Modern Buddy

So, I rejetted from the stock 92 to a 95 when I put on a prima pipe yesterday. It wasn't happy with that, so I took the carb out and bumped to the 100. Too rich, I think. It ran even worse - couldn't get above 40 w/o stuttering and loss of power.

I've ordered the specialized socket to adjust the idle mix. Beyond that and the jet, I'm at a loss of what else to adjust. The carb was also cleaned in a sonic bath, and is squeaky clean. We took great pains to check and double check all channels, circuits, hoses, fuel lines and even tested the carb twice before putting hooking it all up. I feel reasonably confidant that I did not make an error during reassembly.

Maybe I need to keep the 92 in there? Has anyone put the prima pipe on and kept the 92 jet? It's either that or the idle mix. Anyone else have a little experience with this?

you generally should always need to rejet when putting any aftermarket exhaust on any carbed machine.

I've seen this kind of thread before buried in here.

Here at 5k+ feet, our local shop doesn't recommend it unless changing a pile of other stuff. I keep going back and forth on the aftermarket exhaust.

i put my Prima pipe on myself and didn't re-jet . . . had to baby it for the first week and since then it's been fine!

Ok so I am getting a new prima exhaust next week for my 150 and excited about putting it on.

Can someone explain to me what rejetting is and why I would need to do it? Thanks!

Roscoe wrote:Ok so I am getting a new prima exhaust next week for my 150 and excited about putting it on.

Can someone explain to me what rejetting is and why I would need to do it? Thanks!

(Someone feel free to correct me if I misspeak)

Your carb has 3 jets in it that supply gas. The idle jet, the mid and the main. The idle jet supplies gas for when you idle. When you twist the throttle, the next jet comes on. Then when you twist more, the next. It is additive - the idle jet is always supplying gas. When you twist the throttle, the others turn on in addition to the idle jet.

When you put a new exhaust on, it changes the dynamics of the engine and exhaust system. Putting the prima pipe on makes the mix in your cylinder (after the carb, where ignition occurs) more lean. That means it has less gas and more oxygen.

To compensate, you sometimes change the main jet (to make it more rich), and then adjust the idle mix (also to make more rich).

ThreeSheets wrote:you generally should always need to rejet when putting any aftermarket exhaust on any carbed machine.

Thanks, but that's not what I asked.

skully93 wrote:I've seen this kind of thread before buried in here.

Here at 5k+ feet, our local shop doesn't recommend it unless changing a pile of other stuff. I keep going back and forth on the aftermarket exhaust.

Yeah, I thought so too, but couldn't find it.

I'm going to try adjusting the idle mixture and if that doesn't work, I'll yank the damn carb out again and try putting the 92 back in.

I put my Prima pipe on at 3625 miles and I have over 28,000 miles now.
I didn't rejet but I live in Denver where the air is a little thinner so our carb'ed bikes already run a little rich.
My performance is excellent.

If you are having issues after the rejet, just put the stock jet back in and a new spark plug. Ride it for a while and check your plug. It should be a mocha color or light coffee. If it's white and ashy, you are running lean and need a slightly larger jet. If it's black and sooty, you are running rich.

The Prima pipe by itself doesn't change things too much. If you go messing with the air intake, that's when things get sketchy and shouldn't be done by someone not willing to learn how to tweak the carb.

Roscoe - these small motors are designed and tuned to their specific air intakes and exhaust pressure. If you do something to change this balance, the carburetor will usually need some sort of adjustment to compensate. The carb provides the air/fuel mixture and it's flow to the cylinder (along with a couple of other components depending on the system). The main jet is what determines how much fuel is allowed to mix with the air coming in before it goes into the cylinder for ignition.
It's not something you want to mess around with unless you are willing to take the risk. The risk could be running the system rich (too much fuel for the air) which isn't too dangerous but effects performance. The bigger risk is running lean which can do serious damage to your motor.

black sunshine wrote:i put my Prima pipe on myself and didn't re-jet . . . had to baby it for the first week and since then it's been fine!

Cool, thanks. How many miles on your scoot when you did it?

Tocsik wrote:I put my Prima pipe on at 3625 miles and I have over 28,000 miles now.
I didn't rejet but I live in Denver where the air is a little thinner so our carb'ed bikes already run a little rich.
My performance is excellent.

If you are having issues after the rejet, just put the stock jet back in and a new spark plug. Ride it for a while and check your plug. It should be a mocha color or light coffee. If it's white and ashy, you are running lean and need a slightly larger jet. If it's black and sooty, you are running rich.

