problem with my son's bike

[buzzardman]

my boy rides a 97 XR 70.

It is normally a great bike.  For the last couple of trips riding it is getting harder and harder to start.  The last two times we went riding it puffs lots and lots of white smoke for about an hour of riding then the smoke clears up.  It runs fine while it's smoking.

Now, it wont start at all.  I changed the plug first off.  with the new plug in it, I kicked it for awhile and then took the plug out.  The plug had raw gas on it, so it's getting fuel.  It is also getting spark.  The two things that usually make it go vroom vroom.   

Any Ideas.   I'm thinking a stuck valve?  maybe I should do a compresion test.  What kind of compression should a 70 cc have?

thanks.

Clay

[Mars]

Me thinks rings not valve. Compression test will help. Should be at around 90 psi. Sounds like your around 70 psi.

[buzzardman]

my compression tester doesn't have an adapter to go down small enough for that little plug so I just put my index finger over the plug hole and had my son kick it over a couple of times.  I figure at 90 psi it should have pushed my finger maybe not off but at least some right? 

It didn't push at all and the compression noise (rushing air)  sounded like when it came to top dead center the compression sounded like it escaped up into the intake and into the carb???

What do you think,  are the rings shot?

[Fox17]

id say time for a new top end.

[UncleRico]

I'm not familiar with the 70 cc engine, but if we look at what causes compression loss, we can pretty much narrow it down.


1) Burnt valve

The symptoms of worn valves are a general very slow decrease of power over time from an engine that runs fine and otherwise seems normal.

If a valve is unable to cool itself it will burn and parts of it will dissolve so that it no longer seals. If the valve does not seat properly against the head, it will burn faster.

If a valve is adjusted too tight, it will have less time to stay seated against the head and will eventually burn. If the cylinder is running too hot from pre ignition, retarded timing or the wrong kind of fuel, the valve can get hot enough to try and spot weld itself to the head. This creates a ragged edge that keeps the valve from seating and the valve rapidly burns because it can not cool. Since the exhaust valve has the hottest job it usually is the one that burns.

Common operating symptoms include hard starting, loss of power and uneven running with poor idle.

2) Cracked head:

Heads tend to crack then there are large temperature gradients within the head causing different degrees of expansion within the head. This might be caused by a blocked coolant passage or by incorrect fuel or timing causing the inside of a cylinder to get very hot very quickly.


The driving symptoms would be essentially the same as for a burnt valve except that you will almost certainly have coolant loss and overheating problems.

3) Burnt piston

Incorrect fuel and timing causes temperatures in a cylinder to become much hotter than normal. A carburetor providing a little too little fuel or air leak adding too much air to the air fuel mixture leans it out to burn hotter. Usually it is the exhaust valves the burns first but sometimes a hole is burned into a piston.

A compression test shows no compression in a single cylinder.

Symptoms would be the same as a burnt valve but you will probably have a noticeably blue exhaust colour.

4) Worn rings: When the rings wear, or if scratches are made in the cylinder walls they become less efficient and more leakage occurs between the combustion chamber and the central oil area.

Scratches are caused by dirt particles entering the carburetor with the air. The more you get, the faster an engine wears. Running an engine in a dusty off road condition without an air filter can cause scratches to ruin it within a hour.

Hi flow air filters work by having larger air passages in the filter element. This lets more air in, but it also passes more dirt particles through that will score your engine cylinder's walls. Be wary of high flow air filters in a dusty off road environment. Always keep your air filter system intact and clean.

A compression test will show overall lower compression. If you squirt some oil into the spark plug hole and rerun the test, the compression will be higher because the oil around the rings will temporarily provide a better seal.

It's most likely one of the above 4.

Good Luck!

[Chevy1500z71]

is it a 4 stroke or a 2 strok, if its a 2 stroke just get a new piston ring/rings. also jeting is a big part of fouling plugs on 2 strokes but if its stock than dont wory about it.