Author Topic: Tuning carb after jet change  (Read 6509 times)

pauld581

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Tuning carb after jet change
« on: July 14, 2011, 05:40:47 PM »
Recap:
I added an MRP performance exhaust to my 2004 TGB Laser 49cc scooter. After adding the exhaust, the scooter was faster, up to 45mph, but there was a noticeable lag before the power really kicked in. I described it as a pronounced turbo lag. Oilyhans/Harry suggested changing the clutch springs, the main sporing, and the rollers, which I did, to 1500 rpm/1500 rpm/ 6 grams. and the lag basically disappeared.

The last thing I've done is change the main jet in the carb from the stock 60 to a 75. After changing the jet and putting things back together, the scooter started right up and runs fine, mostly, but it definitely isn't right. Although the throttle response is better, by which I mean it revs up quickly and gets off the line faster, at higher speeds/rpms  the engine is stuttering. I am no longer able to get up to 45 MPH, in fact it's barely 40 on the speedo.

My assumption is that I need to tune the mixture to fix this issue, and I'm hoping for guidance with that.

pauld581

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Re: Tuning carb after jet change
« Reply #1 on: July 14, 2011, 09:33:33 PM »
After dwelling on this a bit, I am thinking that the 75 is perhaps too big for my scooter at my altitude (>5000 feet).  I am considering going to the next size down, which for me is 70, but I did want to get some feedback before I do this. I did see in my reading that the symptoms I have point to an over-sized main jet, but I'm really uncertain about this stuff, and in particular whether changing the mix screw is the way to go before taking the carb off again. My understanding is that running too lean can lead to real problems, while running too rich, while not ideal, at least has the benefit of not potentially seizing the engine.

While I'm here, I am also wondering how important the rubber cover for the carb is...? The scooter is garaged, but obviously I occasionally ride in wet conditions. Is the cover necessary or not?
« Last Edit: July 14, 2011, 09:37:41 PM by pauld581 »

harry

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Re: Tuning carb after jet change
« Reply #2 on: July 15, 2011, 06:33:59 AM »
well i would put in the 70 and see where you are with the plug color

tuning is a art and you usually don't hit it square on the head
unless your lucky

all the boot does is to keep your carb clean   .... just put it back when your
done messing about with the carb.

the worst that will happen to your scooter if your too rich
is poor performance and fouled plugs.
 as said the worst that will happen to your scooter if your too lean
you can seize the motor.
 thats why you should go with a bigger jet and work backwards until
you find the sweet spot between lean and rich.

i have a few tuning FYI's at the shop, i don't open for about 3.5 hours 
 so  i'll post them then

oilyhans
i own a scooter shop  with rentals ,sales& racing parts.
i also appraise scooters for KBB.

2003 Vino classic stage 0.75
de-reged stock variator w/8g sliding weights.
1.5k rpm  springs
42mph
2005 veniceLX ~70cc~upjet~1.5k rpm springs & 7gr sliders
46mph
KeyWest de restricted~8gr slide

pauld581

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Re: Tuning carb after jet change
« Reply #3 on: July 15, 2011, 05:17:40 PM »
So I switched it to the 70 and I think I am almost there. The engine runs good all the way through the power band, and in particular, between the springs, the sliders, and now this jet, acceleration off the line is very quick. I get up to 35-40 very quickly. I think I lost some of my top speed with the sliders being 6 grams- I'm going to wait an see if I want to go to 5 gram or some other weight down the line. Right now the absolute max on a flat is 45, but it's really closer to 43.

What I want to do now is fine tune the carb, and for that I need some pictures if anyone has them. I know that the knurled screw on my carb is the idle screw, but I'm not sure what the mix screw is.

Also, I know Harry recommended against it, but I did drill a hole in my air box cover. Without it the scooter just wouldn't rev. I did this yesterday with the 75 when, after changing the jet, the engine would start but would not rev at all.

So anyway, if you could find those carb tuning faqs that would be great.

Paul

harry

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Re: Tuning carb after jet change
« Reply #4 on: July 15, 2011, 05:58:15 PM »
save the 75 jet for the 70cc kit you will want to buy . ;)

if i was you i'd tape the hole shut after the first time the scooter
goes out of tune  or starts to run bad when the weather changes
to fall . just from my own experience its a tuning hassle with holes
or open filters.  also from many posts on scooter forums
its not recommended.

the air screw is the smaller one that is recessed next to the idle screw.
before you adjust the air screw !!!   screw it in and count how many turns
in until you hit bottom  and write it down so you can go back to the
original setting.,  in fact  write down all your steps until you are happy.


turning in the mix screw will make it richer. out  is lean.  do not adjust past
2.25 turns out
idle adjust  to 1600-1700rpm or until the wheel just starts to turn then
back off  just a tad.

this is the scooter tuners bible;; Two-stroke Tuner's Handbook;
http://www.chuckbunnell.com/kart/jennings/jennings.html

how to read spark plugs
http://www.scootnfast.com/scooter/Technical/Spark_Plugs.htm

for fun; how your expansion pipe works...cool animated gif..
http://www.scootnfast.com/scooter/Technical/Expansion_chamber.htm

oilyhans




« Last Edit: July 15, 2011, 06:07:02 PM by harry »
i own a scooter shop  with rentals ,sales& racing parts.
i also appraise scooters for KBB.

