Time to tune - need help.

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Dymaxion

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Jan 1, 2010
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Michigan
I recently rebuilt the top end of my HT engine (new cylinder, piston and rings). I have probably 50 - 70 miles on it since rebuild and it is really running in well - much smoother and pulls stronger now. So now it is time to tune it up a bit.

Here is where we are at:
-motor is a 66cc gt-5
-broken it on opti-2 at 72:1
-all else is stock
-stock jet and needle all the way down (clip at top notch)
-idles great
-4strokes at mid-throttle almost all times unless under high load (such as steep incline)
-wot runs really smooth but will 4 stroke at times
-plugs look good to slightly rich at mid throttle and wot

So I am thinking it is time to step down to a smaller jet and raise the needle a few notches. I ordered the .68mm and .66mm sized jets from SBP and they should arrive in the next day or two.

Do you think I am on the right track?
 
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2door

2door

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Staff member
Sep 15, 2008
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Littleton, Colorado
Sounds like you're on the right track. Let us know how leaning the mix works out for you. Just a word of caution from another Opti-2 user; don't go too lean. Watch that plug color closely and you'll be good.
Tom
 
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Dymaxion

New Member
Jan 1, 2010
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Michigan
Thanks Tom. You hit on my concern; going to lean. It seems strange to me that the motor runs great at wot, but rich at mid-throttle. My understanding is that mix at wot is controlled by the jet; but below that and above idle is the needle. By going a smaller jet do I run the risk of fixing mid-throttle but running too lean at wot?
 
2door

2door

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Sep 15, 2008
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Littleton, Colorado
Yes. Decreasing jet size will effect WOT conditions. I just went back and looked at your specs. You have the needle on the top notch which is the leanest position. Just for grins have you tried lowereing that 'C' clip to the second notch running Opti-2? It's possible that what you're seeing is a lean condition at mid throttle and a little fatter on the mix might help. What's your plug color looking like? Don't be surprised to see a grey color as opposed to the typical brown you'd expect with other oils. Let us know.
Another consideration is the low milage you have on that new top end. I'd give it some time before making too many changes. Remember, you're dealing with the same conditions that you'd have with a new engine. They always run better the more miles they accrue.
Tom
 
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Dymaxion

New Member
Jan 1, 2010
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Michigan
Thanks guys. Yes i have lowered the clip with no noticable difference in plug color although 4stroking increased The plug color has been good - mostly grey with some brown tint to it. Unfortunately the plugs have been fairly consistent regardless of how I ran the bike (different needle positions or throttle positions). I wish there was greater variance to guide me.

The motor really seemed to come on with more power and smoothness in the last 10 miles so I will run a bit longer as is before trying smaller jet. It just seems odd to me that it is 4 stroking constantly between 1/8 and 7/8 throttle but runs smoothly at wot.

I'd like to see some 4-stroking at wot as well to feel better about a smaller jet.
 
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2door

2door

Moderator
Staff member
Sep 15, 2008
16,302
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Littleton, Colorado
It's been my experience that plug color becomes a guessing game on Opti-2. All of my plugs maintain a consistent grey coloration. The only difference being in the shade of grey. Dark, and I know I'm a little rich. Light and I'm either good, or a bit lean.
Others have reported the odd grey color here and I believe it might depend on the geographical area where you live. Gasoline blends seem to differ in parts of the country.

Air cleaner: Is it the stock, kit supplied one or an aftermarket item? Is there any restriction in air flow. Choke plate open completely? Throttle slide opening fully?

I'd still give it a few miles before worrying too much. 50 to 70 miles isn't going to be enough to really seat those rings properly. Run three to five gallons through it and see what happens. As I said; They just keep getting better with age.
Tom
 
mapbike

mapbike

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Mar 14, 2010
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Central Area of Texas
Tom is so right about them running better with a few miles on them, mine all seemed to settle in and start running at there best in the 150-200 mile range and even a bit batter at around 300 miles.

map
 
D

Dymaxion

New Member
Jan 1, 2010
66
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Michigan
Ok, ok, I get the message. You are telling me to be patient. Not one of my strong suits.

I'll keep running it for a few more tanks and see what happens.

Thanks!
 
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