Spark plug reading

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Feb 26, 2005
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Kansas, USA
I’m not good at these readings no matter how many charts I look at. This is on a 2 cycle 80cc scoot. Running a 115 main jet now. It doesn't seem to have the top speed as the original carb, runs and starts better though. Seems to vibrate more with the 115 than the 110 jet but has a little more top end. Almost looks rich to me? I run synthetic Pennzoil and it does the mixing. Anyone better at reading plugs?
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In a 2 stroke dark brown is fine, and safe. Lean is mean, burns pistons. If black then yes too rich or too cold of a plug. Too light of a color on the insulator means you're on the lean side.
Better to be on the rich side to cover lower ambient temp conditions as 2 strokes are temp. sensitive when running wide open.
I've burnt may a piston in snowmobiles in my quest for max power in my younger days.
 
Hard to tell with 2 cycle. I haven't owned one in years but that doesn't look terrible - possibly on the edge of rich but I agree better rich than lean.

How many hours on that plug?
Plug has about 1000 miles on it, 100 miles maybe with the new carb. I really should of brake cleaned before putting it back in for a more accurate reading.

Sounds like it's pretty close with the 115. I only have a 150 jet and that's quite a jump from 115. I might try it just for grins. At least it would be way on the rich side.
 
Testing high speed jetting in a 2 stroke you run it wide open for a stretch to let the plug ''clean up'' and shut it down and pull it out and look at it with minimal idle time that can change the color of the plug.
Exhaust gas temp. is the only sure way to monitor jetting, for me EGT's cost way less in my snowmobiles than pistons in the power quest.
 
Plug has about 1000 miles on it, 100 miles maybe with the new carb. I really should of brake cleaned before putting it back in for a more accurate reading.

Sounds like it's pretty close with the 115. I only have a 150 jet and that's quite a jump from 115. I might try it just for grins. At least it would be way on the rich side.

Does it run okay at wide open? if it does going to a 150 main , likely run like CHT , thats a Huge Jump


As long as the bike runs good, and isnt fouling plugs, that would be a main consideration ,as to messing with it. Now if you are trying to fix a an issue, then differnt story.
 
I’m not good at these readings no matter how many charts I look at. This is on a 2 cycle 80cc scoot. Running a 115 main jet now. It doesn't seem to have the top speed as the original carb, runs and starts better though. Seems to vibrate more with the 115 than the 110 jet but has a little more top end. Almost looks rich to me? I run synthetic Pennzoil and it does the mixing. Anyone better at reading plugs?
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Yeah that's rich AF.
If it's pig rich going to a bigger jet won't help need to be dropping down to like a 107 or something.
 
Does it run okay at wide open? if it does going to a 150 main , likely run like CHT , thats a Huge Jump


As long as the bike runs good, and isnt fouling plugs, that would be a main consideration ,as to messing with it. Now if you are trying to fix a an issue, then differnt story.

Yeah that's rich AF.
If it's pig rich going to a bigger jet won't help need to be dropping down to like a 107 or something.
Good points.. I does run fine at WOT. Just seems to vibrate some with the 115 vs 110. I need to look at the rollers and setup the intake better before I do anything further. Exhaust is the limiting factor on this scoot.
 
Good points.. I does run fine at WOT. Just seems to vibrate some with the 115 vs 110. I need to look at the rollers and setup the intake better before I do anything further. Exhaust is the limiting factor on this scoot.
Since it's a 2 stroke add 10% to 20% more oil while playing with the jets.
If I go pull the plugs in my weedeater that's been in there for 2 years running between 32 and 40 to 1 oil it will still be mostly white.
 
Adjusted the oiler and bumped it down to 110. Runs good and on a flat it’ll run 35-40 without issue. Anything up hill though it has little umph, won’t gain any speed any barely hold it. Wonder if it’s too lean or maybe I’m expecting too much out of a old scoot.
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Adjusted the oiler and bumped it down to 110. Runs good and on a flat it’ll run 35-40 without issue. Anything up hill though it has little umph, won’t gain any speed any barely hold it. Wonder if it’s too lean or maybe I’m expecting too much out of a old scoot.

I presume this is WOT, if so go back to 115 or even 120 main jet. It does look a bit lean, but the mix looks a lot better IMO.
How is it during part throttle? If there are any flat spots there, you will probably need to bump up the slow jet. In most carburetors both the main and the slow jet are active part throttle.
 
I presume this is WOT, if so go back to 115 or even 120 main jet. It does look a bit lean, but the mix looks a lot better IMO.
How is it during part throttle? If there are any flat spots there, you will probably need to bump up the slow jet. In most carburetors both the main and the slow jet are active part throttle.
Part throttle was ok up to 3/4. Only takes 5 min to swap jets so put in the jet I drilled out 1.5mm.. think its 150ish. Seems right on the money now. Has umph to maintain 35 on steep hills now. Rode it for 5 miles or so and the plug looked at bit cleaner. But I’ll pull it a few rides and double check.
 
I'd run a Trailtech temp. guage. Need over about 325F but shut down at 380-400F.

What is it exactly? Seems slow for 80cc. I'm getting 40-45 mph out of a hopped up QT50 with 65cc.

Might want to mix the oil in instead of injection. Easier to jet and performance tune.

What's the compression test?
 
Don’t know much about two cycle except back from the boating days
Plug looks good to me and have to disagree that it’s too rich in your first post and I tend to agree with your second post that it’s too lean.

I think the first post #1 and photo is closer to ideal in a two cycle engine for sure
Second photo in post #13 looks scary lean for a two cycle
 
I'll take a pic of the spark plug again tonight. Does pretty good with the big jet except for doesn't want to run till it's full warmed up, have to feather the choke. I've heard this is the issue with this carb and big jet. It's actually a 79cc but it's a old scoot, even 35mph is pushing it will the drum brakes.
 
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