Yamaha Zuma with poor compression

[KDS4444]

Hey, all! Am the owner of a 2002 Yamaha Zuma 50cc scooter. It has a new cylinder & head, + gaskets, new piston and rings, and new reed valve gasket. The engine is not turning over at the moment, and am not sure why. Spark plug is clean and is generating a nice spark. I have a compression tester that says I am only getting around 75 psi when I kickstart it, which I know is much too low. I recently hooked up an air compressor to the spark plug hole, and turned it on. I found no air escaping around any of my new gaskets or parts, but I could hear air whistling out of my carburetor. Piston is at TDC for this test. I know a leakdown test is the standard approach, but I do not have the tester for this— am hoping to figure out where my problem is with my air compressor and some careful listening. The compressor itself is able to get up to about 75 psi when I turn it on hooked up (it goes much higher than this, but only gets up to 75 psi on my engine) which corroborates what the first compression test told me.

Should I be getting air coming out my carburetor when I pressurize the cylinder? I understand that the reed valve won't close entirely without full pressure in the cylinder, so I expect some air to becoming past it, but the carb too? Thoughts? The carb was rebuilt about 4 years ago and has been working fine ever since.

[KDS4444]

Okay, I put this post up five days ago and so far no one has offered any response. Am not sure if this is because the question is somehow inherently idiotic or because no one knows the answer to the question! Am just left guessing.


I know the reed valve is supposed to seal the cylinder during a regular compression cycle, but my understanding is that it is in part the extreme pressure created by the exploding gasses that creates this seal, and that without it the reed valve is less than 100% effective. So I guess maybe my real question is, does the carburetor also contain any kind of valve that creates a seal against pressure generated in the cylinder? If not, and if the reed valve is a little bit leaky (which it is supposed to be allowed to be, as near as I can tell), then how can I ever get a 120 psi reading from my compression tester? Any ideas? Anyone?? At what amount of pressure will the reed valve create a seal?

[sc00ter]

I thought the scooter was running again. Was it running and stopped? If so, how did it die? Slowly or suddenly? Did you even fix the exhaust gasket?

If you continued to run it without a exhaust gasket there is a chance one of the metal reed pedals broke/cracked/split. If a metal reed pedal fails it will make the motor loose compression and not destroy the piston OR destroy the piston. If the reed chips off behind the piston it will destroy the piston and main bearings.

Or did you have a air leak in the system causing a soft seize? You can rent a leak tester for free from a auto parts store. Watch a few You Tube videos on how to do a compression test and a leak down test. I have a homemade compression/leak down tester made out of one of those pulse check things you put on your arm.

Be aware that reed pedals never seal tight on the reed cage. It's totally normal for them to be warped. I'm currently running super stiff reeds to counter the affect of a 4 stroke atomizer, and they are warped like crazy. The scooter runs 100% and strong.

Two stroke motors are sensitive to air leaks on the intake side. Also to air box being properly sealed. And having the exhaust sealed against the cylinder.

I highly suggest doing a compression/leak down test. Hardest part is finding the right size freeze plug to seal the exhaust port. I can't read the size anymore on my freeze plug because I replaced the washer with a different one, so I can't suggest a size.

[Zappa]