2003 Vino classic stage 0.75
de-reged stock variator w/8g sliding weights.
1.5k rpm  springs
42mph
2005 veniceLX ~70cc~upjet~1.5k rpm springs & 7gr sliders
46mph
KeyWest de restricted~8gr slide

pauld581

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Re: Tuning carb after jet change
« Reply #5 on: July 15, 2011, 11:18:42 PM »
I hear you about the holes. I have read pros and cons, and I take them very seriously, but at my altitude, which is actually closer to 6000 feet where I live, there just wasn't enough air. The engine simply would not rev.

Now, with regard to these 70 cc kits, what would you recommend for my scooter, and how hard are these things to install? Do you have to drop the engine? Frankly there's a cost/benefit thing with me and this whole upgrade scenario, in that for the money I have spent and might spend I could probably buy a used 125 or 150 used, but OTOH it is kind of interesting. I might end up buying a larger 4-stroke at some point, but I would still keep this one to play with.

Percentage-wise, I went from hitting 40 downhill and usually topping out at 37 or so on the speedo, so now I am at 43-45 on the flat and who knows how fast downhill, with much faster takeoff and better speed up hills, so I think I got a 15-20% increase for $350. Whether that's worth it I have to dwell on, but I also think that someone who really knows these things could get another few mph out of the machine, and I may see if there's someone in my area who could help. You've been enormously helpful, but there's only so much you can communicate via forums.

For right now I will try to tune the carb as best I can and see how it goes.

One more thing: I do need new tires...any recs? I would like better wet traction, if possible.

harry

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Re: Tuning carb after jet change
« Reply #6 on: July 16, 2011, 06:48:17 AM »
I hear you about the holes. I have read pros and cons, and I take them very seriously, but at my altitude, which is actually closer to 6000 feet where I live, there just wasn't enough air. The engine simply would not rev.

Now, with regard to these 70 cc kits, what would you recommend for my scooter, and how hard are these things to install? Do you have to drop the engine? Frankly there's a cost/benefit thing with me and this whole upgrade scenario, in that for the money I have spent and might spend I could probably buy a used 125 or 150 used, but OTOH it is kind of interesting. I might end up buying a larger 4-stroke at some point, but I would still keep this one to play with.

Percentage-wise, I went from hitting 40 downhill and usually topping out at 37 or so on the speedo, so now I am at 43-45 on the flat and who knows how fast downhill, with much faster takeoff and better speed up hills, so I think I got a 15-20% increase for $350. Whether that's worth it I have to dwell on, but I also think that someone who really knows these things could get another few mph out of the machine, and I may see if there's someone in my area who could help. You've been enormously helpful, but there's only so much you can communicate via forums.

For right now I will try to tune the carb as best I can and see how it goes.

One more thing: I do need new tires...any recs? I would like better wet traction, if possible.

The 70cc kit is fairly easy to install,  you have to remove the pipe &carb
then 4 head nuts and the head & cylinder will come off.
then remove a 'c' clip and push out the  wrist pin
remove piston and cage bearing in the connecting rod
and rebuild. if you know your scooter its about a 3-4 hr job.
the motor stays in the frame.

you will never find a 4t that will be is as quick off the line and have the power
a 2t has. the only advantage  is that you have a cheaper/less expensive higher
top speed ,,, you can get to the same top speed but you will have to
put on a cylinder kit and replace the final drive gears.  but you will smoke
most 4t's
FYI; if you were to compare the 2,  lets say you have a 100cc 2t
, you will need a 170cc 4t to equal the same power to weight ratio
you could perhaps eek out a 1 or 2mph  but you are about right at 7mph
increase for your mod..

also the 4t motor is a lot more involved , trickier . and you need
lots more specialized tools. then your mods only have half the mph gain
for about the same money and a lot more work if you were to replace
say the cylinder..

oilyhans
i own a scooter shop  with rentals ,sales& racing parts.
i also appraise scooters for KBB.

2003 Vino classic stage 0.75
de-reged stock variator w/8g sliding weights.
1.5k rpm  springs
42mph
2005 veniceLX ~70cc~upjet~1.5k rpm springs & 7gr sliders
46mph
KeyWest de restricted~8gr slide

pauld581

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Re: Tuning carb after jet change
« Reply #7 on: July 17, 2011, 02:06:37 PM »
Hmmm- so which 70 kit is right for my model? I am tempted, I must say. I would want to keep the clutch springs etc that I just changed out? I know I would need to swap the jet again, but that is no biggie. Also, I have read some say that they need to go to a bigger carb, and I have also heard that you need to add extra oil in the gasoline since the 49cc oil pump doesn't deliver enough...true?

