GY6 50cc - Engine Dies/Bogs down when Revved

[wickedpenguin]

Hi All,

I have a 50cc GY6 139QMB engine.

It idles fine, but when I rev the engine just bogs down and I have to throttle off.

I have replaced the entire carb, checked the fuel lines, checked the airbox, the tube from the engine to the fuel vacuum.

I just bought a new CDI unit and spark plug and will fit them tonight, but otherwise I am stumped.

Anyone have any ideas?

[mainepeace]

What's the current state of it?

Is it brand new? Is it old? Did you just purchase it from someone? Did it ever run correctly?

If you've made any airflow changes (air filter, exhaust) you might need to rejet the carb.

If not, you might have an air leak in the intake. You could also have a clogged jet. You could also have too much oil in the crankcase.

Greg

[wickedpenguin]

its seconds hand, average condition but was running fine until a few days ago.

Its a 1996 with 5,000k on the clock

No changes have been made to airflow. All I did was open the airbox and clean the filter, there was a little water/Oil in there.

I have checked the intake and there are no cracks or leaks.

I ruled out a clogged jet, when I replaced the carb.

Regarding the crack case, how would I check this? And would this just happen? as I haven't added any oil at all and the dip stick looks fine.

Thanks for your help.

[kevincro]

It sounds like you need to adjust the carb. You might also check the spark plug. If the engine is running too lean it will bog down under a load. Try turning the mixture screw on the side of the carb all the way in till it stops turning ( do not tighten the screw simply turn till it stops), and then back it out two turns. You will just have to play with it from there. The right setting should be somewhere between two to three turns.

[wickedpenguin]

Thanks for the suggestion but I have already tried that. And replaced the carb with a brand spanker.

So, I dont think that the issue.

[mainepeace]

Well even if it's a new carb it might not be adjusted perfectly. Did you swap the main jet? Since it idles fine you don't have to worry about the idle jet.

Did you check the fuel petcock? Or fuel filter? Perhaps you're not getting enough fuel (which is what it sounds like, but there are a number of causes to that issue).

Greg

[mattack]

Other option could be that its starving for air.
I recently went thru this after an air filter change.
I had something clogging the air line and if I revved the engine it would bog down... if I babied it I could get up to about 25-30mpg but if I went WOT it would bog.

[50seesee]

Hey wicked,

I experienced this, but it wasn't a problem for me because I was messing with the screw on the carb that controls the air and fuel mix. When I had it running too rich (but still able to idle), any throttle caused it to die. Once I tuned it right (in my case, leaning it out...) it ran fine. If nothing above works, mess with that screw.

[wickedpenguin]

Thanks for all the suggestions guys.

I have ruled out airflow as it still has the same issue with free flow filter.

I have checked the fuel filter and it looks fine and fuel is running though pretty fast. However, I have also purchased a new fuel line and filter to rule that out also.

I will have another play with the Ratio Screw in the carb tonight and hope for the best, but I did spend about an hour trying different combinations on the old carb with no results.

[mainepeace]

You should follow Lefty's procedure to tune the carb A/F screw.

[wickedpenguin]

Where can i find that procedure Greg?

[wickedpenguin]

Ok, I tuned the ratio screw on the carb, but still exactly the same this thing. It idles, it even revs and the clutch engages and if I throttle further it just dies.

What else could it be?

I did notice that the back wheel is very hard to turn, is that normal?

[wickedpenguin]

Anyone have any other ideas? I don't want to have to call in a mechanic, but I am running out of ideas.

[mainepeace]

No it isn't. It should spin fairly freely, like a bicycle tire. Perhaps that is your issue.

I had a similar issue with a rental bike. The front brake was so tight that it would barely push.

Check your rear brake. Check the adjustment to see if it is too tight when the handle is released. Loosen it up. Also check to see if the wheel is warped or rubbing.

Greg

[wickedpenguin]

I will have a play with that tonight. Would that cause the engine to bog down and dies then throttled though?

[mainepeace]

Well yes, if the engine tries to rev but can't because the drive wheel is engaged through the clutch and isn't moving, or moves too slowly. At that point, when the revs go up the clutch engages, so the engine speed roughly equals wheel speed (factoring in transmission ratios).

