

Re: compression

Most of your old STEAM gauges used a water colum or oil colum or just straight steam, but calubrated to acertian level of air(but since air compress and LIQUID DOES NOT this was small.

AS for the throttle YOU DO NOT NEED THE THROTTLE OPEN, you get the same air in if you close your mouth as with it open. But iin your case I see a vacuum is there. Air is getting into the cylinder with throttle closed as open. Unless you are sticky your head in the intake.

Re: compression

_But iin your case I see a vacuum is there_

Oh my stars, what a delicious and devilishly crafted insult! However did you come up with such a supernova-sized blast of insightful wit, in only four short days? I just have to show this one to Vonnegut, he'll split in two. What a shame that Twain isn't alive today, to bear witness to such refinement of the craft.

So, how long have you been writing? Have you signed a book deal yet?

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