Jetting a 150cc 157QMJ By ericrockstar (Read 2,303 times)

[ericrockstar]

Hello class, I have some questions. I had no intention of building a fire breather, but my stock exhaust header broke and I bought a performance replacement because It was cheaper LOL!

I still use the stock air box, but I'm now wandering if I should go UNI and go through the jetting steps.

Actually, I'm wandering if I need to re jet with just the new exhaust?

I bought a new carb last year, but just a stock type carb. I have no idea what jet size is in there as its not marked.

I have seen these carb jet tune kits on eBay that claim to get you an additional 8~10% horses. Don't really want to go that route or buy a set of jets...but I might apparently need to.

So, I'm wandering where do I start. I rather stayed away from the whole UNI thing cause I knew this would follow. My new exhaust header is like 1" where the old one was 1/2". So it sounds cooler, but I didn't really notice it being any more powerful.

[ericrockstar]

Ok, so I found a jet kit on eBay for $6. Maybe I start there and ah let everyone know what happens.

[sprocket]

You won't get more power that way.. or very little... the only thing that will give you more horsepower is more displacement...

Don't believe the Ebay hucksters

[ericrockstar]

Thanks Sprocket. I'm not really trying to get more power, just not ruin my engine running too lean. Maybe I can just move the needle up a hair if it is determined to be too lean.

[Bashan]

You may not need to do anything. If the air can't get in any faster through the intake then the exhaust may just sound cool and that's it. However, as pointed out by Sprocket on other threads, the exhaust provides a certain amount of back pressure that the engine actually needs. The best thing you can do is check the plug for the mixture the way it's presently set up. A lot of guys say you need to ride it WOT for a few minutes to do this and then hit the kill switch, I've not found that to be true. I take it out and ride it hard for a few minutes, shut it off, and pull the plug. Don't let it sit and idle before you shut it off and be careful, the plug will obviously be third degree burn hot. Here's a PDF on how to tell if your mixture is rich or lean, and here's Sprocket's preferred method for reading. For those guys to say you could get a 10% HP increase with just a jet kit is patently absurd and I agree, they are hucksters by crackie. Rich

[Gysixxer]

most carbs dont need to be rejetted.....if ur still stock airbox. u dont need to jet. ur exhaust sounds cooler due to the fact that its stainless steel and its a hell of a lot thinner than the cast iron header that was 2 layers thick (ur stock one)....u shouldnt need to rejet...id say check ur plug and go from there...

[ericrockstar]

Thanks for the links Bashan. I'll try that later tonight. Just got her back running yesterday. New cheap variator and belt. Seems if it's not one thing it's something else with these scooters.

[imnts2]

I have to admit my experience is very limited compared to the guys here, but I have had 2 engines in a row that liked a little upjetting. both were 150cc GY6's. Of course, I suspect running lean was more or less built in (perhaps to meet emissions). Of course, both had PAIR systems that may have needed cleaning. I learned a long time ago, most engines like being a little rich at WOT. They run a lot cooler which rarely hurts, particularly with the soft heads common of china 150's. I note that the water cooled 250's rarely require repeated valve adjustments like the air cooled 50's and 150's do. Of course, that could be diferences in the materials used in the valve train.
For 3 bucks, it is to cheap not to try.

Guess I am cautious about rules of thumb.

[sprocket]

Want it richer...just raise the diaphragm needle slightly