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Has anyone had any luck with cheap-ish compression testers, brands? I don't have any specific issues at the moment my motivation to get one is more educational and curiosity of how my gear is doing. So it doesn't need to be really good, but I also don't want to make costly repair decisions about my OPE based on a bad gauge. If it's luck as to whether they are accurate, can I use an air compressor to test the gauge, assuming that the gauge on my air compressor is accurate?

thanks in advance

LH

For small engines you would one design for them. Basically the Schrader valve must have a lighter spring than the automotive type.

The automotive testers are not all that good for small engines. Some testers have the Schrader valve not in the end of the hose. These will give low readings because the hose up to the Schrader valve increases to effective volume of the combustion chamber lowering the measured pressure.
A compression tester like this made for small engines will give more accurate readings. Engines with a compression release complicates things. I have both this one and the automotive type. I usually this this one.

Hammermechanicman said:

The automotive testers are not all that good for small engines. Some testers have the Schrader valve not in the end of the hose. These will give low readings because the hose up to the Schrader valve increases to effective volume of the combustion chamber lowering the measured pressure. View attachment 54508
A compression tester like this made for small engines will give more accurate readings. Engines with a compression release complicates things. I have both this one and the automotive type. I usually this this one.

Click to expand...

So what is the measured difference between the auto and small engine style? Can you test this for us all?

This might explain why my Wisconsin Robin is only reading 50psi.

slomo

StarTech said:

For small engines you would one design for them. Basically the Schrader valve must have a lighter spring than the automotive type.

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Would I need a valve or could I just use a gauge from an air compressor and watch the needle bounce?

Hmmm... since it’s just for play. Use the threads of an old plug and epoxy a tire gauge to the hole in the center
What’s your vote?

The other problem with small volume gauges is the volume of the spark plug.
My tester came with 4 different plug lengths but unfortunately all 14 mm
However it all depends upon on just how accurate you need the results .
What you should be doing is taking compression readings at regular intervals like servicing or every season end etc
You then plot them ( a graph is best ) and watch as the compression goes down over time as the rings / cylinder wears
Some time before this number is 20% lower than the first it is time for new rings.

>What you should be doing is taking compression readings at regular intervals like servicing or every season end etc

That was my hope. And even if the gauge is off, but consistently off it should be OK for me since it would still show a relative decline.

As for my extreme budget tester.... I found a video that was helpful to strip out the ceramic insert in the plug. I didn’t have a holesaw that would fit, but I found that a hacksaw around nut did the trick!

slomo said:

So what is the measured difference between the auto and small engine style? Can you test this for us all?

This might explain why my Wisconsin Robin is only reading 50psi.

slomo

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Here are two different compresssion testers on a 90cc hopped up chainsaw.
The 195 is correct. The automotive tester has a 12" hose from guage to spark plug adapter. The hose effecively makes the combustion chamber size larger when compressing the air like a spring. The smaller the engine the more pronounced the difference is.

And better still are the ones with a short solid tube .
The auto ones can work, but you have to pull the rope around 300 times to get an accurate reading and then you have to take the expansion of the rubber into account as well.
But because the amount pumped each time is so small after the first 10 or so pulls you don't notice it going up .
If you crank the engine with a drill on the end of the crankshaft nut you will gat a lot higher reading, till you burn your drill our.
Then there is the problem of back pressure from the tester between the schreade valve & the plug opening pushing the air backwards into the crank case when the transfer ports open

longhike said:

>What you should be doing is taking compression readings at regular intervals like servicing or every season end etc

That was my hope. And even if the gauge is off, but consistently off it should be OK for me since it would still show a relative decline.

As for my extreme budget tester.... I found a video that was helpful to strip out the ceramic insert in the plug. I didn’t have a holesaw that would fit, but I found that a hacksaw around nut did the trick!

Click to expand...

That will work fine
It will work better if you put the same amount of oil into the cylinder for each test say 3 to 5 drops from an eye dropper so the rings have the same oil seal each time.
When I ran a fleet of light vans & motorcycles we did compression tests bi- monthly.
That way I could catch the wear early and we got 3 to 4 sets of rings out of each piston & bore .
Might sound tight fisted but we ran 8 x 4 cylinder motorcycles and a set of rings & gaskets was under $ 100
A rebore was $120 and a set of pistons $ 200 so it added up quickly.
It also meant that the bikes always ran crisply which is important if you are riding all day every day in capital city traffic .
Did the same thing with the vans but being water cooled, the ring wear was substantially less ( the riders vs Drivers also contributed more than a little )

On small engines i keep it cranking or pulling it until it stops rising.

