QT50 new Hone and Rings. Compression.

After the first heat cycle and cool I got 100 PSI. After full break-in, what should my compression test read?

Should I just start sanding the head now or wait? I'd like to get to at least 150 PSI.

Re: QT50 new Hone and Rings. Compression.

100 psi is pretty low? Usually you should be in the 120 or above range. (edited)

Re: QT50 new Hone and Rings. Compression.

Did you get real aggressive with your hone? Rings gapped?

Re: QT50 new Hone and Rings. Compression.

Not real aggressive, a good crosshatch. New rings at .008 inch. A bit much, can't make them longer.

Re: QT50 new Hone and Rings. Compression.

Of course it still brings up accuracy of compression tests/gauges.

Re: QT50 new Hone and Rings. Compression.

> Peter v. wrote:

> -------------------------------------------------------

> Of course it still brings up accuracy of compression tests/gauges.

Got something else you can test with the same gauge?

Re: QT50 new Hone and Rings. Compression.

Leaf blowers, chain saws, weed whackers, motorcycles, cars. Same problem, gauge wrong or compression off.

Re: QT50 new Hone and Rings. Compression.

Maybe check the compression gauge against your compressor .

Might not be totally accurate , but , it would give you a pretty good idea .

Re: QT50 new Hone and Rings. Compression.

I like that, will have to try.

Re: QT50 new Hone and Rings. Compression.

It usually takes at least a tank of gas or 2 to break in a cast iron cylinder. Sometimes more if it's a re-ring since the cylinder might be a little wonky and who knows how good the hone job was if you did by hand vs a factory machine tool doing it.

My PW (I'm guessing the same or really similar) took a good couple tanks of gas and still doesn't have great compression.

If you want to go hi comp, buy the treats 50cc Puch 'hi hi' head, goes right on makes a noticable difference in performance.

Re: QT50 new Hone and Rings. Compression.

Just did it and it worked! Put on my blowgun attachment (rubber tipped)on my compressor and jammed it in my compression gauge and perfect pressure match. Compressor rated for 125 psi, goes to 120 psi before shut off. Both read 120 psi.

Now that I can trust that reading, do I mill the head more or will I get another 50 psi when broke in? Probably not that much.

Re: QT50 new Hone and Rings. Compression.

Just did another heat cycle then while hot did another comp. check and got 90 psi.

Normal to drop 10 psi for hot check? Thing is starting to piss me off with questionable comp. readings, low readings, etc.

If I ignore the comp. readings it runs great, 1 kick start, instant, fast throttle response, lots of decell. popping, seems lots of power standing still.

Couple more heat cycles then slow riding if it doesn't snow.

Re: QT50 new Hone and Rings. Compression.

If everything else is what you want , I'd relax about the compression until after the motor is well broke in . It may just need some time .

Some motors simply don't need high compression to run great .

Re: QT50 new Hone and Rings. Compression.

Hmmm. Most engines won't run below 90psi. If it's running strong and seems fine I would just run it . Are you doing compression test with the throttle wide open?

Re: QT50 new Hone and Rings. Compression.

Yep, throttle wide open, kick till no more rise in pressure. Usually near max at 3 to 4 kicks.

Still might be something to the whole comp. tests are inaccurate on small engines thing.

Re: QT50 new Hone and Rings. Compression.

Jack Rutherford /

Pretty normal readings for qt50 - 90-100 psi. And they usually run great in that range. If you want to boost the compression, sand the head and cylinder mating surfaces flat and run without a head gasket. That might get you to 115-120. You won’t gain much except higher temps as the clutch fades as it heats up anyway.

The jog piston is slightly taller than the qt50 piston and would give you more compression. Check to see where the top of your piston comes to on the cylinder at TDC.

How many miles are on your cylinder?

BTW, I used to have a bunch of qt50 stock cylinder heads laying around and I examined them in a fit of boredom. I think Yamaha made at least three versions of their stock head to gradually lower compression in later models (that's my theory anyways). You may have a stock head with a deeper chamber resulting in less compression. Or the differences were due to poor manufacturing - I don't know. (edited)

Re: QT50 new Hone and Rings. Compression.

