Help Reading Spark Plug

[bob58o]

If it were me I would get 2 more jets and go 1 size up on each. But yeah it's really close now. Does it start easy ? I know when I drilled my pilot it will start without he choke now and idle. So I know its rich.

In this summer weather I woukd want it a touch in the rich side. It will run a little cooler also

When cold, it starts on the first or second pull with choke on. I don't think it likes to start without the choke, but I have to flip it run quickly.

The 0.028" Low Jet, IIRC, allowed me to turn the fuel screw all the way in and the engine still ran. I'm almost positive this was with the muffler off, but think it was with the colder AR3910X plug.

I can steal the Low Jet from the 212cc carb and I will drill it to 0.027".
I'm 90% sure it fits in the 390 carb.

Not easy for me to find a 1.25 mm drill bit.
But the jet is here.... $5.45 + shipping
http://www.nrracing.com/product-p/99101-zh8-1250.htm

I bought this for $7 free shipping
http://www.ebay.com/itm/190828239150


New 15pc Micro-Drill Bit Mini Set High Speed Metric Sizes
1.05 mm -2.0 mm

15 High Speed Twist Drill Bits Great For Metal smiths, Metal Working & Drilling
Sizes: 1.05, 1.1, 1.15, 1.2, 1.25, 1.30, 1.35, 1.40, 1.45, 1.50, 1.60, 1.70, 1.80, 1.90, 2.00 mm.


What is the saying?

Give a man a misleading horse, or teach a fish to drink the water?

---------- Post added at 10:59 PM ---------- Previous post was at 10:14 PM ----------

And the bits shipped already. I also got a new $10 tachometer. These should be here in a month.

My next test will be with a larger idle (0.027 or 0.028") and the same 0.048" main jet (supposed to be 1.2mm, but HF products' QC is questionable). I measure 0.0475 - 0.0480". It came with two of that size. The other one measures 0.047". The HobbyTown USA 0.0465" bit measures 0.0465"

It makes labeling easy. 0.048, 0.047, 0.046, 0.045, 0.043, 0.042

I will try 0.048 and 0.028 (+0.002") with the AR3911 in the morning.
First I will test just the Idle circuit and transition.
I will start it and see how it starts. I'll try without the choke, then with the choke if needed.
I'll let it idle for 5 minutes and check the plug. Just looking at the tip, no cutting.

Then I'll drive it for 5 minutes at various throttle positions and test the transition and stuff.
Looking for No Hesitation / Bog / Stall.

Then I'll do the WOT runs and cut the plug.

This next $4 Test should teach me the most info per dollar out of all the plugs I sacrificed so far.pu

Also I need a big *** fuel tank. I may need to re-investigate the pump at some point. Or hang tank or shelf for a tank from the roll cage.

 

[Joe-405]

i got the smaller bits #61-#80 from hobbytown usa to be exact fyi.

this exact one..............
http://www.utopiatools.com/61-80-Diamond-Micro-Drill-Bit-Set-Size-0-3-1-6mm-p/u33mi20.htm

 

[bob58o]

34 trees = 1/4 mile
All uphill

1.25mm main jet

Can't find my other pilot jets right now.
I'll check a new plug after idling with the 0.026" pilot .

Then I'll cut the plug after a run with the 0.049 (1.25mm).
AR3911

 

[bob58o]

Brand New AR3911
0.026" Idle Jet
0.049" Main Jet


This Pic shows the Brand new Plug (Notice the pepper).
And also shows the same plug after 5 minutes of Idle with the 0.026"


I think that this Low Jet is right. Because I can't find a 0.027" bit or my 0.028" Jet. LOL

It may be a bit lean?

 

[bob58o]

After WOT

 

[bob58o]

I will try one one last run with the 0.049" (1.25mm jet).
I think it is slightly rich, but doesn't look as good as the previous few chops.
I want another test, but if it looks the same I will go with the previous set up.

0.026" Idle Jet
0.048" Main Jet
Stock gx390 Emulsion Tube
AR3911 Racing Spark Plug
100 Octane
32* BTDC Ignition Timing
(June - September?)

If it gets hotter, the mix should get richer (say in 90-100* heat)

When it gets colder, maybe the 0.049" or bigger will be better.
(October - March)

I couldn't wait for China to ship so I went to McMaster Carr to pick up my drill bit.

 

[Joe-405]

Well I rode the bike all day and night at the track in 100* heat and not a single issue the whole time. I have pulled the stock carb off so decided to look at the plug to see what she looked like. Showed my Efi guy and he about crapped himself for a carbureted plug. Haha.

He said the heat sink and range was perfect. But only a touch lean cause of the conditions we are having right now. Started perfectly all day and night............

Thought you might wanna see it also. My 420cc clone with a bp5es ngk.

P.S. And yeah this is lean for my standards fyi. I just know what works i guess on these little engines. Guess i will relearn something. Im used to my 2-stroke stuff that needs to be rich to stay cooler in racing applications.