more carb woes

[Bill Schmidt]

Back again. The new  throttle cable worked fine , as you all suggested. However, had a pretty good bog when pressing the go pedal this past weekend. So I decided to lean it out a little. Went from a 175 main to a 170. The bog disappeared imediately, but a miss occurred at high speed. I probably should have tried a 172, but ran out of patience in the 90 degree heat. My question is: would being a little too lean cause the high speed miss? I am going to put a new spark plug in it this week, but have a bad feeling that will not fix it.

[Bill Schmidt]

Anyone have an opinion on this one?

[Christiaan Bouhuys]

Bill,

In my experience  it takes more than two jet sizes down to go from a rich bog to a lean pop situation. A rich bog means you are a couple of jet sizes too rich from optimal, and from optimal to lean pop is probably another 1-2 jet sizes.  If the bog with the bigger main jet occurred when opening the throttle after it being closed for >1s,  you may be running to big of a pilot jet and should have gone down in pilot jet instead of main.  But best advice is always to look at the piston top, it should be able to tell you what the mixture (lean/good/rich) is on pilot (intake side) and main jet (exhaust side).

[Matt Dixon]

• ^+1

94y

[Bill Schmidt]

Do you guys view the piston crown by looking through the intake and exhaust ports, or by pulling the head off? (yes, I am a newbie at karting)

[Ray Lovestead]

This is what you want.  Take out your spark plug and peek inside using this light.  It’s a great, bright, well made model.

http://www.amazon.com/gp/product/B0042VILIQ/ref=oh_details_o08_s00_i00?ie=UTF8&psc=1

Christian.  Can you provide a little more detail about what kind of color/etc you’re looking for on the piston top?  I’ve never come across anyone who tunes by looking at the top of the piston like this.  I go by EGT, feel, spark plug color and pitting on the piston (detonation).

Ray

"Karting Expert Since 2015"

[Christiaan Bouhuys]

Ray, I look for the right color on spark plug first (I like to see light cafe latte color), then on piston top. Piston top for me is more a matter of looking for signs the mixture is off (wet/dark = too rich; signs of detonation (pits, peppery metallic looking deposits) then looking for color to determine the optimal jetting (too hard anyway looking through spark plug hole with LED light).

As long as the piston top shows the engine is jetted within the ‘normal’ range (i.e. showing no signs of being too rich or too lean) I then make small jetting changes and decide on what feels best / most strong while keeping an eye on spark plug color and piston top to make sure I am not getting too lean.

I am by no means an expert engine tuner that finds the exact magical jetting every time but what I do at least ensures my engine runs good and safe.

[Chris Hegar]

Remove all jets and blow through the holes with compressed air along with the carb side. Do the same with the inlet needle if you have one, change the fuel filter.

[Alan Sheidler]

Bill the first link I could find was this, for the attachment to the MagLight:

http://www.zieh-fix.com/shop/contents/en-us/d76_Optik-Vorsaetze_fuer_MagLite.html

First one at the top of the page.  I picked up one of those at a local store several years ago.  Advantage is that it is very thin and allows more room to look through the spark plug hole.  They are here in the States, you may have to look around.  Best to try and view the piston in relative darkness, make a “tent” over your head and the engine if you are outside in daylight.

What you are looking to see is not only the color, but the shape and size of the combustion pattern on the piston.

Black all over = rich

Small brown center/black outside = still rich, but better.

Brown circular pattern reaching to within 1/4 to 1/8 inch of the piston’s edge = pretty much prime territory.

Brown all the way out, any lighter/whiter or “pecked” looking spots = Don’t Go There!

+1 for Chris’s advice.

+1 for Christaan’s comments too.

If any of the running issues are happening at part-throttle, it is time to research needles for your carb.

[Ray Lovestead]

Can anyone out there elaborate on what Christiaan was saying about the jetting with respect to the pilot circuit (intake) and main circuit (exhaust port)?  I’d like to know more.
Ray

"Karting Expert Since 2015"

[Matt Dixon]

Be sure to chop the motor to get a good read. Drive it hard and pull the clutch and let the motor die then coast in. Otherwise you will get false readings.

94y

[Matt Dixon]

Ray the pilot controls the bottom end and is the recessed jet the you need a small screwdriver for. The main is more top end and is what you changed.

94y

[Bill Schmidt]

So, on the piston crown reading thing….just pull the plug, rotate engine until piston is at bottom of stroke and view with mini flash light? No exhaust or intake removal to view the piston?

[Matt Dixon]

Bill yes but chop the engine

94y

[Ray Lovestead]

No – I wasn’t clear enough.  I meant, elaborate on the piston crown on the intake side versus the idle circuit and then the exhaust side of the piston crown versus the main jet.  I just have never seen that described anywhere.  How is the intake/exhaust side of the piston crown affected by the pilot/main jet?

 

Ray

"Karting Expert Since 2015"

[Christiaan Bouhuys]

Ray,

The way the piston top looks at the intake side tells you about the mixture of the pilot; the way the piston top looks at the exhaust side tells you about the mixture on the main jet.  I don’t exactly know about the physics behind it, it just seems to work this way…

-Christiaan

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