Carby back fires like a cough

[wiz351]

I bought a second hand Atomix 125 2007 Model a month ago and the idiot who had it had the choke on at all times and had the fuel air mixture screw screwed all the way out and it was idleing very high. I have fixed this by turning the choke off and screwing it back in then fine tunning the mixture screw so it runs and idles propelly.
I thought i would explain all this first before i explain what the problem is.
The problem is when it is running every few minutes the carby would back fire, like a cough, even when your riding around on all positions of the throttle it will do it. It will stall at idle when this happens but when your riding it doesn't stall.
Even after it has been warmed up propaly and you have been riding around for 10 minutes or so it still dose it.
The same thing happens to my YRF 250 but much worse { Carby back firing/coughing }, it is always stalling at low revs.
Dose anyone know what the problem is and how i can fix it?

 

[crazy-latin-steven]

Do they start fine or does it take ages to start?

 

[creole]

id also be interested. i rode my mates atomik through shallow water and mud.
carby started coughing through all throttle positions, eventually spluttered to a halt and wont start.

did you ride through wet weather? these things surely should handle a bit of mud?

 

[Poulsen8r]

id also be interested. i rode my mates atomik through shallow water and mud.
carby started coughing through all throttle positions, eventually spluttered to a halt and wont start.

did you ride through wet weather? these things surely should handle a bit of mud?

I'd like to see any engine that can "handle a bit of mud"... They aren't made to have dirt inside them... hence we have air filters...

Maybe the guy had the choke on all the time to solve this problem... I would suggest cleaning the carby first and check the slide to make sure the E clip is on the middle position... Also check your tuning and make sure its done right

 

[headsmess]

clean your carbie out, could be water in the bowl, and check your damn inlet valve clearance....

 

[wiz351]

Okay it hasen't been threw any water or mud that I am aware of, the bike was in spotlessly clean condition when I got it so it didn't look like it had been threw any.
As for the choke being on when I got it it did the same thing {Carby Cough lol} when I got it with the choke on before I tuned the carby.
As for checking the inlet valve clearance I have no idea how to check that on a bike. I have a drag car and work on V8 engines all the time and check the adjustments on the roller rockers before every race meeting, {I'm not a qualified mechanic just learnt stuff over the past 35 years}.
Is there a web site that explains how to do it or can someone explain to me how to check the inlet valve clearance and what the clearance should be please?
Like I said my YRF 250cc China bike dose it aswell, the engine is a Loncin. Any other ideas what it could be?

 

[headsmess]

pretty easy... take off the caps, 9mm spanner for the adjuster lock nuts, and about .004 for the clearance if you got experience elsewhere, use it

 

[Poulsen8r]

Hahaha... Just havin a go mate lol it was just sarcasm

 

[crazy-latin-steven]

pretty easy... take off the caps, 9mm spanner for the adjuster lock nuts, and about .004 for the clearance if you got experience elsewhere, use it

Arnt you suppost to get the engine to TDC when you adjust valves. Or is that just for timing? I always do the whole thing.

 

[crazy-latin-steven]

YouTube - How To: Adjust your valves

This is the best vid to adjust your valves. I learnt how to do it from this vid. Its pretty simple. May take you a while at first but once you do it once you can do it again easily. I can even remember how to do it without vids now.

 

[wiz351]

Do they start fine or does it take ages to start?

Yeah it starts pretty easy usually

 

[wiz351]

YouTube - How To: Adjust your valves

This is the best vid to adjust your valves. I learnt how to do it from this vid. Its pretty simple. May take you a while at first but once you do it once you can do it again easily. I can even remember how to do it without vids now.

Cheers for that crazy-latin-steven

 

[Cactus Jack]

It can only be caused by a valve being held off the seat when the cylinder fires OR a poorly sealing inlet valve ie bent , burnt valve , warped , cracked seat ... At high revs it can be caused by valve float/bounce ...

The cylinder is only supposed to fire just before TDC on the compression stroke ... just before the power stroke ... and at that point both valves are well and truly shut ...

Things like negative valve to rocker clearance , cam to crank indexing being out of sync or a sheared flywheel keyway can be other causes but either way ... it's because the inlet valve is off the seat when there's combustion pressure in the cylinder ... flame or cylinder pressure should NEVER flow back thru the inlet tract ...

So that rules out the "lean" mixture theory that a lot of people come up with ... the fuel CAN'T ignite if the valves are closed and sealing ... any engine that DOES back fire thru the carb with a lean mixture ... has got something else seriously wrong with it that needs to be rectified ...

 

[headsmess]

my postie got killed by a screw in the exhaust valve still not exactly sure how a screw ended up inside the cylinder, but it was there when i stripped the thing down...

yep. start with the valve clearance, and if that dont fix it.... pull down time

 

[DeanoOz]

That link to the youtube video for valve adjustment doesn't work anymore - here is a similar video for anyone else looking at this thread in the future:
YouTube - Chinese motorbike adjust valve clearance 1

 

[Gawain]

Gota disagree with you Cactus Jack. Your right in that its nt the spark that ignites the incoming fuel mixture, only spark at with valves closed and on exhaust stroke. But under lean conditions at idle combustion is nt homogenous, ie you have burning charge when the inlet valves open. Obviously any problem of this nature should be looked at in terms of a valve closure problem 1st but after that avenue is exhaused jeting is viable. On both my current bikes iv needed to increase the idle mixture via the mixture screws to cure a reversion cough at idle which stalled the engine most of the time.
EDIT: valve clearance was in spec in both cases and engines had good compression and leakdown test results.

 

[Cactus Jack]

You can get reversion in a modified engine with a high overlap race cam ... but it's highly unlikely in a stock engine with a small cam ... Mate IF there's still gas burning in the cylinder AFTER the piston reaches top dead centre on the exhaust stroke ... you've got BIG problems ... I'd be gettin' rid of your super restrictive exhaust ... 'cause that'd mean it's pushin' gas back in ! ...

Look at this link and explain where the "burning" gasses are when the intake valve opens ...

HowStuffWorks "Internal Combustion"

 

[Gawain]

Answer this, engine in good condition, good compression and well sealing valves. When lean there are coughs through the carb, when slightly richened, which gave a better idle too, the coughs disappear. Reversion was a poor choice of words, its more burning mixture pushing through the carb than reversion of gas flow.

 

[Cactus Jack]

I had already answered this question in the other thread before you posted it here ... A lean mixture of fuel and air EXPLODES easily aka detonates like a gas whereas a rich mixture burns slower with lower heat ... even inside the cylinder ...

IF your valves are sealing well , explain how burning mixture can push past them to light the charge in the inlet tract and blow out thru the carb ?

The ONLY true way to pressure test inlet valve seal is with the head OFF the engine ... A block off plate fitted with an air valve is bolted over where the intake manifold mounts ... then the intake port is pumped up with air ... Around the valve head is covered with oil or dish washing liquid and checked for signs of bubbles ... Lapping and blue checking achieves a perfect concentric seal without the need for pressure testing ...

You can leak down test the cylinder of a static engine using the air valve in the plug hole method and it can show minimal leak down ... but in operation the valves rotate and don't return to the exact same position on the seat ... also they can move from side to side if the guides or valve stems are worn and fail to seat effectively when the engine is running ... which results in intermittent pressure leaks past the seat ... Lapping and blue checking gets the valve and seat faces perfectly matched no matter where the valve is rotated to ... that's WHY you use a lapping stick and rotate the valves when re-seating and blue checking them ...

 

[supercross4life]

it some times is the spark plug when it's really dirty.