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Discussion Starter · #1 · (Edited)
Hello from Greece,
I purchased a Yamaha PW50 1992 model for my nephew , and i made some upgrades.
I have put a 60cc big bore kit (44mm piston) modified to 45mm piston, so now the bike is 62,5 cc.
I have purchased the PW BIG BORE CAR KIT with 21mm carb, Aluminum intake manifold and Boyesen power reeds.
When i made the upgrade to 60cc the bike was runing very lean with the stock carburator and also was dificult to tune it due to bad carb condition.
When the 21mm carb arrived, i bore the cylinder andput a 45mm piston.
I have install the 21mm carb and after some carb adjustments i the bike run pretty good.at full throttle.But i i think that is a little rich from the idling to 3/4 throttle.
I have the mixture iddling screw at 1 and 1/2 turn counter clockwise from full closed and set the needle high at the second position from top.
Sometimes when the bike runs at high rpms and the kid close a little the throttle and it open it again the bike bog down and have to close the throttle again and wait with the throttle close to decrese the rpms before open it again or the bike will shutoff...
Also the bike have about 130 psi of compretion but i have stock exhaust with custom muffler.


62,5cc (45mm piston kit)
New crankshaft seals
New crankshaft bearings
Ported cylinder Inlet and Exhaust side
Stock exhaust witn custom made mufler
Stock coil
Stock cdi
2,5% oil mixing
Sparkplug gaped to 0.65 mm


I have attached some videos in case you could understand maby what the problem is.
In the first video you can hear the bike made two times the cuts off
In the second the bike runing good, only in the first 20 seconds you can hear some small cut offs.
“Thank you in advance,
 

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It will be a miracle if anyone who frequents here can help with your situation.
Especially with all the changes to the engineering that you have made.

I think you need help from an "old" mechanic who really understands how engines really work.....2 cycle ones in particular.

The port sizes and valves might not be right for your new piston.
But that's just a wild guess.
 

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Imade the others two because i was not able to upload the two videos.....Now the thread tha has the videos with the problem was deleted...
And now you have it all in the same thread.

GOOD WORK.

P.S. First time posters are not allowed to post links or videos immediately upon joining.
 

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Discussion Starter · #11 ·
Yes you are right, thank you for repying sir, i know that is too many mods.
The bike working very nice at full throttle..
This saturday at the track...in about 50 laps only 2-3 times made this kind of cuts..
The spark plug seems a litle richer than normal...but not to much..And most times that this problem hapens...its when the kid comes from full throttle from a straight line...and close the throttle to take a corner then some timew the bike boging down when is trying to accelerate....But not all the times! It is like lose spark and when he close the throttle and wait the rpms go down the bike find the spark and takes off again...You can see it hapens on the first video..
 

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Based on that latest information, I'd think you have some carb needle and mixture screw adjustments to dial in. Perhaps even may need a different carb slide cutaway or needle taper to match your mods. You really do need someone with serious carb knowledge and extensive experience with porting 2-strokes and setting them up properly once modded.
 
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