Phbg 21 jetting help


New Member
Hello guys, im running a 97 aprilia sr50 that i picked up a while ago, now i do need some advice.

I know my clutch engages a bit early (need it at 8k rather than 5k)

But i always noticed the lowend doesn't sound clear in throttle response.

I ran a 40, it ran a bit boggy, switched to a 37 now and it has cleared it up quite a bit.

The air box has the snorkel removed with a small 5/8 hole drilled near the bottom with a stock foam filter.

The topend is a question in itself its an italkit cylinder with three exhaust ports.

Malossi overrange.

Yasuni c16.

The dellort has

Idle 37

Main 89

Starter 60

Needle w7

1 turn out made the low end much cleaner at idle.
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Staff member
How is the color of your sparkplug? Which sparkplug are you using? Have you have standard membrane plates?


New Member
It looks more like ten.

What are other people running jet wise for a liquid cooled mid race setup with an air box snorkel removed and small hole drilled in the box


Active Member
Your not gonna be able to just copy someones setup there are just to many variables.
So air and gas mixture is nearly right lets look at other things...

Did you mount this cylinder to the block yourself ?
If so did you check how the flush ports matched up on eithother and that the ground gasket did not have any overlap in the ports ?
On the higher rpm's this can give quite a big difference in performance.
This is why tuners match the ports up perfectly by grinding of the acces material.

Did you also check the membrane gives no leaks on higher rpm's ?
Remove airfilter and loosely hold your hand over the carbs intake allowing enough air to go by it.
Then open throttle and check if you get any gas on your hand.
If not the membrane is good , if you do get some gas on your hand at higher rpm the membrane isnt 100%.

You also didnt mention the sice of your carb.. is it stock or did you get a bigger one ?


New Member
I did mention the size, it's a phbg 21 carb, i did that test to check the reeds and no fuel was being thrown out of the carb.

It's 1 and a quarter turned in for the mixture screw.
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