yamaha qt 50 engine tuning...

hi- im new. i have a 1987 yamaha qt 50 which i rescued from the trash. with a carb cleaning and a new piston it hauls. i have noticed it seems to hesitate or actually loose spark at higher speeds i can feel the bike slowing and restarting and slowing again..eventually getting worse at slower speeds.. you get the idea. i had thought it was the spark coil and was right, vibration had loostened the screws and the coil was dangling loose, i tightned with loc washers and still the bike has this wierd behavior at around 20 -25 mph. i checked plug to see if mix is right- the plug is nice and brown in color, perhaps it could be the float? it moves ok up and down. perhaps it is a carb adjustment, does any one have specs( #of turns out or in) for the carb with only two adjusting screws (not 3). i have no other thoughts other than that cdi unit or to buy a new spark unit mine was slightly rusted but has been cleaned... help would be appreciated. can't wait to see how fast i can go. Greg from boston

Re: yamaha qt 50 engine tuning...

welcome to the world of basket case moped rescue.. been there done that and watched a lot of people go there and do that in these forums.

my advice is to check everything you mentioned as well as points adjustment, fuel flow, vacuum leaks around the carb intake, etc, etc, etc..

The best anyone here can do is make wild guesses at more things to try.

Go to the "Resources" at the top of this page and follow it to Fred's Guide and other helpful links.. This will give you more ideas on things to check. Good luck.

Re: yamaha qt 50 engine tuning...

JOEW said- "my advice is to check everything you mentioned as well as POINTS adjustment, fuel flow, vacuum leaks around the carb intake, etc, etc, etc.. "

There are no pints on the qt50- it is a CDI


Re: yamaha qt 50 engine tuning...

oops! i screwed up again.. and to make it worse, greg even mentioned the CDI.. But if thats my only error i was lucky cause i know absolutely nothing about the QT50.

i still sorta thing that the general guides could be helpful since they will suggest things to look for.. and examining everything you can think of will eventually uncover the problem.

Re: yamaha qt 50 engine tuning...

i've owned a few QT50's and both of mine did this same problem,right around top speed it 4-strokes a little, let off the gas a touch and she run's fine. No way to really back this up, a few others i have talked to about this have told me that was a problem with the QT, i tried up jetting to solve this issue but it ran too rich, i finally gave up on them,not a very impressive ped,but fun

Re: yamaha qt 50 engine tuning...

ryan- this is exactly what mine does, common problem? well do you happen to know the carb mix settings on these per chance i think this may help me out. Thanks for the reply greg

Re: yamaha qt 50 engine tuning...

I have the same exact issue with mine, any suggestions?

Re: yamaha qt 50 engine tuning...

Gruff "(OFMC)" - /

this could be many things or combination of things. Mine don't do it but used to when I got em.

Correct new plug gap .026

make sure fuel system has good flow all the way to the carb, clean tank,petcock, filter, carb. most important the carb, do that 3x haha (completely)

if you dont have an inline filter... get one

burnout exhaust

clear cylinder ports

clean head and piston

if you have a stock air cleaner and housing, cut holes in the housing to derestrict the air flow

the screw turnout for stock is 1 1/2 out for mix, I have free flow air filter so mine is 2 1/2 -3 out depending on weather

check for air leaks head gasket, base gasket, intake, carb and look for fuel line leaks, like where they connect to stuff, might be crack or split or lose I use clear lines so I can see the fuel flow.

do plug reads to know how to adjust the mix, 1/4 turns go a long way at full throttle

Re: yamaha qt 50 engine tuning...


adjust it. It's pumping too much with the high rpms. Been said a thousand times on here.

Re: yamaha qt 50 engine tuning...

Bruce Woodburn /

I re-jetted my 1980 with a 65 jet. Put a new oil pump on it, and like Ryan said, too much oil, so I plugged it off and run premix, that it likes very much. It still 4 strokes a little at WOT. Back off just a little and it runs fine.

Re: yamaha qt 50 engine tuning...

Matt Smith /

super common..

get a k&n air filter.. readjust air mixture.. keep turning air mixture screw clockwise until it stops. do plug chop..

that's what cured the two i have dealt with.

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