Possible carb jet issue?

bluegoatwoods · Dec 9, 2015

I have a grade on my commute that is pretty steep. As I'm approaching, I give the bike some gas in order to build up engine rpms.

I've found that if I give it full throttle, then I'll lose rpms and the engine is clearly on the way to stalling. But if I give it about 2/3 throttle, then it'll maintain that particular rpm, or close to it anyway, and the bike will climb the hill.

Does this sound to you as though I'd benefit by swapping my jet for a bigger one?

I've never actually swapped out a jet. But I have some spares on hand and I'd think I can figure it out. I suppose a socket on the end of an extension straight down the carb throat is the way?

And is the jet likely to be my trouble? Or do other things come to mind?

crassius · Dec 9, 2015

could be jetting, but this is kinda normal for a 2 stroke (also why so many folks get bad mileage by over-throttling past the power point)

Greg58 · Dec 9, 2015

If you have a nt the jet screws in from the bottom, from what you described the jet may be too large. If you have to back off the throttle the needle may be entering the jet to restrict the fuel flow thus leaning the mixture. Trial and error is what I've done. Reading the spark plug tells if its rich or lean. The link below should help.

http://motorbicycling.com/showthread.php?t=302

2door · Dec 9, 2015

Greg58 said:
If you have a nt the jet screws in from the bottom, from what you described the jet may be too large. If you have to back off the throttle the needle may be entering the jet to restrict the fuel flow thus leaning the mixture. Trial and error is what I've done. Reading the spark plug tells if its rich or lean. The link below should help.

http://motorbicycling.com/showthread.php?t=302

Blue, I concur with Greg. It sounds like a rich condition. I've yet to see a stock NT main jet that was too small. They all tend to run rich for whatever reason and can benefit from going to a smaller jet thereby leaning the air/fuel mix at WOT conditions.

Keep in mind that when you go to wide open throttle the needle is pulled nearly out of the jet and then only jet orifice size determines fuel flow. The needle is essentially out of the picture by then.

Like Greg said, take a look at your spark plug. It will give you a ball park indication of things. Dark and wet? Too rich. Reduce the orifice size on the main jet. If the plug is very light colored and dry, you're too lean. This I doubt is your problem.

Let us know what you find.

Tom

bluegoatwoods · Dec 9, 2015

Oh!........I do look at my spark plug from time to time. Dry and 'tan' colored.

I've heard that if it looks like toast, that's just about right. Mine isn't quite that brown. But it's definitely not gray.

crassius · Dec 9, 2015

bogging on a flat is different than bogging on a steep hill - if you can go WOT on flat ground OK, I think you're good

on a moderate hill (if you're at good speed before starting up) you shouldn't bog at WOT - if you need to back off a bit to get good 2 stroke sound, you may be just a bit rich

all this depends on your def of steep & moderate

bluegoatwoods · Dec 9, 2015

The hill in question is definitely steep. Picture San Francisco hills. It's in that range. I've ridden a pedal bike there and on this hill. I'll testify; they're similar.

And I'm actually not sure if I can go WOT on the level, since I never try. I suppose I could give it a shot. But from the way it performs under normal use, I think it can do it.

From the sound of the replies --thanks, guys--- I'm thinking that I don't have any identifiable trouble. I'm merely running into the limits of a happy time engine.

Venice Motor Bikes · Dec 10, 2015

It could be too lean!!
If when you give it full throttle & it feels like there's 'nothing there'?, it needs more fuel.

If it feels real 'sputtery'?, then it's too rich.

Venice Motor Bikes · Dec 10, 2015

Here's a little test... While giving it full throttle, (& it's starting to die), reach down & raise the choke lever a little bit at a time... if it starts to run better with the choke slightly on?, it needs a bigger jet.
If it starts getting worse?, it needs a smaller jet.

bluegoatwoods · Dec 10, 2015

Thanks, Venice Motor Bikes.

I've inadvertently run that test already. There was an evening not too long ago when I hit the gas and flipped on my choke when my pant leg brushed the lever. And I immediately lost all power. I don't recall if I actually stalled or if I saved it by pushing that lever back down.

But this would seem to indicate that I had plenty of fuel. So maybe I'll try a smaller jet.

I might as well get some practice fooling around with jets anyway.

Venice Motor Bikes · Dec 11, 2015

You only raise the choke lever up just a little for the test... (maybe 1/4-1/3 of the way up).
Raising the choke any more than that would make any engine run worse.

Basically what this is doing is restricting the air & forcing the carb to draw more fuel through the jet; & if it runs better this way, you know you need a bigger jet.

Good luck & keep us posted. (^)

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Possible carb jet issue?

bluegoatwoods

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crassius

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Greg58

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2door

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bluegoatwoods

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crassius

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bluegoatwoods

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Venice Motor Bikes

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Venice Motor Bikes

Custom Builder / Dealer/Los Angeles

bluegoatwoods

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Venice Motor Bikes

Custom Builder / Dealer/Los Angeles