So it might sound like the engine is bogging but it's really because the drivewheel is forcing the engine to slow it's revs.

Just speculation at this point.

Greg

[wickedpenguin]

I appreciate your help on this Greg..

I will check this out tonight and I might also check the valves and vavle leash and make sure they are set right.

I might also take the clutch and variator apart if that doesn't work and check for obvious damage.

After that, I think I have either replaced or checked every part on the entire scooter except the pistons, rings, head and gaskets, but compression is good so I can't see that being an issue.

i will work on it again tonight and keep you updated.

Cheers,

Rhys

[mainepeace]

Sorry, missed post above.

Lefty's Carb Adjusting Procedure:

ADJUSTING YOUR FUEL RATIO SCREW:

FOLLOW THIS PROCEDURE, take your time DO NOT Be turning the SCREW fast and wait in between turns otherwise you will be off.

Warm Up the SCOOT for at least 5 minutes.

YOU WANT THE HIGHEST IDLE OUT OF THE FUEL RATIO SCREW YOU CAN GET.

ADJUST YOUR RUEL RATIO:

1.Turn the Fuel Ratio Screw Counter Clockwise 1/4 and wait 10-15 seconds to allow the engine to catch up with the new setting.

** IF ON THE FIRST TURN IT DOES NOTHING OR IDLE GOES DOWN- GO to 2.*** Otherwise GO to a. below.

a. If the IDLE GOES up repeat 1.
b. If the IDLE does not go up or it does nothing turn the Fuel Ratio Screw 1/8 Clockwise that should be the highest IDLE you can get out of the Fuel Ratio Screw-- GO to FINISH UP.

2. Turn the Screw Back 1/4 and GO to 3..

3. Turn the Fuel Ratio Screw Clockwise 1/4 and wait 10-15 seconds to allow the engine to catch up with the new setting.

** IF ON THE FIRST TURN IT DOES NOTHING YOUR PILOT JET AND OR PASSAGES ARE CLOGGED*** GO to CLEAN CARB*** Otherwise GO to a. below.

a. If the IDLE GOES up repeat 3.
b. If the IDLE does not go up or it does nothing turn the Fuel Ratio Screw 1/8 Counter Clockwise that should be the highest IDLE you can get out of the Fuel Ratio Screw-- GO to FINISH UP.

FINISH UP:
SET IDLE USING IDLE SPEED SCREW:
USING the IDLE SPEED SCREW set the Idle to 1500-1800 where it IDLES the smoothest AND the REAR WHEEL NOT be Spinning- DONE RIDE SAFE.

CLEAN CARB:
Take Carb Bowl off and clean Pilot Jet and Pilot Jet Passage and Main Jet and Passage.
Then come back and repeat this procedure.

[wickedpenguin]

Ok. I don't want to get my hopes up, but its working. It doesn't run incredible smoothly, but it runs.

I went for a 20 minute ride this afternoon and it dips occasionaly but then recovers.

It seemed the issue was combination of things.

1) Moisture in the engine ( I used from petrol treatment and that helped )
2) Fuel Air ratio - Thanks Greg and Lefty
3) Oil and moisture in airbox - Replaced Airbox
4) Faulty diaphram in carb, and possible blocked main jet - Replaced Carb
5) Spark Plug - Replaced

It has been running this afternoon, but it might be a different story when its cold.

I was going to check set valves to .004, but I have held off as it is now working.

Do you think this maybe the cause of the dipping?
I have also ordered a no rev limit CDI from Scooter Assassin as I thought that might be the cause of the dipping.

Does anyone know the symptoms of a faulty coil or stratos?(sp)

Thanks again all of you for your help and all the awesome tutorials on this site.

Rhys

[pennstump]

Sorry to resurrect such an old thread, but this sounds remarkably like my situation.

His final question wasn't answered. What are the symptoms of a faulty ignition coil?

[chops]

It also sounds like my problem Which I havnt solved yet. My bike only 2 months old amd has only covered 480kms and is unridable at the moment. I'm awaiting a brand new carb coming to eliminate that.