I think I may have a similar problem with my 02" Zuma and I'm suspecting the reeds.
Here's a little history; I bought it 2 years ago from the original owner, with only had 4000 miles on it. He's an older guy who only used it to buzz around the campgrounds, so he never took it on the streets. Later he bought a golf cart and stopped using the scooter. I bought it from him and right away I replaced the battery and the dry rotted OEM tires.
It ran good, but it topped out at 37 mph. I needed more power and speed to keep up with traffic because the roads around here are mostly posted 35-50 mph. Usually people are driving faster than that and I don't want to get run over.
Last year I got an NCY 47mm kit (70cc ?) which came with a pipe, jet set and clutch/variator. It took a lot of trial & error to get the jetting right to where it made good power without bogging down in the mid-range. I check the plug often and it's a nice tan color.
I usually only make short trips around town, 3-4 miles, then stop at the market or wherever for a few minutes, it ran fine and hot restarts were not a problem.
Anyway, I took it for a longer ride a few days ago, it ran good, I got it up to 50 (I read the speed on a GPS) and was cruising around 45 or so, when it suddenly shut off. I coasted to the side of the road and tried to restart it. It cranked normally, but didn't fire. I then pulled the plug, looked good, nice tan color, then checked for spark, which looked OK. I stuck my spare plug in anyway, that also looked to have good spark. I had plenty of gas, and there was gas in the carb, I could smell it, and after trying to crank for awhile, the plug would appear to be a little wet with gas.
Luckily I was only two miles from home, so I walked back, hooked up my trailer and went to retrieve the Zuma.
I just parked it until the next day when I had time and daylight to mess with it. I stuck my finger in the plug hole and cranked it, compression felt good. Checked for spark again, that looked good too. Put a new plug in, cranked, by no start. I put my hand over the exhaust pipe outlet, and I felt flow as I cranked it. Next I pulled the air filter (aftermarket, not stock air box) put my hand over the carb inlet and cranked again. I felt vacuum sucking, but I also felt it spitting gas back onto the palm of my hand. This is why I suspect the reed valves.
Does it sound like I'm on the right track here ???
I just ordered a new reed block, so when I get it, I'll pop that in and see if that's the cure.
Assuming I'm right and it's the reeds, is it normal for the engine to just outright quit when a reed fails ?
There was no loss of power, sputtering or strange noises before it quit running, it was instantaneous, like I had turned off the key.
One more thing, when it quit, it felt like it was pretty hot and I did get a little burning smell. I've heard the term "soft seize" a few times, but never got a clear explanation of what that is, can someone please elaborate ?

[KDS4444]

Thanks for the response! The scooter had been running recently, but never got quite to the point of operability. Then I bought an electric scooter, and put the project on hold. Then I ran into a problem with my property manager regarding how I was going to charge the scooter, and the bastard stole my charging device right out of my parking spot. Am seeking city mediation to have him return it to me. So the electric scooter is now collecting dust, and I want to have SOME form of transportation, so am now BACK to working on the ol' Zuma. Eghad.


At the ol' Zuma still aint working. About two weeks ago I went down to try to start it, and it wouldn't turn over. At all. I had put in a brand new cylinder, piston, piston rings, head, and gaskets, everything but a new crank case. Kicked the kick starter repeatedly, nothing.


So I checked the compression, which should be great on a newly rebuilt engine. 75psi was the max I was getting. WTF? So that was when I hooked my compressor up to the spark plug hole and turned it on and heard air escaping through my carb. A cracked reed would certainly allow that to happen, but so would a reed that was fine, given that they don't seat perfectly in an engine that isn't actually hot and running. I couldn't detect any leaks around my engine block, or around the spark plug hole— then again, I could only get it up to 75psi before all of the air the compressor generated was getting shot out the carb, so I turned it off at that point.



"You need four things for engine turnover: gasoline, a spark, air, and compression." I have plenty of fuel, I have a spark (I could double check that, but I had a spark a couple of months ago and have no reason to think that this has changed), there seems to be plenty of air (the intake certainly isn't being blocked anywhere) so that leaves compression. And when a compression tester tells you you are only getting 75psi, then that CONFIRMS the compression problem!

Oh, and I had been trying to use a new non-oem gasket on my exhaust, which wasn't sealing. I have removed it, and am back to using the original gasket there, which seems to be sealing just fine (for awhile there I was having leakage around the exhaust port of the cylinder, and then I ripped the head off a bolt trying to use a torque wrench to get it to seal in place, so then I had to get a bolt remover and screw out the busted bolt, get a new one, and bolt the exhaust back on, this time with the OLD gasket, and all that took quite awhile to accomplish, though I was hopeful it would all run once I fixed that— wrongo).

