Advice needed. Yamaha Jog R (cs50) lost power.

[cabrinha]

It' s been a week now since my scooter (Yamaha Jog R, almost stock except for a derestricted aftermarket exhaust and a cheap 17mm dellorto clone carb) lost almost all its power. The engine turns on as usual, it idles very low and in order to start moving you have to wot. Its only with full throttle that slowly starts to move (if you are not in completely flat surface you have to help it with you foot) and reaches about 25 km/h (15.5 mph). It behaves as if you turn the throttle to 5% in a good working motor.

Everything started after a small ride with a pillion in heavy traffic.

Changed the spark plug and tried to retune the carb. Both idle and mixture screws are fully tightened now. Even that way the idle is very low and the engine dies. Mixture screw does not respond to any setting, either fully tight or fully open.

I haven't changed anything in the last recent time apart from the belt and the pulleys (bando b4004, bando wr150-12-065-6).

Does the above description gives a hint as what went bad?

[bacon]

If it was working fine (with all those mods in place) prior to your ride in heavy traffic then I can only imagine one of two things happened. Either the engine ran lean/hot and soft seized or you blew out your belt.

[aeroxbud]

It does sound like it could be low on compression. What colour was the plug when you removed it? Does it feel very easy when you kick it over?

[cabrinha]

I thought my problem was either a clogged exhaust or a blocked air inlet. Removed the exhaust and tried the engine without it and it looked liked it worked ok. Did not try to make a full run because the noise was extremely high. So I took off the exhaust pipe and cleaned it (soaked it in paraffin oil, then in sodium hydroxide). Reinstalled the pipe and made a run. For the first 500m everything worked as normal. After that the engine started losing power again and returned to the pre clean stage.

Then I checked the air inlet side. Air filter box ok, air filter ok, inlet pipe ok. Removed the carb and cleaned it. It was in very good condition and all the jets were clean. Installed everything again and tried to fire it up. Same thing. With throttle and mix screws all the way in the engine struggled to idle.

Then I discovered something strange. My carb has an overflow port. When I closed the port idle came to normal. So I plugged this hole. Now the engine idles fine, the idle screw is responsive but the engine (lean) bogs when 1/4+ throttle is applied.

Thats it for now. I ll try to raise the needle one position in order to richen then mix.

Jun 8, 2024 7:37:26 GMT -5 bacon said:

If it was working fine (with all those mods in place) prior to your ride in heavy traffic then I can only imagine one of two things happened. Either the engine ran lean/hot and soft seized or you blew out your belt.

The belt is in good condition. Definetely not a cvt issue here. I thought the same at first but no.

Jun 8, 2024 8:04:56 GMT -5 aeroxbud said:

It does sound like it could be low on compression. What colour was the plug when you removed it? Does it feel very easy when you kick it over?

It feel exactly the same as before.

[bacon]

Is it an overflow or is it the vacuum port for the petcock? Closing an overflow should have no impact on how it runs. If covering it makes it run differently it means there is air entering or leaving there and the carb can't compensate. Post a picture of the carb and the port in question

[cabrinha]

Jun 10, 2024 23:06:09 GMT -5 bacon said:

Is it an overflow or is it the vacuum port for the petcock? Closing an overflow should have no impact on how it runs. If covering it makes it run differently it means there is air entering or leaving there and the carb can't compensate. Post a picture of the carb and the port in question

[bacon]

Is that carb that you have? I thought you had a dellorto clone, but thats a picture of a keihin. If it's actually an overflow, it shouldnt change the way it runs if you plug or cover it. Take a picture of the actual carb on your bike and show us the port in question

[cabrinha]

The dellorto one is the same as the one in the photo above. I am not so sure that the one in the photo is a genuine keihin carb. Also the clone that I have (branded miyazaki or sth like this) is also exactly the same. www.racing-planet.com/carburetor-for-minarelli-cpi-keeway-generic-qj-1e40qmb-p-101976-1.html#lg=1&slide=0

[pinkscoot]

The overflow should be open so that the carb can breath when the fuel bowl fills. This all sounds like an issue with the CVT, that much weight can stress the belt and clutch. Open the CVT and check your belt then be sure the clutch isn't glazed and not grabbing.

[cabrinha]

Its not a cvt problem for sure. I think the problem comes from the carb. All this time I had the overflow plugged and the engine worked better. For some reason with overflow unplugged to much air is entering the carb.

So I decided instead of trying a new carb to go bbk. So I have a new 17.5 carb, new crank, new cylinder/piston kit, new reed (all the above from motoforce). When I ll find some free time I ll split the cases, change crank oil seals/bearings and put the new cylinder kit.

95,38,A2 jets/needle. Sounds good for starting point?