Adjusting Carb for 3/4 to full throttle power

BloodOfAnAkrobat · May 11, 2011

I posted before about loosing power when I went to full throttle, but now I think I have gotten closer to the problem.

When im going "full speed" I can open the choke a little which allows me to pull full throttle and the bike can double its speed. But if i let off the gas or slow down I have to turn the choke off so the engine doesn't die. Is this a sign that the carburetor needs to be adjusted? I read a little bit about it but I want to make sure.

Thanks

Venice Motor Bikes · May 11, 2011

I'm assuming that you have a 2-stroke engine with a HT carb?
First, lets get our terms straight so that we're all on the same page... choke lever 'down' means that the choke is 'off' & or 'open'.
The choke lever up closes the choke & makes it 'on'.
Now... please re-explain what's happening.

BloodOfAnAkrobat · May 11, 2011

Ok. So i turn it on a little just to help it get started and then I turn it off while its running. So to be able to get to full throttle with out the engine dying down I turn the choke on a little bit. Thats when the speed doubles

dan+1 · May 11, 2011

sound like it wants more fuel than it is getting.
turn the srew with the spring all the way in then back out two full truns and play with that a little bit.
It should run at idle with the choke in the off position once it has warmed up.

also check your fuel line to make sure flow is good.

tseekr379 · May 12, 2011

Hello everyone. Question for OP - are you still breaking the motor in? I just began the process yesterday with a china 50cc 2stroke. Running 24:1 for first gallon @ a 9,000 ft. elevation. I am having the same problem. The fuel line is working correctly. I leave the valve in the down position, then lift choke up to start up after a few pedals. After motor starts, I continue pedaling and turn choke down (off) and kick in the throttle. I can't seem to continue getting more speed, although with every start up and ride, (no more than 15-20 min. with varying speeds) it seems to be opening up more. I assume this is until the breaking in is done with.

Alternatively, I have been fiddling with the idle screw, but I'm afraid to turn it in all the way. Is there any harm in having the idle high for a minute or two? Is there any other variable that might be affecting this?

Bob Mac 18704 · May 12, 2011

I had the same problem and I finally changed the main jet from a #70 to a #72 and that solved the problem. It worked for me, others mileage may very

Venice Motor Bikes · May 12, 2011

It's very rare that these engines need a larger jet... there's some reason that your engine needs more fuel though.
Sometimes the jet can unscrew from the carb body... take off the float bowl & have a look.

BloodOfAnAkrobat · May 14, 2011

I'm going to give some things a try today thanks.
I have seen your bikes (venice motor bikes), which are pretty sweet, and noticed that your have single speed bikes that don't have chain tensioners. How did you get the balance between the two chains? Was it just an issue of repositioning the engine and removing/adding chain links?

Venice Motor Bikes · May 14, 2011

BloodOfAnAkrobat said:
I have seen your bikes (venice motor bikes), which are pretty sweet, and noticed that your have single speed bikes that don't have chain tensioners. How did you get the balance between the two chains? Was it just an issue of repositioning the engine and removing/adding chain links?

Look here... http://motorbicycling.com/f23/tensionerless-chain-tensioners-22279.html

Also... maybe your carb does indeed need a larger jet??

DuctTapedGoat · May 15, 2011

I would say adjust the slide needle/jet needle/throttle needle into a richer position.

Looking at it with the clip and clip positions to the top and the pointed tip at the bottom, the higher the clip is the leaner it is. The lower the clip is the richer it is.

The idle screw doesn't affect anything except idling, it does NOT affect when you are accelerating.

tseekr379 · May 15, 2011

If I set my idle screw too high, it will hit a lower top speed and begin revving too much. If I set it too low, not enough power, even slower. But if I set it in in all the way and do 4.5 turns to the left, it makes my MB sing! Magic idle screw or is that normal?

mapbike · May 15, 2011

The screw on the side of the carb. with the spring under it has absolutely nothing to do with how these engines run except for one thing, and that is IDLE SPEED......that screw doesn't effect the air/fuel mixture at all, it just raises or lowers the carb slide piston which gives the engine more or less fuel/air at idle speed, the very instant you twist the throttle that screw become irrelevant to anything that is happening with the way the engine runs from that point on until you release the throttle and allow the engine to idle again.

The ONLY adjustments to these carbs that will effect the air/fuel ratio has to be done by moving the small clip that is on the metering needle, or changing the size of the main carb jet.....this can be done by replacement or by a method many of us have done which is to solder up the jet and then redrill it to a smaller size using a wire gauge drill bit, all of my engines have had jets that were too big originally so I have resized them all to get my engine not to run too rich on fuel.

Once again the screw on side of carb. is for idle speed only and can't & will not make a difference in anything else, it isn't possible at all for the adjustment of the idle screw to effect the way the engine runs above idle.

Best wishes, Peace

tseekr379 said:
If I set my idle screw too high, it will hit a lower top speed and begin revving too much. If I set it too low, not enough power, even slower. But if I set it in in all the way and do 4.5 turns to the left, it makes my MB sing! Magic idle screw or is that normal?

tseekr379 · May 15, 2011

Thanks Mapbike. I was wondering about the original post though - would hitting top speed and then opening choke (up) a little to get more speed be bad? I am really, really curious, hehehe.

