carb problem? starves when accelerating By dubyajayes (Read 1,777 times)

[dubyajayes]

I have a Jonway 50cc 4 cycle with about 450 miles on it, QMB139, CVK carb. I replaced to carburetor, ignition coil and plug and CDI. The engine starts beautifully and idles fine. But when I try to go faster than 15-20mph the engine acts "starved", or "choked out". It fades out until you release the accelerator and reapply the gas. Fuel filter appears full, have new gas in the tank and filters appear clean. I have a new carb so I shouldn't have to adjust it. Any ideas?

[JoeyBee]

Do you use the vacuum operated petcock for the fuel pump. Or do you have a fuel like straight from your tank to the carburetor?

[Alleyoop]

I have a new carb so I shouldn't have to adjust it. Any ideas?

Wrong they set them really lean so you need to adjust the fuel mixture. Look in the tech section and following the 4t carb adjustment thread.

[dubyajayes]

I have the vacuum operated petcock and it operates fine. When I replaced the carb, I checked all the fuel and vacuum lines and replaced those that appeared old and failed my "blow through and suction" test. I guess I need to make some carb adjustments.
dubyajayes

[JoeyBee]

Jan 30, 2015 11:49:36 GMT -6 dubyajayes said:

I have the vacuum operated petcock and it operates fine. When I replaced the carb, I checked all the fuel and vacuum lines and replaced those that appeared old and failed my "blow through and suction" test. I guess I need to make some carb adjustments.
dubyajayes

Alleyoop has a very good walk through for adjusting the carburetor. Give it a try, it's composed in very easy to understand steps.

www.itistheride.com/Help_carb_adj_4T.pdf

[JoeyBee]

And a very good video on the process.

[cyborg]

New carb,, did you check the new carbs jet sizes?

[olblindman]

I am having a similar problem. My scooter idles fine, and revs like crazy with no load, but as soon as there is a load, it bogs down. It has a new carb, fresh gas, fresh oil,and it has bumped up to 72cc's (2 years ago, ran fine, but never had much top speed increase, just more torque). I have played around with the carb adjustments, and nothing seems to make much of a difference...it continues to 'bog down' under load.

I would like to re-jet the carb at some point, but want it to run relatively correct first. I also do not know what size main jet would be the proper choice for more speed. It also has Dr Pulley sliders, but I forget what weight I used. The scooter has less than 300 miles on it.

I only used it to go to and from work for a short time, and would like to use it again.....

[Alleyoop]

That Video of a carb adjustment is for a 2 Stroke Carb Slider type and it that carb HAS AN AIR MIXTURE NOT A FUEL MIXTURE so the adjustments are the OPPOSITE FOR A CVK CARB so do not follow those adjustments from that VIDEO that is not for your carb.

Also if it BOGS it means to much air not enough fuel, and a bog is just that it makes a BOGGGGGGGGGGGGGG sound HOLLOW SOUND. If it makes other sound like POPS, BRAT and or BACKFIRE that is different things depending on where in the throttle range the sounds are being made.

So for those that did a BBK at least a 72cc bbk slap in a 80 MAIN JET leave the PILOT JET ALONE.

AND any carb you buy YOU MUST ALWAYS ADJUST THE FUEL RATIO MIXTURE. Each motor is different and NOT ONE setting will work on all motors it all depends on how much air is being sucked in and the size of the jets as well as motors compression and ignition system how good a spark your coil is producing. So to get a motor to run the best with what it's got you have to adjust the carb.
ALleyoop

[steven]

Alleyoop, I am almost positive that, on a 50cc carb, the air/fuel mixture screw is only for the IDLE. That screw makes no difference once you are on the throttle.

As for the OP, I would bet that your new carb came with a jet upgrade. A bunch of the new "performance" carbs come with an 85 main jet in them. I would pull the 4 hole breather plug out of your air box. See if that helps. You are going to need more air, or a smaller main jet.

The second guy sounds like he is running lean. I would try the 80, or 85 main jet. Both of you are dealing with a main jet issue.

