LF250 Poor Running

[uglydog]

Can anyone help?
I bought an LF250 in October Last year, and quite frankly I love the bike to ride, as it is so comfortable, but mechanically this bike has been a nightmare.
1 month after buying it I had to replace all of the pipe clips as the old ones were leaking, and letting air into the fuel system, and causing poor running., but it still would not run right.
Out of final frustration I then took it back to the importer in England, who replaced the complete fuel system, which then made it run lovely.
However recently I hvae again expereienced running problems:
1) When cold the bike starts on choke but will not rev, but if not tool cold it will start without choke, and then runs fine.
2) I have tried adjusting the mixture, 1/2 turn in which seemed to remove a slight flat spot, but trying to run the bike tonight, I have found once again the bike would not rev when cold on choke (but started and ran on tickover with choke when cold no problem).
When taken for a run once off choke, it kept trying to cut out as soon as the throttle was closed, and having checked the plugs when i got home they were very sooted as if the bike was running rich, but the carb have notbeen touched since the last time it was used when it ran no problem at all. The only difference tonight is that the temperature is approx 2º C, whereas last time the bike was used it was about 8ºC. Does anybody know of choke problems etc, as i am coming to the end of my tether with this bike, which I have had nothing but hassle with from new?
Any other suggestions apart from putting a match to it are gladly welcome!

[countrycruiser]

You don't say how many miles are on it so I will address some of the possible quick fix things to look at.  You may have already done them and if so let us know and we'll see what else we can come up with.

1.  Replace Air Filter

2.  Replace Fuel Filter

3.  Spark plugs may need replaced.  Although black does usually mean a rich mixture, sometimes they can be "Carbon Fouled" and require more secondary ignition than the system is capable of providing.

Finally, When it is cold here I have a heck of a time keeping it going for a bit until it gets warmed up.  You just can't seem to get the choke in a happy position.  After you play with it for a while though it clears and runs ok. 

[uglydog]

The bike has covered 1300Km from new.

Icidentally, I have just tried disconnecting the vacuum pipe where it connects to the connection between the rear cylinder and the caburettor. when the engine side was blocked (covered with finger), the bike ran perfectly on tickover, and revved without any problems at all.
With the vacuum pipe reconnected it still ran fine. Is there anything which could upset this vacuum system when the choke is used?

[uglydog]

Since the last post I have stripped and cleaned the carb. which has at least made the bike rideable, but I am still having a problem with idling.

If the throttle has been shut, the engine will try to die, if not idling very very slowly. Sometimes it will cut out, but when restarted it will idle perfectly normally.

Does anyone have any ideas how to solve this?

Thank you very much

[countrycruiser]

I have thought about your problem now for a couple of days.  The only thing that come to my feeble mind is a vacuum leak somewhere.  While the engine is running try spraying WD-40 all over the intake system and vacuum lines.  If the engine speeds up at a certain point, there's your leak.

[mtliu]

Keep in mind that there are 3 different circuits in the carb. One for idle to about 1/4 throttle, one for 1/4 to 3/4 throttle, and the final for 3/4 throttle to wide open. It sounds like your problem is at the first part of the throttle.

There are two adjustments - one is your idle screw - which jams the throttle slightly open so the engine can get some gas while idleing. If you can get your bike to idle while slightly twisting the throttle, this is what you need to adjust.

There is also the pilot screw which affects the fuel/air ratio. Almost any carbed bike will choke if it's idling and you suddenly open the throttle. However, if you are opening the throttle normally and it is slowing down, you have too rich of a mixture.

In order to get more of everything into the mix, you may need to change your pilot jet.

To adjust the other parts of the throttle, you would mess with your needle and main jet - but to solve your idling problem, these are not part of the equation.

Check for leaks too! The WD-40 trick helps. If you do have a leak, the engine will suddenly change RPMs if you spray WD-40 onto a leaky area.

[mtliu]

When it does idle, what are the RPMs?

[williamh]

I had similar problems with my Lifan 250. The bike is very good and runs very well in warm weather. After the first service I had problems with running in the cold (less than 5C). Because this is rare here it was difficult to replicate with dealer. The plugs would foul after running 12-15K and the bike run very poorly at low speed in these conditions. Fast running was fine. I have adjusted the pilot mixture so the screw is only open about 3/4 to 1 turn from closed. I haven't had any problems since but it also hasn't been cold enough to really test it. In every other respect the bike is great and cruises well at 100Kph with good acceleration. I did also find low grade fuel was no good very quickly. I run on 95 Octane. Good luck.

[lardie]

i do not own a lifan i joined this forum as i considering buying the 400 cruiser shortly.on saying that i am a member of the jinlun owners forum as have had a jinlun cruiser for a year which i have just sold to pay for test.
this is a fault i have read similar problems on the jinnie's and the answer with them tends to be
Silkolene Pro Fst in your tank as you fill.
it stops alot pf problems with carb freeze dodgy starting when cold as described in first post not gonna promise but it def the first thing i would do
cheers
lardie

[uglydog]

Thanks everybody for your help and interest. Unfortunately I have not been on the site for a while, and there are now some new ideas here for me to try if the problem is still there.
However, during the summer I thought I had run out of options along the carb route, and fitted new leads, plug caps, and re-routed the fuel line and installed a shorter fuel filter such that the fuel was effectively running down hill to the fuel pump from the fuel tank. I am pleased to report that this was an instant cure for all the previous problems.
As the weather is now getting colder it will be interesting to find out if any cold weather problems begin to develop again.