Suzuki FA50 Low Compression

Tyler Fritz /

Jul 26, 2020 12:41PM

I have finally gotten the moped to run after it sat for about 8 years outside. When I hopped on and rode, the speed peaked out at about 20mph (which seemed low to me) and it was incredibly hard to start cold. I went to Autozone and rented a compression tester and the reading was 65-70 PSI which I assume to be very low and the main reason it is so hard to start and why it's performance is subpar. There is some scoring on the cylinder walls but I wouldn't think they are large enough to cause issues (images below), do you think they are enough to cause poor compression? I worry that if I buy new rings and there is an issue with the cylinder, the cylinder will tear them up. Would it be worthwhile to get a new cylinder and piston? Are new piston rings the way to go? Is there something else I'm missing that would affect compression? Thank you in advance.

Re: Suzuki FA50 Low Compression

live ɘvil /

themotorhead

Jul 26, 2020 01:49PM

The rings may simply be carboned up, stuck in the piston grooves and not expanding. Worth looking at before buying new parts. (edited)

Re: Suzuki FA50 Low Compression

pat splat /

ggspeed

Jul 26, 2020 01:57PM

if u can feel em with your finger nail they r too deep.

Re: Suzuki FA50 Low Compression

live ɘvil /

themotorhead

Jul 26, 2020 02:00PM

This is what a piston looks like with stuck rings. Obviously they should be expanded out of the grooves but also notice the combustion "blowby" below the rings on the sides of the piston. The fact that the FA50 uses an oil injector (which errors on the heavy side of mix) makes those types of motors more prone to carbon build-up around the ring grooves.

(edited)

Re: Suzuki FA50 Low Compression

Tyler Fritz /

tylerfritz OP

Jul 26, 2020 03:48PM

Fortunately they're not that deep, seems to be more aesthetic than actual scratches.

Re: Suzuki FA50 Low Compression

Tyler Fritz /

tylerfritz OP

Jul 26, 2020 03:54PM

I took apart the cylinder and the piston definitely has experienced some blowby (see images). I assume that the only cause of blowby is the piston rings not sealing correctly. The rings didn't seem to be stuck but they could have been freed from the cylinder removal process. The piston seems to have taken a beating in terms of coloration and marking, is it too far gone? If not I think a new set of rings and gaskets may be in order. (edited)

Re: Suzuki FA50 Low Compression

cheetahchrome . /

cheetahchrome

Jul 26, 2020 10:29PM

The relaxed gap in that ring seem way excessive. I would be pulling off those rings and then putting them in the bore by themselves to see what the gap is using a feeler gauge.

Re: Suzuki FA50 Low Compression

Christian Kyrmse /

esmryk06

Jul 27, 2020 01:03PM

I would probably replace the rings just to make sure that’s not the issue and I would also make sure to torque the head bolts to at least 11 lbs. when reassembling the motor.

Re: Suzuki FA50 Low Compression

Mark Anderson /

m_anderson

Jul 27, 2020 01:06PM

The rings look pretty beat to me. Heavy chamfer on the top edge shows its been really broken in. But the side where the "blow by" is the worst, the ring edge is practically gone. The should be nice and even all the way around with nice sharp, machined edges. Your squish looks shot too. I'd start with cleaning the cylinder, head, piston and ports and slap some new rings on that thing and recheck compression. Don't forget to cut yourself a new cylinder base gasket. Over time and breaking in, the leading edge of the rings will get a very, very slight chamfer to them. Some people will even file and ever so slight chamfer on them before installing to cut down on break in time.

Re: Suzuki FA50 Low Compression

Mark Anderson /

m_anderson

Jul 27, 2020 01:09PM

Check your wrist pin too, for slop. Looks like your rings are wearing uneven. Which is somewhat normal. Yours just looks to be excessive on one side

Re: Suzuki FA50 Low Compression

P D /

pdcomm

Jul 27, 2020 03:50PM

That piston and rings look like royalty compared to what I have running very nicely in my FA50 . And , have been for years . It's a very reliable 30+ mph machine .

I'd ease those rings out . Clean the ring landings and the rings ( do not chamfer the rings ) .

Put the rings back on the piston , only reverse : top on the bottom and bottom on the top .

Then run a hone through the cylinder just enough to clean it up , but , not enough to take much if any material . Check that all port edges are chamfered .

New base gasket and run it .

While you have the piston out and in hand , consider relieving the piston crown at the ports by about half the distance from top piston edge to top og the top ring landing .

That does two things , mainly , advance the port timing and lowers compression .

Lower compression is what you think you want to avoid , but , once you have something close to normal compression , the relieved crown will help prevent detonation .

Anyway , try 'stuff' . Like piston exhaust skirt holes for lube and cool or maybe shaving the intake skirt or widening the exhaust port or sanding the head or ...

Even if you screw something beyond use , everything is pretty cheap to replace .

Mostly , have fun . :)

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