49cc 2 stroke compression

[tml4ever]

I am having problems with my sons bike. It was running great until recently. On the weekend he was riding it and it started to bog out, I noticed that the choke lever was moving with the vibrations of the motor, and the bike was being choked. I bent the choke lever a little so that it stopped moving and I thought the problem was fixed.

After about 20 minutes the bike just died. It seemed like it over heated, it was really hot. He had only been riding for about 30 minutes total.

Now I can't get the bike to start! I have taken the carb of and cleaned it. If I put gas down the head the bike still won't start. The bike has spark and I put a new plug in too.

I checked the compression using a gauge and held the throttle open and pulled the bike over 3 times, the psi was 75. Is that good compression? Did I use the correct method to test it?

I am using gas mixed 25:1 with stabilizer in it.

I don't know what to check next?

 

[Patman]

It's an air cooled 2 stroke right? If so just pop the head and take a peek inside.

 

[tml4ever]

air cooled

Yes, it's air cooled. What should I look for when I take the head off?

I have never worked on a motor, I know there will be a piston, with 2 rings i think. But how to tell if the rings are warn out?

 

[Patman]

Let me guess no service manual for the bike either. I would suggest making friends with somebody in the area that has a similar bike or some mechanical skills to help you. Even then they will need information contained in the service manual to help determine what the specifications for the wear limits are.

 

[tml4ever]

no manual

Yes, you are right, no manual. I would like to get the rest of this season out of the bike, then scrap it.

Should the bike start if the compression is 75 psi?

There is fuel in the carb and it has spark.

 

[Patman]

I'll guess that 75psi is well below the necessary number which I'll bet is a lot closer to 120psi.

 

[tml4ever]

When I check the compression, how many times should I pull over the motor to get an accurate reading?

 

[rmc_olderthandirt]

Throttle wide open, kick it at many times as necessary, until the reading stops going up.

75psi is really low.

I am not sure I understand the choke lever thing. When you bent the lever to keep the choke off did the bike run OK, or did it still bog?

It is likely that the bog was because it was running lean, which could be from an air leak. Running lean would make it run hot. Running hot would make it seize. Seizing would make it stop, and ruin the compression.

Rod

 

[tml4ever]

When I bent the choke, the bike ran better but it still was lacking top end. Then it died.

Where would the air leak be? Between the carb and motor?

 

[rmc_olderthandirt]

Between the carb and engine is a likely spot. It is also possible that the main jet in the carburetor clogged and it was running lean as a result. Its also possible that it was something totally unrelated to the fuel mixture.

What I suspect, however, is that what ever the original problem was it has now gotten lots worse.

The first symptom was that it wasn't running right.
The second symptom (which may have gone unnoticed until it was too late) was that it overheated.

It may be hard to establish what the original problem was, but I suspect that the major problem now is that the piston seized.

It's time to remove the cylinder head and take a look at the piston and cylinder.


Rod

 

[SHSPVR]

Most avg air cool 2 stroke motor have do low compression ratio any where for 6.0 to 7.5 so 75 PSI on a 49cc 2 stroke is about rigth.
Like what Rod said pull cylinder head and pull the cylinder off so you can look at cylinder bore and piston and rings post screen shot here I sure we can help with it.

 

[tml4ever]

OK, this could be a good learning project for my son and I. The bike may never run again, but that's OK. I am shopping for a new bike for him now.

Anyone know of a good book I could buy to help me along the way?

Thanks!

 

[SHSPVR]

what make and model of your son bike?.

 

[Patman]

Yes check out your options here http://www.eric-gorr.com/ Eric makes it painfully easy to understand what's going on with engines then if you need minute details get a factory service manual.

 

[tml4ever]

what make and model of your son bike?.

It's a Giovanni mini pocket dirt bike.

http://www.giovannibikes.com/Dirt-Bikes-23732/3544328-NEW-GIOVANNI-49cc-GAS/ProductInfo.aspx

I was hoping it would last 1 season, it almost made it! This was his first bike to see if he was into it. Now I am looking at either a crf70 or 80.

 

[SHSPVR]

That should be super eazy to work on not all that much diff from a chian saw, weed eater or R/C motor so any book on 2-stroke will do.

 

[tml4ever]

If I do get it running again, how do I find the air leak so that the motor doesn't melt down again?

The carb is mounted directly to the motor, there is no rubber boot there.

 

[SHSPVR]

Must common air leak in your case would be the crank seal