Carb problems, scooter boggs By rsw1158 (Read 2,947 times)

[rsw1158]

I have a Jonway YY250T, 2013, that I think has a carb problem. I noticed from day one that the scooter has a flat spot in the throttle response between 4k and 5k than again at 6k and over. I have rejetted this carb anywhere from a 110 main jet to a 138 main jet and tried different air intake variations. I also tried different pilot jet combos from a 38 to a 42. I always have the same flat spots. These flat spots really didn't bother me until the other day when I was up against really strong head winds. The winds where restricting the scooter where it couldn't get up to the 5k range where it likes to cruise at 60mph. It was making me hang around in that 4 to 5k flat spot area and wouldn't unleash any more power to get out of it. It was bogging, spitting, and back firing if I would open the throttle then would recover when I backed off a little and run ok but was on the low end of the 4k range and only achieving about 52mph. I now want to try and resolve this problem and get this thing running great. I did disconnect the charcoal canister and vented my gas cap. I also added the pcv valve and filter on crank case vent hose and mounted higher than the cylinder head incase you wanted to know about any mods. Also I am running a Uni style foam filter straight off the front of the carb. There is a hose that ran from some sort of gizmo that is attached to the header pipe on the exhaust and vents to the air box. What is that gizmo and what is the correct thing to do to the hose? I just have it plugged off.

[Alleyoop]

It is running really lean if it is Backfiring. That GIZMO attached to the pipe is an EGR emssions what that does it is sends unburned fumes back into the intake to be reburned. So since you now have a UNI just let it hang down zip tie so it does not flop around.

Check your exhaust pipe for any leaks especailly where it connects to the motor.The flange gasket might be blown out.

30mm carbs ususally come with pretty big main jets so put the #138 main back in and at least the #38 pilot jet.

Now I did not see in your post about adjusting the carbs FUEL RATIO MIXTURE so do that. And you have to do it while the motor is running and after it has warmed up at least 5 minutes to allow the ENRICHER to shut off the extra fuel.

Also take a picture of your carb area and hoses.

Here is how to adjust your carb: And if you have the one with the fuel ratio screw in front underneath you will need something go reach under there to adjust the Carbs Mixture.
Alleyoop

scooterdoc.proboards.com/thread/1499/4ts-carb-tuning

[rsw1158]

Thanks for the response. I did adjust the mixture screw and that seemed odd to me. It runs best all the way turned in. The mixture screw is between the carb and the head underneath. I did put the 138 jet back in and I either have a 40 or a 42 in for the pilot. I didn't check that. I did notice that my exhaust is leaking at the flange. I will see if I can fix that. Will that make a difference in the performance that much? I have pics but I can't figure how to post them.

[Alleyoop]

OK, if the Fuel Ratio screw is all the way turned in THEN YOUR PILOT JET is TO BIG go down to the next size smaller on that jet. How it should work with the PILOT jet is if you start to turn it CLOCKWISE LEANING IT OUT the motor should die when you have about 1 full turn before it stops.

Good if you know it is leaking by the FLANGE that is the most common place that causes BACKFIREING it is sucking in to much air and making it run really lean. Also you do want to fix that right away because BACKFIRING will damage your exhaust valve then you have bigger problems.
Alleyoop

[rsw1158]

OK Thanks. I will take care of both things right away. How do I get the attachments button turned on for me so I can post pics?

