compression

[bhult]

What should the compression be at for a 90 trail?

[bhult]

I am having trouble getting the bike to run decent at an idle without using the choke and throttle. The gas line doesn't always seem to stay full up be the fuel cock.  I did put a new fuel cock on the bike and took the carb apart and cleaned it really good.  I'm getting a little frustrated

Any input would be great.  Im wondering if compression could be an issue?  It has about 3400 miles on it and is a oil injection bike.  1968 BS90  Trail.  I have not tested it yet and am not sure what it is supposed to be. 

[old smokey]

http://bridgestonemotorcycleparts.com/index.php?action=downloads;sa=view;down=18

I didn't see any compression value in the service manual (link given above).
Sort of sounds like a fuel supply problem (dirty carb idle jet?) if you're not able to get a good idle.

[bhult]

Which is the idle screw?  Sorry Im not the best with the Carbs.  Would that explain the fuel line not being full up by the petcock?

Thanks

[reed]

bhult,
The compression would be around 100 psi.
Thanks.
Steve.

[old smokey]

Reed to the rescue.
Bhult - familiar with doing compression tests?
Best to have a warmed up engine (but wouldn't hurt to take a cold reading first).
Leave the fuel petcock off for check.
Spark plug removed.
Hold throttle wide open when kicking over.
and obviously key can be left off so you're not making a spark
if after the warm test the reading seems low, you could put several drops of oil inside the cylinder and repeat to see if the reading goes up some.

[BS Mechanic]

bhult,
Unless your compression is way too low, from your description, it sounds like a carb issue, especially if the choke seems to help it run at low speed.  My guess is something with the idle/slow speed circuits of the carb.  If so, you'll need to disassemble and clean out each of the tiny drilled passages and holes in the pilot jet, needle jet, and air screw jets and passages.   These commonly get plugged up with "varnish", and crud from the tank from sitting over the years.  Be absolutely sure you are getting clean fuel from the tank and petcock.  Add a fuel filter to the line if there's any question on the tank.  If you aren't comfortable with the inner carb workings, I'd suggest you contact Steve Reed, he's the carb expert.

[bhult]

Thanks for the help everyone.  Carb is cleaned...just waiting on the new bowl gasket.  compression check  tonight.

[bhult]

I did a compression check and it read 70psi and put a few drops of oil down the cylinder and it brought it up to about 75-80psi.  Still need to find a carb bowl gasket.  What does everyone think...rings?

[Bikenstein]

Take the top end off (easy) and check your cylinders, pistons, and rings. I still think you might have a carb problem because it runs better at idle with the choke on indicatin that it ain't gettin enough fuel or suckin air one.

[bhult]

I took the top end off and it looks like I should replace the rings.  Looks like some has been getting by.  The piston has EX on top of it.  Is there an easy way to tell if it is standard or oversized? 

[slawsonb]

the EX indicates the side that is installed toward the exhaust port (front).
...bert

[bhult]

Yeah I found that after I posted in the service manual.  I'm guessing that it is a standard piston but just want to be sure.  Is there a way to tell?

[Bikenstein]

If it's not stamped on top or on the rings, then you'll have to measure it or compare it to a known piston. I don't know if these had chrome cylinders but if it does that might indicate standard if it has low mileage. Usually a low mileage bike has never been bored. The speedo could have been replaced, disconnected, cable bad, or drive bad so you can't positively go my mileage either. If the piston is oversized it should be stamped on top like normal.

[reed]

bhult,
Measure the piston 50cc is std, and the bore is high grade cast iron and there are two over sizes .25 and .50 etc
Thanks.
Steve.