Power Loss when engine is warm

[rawket]

Guys--

I have a bone stock 1989 Zuma with ~430 miles on the clock. I purchased the scooter 5 or so months ago with ~100 miles on it. for the past ~320 miles it has run perfectly aside from a slightly slow idle which I adjusted up about 300 miles ago.

About 2 months ago I was scooting up a moderate hill and my speed started gradually dropping. This is near my home and I usually can hold ~30mph and was going closer to 20. Upon stopping at a light, the scooter could not go at all without my kicking and pushing. I made it home and parked the scooter for a couple of hours. Upon restart the scooter had roughly standard power for the first 5 or so minutes then the condition returned. I ordered 2 new spark plugs and the snow fell.

Last week I installed a new plug. The old plug looked great but I figured I would give this simple fix a shot. I have the exact same symptoms now. I did a plug test per forum postings and compared to http://www.myvento.net/kbarticles/spark50.html . I feel that the plug falls into the 'Best' category most closely resembling number 15.

The scooter has plenty of fuel, fresh oil, and the exhaust and intake/air filter all seem quite clean and unobstructed. I am inclined to feel that I may need to tweak my mixture a bit, but thought I would run the scenario past the experts before I went non-stock on the carb.

Again, this is a 100% stock scooter with a fresh spark plug.

Thanks,

[StepVino]

Fist thing I would do is check hot compression, then
disassemble and clean the carb.

[kaizen]

kaizen watching...

[fastrazz89]

Sounds to me you are slightly lean seizing your engine.
This is likely due to the colder weather. When you are running a 2 stroke engine hard in colder weather, it needs a richer fuel mixture, otherwise it will overheat and seize.

[kaizen]

rawket, were you able to check the plug?

[rmehuron]

I think your problem lies in the CVT. I very much doubt that your stock jetting is the cause. Maybe your belt is slipping......

[rawket]

Roads are clear, Compression tester in hand, freshly rented from 'Auto Zone' but I am unable to get it threaded into the cylinder. It is affixed to the hose, trying to locate a more sophisticated tester or pull some panels to get it to work.

Condition is same otherwise.

[robotguy]

I mention this to all prebug vertical minarelli folks all the time.

That Intake boot that goes from the carb to the cylinder
might have a crack in it.
My 89 did, just from age.
That gives you an air leak, and can cause this very problem.
Inspect the boot. Any cracks at all?

[rawket]

All---

Thanks for the help I went through the following steps:

compression test
intake 'manifold inspection'
carb cleaning
pulled exhaust off to check for low flow

Eureka! The screen on the back of the exhaust was completely blocked. Left end piece off and bam! Full power even on a long ride. Ripped blocked screen out of end piece and am back to full speed!

Thanks for all the help guys!

[rmehuron]

glad to hear when something work out, not to mention didnt cost anything to replace :)