spark plug choice and carb tuning

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jpullen

New Member
Sep 18, 2013
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Damascus GA.
I have been reading different threads and I have throughly confused myself. I run a Grubee skyhawk angelfire with a cns carb. I had it running and idling good at 19:1 fuel ratio but spark plug(autolite 4123) was nasty oily and heavy carbon build up. I went to 32:1 same fouling idles really low or chokes out, but sounds and runs great at speed. I went to a ngk b6hs at same 32:1. It smokes like **** but the plug is brown/black and dry no carbon build up. However now it doesn't run nearly as fast as doesn't sound as good at speed and wont idle. I screwed idle in to get it to idle but it still sounds sick while at speed. My question is do I need to rejet the carb to get less fuel and go with a cooler plug or use the hotter plug and try to figure out the correct idle and air/fuel mix?
 

xseler

Well-Known Member
Apr 14, 2013
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OKC, OK
Have you removed the brass plug on the left side of the carb? There's another adjustment screw that has helped mine a lot. I also use 50:1 Echo 2 cycle oil.

 

jpullen

New Member
Sep 18, 2013
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Damascus GA.
I checked the gap when I put the plug in and it was .25. I have removed the cap and adjusted the air fuel screw to change idle but honestly I have no idea how it should be set. All I know is screwed in it idled lower and chokes or screw it out and it idles higher. Any clarification of how to set air / fuel mix screw would be very welcome . I have it ideling now but it wants to bog when I first take off and still wont go as fast as it was.
 

jpullen

New Member
Sep 18, 2013
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Damascus GA.
Thanks i bought an ngk bpr7hix plug today I will try it next week once my shifts are over. Will a hotter plug help with the bogging at take off or is that issue related to cns float letting to much fuel into the bowl?
 

KCvale

Well-Known Member
Feb 28, 2010
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Phoenix,AZ
Thanks i bought an ngk bpr7hix plug today I will try it next week once my shifts are over. Will a hotter plug help with the bogging at take off or is that issue related to cns float letting to much fuel into the bowl?
You will know if it is a float issue if you see fuel in the bottom hose.

I suspect you just have a really poor ignition problem and with enough spark you will burn all that fuel going in and get the performance you expect with the carb set stock.

It is not just the plug though that is paramount, your plug wire and wires to the CDI from the mag make a difference in how big a spark you can get to burn all that fuel.

The plug will help but check your wires, personally I don't dick with it, I put an Iridium in and replace the stock wiring on all my builds with nice stuff but that's just me.

In short play with ignition first is my suggestion.
 

jpullen

New Member
Sep 18, 2013
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Damascus GA.
I will grab a hotter plug (bpr6hix) this week and see if it helps before I go digging my plug wire out of the cdi. I do have some fuel leekage in lower overflow tube it doesn't pour out, but it does regularly drip. I turn off my fuel 200 yards from the house to run the excess out so it doesn't drip and makes a mess in the house. If I let to much run out the motor idle gets higher and higher to the point the kill switch has trouble stopping the motor.
 

stonegrinder

New Member
Sep 29, 2013
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Walla Walla
So I am running a Sky Hawk 66cc with a cns carb and am I understanding correctly that there is a fuel/air mixture screw under the plug the factory put in? If so I am taking that stupid plug out asap.
 

jpullen

New Member
Sep 18, 2013
27
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Damascus GA.
It did make make setting the idle much easyer. The cap it not put in there very well I twisted it out with needle nose pliers.
 

KCvale

Well-Known Member
Feb 28, 2010
3,966
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Phoenix,AZ
So I am running a Sky Hawk 66cc with a cns carb and am I understanding correctly that there is a fuel/air mixture screw under the plug the factory put in? If so I am taking that stupid plug out asap.
The covered screw to set the air/fuel mix is ONLY for the secondary idle system and unless you are not between about 2000' to 5000' altitude you really doesn't need to mess with it as it can really harm mid-range performance.

The idle air mix circuit functions along with the main jet/barrel circuit above idle kind of like this if you can follow the diagrams of of how they interact and overlap.



As for fuel in the overflow tube I found out the hard way that more than the float level can cause it on a CNS and both are really easy to check.

Pull off your float bowl and fill it with liquid, you can use gas but water is OK for testing it for leaks if you wipe it out when done.

The first thing that can an overflow is the drain screw, the other is the brass overflow tube that goes up from the drain screw, they can come loose and let fuel drip out where it attaches but only to the level of the bowl itself.

To fix the drain screw just tighten it or replace the gasket if necessary, for the tube just a tap or two down on it will re-seat it, note that goes for the fuel intake tube, they can get loose and leak fuel there too ;-}

In short when you can fill the float bowl off the carb, let it sit awhile as it usually very slow, and nothing drips out your test is complete.
 
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stonegrinder

New Member
Sep 29, 2013
17
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Walla Walla
I removed the plug over what I thought was the air/fuel mixture screw, but now I am understanding that it isn't. I tried to get this Chinese motor running right and changed the setting on this screw. Does anyone know what the proper setting is for the 'secondary idle screw'? I am slightly below 2000ft. elevation.

I am trying to figure out why I can't accelerate over 10-15 mph unless I pull the air filter off, then it runs great.
 

jpullen

New Member
Sep 18, 2013
27
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0
Damascus GA.
Oops! In two other posts had read to get your basic idle with the rpm screw then set the air/ fuel so you had a higher idle? Now I have to find out what the factory settings were. I read to screw it ( fuel/air)in till it wants to choke then out a .25 turn?
 

jpullen

New Member
Sep 18, 2013
27
0
0
Damascus GA.
Ngk bpr7hix works great! Plug isn't fouling very much and it isn't boging as bad. Im still getting a small amount of gas overflow but it is negligible. Thanks tor the help and clarification.dance1
 

KCvale

Well-Known Member
Feb 28, 2010
3,966
57
48
Phoenix,AZ
Ngk bpr7hix works great! Plug isn't fouling very much and it isn't boging as bad. Im still getting a small amount of gas overflow but it is negligible. Thanks tor the help and clarification.dance1
You are welcome, I just pass on what works for me.