GY6 Wont start

[galileo2]

Hello everyone. After quite a bit of work my GY6 is still not running. It is not getting fuel to the spark and I've run out of ideas to correct the issue.

I have received a lot of good feedback on the issue (see below). Here is what I have done to correct the problem.

1. Applied copper sealant to the cylinder head gasket, that alone increased my compression from 90psi to 120psi
2. Lapped new valve to properly seat it. The valve seals well, bit compression didn't change.
3. Made sure that all bolts are torqued to spec.
4. Confirmed a good spark.
5. Confirmed that the timing is good.
6. Confirmed that the pistons rings are not leaking (oil does not smell like gas and when oil was sprayed into the cylinder, compression didn't change.
7. Cleaned and inspected the carb. All seals and diaphragms look good.
8 Confirmed that the carb is getting gas.

I may have done things wrong, so please let me know.

[WildMan]

galileo2
Do you have spark? If you do have spark, check your cam timing on Top Dead Center of Compression Stroke..

David Sr.

[galileo2]

Thank you for the reply. I have a good spark, I tested it by grounding it on the engine. The timing looks good as per the manual. The cam shaft has the two small holes lined up with the cylinder head while the other thing is lined with with the "T" mark. I was a little confused because in the manual there is only the t mark and mine has a T, F, and some arrow thing with a solid line and a dotted line leading up to the T and F marks.

[WildMan]

galileo2
First when you have the T on the flywheel lined up w/ the tab on the crankcase & the holes in the cam sprocket lined up w/ surface of the head, are both valves closed & both rocker arms free?? Also, w/ spark plug removed & you stick a small screwdriver in spark plug hole can you feel the top of the piston?? Piston should be all the way to the top of it's stroke w/ both valve fully closed & rocker arms free w/ all timing indicators lined up, if not then you're 180 deg. off time.. The arrow & dash box marking to the left of the T on the fly wheel is the advanced side of top dead center.. If your fly wheel lines up slightly inside that advance box marking you still maybe all right.. W/ cam timing you're better off being advanced then retarded..

Is your spark plug wet or dry after cranking on it trying to start engine?? If spark plug is dry, that's an indicator that you're not getting gas to the cylinder.. If you've got spark & spark plug is wet w/ gas, then that's an indicator that your cam is out of time still..

Did you replace just the valves or the whole head, or complete cylinder & complete head??

David Sr.

[galileo2]

I replaced the valve, the complete cylinder and piston. I have the flywheel set directly on the T mark and the rocker arms are free. The spark plug is pretty much dry after trying to start, although it smells like gas. When I reassembled the cylindar I made sure that the piston was TDC and that the flywheel was on the t mark, I just checked the rocker arms for spacing so I'm pretty sure that they are free. I double checked the spark and its nice pretty blue. So I'm not getting enough gas to the cylinder, does that mean it's a carb issue? I guess it could also be a vacuum compression issue. Is there a way to test these without buying specialized tools?

[tonjon1]

If you have a Harbor Freight in you area, this compression tester is only $8.00.

www.harborfreight.com/flex-drive-compression-tester-92697.html

I didn't read your posts carefully enough to know if you already said, but did you space the ring gaps at 120 degrees apart?

[galileo2]

i did place the ring gaps at 120 degrees. i'll try to find a compression tester. do you know what the compression should be?

[tonjon1]

My Haynes Chinese Scooters manual shows examples of 174 psi and 184 psi for a 125cc scooter but interestingly has no example for a 150cc scooter. 200 cc it shows an example of 213 psi.

The users guide for my Xingue 150 does not give a spec.

[second2none]

There was a sticky on this site that said compression should be no less than 150 psi on a 150cc.

[JR]

no No NO! The Haynes manual lists 190PSI on the 150cc engine. JRR

[tonjon1]

Jun 5, 2010 21:17:48 GMT -5 JR said:

no No NO! The Haynes manual lists 190PSI on the 150cc engine. JRR

No doubt, I found it. "A reading of approximately 190 psi (13 Bars) indicates a cylinder in good condition"

The sample selection of specifications, at the end of the manual, has the numbers I saw first off.

[second2none]

Jun 5, 2010 19:33:41 GMT -5 second2none said:

There was a sticky on this site that said compression should be no less than 150 psi on a 150cc.

www.dansmc.com/compression_test.htm

[WildMan]

galileo2
The compression for a 150cc should be as follows, 100 - 125 lb is poor but doable, 150 lb is fair to good, 175 lb + is excellent, on a 150cc engine YOU DO NOT WANT the compression to go over 200 lb because that's a High Compression engine range which requires higher Octane Fuel..

Now I'm going to tell you from my experience you need a compression tester that has the rubber cone type seal that you hold against the spark plug port & NOT the quick disconnect air fitting type, other wise you will not get an accurate reading do to pressure leakage, trust me I know..

David Sr.

[galileo2]

Thanks everyone for the info. So far finding an adapter small enough to attatch my compression tester to my GY6 is harder than I thought. Besides online, is there an auto parts store that has a decent selection for small engines?

[WildMan]

galileo2
I think NAPA Auto Parts would have a 10mm adapter..

David Sr.

[galileo2]

I found a universal compression tester. It tested around 90 psi. Unless I did it wrong, I would say that I have a compression issue. I already replaced the intake valve, cylinder, piston, piston rings. Do I have to replace the cylinder head? I put in a new cylinder head gasket when I changed out the cylinder, I didn't use any adhesive, was I supposed to?