Scrubcadet10 said:

On small engines i keep it cranking or pulling it until it stops rising.

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Correct. On any engine you crank till it stops rising. At a certain point it will stop rising and just bouce a couple PSI.

Scrubcadet10 said:

On small engines i keep it cranking or pulling it until it stops rising.

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Try it by pulling the rope then by spinning the crank.
You will not notice it going up 0.5 psi ( or less ) from 90 to 190
There was a sub 50cc kart class down here a while back & one of the cart tuners showed me that .
On bigger engines you are incresing 1 to 20 psi each pull but on small ones easy to see a very small rise as a "bounce"

Ok I’m sure it’s not accurate, but for a proof of concept it’s “kinda” OK. I already had the abandoned .97¢ tire gauge, the old spark plug, and a couple drops of 5 minute epoxy... so there’s no loss.

It’s an old edger that hadn’t been started in many years after the fuel lines crumbled that I picked up at a garage sale as a challenge to see if I could learn a little more about small engines. After a new carb, fuel lines and some new gaskets my edging looks much nicer

But even though it runs great, from pulling the starter the compression felt light (most likely my lack of experience). And putting my thumb over the hole didn’t seem accurate. It’s only a 22cc engine but the edger works great so I’m assuming it’s OK. Although I am still curious about finding a good tester for small gear.

Here’s the mini tester V1 ... at least I’m learning along the way and thanks for all the helpful tips/info! If I find a more accurate tire gauge maybe there will be a V2?

Enquiring minds need to be fed .
And thanks fr sharing.
Good compression testers cost big bucks
Mine was around $ 250 ( Aus ) and even then I have suspicions about it's accuracy .
OTOH from using it over the years I can be fairly sure if an engine won't pull 90 or better then it is not going to run .
However I rarely use it now days and have gone to leak down testing which is a lot easier and works on all engines .

bertsmobile1 said:

Enquiring minds need to be fed .
And thanks fr sharing.
Good compression testers cost big bucks
Mine was around $ 250 ( Aus ) and even then I have suspicions about it's accuracy .
OTOH from using it over the years I can be fairly sure if an engine won't pull 90 or better then it is not going to run .
However I rarely use it now days and have gone to leak down testing which is a lot easier and works on all engines .

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I am with you on the leakdown test instead of compression test on 4 stroke engines. I still like to do compression test on 2 strokes. My good automotive compression tester died a few years back and i picked up a cheapo asian one (big mistake) not realizing where the Schrader valve was located. Not that i need one but i think the china cheapo is going in the bin and going to get a better one.
Strangely enough had a Shindowa 488 in the shop last week and felt low on compression. Sure enough i measured 95psi. I though it would be a dog if i even got it started. Fixed the fuel line problem and it fired right up and had good power for a 48cc saw. Spec is 140-156psi. I have tested a lot of saws with my good tester and i have measured other saws at 90psi that barely ran. Just when i think i know 2 strokes i find out i don't.

The Shindawa is a 2 ring piston
Two ring engines will run at lower compression ratios than single ring engines
Usually a 2 ring runs a couple of k slower than the equivalent single ring.

bertsmobile1 said:

The Shindawa is a 2 ring piston
Two ring engines will run at lower compression ratios than single ring engines
Usually a 2 ring runs a couple of k slower than the equivalent single ring.

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Good to know. Of course both my hopped up kit saws run 2 ring pistons and the 60cc turns 13500 and the 92cc turns 13000 WOT. But they aren't stock either. Always fun to show up with a chineese knockoff kit saw to cut and buck storm trees and cut rings around the guy with the same model Stihl brand $1500 saw.

Hammermechanicman said:

Good to know. Of course both my hopped up kit saws run 2 ring pistons and the 60cc turns 13500 and the 92cc turns 13000 WOT. But they aren't stock either. Always fun to show up with a chineese knockoff kit saw to cut and buck storm trees and cut rings around the guy with the same model Stihl brand $1500 saw.

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A stirrer after my own heart
I run a couple of old WWII motorcycles, bored & stroked from 500cc to 760cc & 860cc .
They live in a lean to on the side of the shed so look really tacky and I really love sitting right on the back wheel of a pristine over restoration then retard the ignition & glide past them on long hills as they shuffle back through the box.