[Alleyoop]

Use this it is easy and free:
Alleyoop

1. Click on this link:
tinypic.com/index.php

2. Click on Browse, that will link to your computer.
3. Find what you want to load.
3. Click on the name of the picture
4. Click on OPEN
5. Click on UPLOAD
6. Click on the LINK that starts with IMG.
7. RIGHT CLICK and CLICK on COPY
8. THEN Come here to your THREAD ON DOC.
9. RIGHT CLICK and CLICK ON PASTE.
Alleyoop

[rsw1158]

I just came in from working on my scoot. I have a 138 main and a 40 slow. Before when I thought I had a 138 in, it was actually a 118. I miss read my jet when I put it in. It runs better and I got rid of the flat spots, but when I run it at 5k or 6k for any length of time, it will begin to buck and start backfiring. I let off the throttle and let it recover and will straighten out. If I rev it, it will act ok. No backfire on decel. Only if I hold it at high revs. I noticed my muffler came apart inside. I cut the back off to se if any parts would fall out but it is all the other end. I left the cap off I cut off and it sounds pretty good that way. I probably derestricked it also. Any thoughts?

[Alleyoop]

Yes I think you have a performance muffler now But since it is now open it probably needs more fuel at the top end.
But lets find out what it needs Less Fuel or More fuel so do the following:

Bring it up to where it acts up and when it starts to act up BACK OFF THE THROTTLE 1/4 of an INCH and HOLD IT THERE.
Now pay attention to what it does:
1. If it Speeds up it is LEAN.
2. If it stumbles and or then settles down it is RICH.
But I think it is LEAN.
Alleyoop

[rsw1158]

I thought I might post the results of my progress with this scooter incase anyone that has one of these with the CN250 or CF250 vertical engines might be reading this post. I ended up with a 138 main jet and a 40 pilot jet and adjusted the carb after all the other mods I did. These mods are removing the air box, charcoal canister, and installing a foam UNI style pod filter directly on the front of the carb. I routed the crankcase vent hose with a pcv valve and a fuel filter, to keep dirt out, high on the back of the frame. Blocked off the gas tank vent hose and drilled a 1/8" hole in my gas cap because I suspect my tank vent is plugged. I fixed an exhaust leak and in the process of doing that discovered that my muffler had come apart inside. So I cut the end cap off with a hack saw so see if any parts fell out. No chance, everything is rattling on the other end. So I left the cap off and just pushed on the end cap that bolts on with two screws and used that for the end cap. You'll have to use a couple of screws to secure it. The muffler sounds great and I'm sure that frees up some restriction. On my road test drive this thing runs great. I used to cruise 60mph at 5000rpm. Now it does 60 at 4200rpm. When I'm doing 5000rpm, I'm doing 70mph. Big power difference. No flat spots like before and better take off speed. I still think I could use a bigger main jet but 138 is the biggest I have. I am having problems finding bigger ones on the net. Any input or suggestions are welcome. I'm still fairly green at working on these 250s.

[Alleyoop]

Here you go Bigger main jets:
Alleyoop

www.buggypartsnw.com/index.php/buggy-parts/250cc-parts/air-carb-and-exhaust/main-jet.html

[rsw1158]

Thanks for the link for the jets. I just had my first real test for my scoot after all the adjustments. I'm impressed. I have a 27 mile commute to work and when I first noticed having problems this year was just 5 days ago when I rode to work against very heavy head winds. Couldn't get up to speed and even the rpms would not respond. I will mention that I have a Givi tall replacement wind shield. Yesterday was a copy of that same day with different results. The scoot was unaffected by the wind. I was able to maintain 65 to 70mpr with no problem at all. No flat spots, no bogging, no backfiring, and loss of power. This is way they should run out of the box. Thanks for the help.

[Alleyoop]

FANTASTIC!! I also have the GIVI windshield on mine the tallest they make I SEE THROUGH it not over it works great.
Alleyoop

[roketa250]

Very helpful post! I will be trying this out on my Roketa 250 tonight. It seems like it is running very lean!

[roketa250]

For my particular scooter.... Which carb part # or type/size do I look for? Thinking about getting a new one and most stock carbs all look the same with some diff's. I don't see any one that looks like mine! Any help would be really appreciated!

[rsw1158]

Are looking to rejet your carb or want to replace the carb? What engine do you have? The Yamaha clone or the Honda clone? The Yamaha is a horizontal engine and the Honda is a vertical engine.