Question for thoses who swapped for bigger carbs

[coltc7]

Hi everyone ,

When you swap for a bigger carb ... do you need to rejet !??

Let's say i swap my 2001 prebug factory carb (think its a 15mm or 16mm) for a 19mm ... MJ is a 93 so it should stay like this on the new carb !

My resoning is ... the mixture is perfect with the 16mm carb .... by swapping to a bigger one ... i increase the flow ... but the mixture should be perfect too with the same MJ .

I'm i right !

Educate me guys !!

[Zino]

You have to rejet .
Dellorto 17.5 mm has different diameter jets then 19 mm jets for example .

Here is a article on picking jet sizes for Dellorto carbs
www.tuningmatters.com/how-tos/how-to-quickly-determine-the-carburetor-main-jet-size/

Buy a jet kit start way too big and work your way down till It runs best

[Mech Warrior]

Not necessarily I have swapped to larger 21mm carbs from 19 and i kept the 95 main jet because with the air box on it does not matter that have a larger carb and intake if you are still taking in the same amount of air. I like to keep stock air box on and i was able to keep my original jetting. Runs great no over heating , rana few plug chops to make sure because even though it seems great better safe than sorry. But like zino says is really the best way to do it for most.

[coltc7]

Nice read !!

Thank you ... i was sure that all jet numbers were the same diameter from brand to brand .

If i applied the formula to my daughter sym jet(keihin carb 15mm venturi) its perfect for the stock config ... 15mm x5x0.9=67.5 ... factory Main was 68 ... but doesnt work after tuned up (air box mod and exhaust) 85MJ (formula gives me 71)

And if i reverse the formula for my prebug(all stock for now)... gives me a 20mm carb ... that makes no sense ... prebug have a damn big factory MJ from what i see

[Mech Warrior]

One things to consider though is if you go with a larger carb the advantage would be to let more air in there for possibly modifying the airbox a little which in turn will make the scoot run leaner ,so just a few things to consider like zino said though best to start hi and go lower for most.

[aeroxbud]

When I went from the 12mm to 17.5mm Dellorto, I had to change the jetting.

[coltc7]

That's what i've done with the sym ... after exhaust and air box mod ... went from a 68MJ to 80Mj to begin ... i've decided to go up till it begin to bog at WOT

i was barely hitting 55kph stock ... then i modify the exhaust ... almost no change ... drill 4 one inch hole at the bottom of the air box and upjet to 80 ... was barely hitting 60KPH ... 82Mj no change ... 85MJ OMFG ... was hitting 72KPH but was long to get there !! Try till 88 and it begin to bog at WOT so i backed up to 85 .

Then i've decided to go with Technigas Q-Tre ... and holy shit ... night and day ... (200 pounds guy here) i hit 80kph with it !! 60 65 at quarter throttle ... it run so great now !!

The stock exhaust



After Mod




And now




So now i'm wondering if the Sym could benefit from a bigger carb

I'ts like a preheat exercise for me hahaha ... my prebug is gonna be the real project this winter !!

[Zino]

You have a good thought process on the main a size or 2 smaller from where it bogs is a good safe range with plenty of performance.

A bigger carb does help for keeping a cylinder cooler , power in the low and mid range goes up the most a little bit extra on the top end.

[Mech Warrior]

I like your homemade exhaust heat shield Colt.

[harleyracer59]

Jul 11, 2020 12:23:00 GMT -5 Zino said:

You have a good thought process on the main a size or 2 smaller from where it bogs is a good safe range with plenty of performance.

A bigger carb does help for keeping a cylinder cooler , power in the low and mid range goes up the most a little bit extra on the top end.

    my experience has been that with a bigger carb,  you lose bottom end and throttle response,  major gains are on the top end  due to the bigger carb out flowing the smaller. bigger carb has also given me more rpm aswell.   
   with a bigger carb it seems to run warmer at low rpm.   usually due to the extra air and the lack of vacuum in the manifold to pull the fuel from the bowl.   oko carbs with the power jet are great for cooling at wot.   but they can be a pain in the ass to tune with out an air box.