Also: I checked the reed valve about a month ago when I was working on all the other parts. There were some gaps, but the petals looked fine. I flipped them over to get closer to an total seal, and put them back in the engine with a new gasket to boot. I guess maybe I will take them out and have a second look, because I can't figure what else could be wrong.



I suppose I need to create a home-made leakdown tester of some kind. I am too broke to be going out to buy one.

[sc00ter]

Are you 100% sure the piston ring(s) didn't break during reassembly? Could the snap-ring for the piston popped out (Be certain its FULLY seated before reassembly!) as it will pop out and tear stuff up. We use "G" style snap-rings because their easier to install.

I doubt its the reed cage. If the reeds are not broken then all is good there. Reeds rarely sit tight on the cage.

Again, compression test. Make sure everything is tight, as compression testers are a PITA to use sometimes. Do 3 test, from start to finish, and average the numbers.

No matter what the results are, you need to determine what caused the problem(s) to begin with or they will just repeat.

We see air leaks in the system (mostly around the reed cage/case seam) that will eventually lean seize the engine. Poor torque values on the cylinder head can warp the head, causing a air leak. Torque wrenches are a must!

I'm sorta done with 2 strokes as a whole. And I think my riding buddy is selling all his stuff as well. I might hang onto my 98 pre-bug Zuma but its staying mostly stock/Stage 1. But that being said, keep us posted and I'll attempt to help as much as possible.

Side note: Did you ever get the carb setting back to factory? A lean setting on the carb will slowly lean seize the motor, eventually killing compression.

[sc00ter]

Zappa, a soft seize sucks. Not as bad as a lean seize, but still not good. If you pull the exhaust and look up into the cylinder (with the piston at top dead center/ up position) look for smear/scrape marks on the bottom/exhaust side of the piston. That's the start of compression loss.

A straight up lean seize will finish off all compression. Usually scrapes on the bottom/exhaust side of piston and/or a hole in the top of the piston.

Favorite causes of lean/low compression issues:
Leaks around the reed cage assembly.
Wrong settings on the carb.
Wrong heat range on the spark plug, OR plug reach. From buying the wrong plug!
Riding with no muffler or with a serious exhaust leak at the head. Exhaust pulls heat off the cylinder head.
Pod style air filters! No one ever re-jets for them properly.
Other causes, but these are my favorites.

[Fahqu2]

Originally Posted by sc00ter

Are you 100% sure the piston ring(s) didn't break during reassembly? Could the snap-ring for the piston popped out (Be certain its FULLY seated before reassembly!) as it will pop out and tear stuff up. We use "G" style snap-rings because their easier to install.

I doubt its the reed cage. If the reeds are not broken then all is good there. Reeds rarely sit tight on the cage.

Again, compression test. Make sure everything is tight, as compression testers are a PITA to use sometimes. Do 3 test, from start to finish, and average the numbers.

No matter what the results are, you need to determine what caused the problem(s) to begin with or they will just repeat.

We see air leaks in the system (mostly around the reed cage/case seam) that will eventually lean seize the engine. Poor torque values on the cylinder head can warp the head, causing a air leak. Torque wrenches are a must!

I'm sorta done with 2 strokes as a whole. And I think my riding buddy is selling all his stuff as well. I might hang onto my 98 pre-bug Zuma but its staying mostly stock/Stage 1. But that being said, keep us posted and I'll attempt to help as much as possible.

Side note: Did you ever get the carb setting back to factory? A lean setting on the carb will slowly lean seize the motor, eventually killing compression.

I agree, I think either broken ring, or you did not seat the ring properly on the piston.

[Big Bill Gilesbie]

looks like you have more than enough answers from these gentleman and appear to be on right track

[Fahqu2]

You could have an air leak in the crankcase. Scooter swap shop sells a compression test where you plug exhaust port and pump air into the engine from intake. You can go around crankcase and cylinder head with spray bottle filled with water and soap. If you have a leak the soap will bubble.