Surging ? Revs go up and down

[Rizzo97]

Revs sort just drop off then pick back up and drop off again when I give it more than 3/4 throttle, doseit in gear and nutrle, engines a lifan 125 with a mikuni copie on it.. Plug is black ( again ) replacing fule filter and switch got it running good again but now after running fine, it's wayyy worse and is surging, has happend before on a ducar 125 but can't remember how I fixed it or what caused it.. Any ideas? Carbie is spotless and soils the air filter. Valves and tinmimg is within spec, havnt checked for air leaks yet but it wouldn't make any sence anyway because the plug is black not white. Has never had this problem with the stock jetting.

 

[gottagetacheaperhobby]

Blow chaff? Rings could be gone, or a seal is shot letting to much oil in. Have you pulled the head off and had a look at your valves and combustion chamber?

 

[Rizzo97]

Yeah I had to replace the exhaust valve not long ago because the original got hung up and bent on a helicoil that was to long for the threaded spark plug hole the new one looked a lot thicker (not seatedi think) than the original could that be a problem? Should i have replcaed them both at the same time ? Piston and chamber had some carbon build up but besides that and the "unseated?"valve it looked fine and has run fine since I last habit apart.. Dosnt blow any smoke or anything and its not the rings i had a old ducar do that and it had a wet plug from all the oil the plug in this is dry fouled Andi blew some regularly

 

[my67xr]

surging is lean, air leak i'm guessing ?

 

[gottagetacheaperhobby]

Probably is, I was just wondering why his plug was black again. Guess un oiled filter might explain all the carbon and the black plug. Would think the air leak would make the bike show lean symtoms earlier in the rev range to..rev hang of idle etc unless he tuned the carb around those maybe?

 

[my67xr]

and also the plug wouldn't have been cleaned, and a plug chop done from wot in 3rd or 4th.

 

[gottagetacheaperhobby]

Yeah fair enough, explains it I used the tiniest smear of gasket goo over my inlet manifold on the gaskets i just did on my bike and havnt had a problem. Could help with your problem rizzo, but if your exhaust valve snagged the helicoil, I wouldnt trust that to be airtight either. I may be wrong, but I imagine there would be sufficient force to cause a slight air leak around the helicoil.

 

[Rizzo97]

Yea that's why I am so confused, lean symptoms and rich plug, it is like jet black! I put a new onein and same thing and the plug went black to, yeah I know for sure that's why the carbons there.. My old filer had like a steel mesh screan that went between the filter and carb.. Always let dirt threw! I tossed it and it's always clean inside now, and when I take the filter off to clean its usealy spotless, so yeah I figuer that's where all that carbon is from ( bit dusted from that metal sheet and the time I rode it with a ripped filter ) , Going to double cheak valve Clarence's while it's cold and then test for an air leak..

 

[Rizzo97]

In nutrle it revs reallly high at half throttle like it shold at wot ad then drops then Drematicly then picks up again to nothing near the rpm that it peaks at before dropping.. Dose that ring any bells fellas?

 

[Rizzo97]

Just removed old helicoil and put a new one it because it was carving into the sp gasket/ washer and there was black stuff in there to! So I put a new one it and used a new plug and same thing.. No better or worse.. Not even going to bother checking for a air leak seeing as it's fouling plugs after plug chops.

 

[Rizzo97]

Just took the air filter off completely because a old mechanic told me to and it came back to life! Screamed all the way through the throttle but it should be able to do that with a filter on there! And it was clean as that filter, bit lost because carbie is spotless and Has always run perfect with the stock jetting..

 

[my67xr]

ok, so maybe there is too much oil on your foam filter ?

 

[Rizzo97]

Nah no oil at all, put a old ( but clean) uni filter on and it works fine.. Just stalls when I crack it to WOT in nuteral, I forget what that means.. But I'm pretty sure it's a rich piliot circuit? Or a least somthing to do with the pilot circuit

 

[Rizzo97]

My brothers bike started doin the same thing, so I assumed it was the hot whether leaning the jetting but I took his filter off and ranit hard in nuteral ( did the same thing) put it back n and wallah it gose perfect again, But mine gose fine with no filter or the uni one and dose that surging with my paper filter on it, Very strange why the just cleaned paper filer won't work.. might soakit in petrol then wash it again and see if it runs okay. And it's weird that it oly affected wot and not the rest or the throttle positions

 

[my67xr]

stalling when you open the throttle is lean, if it were too rich it would bog and sound deeper

 

[gottagetacheaperhobby]

I didnt think the paper ones were washable, I thought they were a throw away item. I reckon the paper has broken down and wont let enough air through. Uni filter needs to be oiled as well, or your sucking dirt and sand into your engine. The head and bore will NOT like that one little bit.. I think you may be struggling to get whats happening into the right words that accurately describe whats happening, hence the difficulty with diagnosis on our end

 

[Rizzo97]

If that's the case then that's defently the problem, some filter oil+ uni filter and pilot tune and she'll be right. cheers ay, well use have already figured it out so must not have been to bad of a discription. If its rich on the fule screw will it blow black smoke and grey smoke ? Gona start it up tomorow with the mixture screw at 1-1/4 turns out and see if it dosnt blow any more then go from there getting a crisp throttle snap, I think it was at like 2 turns then I thought the stalling was because it was lean and turned it out more and it started blowing black grey smoke so Unless it's dusted its got to be on the rich side. Could very well be dusted

 

[my67xr]

we can't tell you what to set your mixture screw to, it needs to be adjusted by you, to where it runs the best.

generally a mixture screw is set at 1.5 - 2 full turns out from when it is made at the factory, then once it's on the bike it needs to be adjusted to suit the engine/filter, exhaust, ignition combination

 

[Rizzo97]

Yeah i know haha Just saying cheers for the info bout paper filters and was curios about the grey in the smoke runs nice and crisp now, a little on the lean side but it still stalls when I wack the throttle hold it open

 

[my67xr]

That could be a Pilot jet blockage, a Pilot Circuit blockage, or that the Pilot Jet is too small.