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Bike starts instantly, if throttle is wide open, WHY?

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Good morning,

So I completely rebuilt 2002 CR125, I have replace every internal engine bearing, seal, etc.  I have done compression test and I am good, I have done crankcase pressure test, lost only .5 psi after 2 hours, so no air leaks!.... The bike in my opinion runs great, I have a little bit of spooge dripping about halfway down the outside of the muffler after 5 hours of slower 2-3 gear, 1/8th - 1/2 throttle yesterday.  Now the problem I have and I think it is jetting related is my bike is tough to start if I stall/stop/dump/kill for any reason while riding.  It generally wont start unless I give it 1/2 to 3/4 throttle while kicking, if I do that is starts instantly.  My brothers KX125 that I just rebuilt for him also, just easily anytime with just a slight kick of the starter...

So I feel like it must be jetting, but with the throttle open that means I am letting more air in through the slide/bore, but based on the throttle position I believe at that point I am also pulling from the main instead of the pilot?  So theoretically, I feel that since there is some spooge from my riding (mainly 1/8th - 1/2 throttle) and it starts instantly at 1/2 throttle and above...My pilot jet/air screw must be off assumedly too rich?  What are your thoughts, does this seem right?

Edited by Ss65supernova

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Maybe your pilot jet is clogged. Than you need more throttle to activate main circuit.

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Pilot jet is not clogged as the carb has maybe 5 hours on it, but even at that I completely disassembled and cleaned prior to the weekend ride.  Plus I would assume if it was clogged, that riding in the low throttle position would be off also?  But that's my assumption, I am still learning.

 

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Check the float needle and float needle seat. Make sure it seals/closes 100%, no signs or wear and the float height is correct.

You also may be rich on the pilot jet. Stock jet, never drilled or reamed? Air screw causes correct response? Choke fully seats?

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I will double check those item, William1. 

 

-  I assume I can check if the float needle seals 100% by blowing in the fuel hose and chek if when the float closes I can no longer do so, similar to how I set the float level.

-  When I have adjusted the Air Screw on this bike (unlike my brothers kx125) I noticed no changed in RPM....could the issue be the pilot is so far off that it is not even affecting it?  Or could it be since the 2002 CR125 has an open vent for the exhaust valve (which I plug for crankcase pressure tests) that it is not adjusting bc it is treating that like an air leak and I need to plug just while adjusting the air screw?

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Choke seems to fully seat (plus carb is brand new).  Float and float needle all in good shape and float height is set at 8 instead of stock 6.5.

 

Ok so more testing today....and not that it matter but its a Keihin on a CR125 (stock is Mikuni)

Scenario...

Temp: 50 F

Elevation:  500 ft

Carb:  36mm AIrstriker

MJ:  170

PJ:  42

Starter jet:  85

Needle:  NECG #3

 

Bike Starts up cold or hot with throttle.  I got it to temp and played with idle screw as it would not idle without throttle, until the idle screw is almost all the way in.  Next I adjusted Air Screw to try and check the pilot jet, it did not seem as sensitive as I have seen on other bikes (brothers kx125), yet for sure I can say the RPMs dropped off as I turned the screw in.  I also rode it around a bit, it seemed to have a slight bogging sound and hesitation off idle, cannot tell if its a lean bog, but it doesn't sound like it.  

 

Between the fact that I have to have the idle screw almost all the way in to idle (therefore leaning it out if I am correct???)  And the RPMs dropped off as I turned the AS in...I think the pilot is still just too big.  Unfortunately I don't have any on hand....and need to order a few smaller.  

 

 

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Ok I think I have that at home...I bought a handfull of needles....

Now a more general question, if I follow all the jetting guides the very first thing to do it set correct pilot screw based on the turns in/out of the air screw...It seems like the AS/Pilot is the main factor at idle to 1/8th and 1/8th to 1/2 has more needle and almost no AS/pilot. 

 

So what do you have to have set first, needle diameter or pilot size?  Or is it not that simple and I should keep playing around, but since I am at work and not playing with my dirt bike I am asking dumb questions...

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iF IT IDLES; then the pilot circuit  is fine

IF THE AIR SCREW IS MORE THAN 1 AND LESS THAN 3 TURNS to reach maximum idle (subtle) then you have the right pilot size

IF THE CHOKE KILLS THE MOTOR once it is warmed up, then you have the right power jet size

 

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YOUR NEXT MOVE SHOULD BE TO LET US KNOW HOW MANY TURNS OUT FROM LIGHTLY SEATED YOUR AIR SCREW IS AT WHEN YOU FIND YOUR BEST IDLE FROM TUNING THE AIR SCREW.

