BBk that runs but doesnt ride

[TikiMan]

Hello,
MY friend has a 1998 sa50 lx. The problem at hand is the scooter ran fine until the piston rings went out. Since then, the piston and Rings have been replaced but the scooter will not drive under load. the scooter runs completely fine on the center stand but once the rear tire is on the ground it seems powerless. What should the compression be around for a 72cc kit? Also it has oil coming from the exhaust pipe where it bolts to the cylinder. But that just started happening since we swaped the piston out.

[Bear45-70]

125 to 150 PSI, any high than that and there will be issues.

[killerpancake]

was the head gasket replaced correctly and torqued to spec? that could cause low compression. also, maybe the carb is either incorrectly adjusted or dirty.

[eliteguy50]

Hello,
MY friend has a 1998 sa50 lx. The problem at hand is the scooter ran fine until the piston rings went out. Since then, the piston and Rings have been replaced but the scooter will not drive under load. the scooter runs completely fine on the center stand but once the rear tire is on the ground it seems powerless. What should the compression be around for a 72cc kit? Also it has oil coming from the exhaust pipe where it bolts to the cylinder. But that just started happening since we swaped the piston out.

The LX was not made in 1998. Might it be the 1988? Hopefully the proper parts were used.

[TikiMan]

Im sorry for the confusion... It is a 1998 sa50 elite. We tried running it today and everything is checking out fine. It ran today and started running better after a short time. It has the proper compression and the carburetor setting were not changed when we changed out the piston. What torque would be optimal for the kit. It did not come with any paper work besides shipping information.

[Wheelman-111]

Greetings:

If oil is coming out the exhaust gasket, air is very likely getting in. Fix that for sure or risk a lean seizure.
Air-cooled VW gaskets can be adjusted to fit. Don't use the stock "donut" gasket with most BBKs.
Head bolt torque to 12-ish pounds in an X pattern with threads clean and lightly lubed.

[Bear45-70]

I would expect the BBK to need more main jet than the stock cylinder. Do a plug chop to check mixture.

[TikiMan]

So we installed a new kit (the old one was damaged) and we have the same problem. It will run and makes 120-125psi. But it loses psi quite steadily. We did a leak down test at tdc and it had a lot of air exiting the exhaust pipe. I believe its the piston rings, but my friend is guessing exhaust gasket or exhaust pipe itself. What other test can I do to narrow it down? Or does anybody have any suggestions?

[eliteguy50]

So we installed a new kit (the old one was damaged) and we have the same problem. It will run and makes 120-125psi. But it loses psi quite steadily. We did a leak down test at tdc and it had a lot of air exiting the exhaust pipe. I believe its the piston rings, but my friend is guessing exhaust gasket or exhaust pipe itself. What other test can I do to narrow it down? Or does anybody have any suggestions?

Plug the exhaust at the head and do the test again.

[Bear45-70]

So we installed a new kit (the old one was damaged) and we have the same problem. It will run and makes 120-125psi. But it loses psi quite steadily. We did a leak down test at tdc and it had a lot of air exiting the exhaust pipe. I believe its the piston rings, but my friend is guessing exhaust gasket or exhaust pipe itself. What other test can I do to narrow it down? Or does anybody have any suggestions?

You are correct, leaking thru the exhaust when doing a leakdown test means weak or bad rings. The exhaust has nothing to do with a leakdown test It checks the rings and plugging the exhaust will not prove anything. Pull the top end and check the rings.

[eliteguy50]

So we installed a new kit (the old one was damaged) and we have the same problem. It will run and makes 120-125psi. But it loses psi quite steadily. We did a leak down test at tdc and it had a lot of air exiting the exhaust pipe. I believe its the piston rings, but my friend is guessing exhaust gasket or exhaust pipe itself. What other test can I do to narrow it down? Or does anybody have any suggestions?

You are correct, leaking thru the exhaust when doing a leakdown test means weak or bad rings. The exhaust has nothing to do with a leakdown test It checks the rings and plugging the exhaust will not prove anything. Pull the top end and check the rings.

It will prove if they are doing the test right or if there is a top end leak.

[Bear45-70]

So we installed a new kit (the old one was damaged) and we have the same problem. It will run and makes 120-125psi. But it loses psi quite steadily. We did a leak down test at tdc and it had a lot of air exiting the exhaust pipe. I believe its the piston rings, but my friend is guessing exhaust gasket or exhaust pipe itself. What other test can I do to narrow it down? Or does anybody have any suggestions?

You are correct, leaking thru the exhaust when doing a leakdown test means weak or bad rings. The exhaust has nothing to do with a leakdown test It checks the rings and plugging the exhaust will not prove anything. Pull the top end and check the rings.

It will prove if they are doing the test right or if there is a top end leak.

You have no clue how a leakdown test is done or what it tests. It test the rings and how well they are working, ONLY. Plugging the exhaust will prove you plugged the exhaust, but you know that when you plug the exhaust.

[eliteguy50]

Really bear? A leak down test is only used to test rings? So what do you use to test seals?

[konaking]

Let's not get into another "hondaspree burn debate". If you installed a new cylinder you should know if the rings are good or not. Like wheelman said try putting a new exhaust gasket on and go from there

[Bear45-70]

Really bear? A leak down test is only used to test rings? So what do you use to test seals?

Yes, a leakdown test is not the same as pressure testing the crankcase. I've probably done several thousand leakdown test over the years.

You set the motor at TDC and pressurize the combustion chamber. I guess you could find a blown head gasket or a holed piston but a simple compression test will show those and it is 10 times easier to run.