My fling at a motorcycle was turning an old Kawasaki 350 Bighorn into a psuedo cafe racer. Completely rebuilt the frame with clip on bars and an engine that was used to flat track race till the new best engine was the Honda 500 thumper. No top end but 0-60 was holy s-word fast. Gearhead buddy had a Denco prepared H3 tripple. I didn't have the testicular fortitude to ride that monster. Of course my buddy had more balls than brains.

H 3 brave man
Got lots of friends with limps, funny shaped arms and sorts of dissabilities from not being able to handle the fierce powerband on those .

I clearly don’t have the experience ( or capacity) to understand very much of it, but it’s an honor to listen in, great conversation!

longhike said:

I clearly don’t have the experience ( or capacity) to understand very much of it, but it’s an honor to listen in, great conversation!

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The H3 was kawasakki with a fierce power band that kicked in a instant so you would be tootleing along in traffic at 40 mph with the engine putting out 15Hp .
The traffic opens up a bit , so you accelerate , but the slight movement of the throttle raises the revs to where the power band kicks in and you instantantly go from 15Hp to 45Hp
The front wheel points skyward and the bike ends up enbedded 2' into the boot of the car that was 200 yards in front of you before you moved the throttle.

And to that my buddy's H3 was race prepped by a company called Denco. The engine is a 750cc 3 cylinder 2 stroke that stock was pretty much the fastest bike around. His Denco H3 could bury the 150mph speedometer with ease. Bert is right about the powerband is like an on off switch 10hp or 100hp. Not much in between. The Kaw H1 H2 and H3 were called the widowmakers and many insurance companies would not insure them. In the 70's kaw tripples were the fastest stock bikes 0-60 and 0-100. Unfortunately they also had 70's front fork design and geometery. Things went from a straight line to dead real quick.

Same story down here
By the time the H3 was released the insurance premium was near the cost of the bike
But to race production class kawaasaki had to sell 200 each season so there were a lot of really good deals if you had quick reflexes and a death wish.
I rode BSA's ( still do ) so lightening fast throttle response was not my biggest worry on the road .

After my second time i got put in the ditch by a stupid car driver i stopped riding. Walked away from the first one. Second one i limped away. Ground the toes off my boots, the knees out of the jeans and ruined a nice Bell Tourstar helmet.

better to ruin a helmet than what's underneath it.

Scrubcadet10 said:

better to ruin a helmet than what's underneath it.

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Oh yeah. I slid a pretty good way face down then went under a guard rail and wound up in ditch full of water. The handlebars hung on the guardrail and kept the bike from following me in the ditch.

Well I am a founding member of the "Previously Deceased Motorcycle Riders Club" having been triarged as DOA at the emergency department .
I got taught to ride by a speedway rider so learned how to fall off without hurting yourself.
Should be compulsory training for all motorcyclists
And to push things further, no one should be allowed to drive a 4 wheeler untill they have done a couple of years on 2 wheels and have learned to read a road.
The again a couple of years in sedans before a 4WD urban assult vehicle or heavy truck.

I was reading the road pretty good going around a blind curve about 40 mph in the rain when a car came around the corner well left of center. Touched the brakes and the bike slid out and i Supermanned down the road. No skill, just luck the car missed me by inches and i went between the guardrail posts. Of course the car didn't stop.

Combining motorcycles and lawnmowers....why do people throw their grass clippings into the street?

cpurvis said:

Combining motorcycles and lawnmowers....why do people throw their grass clippings into the street?

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BEcause they are selfish greedy lazy consumers who were brought up in an apartment .
I knew a council inspector who was livid about people doing this so he would force a ball up their stormwater outlet into the street drain .
Then when it rained and their guttering fell off the roof & the yard turned into a bog they would call the councit to complain.Several hundred dollars latter he would tell them it was grass clipping blocking the outlet .
This worked very well
Telling them it was illegal garbage dumping or even fining them made not difference.
And he was a motorcyclist too which is how I knew him.

Don't forget the ones who blow leaves into the street. Wet leaves are the best

These same people would undoubtedly throw a FIT if someone threw a hamburger wrapper out in front of their house, despite that it poses essentially no risk, while a street coated with grass clippings is about as slippery as ice.