Weird Hard To Start Problem??

[Cactus Jack]

It could only be the pilot jet since the main jet does nothing at idle ... and keep it in mind that the float level affects all of the jetting right across the rev range ... So any carb rejetting should start off with making sure the float level and fuel height in the float bowl are right first ....

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OK ... I wasn't going to post this info .. BUT since this IS Tech Talk , I'll post it for the benefit of others who are interested ... and for others who have a similar problem in the future ... those who aren't interested in it or want to argue ... DON'T bother reading it ...

As I've stated in another thread ... We had a brand new Lifan 120 BVH motor ... To run it in I took the carb off a near new 120 Loncin which was tuned spot on , started flawlessly (1 kick , no throttle , no choke when cold) and ran perfectly ... The LF120 was put into the same new bike that the Loncin came out of so we knew everything was right with the coil , CDI and wiring ...The Lifan 120 was hiccing and popping until it warmed up ... then it ran perfectly ... The plug colour was right and the ignition timing was right ... the engine temp was spot on and the new pipe wasn't glowing or changing colour ...

My relo , who can be a bit of a scammer , bought it off dirtbikesonline as a Mojo Super Motor and not long afterwards ... sold it to me ... so he probably tried it and found the annoying hic and pop problem , got cold feet , and decided to palm it off ...

I knew the hic and pop HAD to be a valve sealing problem because both valves are totally shut at TDC on the power stroke when the plug fires ... The exhaust valve opens 45 degrees or later after TDC and the inlet valve doesn't open again until nearly two crank turns after that !!! So knowing that I decided to pull the head off to port it and check the valve seats while I was at it ... BINGO ... the valve was only touching half way around the seat ... And on checking with bearing blue I found the valve itself to be running true but the seat wasn't ... After porting the head , lapping , seating , and blue checking the seats for a perfect all around seal ... I put the engine back together , put it back in the bike and it started first kick and ran perfectly just like the Loncin but with heaps more torque ... The hiccing and popping totally disappeared and I could get the engine to idle as slow as at a constant steady rate ... a solid thump , steady as a watch with no fluttering ...

I'd like all the people who think a lean carb is the cause of hiccing , popping or back firing at idle to logically explain how the flame or even the sound can get past a sealed intake valve that is shut ...

IF enrichening or cleaning the jets stops the problem , then all that indicates is that the leak is small and the richer mixture doesn't ignite in the inlet tract ... But the leak is STILL there and should be fixed ... You'd have to do a leak down test with an air fitting screwed into the spark plug hole , piston at TDC on the power stroke (cam on the base circle with both valves shut) ... then brush thick soapy water over the intake manifold mouth and exhaust port to check for bubbles ... (NO not Jacko's pet chimp) ... which can only form if there's a valve leak ...

When an engine fires ... the cylinder is a sealed chamber ... if it's NOT then it won't produce the maximum power and efficiency ... and the escaping flame can be dangerous and set the carb on fire ... At the minimum , the carbon from burnt fuel (exhaust soot) getting blown back to the carb can block and foul jets ... And (logically) the escaping pressure will blow back up the inlet tract and make it hard for fuel to make it to the cylinder in the first place ...

In a nutshell ... IF the inlet valve is sealing ... ANY engine that is timed properly at idle can't hic or pop ... no matter how lean the carb is ... So if your engine hics , pops , backfires thru the carb or shows ANY weird symptoms like that ... then you either have a poorly sealing inlet valve or your timing is way out of whack ...

An engine that has a pressure leak past the valves can STILL have good compression and can still run great once started ... but it can be hard to get started and won't idle smoothly ...

On top of that , they can be easy to start at times and hard to start at other times because valves rotate as they lift ... the springs twist as they are compressed ... So a worn or faultily manufactured valve can be in a good position on the seat where it's ovality matches the seats ovality and gives a good seal , and at other times it can seat with the ovalities at right angles ... Worn guides or stems allow valve heads to move off the guide centreline ... which causes the seats to wear out of round so it's important to fix valves that can be moved around too much in the guide ... New chinese engines can most certainly have leaky valve seats even IF the valve clearances are set right ... I've experienced that fact first hand ... The fact that valve lapping sticks , valve lapping paste , and checking blue exists is solid proof that seat cutting and machining alone is far from perfect ...