That cylinder has less than 500 miles on it.

I'm going off of xrongor posts on the MRA site where he has 155 psi with all Yamaha parts and stock QT head.

others talk about 255 psi by sanding down a QT head.

I was going to buy a new comp. guage this morning but now I don't know. It does match my air compressor perfectly but I still wonder if it reads right.

Something is not adding up.

Re: QT50 new Hone and Rings. Compression.

225 psi is just not true. If you are in the 100 to 140 range you are doing well. As said above seems like 100 is fine for that bike. I am no expert on that bike by any means but just in my experience with other engines that's a common psi. Please show us a post where someone had 225psi on a qt. Haha

Re: QT50 new Hone and Rings. Compression.

Jack Rutherford /

That dude was using a stock yt60 head.

You don’t want a lot of compression with the stock qt50 head. You’ll just burn up the engine with the extra heat because the stock qt50 head is tiny and thus does a poor job dissipating heat.

Sanding the head and cylinder will never get you 100 psi extra in compression. The piston will hit the head before that happens. You may be able to bump it up 20 psi or so. I haven’t done it in a long time because you don’t gain much in performance.

Re: QT50 new Hone and Rings. Compression.

I'm using a YT60 head sanded down already, probably not much room left to sand further.

I've loaded and watched many Youtube videos on compression testing on small 2-stroke engines. 27cc weedeaters, leaf blowers etc. Most getting 130 psi. 100 psi seems low for QT's in general. I've got to check some of my equipment and see what I get yet.

Re: QT50 new Hone and Rings. Compression.

Jack Rutherford /

what cylinder jug are you using?

Re: QT50 new Hone and Rings. Compression.

Jack Rutherford /

ok I see the issue from your other post.

With the copper spacer you are using, is your piston returning to the top of the cylinder at TDC? It probably is not if you are using a 2mm spacer and 2 base gaskets. This accounts for your low compression reading. You need to sand the cylinder jug so that the top of the piston reaches the top of the cylinder jug at TDC.

1mm of iron is ALOT to remove via hand sanding even with the coarsest sand paper. I use a belt sander with a belt made for metal removal. Take proper precautions by wearing a mask approved to filter out fine metal dust, eye protection and you don't want to get any on your skin either. In fact use your belt sander outside as well. I stuff a paper towel and soak it with WD-40 in the cylinder jug and that traps a lot of the dust and/or attach your shop vac to the belt sander. Many have a space just for that.

Re: QT50 new Hone and Rings. Compression.

I don't know why Peter is being so persistent on thinking that he needs more compression .

Like Jack states , only destructive heat results .

Mountain from mole hill .

Re: QT50 new Hone and Rings. Compression.

Just ran a compression test on my Husky leaf blower, got 125 psi. Tester is good.

100 psi on the QT still seems low.

Re: QT50 new Hone and Rings. Compression.

OEM Yamaha 60cc cylinder, already taken down with belt sander, OEM Yamaha YT60 head shaved down, OEM Yamaha piston, 2 mm copper along with 2 base gaskets.

Re: QT50 new Hone and Rings. Compression.

If it runs how you want it to, I'd leave it alone. 2 strokes dont need much compression to run at all. My brother had a 70s rm100 with stuck rings. Read like 30psi but started first kick and ran like a r*ped ape. My old it250 was reading 60ish before reringing it also started with ease and ran great. I don't even bother to test mopeds with a gauge. Its either got compression or it doesn't.

Squish and combustion chamber profile are more important.

Re: QT50 new Hone and Rings. Compression.

^ this. Get it good and broke in, and do the hc Puch head.

Re: QT50 new Hone and Rings. Compression.

How does it run? If it runs good, then don’t obsess about compression numbers.

Re: QT50 new Hone and Rings. Compression.

> Ron Peek wrote:

> -------------------------------------------------------

> How does it run? If it runs good, then don’t obsess about compression

> numbers.

Yeah you are beating a dead horse.

Re: QT50 new Hone and Rings. Compression.

Here are the posts. I also think you can't hit these numbers without piston/head interference.

Screenshot_20220103-171515_Gallery.jpg
Screenshot_20220103-171504_Gallery.jpg

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