DuctTapedGoat · May 15, 2011

That's not how you should run your bike. If you do that, all you're doing is starving it for air.

would hitting top speed and then opening choke (up) a little to get more speed be bad? I am really, really curious, hehehe.

That's not "opening" the choke. The choke literally "chokes" your motor. It starves it for air when in the closed/up position. Honestly, if you're running an NT carb, you shouldn't ever need to touch the choke after your bike breaks in. If you are messing with the choke to adjust performance, then you have something going on where you have too much air for the amount of fuel being delivered - so you'll want to increase the amount of fuel to air - NOT decrease the amount of air to fuel, especially with the choke.

tseekr379 · May 15, 2011

Thanks DuctTapeGoat. I haven't, and will not do that, but I had to ask to understand. All I do is turn choke up, pedal, release clutch, continue pedaling for a minute to warm up and rev it a bit, then put choke down and I'm off!

I must mention that I am still breaking my 50cc 2 stroke in, and today I will be starting on the second gallon. I must also mention that I live @ a 9,000 ft + altitude in the mountains! So after break in, from what I understand, I must re-jet so I can get more air into the mix.

DuctTapedGoat · May 15, 2011

Definitely - we learn by asking. Downjetting will allow you to decrease the amount of fuel to air - which would be very valid with you being almost a mile an a half above sea level.

mapbike · May 15, 2011

Right On Right On Roght on...........

DuctTapedGoat said:
That's not how you should run your bike. If you do that, all you're doing is starving it for air.

That's not "opening" the choke. The choke literally "chokes" your motor. It starves it for air when in the closed/up position. Honestly, if you're running an NT carb, you shouldn't ever need to touch the choke after your bike breaks in. If you are messing with the choke to adjust performance, then you have something going on where you have too much air for the amount of fuel being delivered - so you'll want to increase the amount of fuel to air - NOT decrease the amount of air to fuel, especially with the choke.

Egor · May 17, 2011

Wow 9000 ft, the last time I tried to run a two stroke at 9000 ft I had to push. It was a Yamaha trail in the 60's and I have to admit I was still climbing and not able to change the jets for each 1000 ft, but it did not make much power up there. Tseeker I think I would look into a CNS carb, I think you will need a better carb than these kits provide. I'm at sea level and it takes a while to get them going correct for me also, and I need to lean them out. At your level I think you will just need to wave the gas can near the bike and it should run about rite, LOL. You have your work cut out for you, I have a feeling unless the outfit you got the bike from helped you out by knowing about what jet you need and got you started you will have to drop the jet size considerable. The stock carb has no slow speed circuit, its just a sloppy carb and depends on probably the vibration to get fuel up for idle. Map & the Goat are dead on with there advice. I look forward to hearing how well you do on getting your mixture right. Have fun, Dave

tseekr379 · May 17, 2011

Hey Egor. You know, I've done lots of reading on this - and from what I've learned, I may need to lean out a bit. Since there is a bit of lack of air up here, I feel the needle may need to be put on the first position for more air, less fuel ratio. Also, I am still breaking it in, so there won't be any significant changes to performance for at least another couple of hundred miles (a bit more than 100 miles put on it so far). Besides that, I DO need to change the jet size, as there is way too much fuel going in. Reducing the jet size and leaning out the needle will help me. No need for another carb. Especially not a CNS carb, as I've read many many posts referring to them simply as "unreliable". I'm quite happy with the NT carb I've got. There is a link to a youtube video in my signature that shows my MB in action, check it out.

Edit: I wanted to throw in the fact that going anywhere over 25 mph begins to make me a bit nervous. The vibrations are not pleasant. At those speeds, I just ease off the throttle and pull in the clutch to cruise. I am hoping my gas mileage improves though. I am looking at 60-70 mpg so far (still breaking in).

mapbike · May 17, 2011

Just watched the video and you are needing a smaller jet bro. that things sounds good but is 4 stroking it's tail off, way too rich on fuel for sure..

tseekr379 said:
Hey Egor. You know, I've done lots of reading on this - and from what I've learned, I may need to lean out a bit. Since there is a bit of lack of air up here, I feel the needle may need to be put on the first position for more air, less fuel ratio. Also, I am still breaking it in, so there won't be any significant changes to performance for at least another couple of hundred miles (a bit more than 100 miles put on it so far). Besides that, I DO need to change the jet size, as there is way too much fuel going in. Reducing the jet size and leaning out the needle will help me. No need for another carb. Especially not a CNS carb, as I've read many many posts referring to them simply as "unreliable". I'm quite happy with the NT carb I've got. There is a link to a youtube video in my signature that shows my MB in action, check it out.

Edit: I wanted to throw in the fact that going anywhere over 25 mph begins to make me a bit nervous. The vibrations are not pleasant. At those speeds, I just ease off the throttle and pull in the clutch to cruise. I am hoping my gas mileage improves though. I am looking at 60-70 mpg so far (still breaking in).

Adjusting Carb for 3/4 to full throttle power

New Member

Custom Builder / Dealer/Los Angeles

New Member

New Member

New Member

Member

Custom Builder / Dealer/Los Angeles

New Member

Custom Builder / Dealer/Los Angeles

Active Member

New Member

Well-Known Member

New Member

Active Member

New Member

Active Member

Well-Known Member

New Member

New Member

Well-Known Member