You are taught that fact as part of a fitting and machining course ....

 

[Zirtech]

cheers for the info cactus!

ill do a leak down test first...as its the simplest for me

 

[pickles09]

howd you advance the timeing zirtech?

 

[Zirtech]

motor has an inner rotor kit...and theres a small amount of timming adjustment on those kits

 

[pickles09]

yeh how do you do it?

 

[Zirtech]

theres 2 screws with a slide on the right side of the inner rotor...you under the screws and push down to advances and push up to retard

im still up in the air to whether its helped the starting problem...i hink it had at least a little bit

 

[Cactus Jack]

When starting four stroke engines ... don't twist the throttle as you kick ... When you do that you're reducing the vacuum suction which carbs rely on to pull fuel thru the jets and start it flowing ... so in lifting the slide ... the cold unprimed engine gets no fuel ... just gobs of plain air ...

That's why 4 stroke specific carbs that have no accelerator pump usually have a butterfly or slide choke which cuts off the air flow (similar to putting your hand over the carb mouth) ... as a result , the jets cop all of the suction and the engine starts easily ... Mowers have a manual rubber primer to squirt fuel into the cylinder ...

 

[Zirtech]

in this case whether my choke is closed or not changes nothing...

makes all the same noises...and in all my trying i cant seem to get it to actually start with the choke engaged

sometimes if i pump the accelerator 10 times...then close it...then kick it..it will start....other times i try that and it wont want anything to do with it

 

[Cactus Jack]

I sent this info to guy who PM'd me about a breaking up under load problem he couldn't solve ... IF you have a Mikuni carb with the screw underneath , then it's important to get that screw set right ... Those carbs bypass the slide and needle jet and trickle fuel in all the time to help 4 stroke engines at low revs when they have poor vacuum and air velocity . So if the mixture isn't set right they can flood the engine and affect the mixture at all revs ... What you have to work out first is if you are getting fuel to the cylinder ... is the plug wet and smell of fuel when you pull it or is it dry ??? If it's dry , squirt some fuel into the plug hole before putting the plug back in ... or squirt the fuel into the carb mouth to see if it tries to fire ... that way you'll know if it's a fuel starvation problem if it fires or an ignition / plug problem if it doesn't ...
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It all depends on what type of carb you've got as to how you tune it ...

Do you have a Mikuni with the mixture screw underneath ???? If so you tune it like they state in this link ....

ThumperFaq: MXA Fuel Screw

And here's some general carb tuning info ...

Motorcycle Carburetor Theory 101

First off ... Check for air leaks between the carb and intake manifold and the intake manifold and head ... otherwise you'll be wasting your time messing around trying to get the mixture set right ...... THEN you have to get the air/fuel mixture set right at idle ...

After you do that it should start to idle and run good ....

I'd also check the coil earthing point ... You can also hook an inductive pick up power timing light to your spark plug lead and look at the light while you both idle and rev the motor to see if the pulse to the plug is constant and steady ... IF it isn't ... then there's your source of the breaking up ...

But since you've said the plug is white then it's way too lean and that can cause misfiring and detonation under load ... Check to see that your float level is high enough as the fuel level in the float bowl affects the mixture right throughout the rev range ... also check the fuel flow from the tank and make sure there's no bits of plastic in the line or tank as they can restrict the proper fuel flow to the carb ...

If you still have problems after that ... contact me again and we'll try a few more things ... NO probs ...

 

[Zirtech]

i undid the screw almost untill the head was sticking out of the hole...kicked 3 times without even hesitating

ill ride it tomorrow and give it a go hot and cold

 

[Panda_ET]

ThumperFaq: MXA Fuel Screw

And here's some general carb tuning info ...

thanks for that just the info i was after haha saw that screw and thought mmm only screw other than idle so it must be fuel, that just confirms it. Thanks again mate