Motorcycle Repair: honda 1989 sa50 poor performance at high speeds, vacuum leaks, electrical diagnosis

Question
Hi Mark.  I have a 1989 honda sa50 two stroke with a high performance problem.  It starts fine idles great and has great pickup.  Upon reaching around 25 mph it starts cutting out but will accelerate roughly and achieve about 37mph especially if I get a down hill stretch where I can run it at full throttle.  I have cleaned all the jets and tried moving the needle up and down a notch which made it worse.  The thing ran used to run fine for about 600 miles after I did a new top end and piston job and would achieve about 40mph.  Smooth acceleration all the way to 40mph.  This started happening when tempertures were dropping below 50 degrees.  The trouble shooting manual says it could be the cdi unit.  I have almost eliminated everything else.  It seems to have good fuel flow through the fuel line when it is disconnected and running the starter.  What could be my problem and is it the cdi and is there a way I can test that to see if it has a fault.  

Answer
Hi Henna,

The CDI either works or it doesn't. Test CDI with a known good CDI (dealership luxury).

I suspect the problem is carburetor tuning.

The first thing to do is check for obvious problems such as vacuum leaks, defective or loose wiring connections.

Next, read the appearance of the spark plug. Learn about reading plugs at NGK's website: www.ngk.com

Perfrom a compression check followed by a leak-down test if necessary.

Perform electrical diagnosis if necessary.

Get a copy of the service manual.

Do you have email? If so, include it with your reply.

I return test and repair files by email There was no email address with your question as requested.

Standing by...


Respectfully,
Mark Shively