The Prima pipe by itself doesn't change things too much. If you go messing with the air intake, that's when things get sketchy and shouldn't be done by someone not willing to learn how to tweak the carb.

Roscoe - these small motors are designed and tuned to their specific air intakes and exhaust pressure. If you do something to change this balance, the carburetor will usually need some sort of adjustment to compensate. The carb provides the air/fuel mixture and it's flow to the cylinder (along with a couple of other components depending on the system). The main jet is what determines how much fuel is allowed to mix with the air coming in before it goes into the cylinder for ignition.
It's not something you want to mess around with unless you are willing to take the risk. The risk could be running the system rich (too much fuel for the air) which isn't too dangerous but effects performance. The bigger risk is running lean which can do serious damage to your motor.

Yeah, I bought a pack of six plugs for this. I already blew one - after putting in the 95 and suspecting it was lean, I pulled the plug and it was totally lean. I'm strongly leaning towards putting the 92 back in. Which doesn't make sense ... that would cause it to be MORE lean, right?

Yeah, not sure what else may be going on. I've read many posts on jetting and the Prima and it seems like most folks at lower altitudes end up fine going from the stock 92 on the 125 cc's up to 95 or 98.

I wonder if there is an air leak somewhere or if the air/fuel mixture screw needs some additional adjustment. I can turn mine with just a small screwdriver; no special tool needed. But that's on a 2009 150. Not sure if they are really that much different.

Tocsik wrote:Yeah, not sure what else may be going on. I've read many posts on jetting and the Prima and it seems like most folks at lower altitudes end up fine going from the stock 92 on the 125 cc's up to 95 or 98.

I wonder if there is an air leak somewhere or if the air/fuel mixture screw needs some additional adjustment. I can turn mine with just a small screwdriver; no special tool needed. But that's on a 2009 150. Not sure if they are really that much different.

The mixure on mine could be jiggered with a needle nose. It's a circular peg with one flat side. I'm going to order a special socket today, just so I can have the right tool. Also because I won't be able to work on it this weekend (friends in town).

I'm in Chicago, no high altitude here. We are pretty confidant that there is no air leak, and that the carb is put back together solid, all hoses. I could always check again, but we've checked and double checked. I am in favor of putting the 92 back just to remove that variable from the equation. But in theory, I SHOULD have rejetted. So ... blergh!

Tocsik wrote:I put my Prima pipe on at 3625 miles and I have over 28,000 miles now.
I didn't rejet but I live in Denver where the air is a little thinner so our carb'ed bikes already run a little rich.
My performance is excellent.

Did you notice any real difference after the pipe? or just a tad louder?

I've often wondered if it would give a mph or so at top end, or if it's cosmetic.

skully93 wrote:

Tocsik wrote:I put my Prima pipe on at 3625 miles and I have over 28,000 miles now.
I didn't rejet but I live in Denver where the air is a little thinner so our carb'ed bikes already run a little rich.
My performance is excellent.

Did you notice any real difference after the pipe? or just a tad louder?

I've often wondered if it would give a mph or so at top end, or if it's cosmetic.

Supposedly it will add a HP or two.

yeah, I call marketing nonsense on that though. I mean, even the 150 comes in at what, 11 hp or so? I can't see it truly adding that much. I want it for cosmetic and top end reasons though, and next service I want to get better tires, figured why make the mechanic take it off twice?

On a 4T the pipe will not do much, but it does add a little extra. You may see anywhere from 1 to 5mph extra on the top end. For a 2T the pipe makes all the difference.

Make sure the nuts holding the exhaust pipe to your manifold are tight. Did you use a gasket or gasket maker? Air leaking at that interface could be a potential problem.

Hwarang wrote:

black sunshine wrote:i put my Prima pipe on myself and didn't re-jet . . . had to baby it for the first week and since then it's been fine!

Cool, thanks. How many miles on your scoot when you did it?

hmmm. around 6500, i think.

I think it would make more difference for the 125 than the 150 anyway, just to give it some air!

I got he Prima pipe put on my 170i. No jetting because its fuel-injected.

I put a 95 in my BJ throttle was response wasnt as crisp went back to 92. Thinking I might put 95 back in for the summer think I read somewhere it'd help it to run cooler

I up jetted to be safe on my prima install. You can install the 95 and open the airbox to check for improvements. First remove the divider it's in the air box cover. You can try disconnecting the snorkel too. If its still too rich back the air box cover screws out 3/16" for testing only.

For those of you that upjetted, did your mpg's suffer? If so, by how much?