the right compression tester for mopeds<<<<<<<<<<<<<

I was scratching my head for a couple days as my combustion chamber cc measurement and math were not coming close to what my compression gauge was saying. Changed pistons, changed rings, changed cylinders, even checked the gauge against an air compressor, finally borrowed a tester..... still reading low. I was about to deck my head some more, then it hit me

^this kind is the wrong kind to use on a small displacement engine. The hose attached to the gauge has a schrader valve at the coupler but the detachable hose that screws into the head has no valves in it at all. Therefore the detachable hose has to be counted as combustion chamber volume. No big deal on a large engine but when you're dealing with a combustion chamber of around 4cc's it greatly affects what the gauge shows.

^So I dug out one of these, took off the rubber thing, gutted a sparkplug and soldered the sparkplug remnants to the compression tester. Schrader valve now sits where the electrode would go and combustion chamber volume is dead on. Gauge now shows 150% of what it was reading before.

So before you go decking your head and seizing your engine make one of these up to throw in your tool box. Takes about a half hour and only took a propane torch and some solder.

Re: the right compression tester for mopeds<<<<<<<<<<<<<

Thanks Ryan. I'll be making one of these ASAP.

Re: the right compression tester for mopeds<<<<<<<<<<<<<

Good idea. It always bothered me how much 'loss' I knew was happening in that hose.

Re: the right compression tester for mopeds<<<<<<<<<<<<<

Thanks! Always wondered why my checker was weird on my lil motors but fine on my 250.

Re: the right compression tester for mopeds<<<<<<<<<<<<<

Thats what I use but I epoxied mine

Re: the right compression tester for mopeds<<<<<<<<<<<<<

thanks for the tips, i may just round up for a while but good to know, i wonder if i can convert the gauge from my existing hose type to the only screw in type......

Re: the right compression tester for mopeds<<<<<<<<<<<<<

Very nice, Thanks.

Re: the right compression tester for mopeds<<<<<<<<<<<<<

So my rings aren't all jacked up after all! YEA!

Re: the right compression tester for mopeds<<<<<<<<<<<<<

This makes so much sense and totally explains what I thought was a bizarre compression problem.

Re: the right compression tester for mopeds<<<<<<<<<<<<<

matt boda1nz /

so, i can just mod my current compression gauge to be used without the hose, with the valve in tact, soldered to the gutted spark plug metal and i should be good to go..

i wondered why i was only getting 90 psi on a new top end, .025 off of the head on a qt50...

Re: the right compression tester for mopeds<<<<<<<<<<<<<

what mine looks like.

added to the wiki http://www.mopedarmy.com/wiki/Compression_gauge

though my wiki skills suck. someone should help me out

Re: the right compression tester for mopeds<<<<<<<<<<<<<

I noticed on my compression tester it actually has a Schrader valve at the very tip end of the hose right where it screws into the spark plug hole as well as a pressure release at the gauge so I think I'm getting a good reading as is. Don't you?

Re: the right compression tester for mopeds<<<<<<<<<<<<<

matt boda1nz /

yeah.. i did some reading and i saw if you have two valves you should be good to go

Re: the right compression tester for mopeds<<<<<<<<<<<<<

Ok then, I too have been using a Matco tube style tester. So when I have it screwed in and it reads 150 psi, then the actual compression is higher or lower than the gage reading?

Re: the right compression tester for mopeds<<<<<<<<<<<<<

my two piece tester had one schrader at the couple for pressure in and another schrader at the gauge for pressure out(when depressed of coarse). The first tube that screws into the head didn't have a schrader valve in it nor did it have threads for one. Therefore that first tube's volume between the head and first schrader(in the coupler) has to be added to the volume of the combustion chamber. Therefore what the gauge shows is lower than actual pressure with the sparkplug in.

Re: the right compression tester for mopeds<<<<<<<<<<<<<

matt boda1nz /

this is how a proper tester should look...

untitled.JPG

Re: the right compression tester for mopeds<<<<<<<<<<<<<

Using one of the shraders with the light springs will give a more accurate test

Re: the right compression tester for mopeds<<<<<<<<<<<<<

if i rig mine to work w/ no tube, can i just unscrew it from the head to release the pressure, or is that gonna fuck shit up?

Re: the right compression tester for mopeds<<<<<<<<<<<<<

so what kind of gauge is that called?

Re: the right compression tester for mopeds<<<<<<<<<<<<<

matt boda1nz /

p.s. excellent rig ryan..

i'm going to do the same thing.. nice.

Re: the right compression tester for mopeds<<<<<<<<<<<<<

That's why they call it displacement. If your compression tester didn't max out that means you didn't crank the engine over long enough. If you cranked it over long enough and there was a difference in readings between using the hose and a more direct connection then the hose leaks.

Using full throttle, crank the engine over quickly as you can. On a ped that means pedal hard. Do that until the reading will rise no more. That is the cranking pressure of your engine.

Re: the right compression tester for mopeds<<<<<<<<<<<<<

derek from black pipes moped gang /

I had a mk65 e50 that read 225lbs with one of those hose comp testers. I wonder what it actually was. :|

Re: the right compression tester for mopeds<<<<<<<<<<<<<

will they blow out and screw up threads w/ out a release valve?

Re: the right compression tester for mopeds<<<<<<<<<<<<<

matt boda1nz /

without a shrader or what you are calling a "release" valve.. the air will just come back out of the gauge and out of the exhaust. in otherwords.. won't work.

Re: the right compression tester for mopeds<<<<<<<<<<<<<

tim is talking about a gauge that has only one schrader valve and not two. Yes a single schrader valve gauge works just fine. The only thing is you have to unscrew the gauge to press on the single schrader to decompress the gauge instead of conveniently pushing on the second schrader valve on the side of the gauge to release the pressure. My gauge I made only has one valve.(edited)

Re: the right compression tester for mopeds<<<<<<<<<<<<<

Road Rash Dave /

I'll post a pic of mine...its old and is on par with your shorter is better plan...and it comes apart...hard to explain....pics will be better...

yes back from the dead is this post...lol

Re: the right compression tester for mopeds<<<<<<<<<<<<<

I did that once, it didn't hold up very well but it worked for a year or so. probably right around the time this was originally posted - hmm

I kinda prefer tha hosey ones - i mostly just look at compression in a relative way, like oh i did this - now i have this.

Re: the right compression tester for mopeds<<<<<<<<<<<<<

bump for good thread.

I have a screw-in one with a permanent hose, going to have to check where the valve is at..might be making more than the 150psi i thought.(edited)

Re: the right compression tester for mopeds<<<<<<<<<<<<<

my MAC brand one from about 150 years ago has the schrader valve in the base, it takes a few more revolutions to 'pump up' the hose, but eventually it levels off and that is dead on measurement.

i don't use it for performance stuff, though, because static compression is mostly useless in light of the pipe supercharge and other factors affecting dynamic compression... i just use it for checking rings and stuff.

Re: the right compression tester for mopeds<<<<<<<<<<<<<

Mine acts the same way. It's a cheap one I bought it in the 90s, always served me well on bigger engines. I also mainly use it to see if the bike should start, how the head seals, and monitor ring condition. If it's significantly lower than it was/should be all of a sudden I know something's up in there. That and compare to "stock" numbers.

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