[WildMan]

galileo2
Describe to me what the the compression tester looks like..

As I stated above of this thread, compression testers are not all equal.. I have 4 different compression testers, 2 w/ quick disconnect air type couplers & 1 w/ just a hose that threads into the spark plug port, & 1 w/ the rubber cone type seal that you hold tight in the spark plug port & each give a different reading.. The tester w/ the rubber cone type seal is the most accurate & gives the highest reading.. The tester w/ just the hose that screws into the spark plug port reads 25 lb less than the 1st tester.. The 2 testers w/ quick disconnect air type couplers read 50 - 75 lb less than the 1st.. So if you didn't follow my recommendation, I'd say your compression tester is likely giving you a false reading.. I'd really think hard about this, you could end up unnecessarily tearing your engine apart & spend money & time that you can use on riding or on some other mod.. Good Luck!!

David Sr.

[galileo2]

The tester that I used has the rubber cone type connector. I got 90psi fairly consistantly while testing. I did see on a different thread that replacing the cylinder head fixed the compression problem. Is that what you would reccomend?

[sprocket]

You are using a new head gasket?
I would have used high heat cylinder gasket spray
( Permatex Copper Form-A-Gasket Sealant) or Loctite 819 Hylomar Gasket Dressing-Sealant (spray)
You did torque the head bolts to spec?
You did seat the replacement valve using grinding compound?
You do have oil in the engine?
You have the valves gapped to .004" ?

If the 90 psi is accurate your compression is going one of three places.

It is either getting by the rings and going into the sump
OR
it is getting past the valves
OR
it is leaking between the head and the cylinder

I would remove the valve cover and see if it is the valves first. You should not be able to
feel air escaping from the values when you turn over the engine. You may want to
turn the engine over slowly by hand with the spark plug tight.

Then get some soapy water and run it around the joint where the head gasket is. If air is leaking the soap will bubble when you turn over the engine

To check the rings, spray about 2 tablespoons of new oil into the spark plug hole
and re-test your compression. If it goes up then your rings are leaking...

Be certain, once you get it running - and you will, to re-torque the head bolts.

HTH

[second2none]

Jun 7, 2010 13:29:28 GMT -5 galileo2 said:

The tester that I used has the rubber cone type connector. I got 90psi fairly consistantly while testing. I did see on a different thread that replacing the cylinder head fixed the compression problem. Is that what you would reccomend?

I replaced my head and made sure the valves were adjusted right after to .004. Went to go fire it up and voila its alive again.
Follow galileo2's guideline which is what i did similarly when reassembling.

[WildMan]

galileo2
It sound like you're using the right style compression tester.. I agree w/ sprocket fully, accept you should do the compression test w/ oil first before taking anything apart.. The only other real problem I have w/ this, is if 90 lb compression is an accurate #, is that even though 90 lb compression is not good, the engine should still start & run.. Hell, I've had engines (motorcycles & cars) w/ only 75 lb compression start & run, & run fairly well for being worn out!! But no questions there's some trouble shooting to be done here..

David Sr.

[sprocket]

Great you got it running! Run it for a few days and then re-torque the head bolts to spec.

Hopefully your compression is up to at least 180+ psi.

[galileo2]

Thanks everyone for the info and advice. I have some Copper Spray-a-Gasket, lapping compound and a torque wrench. I'll let you know how it goes.

[galileo2]

I sprayed the gasket with sealant and torque the nuts to spec. The compression went up from 90psi to a consistent 150psi. After dinner however, the best I could get was 120psi. The barometric pressure did drop due to a pending storm, but I’m not sure if that would affect the engine. I didn't have time to get to lapping the valves due to the weather, but it is on the agenda for tomorrow. I did have time to test the valves by removing the cylinder head, filling it with engine cleaner and looking for leaks. The intake valve did leak a little bit, which makes me hopeful that properly seating the valves will do the trick. Thanks again everyone for the help.

[sprocket]

I know it is a pain in the butt, but you have to seat the valves and test them. ANY leaks and you will loose
lots of compression. These engines are so small, that even a very small leak will make a BIG difference.

No barometric pressure won't make that kinda difference. Something else is going on. You need a compression
of 150 PSI at minimum.

[galileo2]

I got the valve lapping done today but I will not be able to test the valve untill i get a replacement valve keeper. It turns out that they are very difficult to find in the grass. Hopefully it will be here on monday.

[unorthodoxneon]

I wonder what caused the bent valve to begin with.

[sprocket]

Good question unorthodoxneon!

Valves are seriously hard to bend. The only time I have seen it is when the valve drops into the cylinder
and the piston hits it. After that the engine is usually finished.

[unorthodoxneon]

Jun 13, 2010 15:17:01 GMT -5 @sprocket said:

Good question unorthodoxneon!

Valves are seriously hard to bend. The only time I have seen it is when the valve drops into the cylinder
and the piston hits it. After that the engine is usually finished.

On the scoots oh yeah these engines would be toast if it was to suck a valve in.

If hes using the same head from the bent valve i would of also checked the valve guides as those too usually get messed up along the way.

Also now i think about it you might want to look at your valve springs too. Sounds like you might of had valve float that could of caused it.

[galileo2]

The engine actually over heated, causing a multitude of problems including a bent valve. The cylinder head, valve springs, valve seals, etc. all look good.