KEEP IN MIND WHEN DOING THIS IN IS RICH AND OUT IS LEANER- 1/2 TURN OR LESS INDICATES YOU COULD USE A BIGGER PILOT JET. MUCH MORE THAN 2 TURNS OUT TRY A SIZE SMALLER PILOT JET, 

WHEN YOUR BIKE WARMS UP IS IT SMOKING LOTS AT IDLE? DOES IT START WITHOUT THE CHOKE EASILY?

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Yea I am hoping to have some time tomorrow to play more...

I was waiting for more pilot jets to come in, but it seem the RPM went up as I reached 3 turns, never went down, so I ordered a 40 and 38 pilot to try.  Additionally yesterday I replaced the NEDG with the leaner NEDJ needle.  I only had a few minutes so I didn't get to let it fully warm up and play with the air screw...but....

At 1.5 turns out of the AS, no choke, the bike started instantly (cold).  I opened the choke while it idled just to see what it did, it revved up for a few seconds and then settled, then I closed the choke again (just to see) and the bike died after a second or two...wife called and never made it back out in the garage for the night...

Based on the above and it starting cold with no choke, I still think my pilot is too rich, just have to wait for those pilots to come in to try it out. 

Once they come in I will swap out the pilot, warm up the bike and adjust the AS to find the ideal RPM, hopefully greater than 1/2 and less than 2.5, then ill take for a ride and check the rest of the range, and the starting ability once warm....more to come, thanks all.

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Now I'm lost...

today is 65 out with 30% humidity.

 

I have tried both 40 and 38 pilot jets.

 

Still starts first kick cold no choke and first kick hot but only with some throttle.

 

I started with the 40 pilot 1/2 turn at a time. Both 38 and 40 dropped off rpm at about 1-1/2 and would stall out. Above that the RPM rose gradually as I turned the screw out. Rpm rose and settled on the 1850-2100 range per my hour meter/tach. As far as 4 turns out the RPM never dropped off again. So I don't understand, as that would mean yet a smaller pilot, yet 38 is the smallest. There doesn't seem to be any lean bog when I give it a quick throttle of idle while on stand. And the bike will still start cold no choke...

 

Also the bike just sat and idled on the stand for at least 40 mins while playing with the jets and AS. Engine not overheated, pipe I could touch, coolant temp never went higher than 150f. So it doesn't seem to be overheating either.

 

So now what?

 

 

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Krannie,

Well, two reasons I thought it should start with the closed throttle, (and they may both be not valid), its my first bike so maybe I made an assumption, and my brothers kx125 didn't seem to need it to start as much once warm.

So that started me down this path along with me wanting to understand jetting better.  So if once warm some throttle is normal for starting, great, I got that.   But in doing this it has uncovered two other questions....

1)  I almost always see the statement on jetting guidelines, that if the bike starts cold with no choke, the pilot is too rich, so I have the smallest available pilot, what is going on there?

2)  Doing the AS adjustment to determine the correct pilot seems to guide me to a smaller pilot, yet once again I have the smallest pilot available, what does this tell me?

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your running a NECG needle the last letter (G) indicates starting diameter or the top size of your needle- what you should know is that as the letters progress its getting leaner so if the last letter of the sequence was an (H) or say a (J) you would find that air screw going in farther before your motor stumbled, and you probably would get away from the smallest pilot jet sizes.

NECG - of these 3 this will be richest on start and idle

NECH

NECJ leanest on the start idle circuit of these 3 needles

On 5/11/2017 at 9:06 AM, peaceofwork said:

NECJ 

40 OR 42 PILOT IS WHAT I WOULD TRY

perhaps your just being anal about jetting this bike  ? really if everything is great and you just have to crack the throttle a bit for it to start any old time thats what i would be doing , riding instead of tinkering n tweeking is ware its at, pipe drool and twisting the throttle to start the bike is really no big deal at all. 

for now its fine perhaps in the summer it may need a needle change if it turns into a plug fouling blubbering mess

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Peaceofwork,

Yes it is 100% possible I am being far to anal about the jetting.  I did switch the "J" diameter needle (NEDJ, is what I have on hand) as you suggested and now have a 38 pilot in there.  I plan to take it off the stand and just go ride this weekend and see how it goes.  It starts instantly cold and instantly warm with a little throttle.  Now it just confused me why the RPM would never reach a max value and drop off by turning the AS out all the way to 4 turns (as the jetting guides state).  Maybe it is happening and my meter is nor sensitive enough to pick up, or maybe since I run TruboBlue 110 its giving different jetting results....either way I will try to be less anal and just go ride...

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