Idle and jetting

[johnwohl]

Hello all.  I picked up a friends scooter for really really cheap because he got bored with it.  It is a chinese 50cc gy6 scooter, and has an upgraded foam filter intake, upgraded exhaust, and upgraded ignition system along with transmission upgrades. I've seen it run and drive with only the exhaust done so I'm pretty sure it works and he is a close enough friend that he saiud he would take it back if I couldn't get it working this spring.  I've got it to start for a few seconds now and then, and one time it ran for a good 5 minutes but mostly it won't start.  The time when it ran, the idle set screw was on its very last 1 or 2 treads before falling out.  The jets on it now are a #34 idle jet and a #85 main jet.  He gave me a #38 idle jet and #88 main jet he never got around to using.

[90GTVert]

My first thoughts are to check the smell of the fuel. Not sure if you've ever been around old gasoline, but it gets sort of a varnish smell. If you smell that, drain all of the fuel from the carb and tank. Clean the carb. Check the fuel lines. Then refill with fresh fuel and see how it does. I think those are good steps any time a bike sits for very long or it's past is not totally certain and you're having this sort of issue.

[Moat]

Yes - as 90GTVert mentioned; fresh fuel & clean the carburetor.

I'd also;

1) Install the #38 idle jet and #88 main jets you have while the carb is apart. Considering the exhaust and air filter mods - and the mention of the idle mix screw being set so far out - those bigger jets should get you closer than where you are now.

2) Check valve clearances and adjust as necessary, being the history of the scoot is a bit unknown. Overly tight valves will definitely cause hard-starting issues.

Bob

[johnwohl]

I drained the old gas and it was definitely off color. Put in fresh gas but no dice on the starting front. I will take apart the carb and clean it tomorrow.

[johnwohl]

I have the carb torn apart and the idle and main jets are soaking in carb cleaner. While thats percolating, I did notice that while the main jet on the carb was indeed marked 85, the other one that i was given isn't marked so I really have no idea what size it is. My friend said it was an 88, but is there any other way to tell? I've borrowed a jet kit from him and the next size up from 85 I know for sure I have is 90.

[johnwohl]

Progress! So after no end of screwing around I got it to start and run by holding the starter on and giving it a bit of gas. This was with the 38 idle jet 1 and a half turns out. It would not idle by itself until I put the idle set screw all the way in and set the throttle screw a bit open at idle. If I backed that screw off it would still not idle without any throttle. Moving the idle screw out at this point made rpms drop and I couldn't go any farther in. I drove it to the gas station for air in the tires and back and as soon as I shut it off, it never started back up again no matter what I try. I'm confused haha.

[GrumpyUnk]

QUOTE: "Moving the idle screw out at this point made rpms drop and I couldn't go any farther in."

If the idle mixture screw, on the same side as the electric choke, is screwed in all the way, that should cut off the fuel flow to the idle circuit. To have it run, you must be running on the 'bottom end' of the intermediate circuit. I suspect the fuel flow is too much, or the 'air ports' on the filter end of the carburetor inlet may be clogged. All those little holes around the air intake to the throttle plate are air 'bleeds' to the other circuits. They have brass fittings with precision sized holes to allow a specified amount of air into the individual circuits. If clogged with 'stuff', no air gets in, and the idle will be poor. With no air, any amount of fuel would be 'more than enough'. Worth squirting a bit of carb cleaner through to make sure they're not plugged. Wear eye protection as a pressurized fluid will fly back at you if totally or partially clogged, even if not clogged at all and you spray too much.
tom

[johnwohl]

Thanks for the advice. I cleaned the whole carb let it soak etc but I'll go ahead and spray it out again paying special attention to those air ports. I have a set of different idle jets coming though it has a #35 and #38 no #36 or #37 so I'm hoping the #35 will work better than the #34 but I'm not holding my breath haha

[johnwohl]

Well, I have the #35 pilot jet in and try as I might it still won't run on the idle jet alone. If I adjust the throttle set screw it will run fine, albeit at a higher rpm than intended and it won't start again for a while if I shut it off. I'd really like to get this running reliably. Is it possible that since I couldn't get either the #35 or #38 to idle by themselves at any idle mixture screw setting (always have to add a little throttle), that I might be still too lean with the #38?

[GrumpyUnk]

Might be too rich with a #35. You mentioned it would not run with the idle mixture screw opened any amount. That indicates it already has enough fuel from some other source.
A couple possibilities come to mind.
It may be that you are not running on the idle circuit, but on the bottom(lower speed) end of the main. If you close the throttle plate completely, it will not run. Yours dies if you close it past a certain point, and the engine is running too fast but dies if you close the throttle any more. If you had an air leak, the engine would not idle well, if at all. If you are not running on the idle circuit, or it is not working, the main will only be able to work to a certain point, and below that it will not run.
I think your idle circuit is not working. Check the pilot jet for blockage, and the tiny ports on the side of the venturi just 'after' the throttle plate. Run the idle MIXTURE screw on the side in all the way GENTLY, and then back it out 1 to 1.25 turns. Set the throttle plate hard stop so the throttle plate is just cracked open. See if it will run at lower rpms with all that. You may(likely will) have to fiddle with the mixture screw and the hard throttle stop.
tom

[johnwohl]

I'm very confused. I set the idle mixture screw and throttle screw as you described with every idle jet in a set from 32 to 55 and nothing seems to make any difference. im trying them from 1 turn to 3 turns out and then going up another size. ive kept the main jet on the stock one for this round. it won't run on any of the idle jets I have but if I set the throttle open a little it will start and run on the main so at least I know it does run. Could I be doing it wrong?

[scootnewb]

Is it possible something is restricting fuel delivery to the carb? A clogged fuel filter, a cracked fuel or vacuum line?

[GrumpyUnk]

Have you removed the idle mixture screw completely and run a burst of carburetor cleaner through it? If the port behind the screw is plugged, your engine will not run on the idle circuit. I think there may also be an O-ring on the mixture screw to keep things sealed. If that's missing, dunno the effect, but I think extra air can get sucked in.
The pilot jet should work, no matter which size you use. The larger numbers seem to add a bit more fuel once off idle speed, sort of the 'low-medium', but the mixture screw should affect things at the bottom of the rpm range.
If the fuel delivery was compromised by the filter float, bad vacuum line, then I think it would not run above idle speed as more fuel is required for higher rpms.
Have you removed the mixture screw and squirted carb cleaner 'backwards' from inside the venturi through the tiny holes near the throttle plate? The hole(s) are on the side, near where the mixture screw is installed. If you can squirt in some juice, cover the port with a thumb, while squirter tube is still in the carb, using the thumb as a partial seal to pressurize that volume, you may force some cleaner back out the port. The holes are so tiny they are almost invisible.
Have you squirted carb cleaner spray up into the area the pilot jet is screwed into? That is 'the idle circuit' in general. Air comes in at the air cleaner end of the carb, you can see the individual holes around the periphery of the air horn. If no air gets in, it won't mix with fuel, and there'll be no 'idle fuel:air' emulsion to feed to the engine. Check for pluggage of the air ports. Could be gunked with oil/dir mixture if an oiled-foam air cleaner was installed.
tom