Resend my activation email : Register : Log in 
BCF: Bike Chat Forums


Weird throttle issue on a scooter

Reply to topic
Bike Chat Forums Index -> The Workshop
View previous topic : View next topic  
Author Message

jug5525
L Plate Warrior



Joined: 02 Apr 2021
Karma :

PostPosted: 18:34 - 02 Apr 2021    Post subject: Weird throttle issue on a scooter Reply with quote

I have a 2 stroke Kymco agility 50cc scooter that i have recently upgraded with a 70cc malossi kit. I have also installed a racing exhaust and have increased the main jet size from 70 to 82.

After riding it for a while I have noticed a strange thing that happens when driving it on a straight at full speed and wide open throttle. If I release the throttle from wide open to about 90%, the scooter actually speeds up. Continuing to drive at that position for about 10 seconds caused it to suddenly cut out. The speeding up when slightly decreasing from full throttle seems to be happening quite consistently but only at top speed. Other than that I don't see any obvious problems.

Any idea what it could be?
I'm guessing it's carburetor related but not sure how to fix it Rolling Eyes
 Back to top
View user's profile Send private message You must be logged in to rate posts

Easy-X
Super Spammer



Joined: 08 Mar 2019
Karma :

PostPosted: 18:55 - 02 Apr 2021    Post subject: Reply with quote

Sounds like overrun due to a lean condition. You've mentioned all sorts of... dubious upgrades Wink but what did you do with the air intake?
____________________
Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter
 Back to top
View user's profile Send private message You must be logged in to rate posts

Nobby the Bastard
Harley Gaydar



Joined: 16 Aug 2013
Karma :

PostPosted: 19:03 - 02 Apr 2021    Post subject: Re: Weird throttle issue on a scooter Reply with quote

jug5525 wrote:

I'm guessing it's carburetor related but not sure how to fix it Rolling Eyes


Put it back to standard. As well as it working properly, you'll be licensed, insured and taxed correctly.

The mot for anything bigger than a moped is much more stringent as well.
____________________
trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050
 Back to top
View user's profile Send private message You must be logged in to rate posts

jug5525
L Plate Warrior



Joined: 02 Apr 2021
Karma :

PostPosted: 19:13 - 02 Apr 2021    Post subject: Reply with quote

I left the stock air filter on and didn't do any changes other than trying out different main jet sizes and tuning the idle mixture. I did a few plug chops initially running at full throttle and I think I chose roughly the correct size. Larger sizes than 84 made it run worse. I didn't change the needle or pilot jet in any way so I guess it could be running lean in lower throttle positions. The 90% position I stated earlier could actually be closer to 75%. Not sure why that would cause to speed up though.
 Back to top
View user's profile Send private message You must be logged in to rate posts

Robby
Dirty Old Man



Joined: 16 May 2002
Karma :

PostPosted: 19:44 - 02 Apr 2021    Post subject: Reply with quote

I'm assuming the plug chops were done revving it at a standstill, not with the engine under load. That won't give you an accurate reading, you can rev and engine up on tiny throttle openings with no load on it.

You've got some fiddling to do in order to get it vaguely tuned right. Best guess here is that the main jet is too small. Close the throttle a bit so the needle is doing the work to control the mix and the mixture is right to make power. Of course, you could get the same problem if the main jet is too big and its too rich to make power at wide open throttle.

The only way to to get in dialled in is to make a change, write down the change, test ride. Keep going until you get it right. Main jet size and needle height are the main variables to play with here. I would start by going up a size or two on the main jet, and move the needle down one position.

Or sell it, get a 125, do your test, get a proper bike.
 Back to top
View user's profile Send private message You must be logged in to rate posts

jeffyjeff
World Chat Champion



Joined: 02 May 2020
Karma :

PostPosted: 00:36 - 03 Apr 2021    Post subject: Reply with quote

Robby wrote:
I would start by going up a size or two on the main jet, and move the needle down one position.


Do you literally mean move the needle down, or move the needle clip down one position?
____________________
History shows again and again how nature points out the folly of men - BOC
 Back to top
View user's profile Send private message Send e-mail You must be logged in to rate posts

Easy-X
Super Spammer



Joined: 08 Mar 2019
Karma :

PostPosted: 12:22 - 03 Apr 2021    Post subject: Reply with quote

Not changing the airbox in any way makes the exhaust swap a little pointless - you've not got more air flowing in so you can't have more fumes flowing out! That being said I found cheeky barrel swap and de-restricted CDI gave a little more useful power down low. Got it up to 40mph. That was on a 4-stroke though but you get the idea.

Bear in mind I have a full licence, no L-plates, so going >28mph on a little scooter is perfectly fine. If you're 16 these sort of mods are 100% going to get you into trouble! You have been warned.

Thinking again on your problem I still wonder about an air leak. On full throttle, everything is fine but there's more air coming in than should be. You've "covered up" this problem with the up-jetting. Easing off drops the fuel but the air's still flowing as wherever the leak has opened up the vacuum pressure is still holding it open
Thinking

Is crankshaft seals going a thing on scooters? How old is this Kymco?
____________________
Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter
 Back to top
View user's profile Send private message You must be logged in to rate posts

Robby
Dirty Old Man



Joined: 16 May 2002
Karma :

PostPosted: 21:18 - 04 Apr 2021    Post subject: Reply with quote

jeffyjeff wrote:


Do you literally mean move the needle down, or move the needle clip down one position?


I mean move the needle down. I find it counter-intuitive to talk about moving the needle clip, because moving the clips down moves the needle up. Get that wrong and you do the wrong thing.

My thinking - currently it seems to be fuelling OK on the needle, but not on just main jet. A bigger main jet will also allow a bit more fuel when its on the needle. So dropping the needle should keep the fuelling in the right place between 25-75% power, while the bigger main jet allows it to make peak power.

Of course I should have mentioned in the first place - how did it run before fucking around with the carb? May be that the original settings were fine with the 70 kit.
 Back to top
View user's profile Send private message You must be logged in to rate posts
Old Thread Alert!

The last post was made 2 years, 359 days ago. Instead of replying here, would creating a new thread be more useful?
  Display posts from previous:   
This page may contain affiliate links, which means we may earn a small commission if a visitor clicks through and makes a purchase. By clicking on an affiliate link, you accept that third-party cookies will be set.

Post new topic   Reply to topic    Bike Chat Forums Index -> The Workshop All times are GMT + 1 Hour
Page 1 of 1

 
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You cannot download files in this forum

Read the Terms of Use! - Powered by phpBB © phpBB Group
 

Debug Mode: ON - Server: birks (www) - Page Generation Time: 0.11 Sec - Server Load: 0.21 - MySQL Queries: 17 - Page Size: 59.92 Kb