Top speed and rpm loss?

danlandberg

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Apr 25, 2009
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I have a loss of top speed and rpm. Low speed and power are good, A little more vibration on acceleration than before. I have changed the needle setting both directions, checked muffler for problems, changed spark plug and cleaned air filter. I have good compression! (Slat head,and tough to start with out skidding rear tire) So far still no difference. It acts like I am in the break in period again 16:1 (I run 25:1, Had it for a few months) I used to be able to hit 35mph + on a good day! Now I'm LUCKY to hit 21mph:( And 4 stroking alot when it reaches its max):cry: Any one have any ideas on what to check next? :confused: Or did I wear it out already? Could I have got the RAW deal:sick: I have read about in other threads?:mad:
 
check that mounts and bolts are not loose,make sure chain is not loose.
add more oil and go back to atleast 20:1 and see if that works,if that dont work go to 16:1 for awhile.
 
Drop the fuel bowl and make sure that the main jet didn't loosen up and fall off in the bowl. Also check to make sure the needle jet is tight. (the part the main jet screws into. the jet needle is the part you adjust with the c clip)
 
Thanks C.F. I Was thinking of doing some thing with the fuel/oil mixture next, I Did get fuel from a different gas station last time (there are only two in this town). The thing that got me was, it would start to run better when I forgot to turn the fuel valve on, (until total starvation) carb ran out of fuel. And I am not to sure about a rich/lean mixture. Some say more oil causes a lean condition. To me it would be opposite (checking plug condition)
 
And I am not to sure about a rich/lean mixture. Some say more oil causes a lean condition. To me it would be opposite (checking plug condition)

More oil equals lean because there is less gas per drop... thus creating a lean mixture. Less oil equals rich because there is more gasoline per drop. If you just ran oil there would be no gas in the fuel... a very lean condition. A lean or rich condition is in relation to the gas not the oil.

Sorry if too off topic
 
A.G. I have two carbs (stock ones). I checked both for any problems (clogging, cracks, ect.) Found nothing wrong. Runs the same with both carbs. I have a GOOD understanding of the workings of engines, (mechanical) it's the oil/fuel & air mixture that has me baffled. It ran for 3 weeks fine on a 25:1 mixture then on the last 1/4 gallon (I mix 1 gal. at a time) I lost top speed and high rpms. I thought the fuel lost its uumff. Got gas at the only place open on Sundays, no change. After riding at fair speeds and power in the higher rpm range I feel like a snail now!! Thanks for the input! DANgerous Dan
 
G.M.V, I see what you are saying about the fuel/oil mixture amounts. But in the four stroke world lean/rich is the fuel to air mixture. When I got this thing I ran 16:1 for break in and I had to to drop the needle setting one notch to lean it out, was getting to much fuel/oil to air. Doing this allowed more air to fuel. I'm at 5500 Feet asl.
 
He IS describing fuel/air mixture, dan.

If you have more oil (say 16:1 vs 32:1), there is less gasoline in the same amount of liquid being atomized through the carb while air volume remains the same. Since the same amount of LIQUID is pulled through the jet, regardless of the fuel/oil ratio, more oil means less gas so the air/fuel mixture is leaner. Since the oil doesn't burn, there is less burnable fuel per unit air - leaner mixture.

Changing your needle changes the LIQUID to air ratio. Only the gasoline part of the combined liquid contributes to the fuel/air ratio. If it is all gas, then it IS the fuel/air ratio (as in a 4 stroke). If it's a percentage oil (as in a 2-stroke), then the fuel/air ratio is a combination of needle setting AND gas/oil ratio and changing either will change the fuel/air ratio accordingly.

Granted, changing from say 16:1 to 32:1 fuel/oil ratio only makes a small overall change in the fuel/air ratio (about 3% richer between 16:1 and 32:1). Or in stochiometry terms if you were at 14:1 air/fuel it would change to about 13.7:1. BUT this can be enough to make a noticeable change in performance.
 
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Another chap on the forum said he had to solder the main jet shut and re-drill it a couple of sizes smaller to get his engine to run properly at about 4700 ft.

I don't know why your engine ran so good earlier, unless your weather was considerably colder and your air was more dense? Maybe air density + fuel/oil ratio changes added up to drastically change the way your engine runs?

Where I live, I am at about sea level and the 66cc engines run a little lean near top speed, but the 50cc engines are fine. In colder weather, both need a larger main jet for best performance.
 
Q.M. Thanks, It does make sense to me now. But some oil has to be burning (smoke) the more oil the more smoke. I go plug color, lean (white) rich (black). When mine was running good plug was brown. Now it is black but dry. And it does not hardly smoke like it used to at 16:1 during break in. I am going to change float level because there was a small puddle of fuel this morning, I thought the fuel vale was completely off, maybe it wasn't.
 
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