[olblindman]

OK Quick update...I remembered that I did change the main jet in my old carb....I do not know what size though. I put that jet in my new carb, and it is running MUCH better...not lagging. I am still topping out at around 30-32 mph, but I do not know if I should be getting any better than that. I am sort of a big guy...205 lbs. I did also change the CDI,the exhaust, air filter, and the cam was supposedly a performance model, but it looked identical to the stock unit. If it is different, it is just slightly different.

So, my question is can I get a few more mph out of it (without spending a fortune)and how....bigger jet?
Better carb?

Down-hill she'll do 45 no problem..uphill 20-25. We have lots of hill here in Western NC!

[Alleyoop]

Most folks believe that the Fuel/Air mixture screw is only for idle but the Pilot Jet passage is always having gas sucked out of it along with the main jet. There is no CUT OFF from the pilot jet it is an open passage and the screw if its a CV CARB cuts off SOME of the fuel passage when turned clockwise having the screw tip go more into the passage. As a matter of fact if you look by the butterfly and open it a little you will see about 4 tiny fuel outlets, the back outlets are ALWAYS open those the screw does nothing to them because the passage for those outlets is BEFORE the fuel Ratio Screw. The Fuel Ratio screw controls only the front outlets not the ones further back.
So contrary to what most folks believe sorry to say but they are wrong.
Here is a chart of when things get going on a carb.
Alleyoop

[Alleyoop]

Jun 28, 2015 16:06:12 GMT -6 olblindman said:

OK Quick update...I remembered that I did change the main jet in my old carb....I do not know what size though. I put that jet in my new carb, and it is running MUCH better...not lagging. I am still topping out at around 30-32 mph, but I do not know if I should be getting any better than that. I am sort of a big guy...205 lbs. I did also change the CDI,the exhaust, air filter, and the cam was supposedly a performance model, but it looked identical to the stock unit. If it is different, it is just slightly different.

So, my question is can I get a few more mph out of it (without spending a fortune)and how....bigger jet?
Better carb?

Down-hill she'll do 45 no problem..uphill 20-25. We have lots of hill here in Western NC!

I would expect it to do little better than 30-32mph(that is what it should have been doing STOCK).

So I think you problem now is in the CVT and the RPMS it is producing.

So what are the RPMS at WOT? and also going uphills do not expect to much your actually doing pretty good at 20-25 most on a good hill will crawl up.

But the important thing is the RPMS the motor is putting out that will tell what you may need to get a little more out of without to much more money. With what you have I would expect that on a flat it would be anywhere from 35-40mph more like around 38mph.
Alleyoop

[olblindman]

My scooter does not have a tach, so I do not know what RPMs I am pushing. The speedo may also be inaccurate (it IS a cheapo Roketa), and I am unsure of what Main jet I have. I know it is visibly larger than the stock.... and I do not recall the weight of the sliders.....

Also, find a flat around here is almost unheard of...nearly everything is up, or down to some degree.

How difficult is changing out the tranny gears? I am mechanically inclined, and did the big bore kit (Hell, I used to own a Jaguar XK-E ....one HAS to be able to fix things!).

[Alleyoop]

I would first get a tach and work on the weights and RPMS then MAYBE think about gearing up or down.
Without a tach you are just guessing on how your motor is running. A tach is like a fortune teller it will tell you the past present and future. Once you get your motor and cvt purring the tach will tell you when things are starting to go south and you can then get on it before it leaves you stranded on the side of the road.

You do need to know the RPMS so you can tune your CVT with the proper gram weights. Right now they may be to heavy and causing low rpms and loosing climbing and top end or to light causing high rpms and also causing you to loose top end. You need to balance the rpms and weights to get the best top end and power going up a hill. Going up a hill the motor should not loose much RPMS maybe a few hundred when getting to the top, you want the power to keep pulling up the hill and just loose some MPH due to the weight it has to haul up the hill slowing it down.
Alleyoop

Here is a good little tach installs in 5 minutes registers immediately not like some which have a lag registering:
www.trailtech.net/digital-gauges/tto?cat=117