engine leans out mid throttle

B

BSA

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My bike runs sweet, but at mid throttle were the powerband really kicks in, I tend to think it's leaning out.
As soon as I hit the choke full when warmed up it kills the engine strait away. It smokes like a chimney in lower end and also in higher end, but in mid throttle there is no smoke. Also at idle or low power when you put the clutch out the RPM stays pretty much the same, high end is the same but mid throttle the RPM skyrockets when I let the clutch out.

BSA
 


L

Large Filipino

Guest
Have you tried playing with the needle adjustment? Is this a Happy Time?
 
B

BSA

Guest
Yeah it's a happy time. Yeah it's on 3rd slot so quite rich, it's rich enough on most of the time it's just that it seems to lean out at mid throttle. Might try the 4th slot tommorow, see if that improves it. I think it might be to do with the exhaust, I know some people who did the baffle mod reported cut outs at he top end.

BSA
 

sparky

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Mar 19, 2008
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I'm having similar problems. Kerf replied in this thread, saying that the exhaust is clogged... I must try cleaning it out tomorrow.
 

sparky

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I looked inside the exhaust, and it's not clogged. Even with 20:1, I haven't had enough time with this new bike/engine clog it up.

To be exact, my engine is bogging out at mid throttle as well. Sometimes if I keep it at mid throttle for a good distance, I can make it to the upper RPMs where it runs great. Sometimes if it's colder it can make it past this mid throttle "hurdle".
 
S

SirJakesus

Guest
If it bogs couldn't that mean you're running way too rich? Like when an engine bogs when the choke is on... Check your sparkplug. If its really built up with black greasy carbon your too rich, if its dry and light grey/white you're way too lean.
 

sparky

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It was definitely not greasy black. It was more tan/brown with a bit of grey/white. If anything, I'm guessing I'm too lean. I guess I'll have to open up the carb & figure out how to adjust my fuel/air mixture later today.

I think I'll just try a 27:1 gas/oil mixture first, then start messing with the carb.

Thanks for the help, guys!
 

BoltsMissing

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Jan 31, 2008
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897
BSA
but at mid throttle were the powerband really kicks in, I tend to think it's leaning out.

I had a similar issue, I think we had a long thread on it a few days ago, but this is how mine has panned out so far.

I replaced the gasket with a new standard HT exhaust gasket and a standard HT muffler. It went like ****e when uphilling, no power.

I lost the baffle,it fell of,so the next day I did the Coke can trick, and I had that power up the hill again. It eventually blew the Coke Can while traveling to get the new mufler I had in stock at some place.

This time I trimmed the same standard exhaust gasket exactly the same size as the exuast port on engine.
On the new baffle, I drilled with a 1/8th pop rivet drill bit holes 1" apart all the way along the baffle's pipe right thru. Then 1/4 turn, another set of holes all the way thru along the entire length of the inner baffle pipe.
Is a touch louder, but "respectable".
Goes up hill better with none of that fade out.
Fuel Ratio, 20:1 Castrol 2 Stroke, 95 Octane fuel.
If that exhaust port shamozzle messes up again, I'll be making my own custom header for the muffler so it matches the ex.port, and cut off the header that comes with the standard mufler and weld on my own, rather than wait around for it to happen in HT Land.
 

sparky

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Mar 19, 2008
Messages
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I rode around for a good bit yesterday, but I still couldn't use my last quarter gallon of mixed gas. So, instead of waiting to come up with a new batch, I went ahead and tried to figure out how to adjust the air/fuel mixture.

This is the first time I have adjusted anything on my carb. There are two screws (one which I didn't see before) -- the one I knew about is for idle, the other is for gas. I'm assuming this one for gas is my mixture screw, and I don't have hi & low screws. I'm also assuming I don't have that slide needle thing, either... as the only place it could be is in the diaphragm, but I'm not opening that up just yet.

Anyway, I finally got rid of the mid-range bogging, but it came at a cost. Adjusting the fuel screw finally led me to choosing between good bottom-end & mid-range -OR- good top-end only. I'd adjust at 1/8th turns or less & there were pretty big differences in results. Just a quarter turn could add soo much vibration, or it could run sweet until I get to the top-end where it wouldn't bog, but it sure doesn't have the top-end oomph like it used to.

I think I'm really gonna waste this gas today & then start fresh.
 

sparky

Active Member
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Mar 19, 2008
Messages
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Well... with the tuned carb & freshly mixed gas, it seems to be running about the best it can, but the top end is still not quite where it was before. Seems like I got 2 or 3 mph better outta this thing before.

The only thing that seems to be different is the spark plug boot being fairly corroded. It looks pretty beat up for being less than a month old. Seems strange that it would only affect a portion of my RPMs based on my air/fuel mixture than just sputtering all the time. If the small engine man can't find something cheap for me, I'll just get an expensive 8mm hi-performance one from the auto parts store.
 
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