Finally, about the tires, what do you think of these:
http://www.americanmototire.com/catalog/product_info.php?manufacturers_id=590&products_id=4173

harry

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Re: Tuning carb after jet change
« Reply #8 on: July 17, 2011, 06:12:52 PM »
yes , stay with the 1500rpm springs those are the ones you want.

some say a bigger jet in a smaller carb has more turbulence and
mixes the fuel  better...
but with a bigger carb like a 17.5mm your already closer to the
right air/fuel ratio and maybe at your elevation you wouldn't have to upjet
like with the stock 13mm carb

as for the cylinder kits  the difference is  really the longevity
of the 70cc  vs. the full power of the 72cc .
the 72cc kit has a thinner cylinder skirt and has a higher potential
of overheating if your too lean and revving  the motor too high
too soon in the break in and of course the prolonged time at WOT .

i use a 45 idle jet and a 75 main in a 72cc at sea level.
also i put 1oz of amsoil saber 100:1 in the tank  one bottle
will last about 2 riding seasons and they make handy little
packets like  fast food catchup  you can keep in your seat
for when you have to gas up away from home.

thats a good brand of tire  and should work .
i noticed that the "also purchased" tire was a 130/70
perhaps they are using the fatter tire on the rear?
for the sport bike look?

the good tires are what they call a 'sticky' tire, they grab the road
better, but the downside is they wear faster so you replace sooner.

 oilyhans


i own a scooter shop  with rentals ,sales& racing parts.
i also appraise scooters for KBB.

2003 Vino classic stage 0.75
de-reged stock variator w/8g sliding weights.
1.5k rpm  springs
42mph
2005 veniceLX ~70cc~upjet~1.5k rpm springs & 7gr sliders
46mph
KeyWest de restricted~8gr slide

pauld581

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Re: Tuning carb after jet change
« Reply #9 on: July 19, 2011, 05:01:42 PM »
So obviously the fatter tire fits the wheel, yes? I think that would look cool.

Now, I called a local repair shop today to ask about getting the tires changed, and they said that I would have to buy the tires from them (understandable), and they said that the tires + install would be $220 or thereabouts if left the wheels on, and $180 if I brought in just the wheels, and I was like, "Whoa!".

These are the tires that would cost $90 delivered, so the install and local purchase doubles or more than doubles the cost!

I don't mind paying more for the tires, but that seems awfully steep. Any opinion on that? How hard is it to change the tires myself, and what tools would I need?

Still mulling the 70cc, BTW.

harry

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Re: Tuning carb after jet change
« Reply #10 on: July 20, 2011, 06:19:44 AM »
yeah, lots of places want the big bucks for changing scooter tires.
i used to take all my tires to the local Les Schwab and they
did it for $12 a tire.

when they old manager left and the new guy came he said
they didn't do it anymore ,i went to Harbor Freight and
bought a mini motorcycle tire changing kit for $49. you will have to
mount to a bench to stabilize it.
i also bought a bead breaker which i like better than the one
that came with the kit, but the kit one does work.

the only issue is with some  front rims , the wheel bearings have to be
removed from the wheel so that the 'rod' that holds the wheel down can go through,
which takes about 10 min to remove them.

the tool works good, i've  changed many tires with it and it takes about 30-40min
per tire , with removal, changing the tire and remounting on the scooter.

oilyhans
i own a scooter shop  with rentals ,sales& racing parts.
i also appraise scooters for KBB.

2003 Vino classic stage 0.75
de-reged stock variator w/8g sliding weights.
1.5k rpm  springs
42mph
2005 veniceLX ~70cc~upjet~1.5k rpm springs & 7gr sliders
46mph
KeyWest de restricted~8gr slide

pauld581

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Re: Tuning carb after jet change
« Reply #11 on: July 24, 2011, 03:34:11 AM »
For the tires, it doesn't look like HF sells that any more, and that's ok since  it is getting into that area where you spend $50 or whatever to save $50..eventually, i.e., who knows how often I will actually change the tires on the scooter? My gut is saying pay someone else to do 'em and let it go.


For the 70cc thing, I am spinning that off to a new thread.

The Professor

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Re: Tuning carb after jet change
« Reply #12 on: August 29, 2012, 01:56:49 PM »
Paying $50 to save $50 is good in your situation because you would actually be paying someone $50 and getting a $50 tool out of the deal that will save you time and money again down the road, or you could even return it and get your money back. Let your other local buddies use it so it saves them money and hopefully they return the favor in another way somehow.