Brozz too lean fix
 

I have a 2020 Brozz 250, everything is stock, except for an NKG iridiumn spark plug. I have got about 1250 miles on it since June 2020. Bike runs fine, not worried about speed or power , just ride back roads and and rarely go over 45 mph and ride about an hour when i go out.

I am concerned that the bike is too lean and may ultimately hurt the engine, so i am looking for some way to change that around. I have the stock Keihin carb and would like to take it off and keep it stock just in case i ever want to go back to a straight stock setup.

I am thinking of getting another Kehlin carb from Peace sports and rejetting it to a 115 main and maybe try and increase the airflow mixture screw, or another idea would be to get a VM26 Mikuni and rejet it as well or is there something else i can do to my stock bike besides rejetting to make it run less lean,

i live in Ohio elevation about 1000 , always run premium 93 octane, just a shade tree mechanic skill level.


Any ideas would be greatly appreciated

You should be able to rejet that stock Keihen clone carb. Remove the stock main jet. Note the number. Install the next higher number jet. They usually go up five numbers to the next size jet.
I have not rejetted a 250, but I have rejetted a 125cc mill with a Keihen clone carb.

You can rejet the stock carb. The main jet is easy to locate, the trouble is finding the correct pilot jet. I was able to finally find the correct pilot jet and ordered from China online, there is a guy on YouTube that did it and that showed me how to find the replacement pilot. It idled much better increasing the stock pilot size and ran much better full open with a bigger main jet. For the midrange you will want to put one brass washer under the needle jet.
The other option would be going with an aftermarket carb if you plan on doing any modifications in the future.
I also live in Ohio, I have a pilot jet and a couple brass washers left over from when I did the stock carb rejetting, if you PM me your details I can mail them to you.

Switch to regular octane. The engine isn't designed to use 93 octane fuel and will eventually carbon up as a result. It may even ping. Do this and see how it runs before doing any rejetting.

Then take the carb apart and order the next size up jets based on the numbers stamped into the factory jets.

I hadn't heard of issues running premium fuel before.
I always run it because the only ethanol free fuel available to me is 92+ octane.
I would think that if the carb is jetted correctly on a higher octane fuel, it shouldn't matter. Rich is rich and lean is lean. A couple octane points shouldn't change it too much.
But maybe I am ignorant to something I'm not aware of.

Oh and I thought engine pinging was caused by pre-detonation of lower octane fuels.

Usually running a higher octane will result in more of the fuel being combusted in the chamber which will prevent pinging.

High octane in any of the standard compression Chinese motors around here (99.9%+) is a waste of money for the low compression motor you've got.

More likely than not, since higher octane fuel needs a hotter combustion chamber to burn completely, you're just sending more fuel out the tailpipe.

If you're not already close to pinging, it won't prevent donation.

Even getting ethanol free gas doesn't make much of a difference, if you use the bike.

If it is going to sit more than like 2 months with 87 E5/10/15 ethanol in the tank, drain the carb and the tank, and put the fuel in your car, it won't have any problem with it unless you drive something +25 years old.

It might say 91 octane in the owner's manual, but that's RON, not R+M/2, which in this case most likely works out to 85-87 american.

91 RON is 87 Octane.

Remember. Octane rating is a fuel's resistance to ignition. You don't want it to resist ignition. You want it to resist ignition just enough that it doesn't preignite. These engines are designed and tuned for 87 octane (at best, if not lower).

The higher octane results in incomplete combustion. That's where the carbon buildup and deposts begin. It's impossible to jet or tune for higher octane fuel without also changing physical parameters of the engine. An engine with an 8.5:1 compression ratio and modest spark advance is going to be the same no matter how much fuel you feed it.

I'm gonna agree and disagree with some of the higher octane/lower octane/ethanol/non ethanol conclusions....
1st everyone is right, these low compression motors were designed to run on cheap low octane fuel. Higher octane tends to burn slower and could cause a very minor loss in power (but probably not significant). However it's resistance to detonation is probably a good thing with the overly lean stock jetting, which is more prone to knocking due to higher combustion temperatures.
Ethanol has a slight cooling effect but also requires more volume due to its lower btu which tends to make the slightly lean stock mixture act slightly leaner.
Lastly, whether it's true or not, most gas companies tout their premium fuels have more cleaning additives which if true should slow the deposit buildups.

Personally, I'd run premium with the stock motor until I rejetted for safety against detonation, and after rejetting I'd run whatever is available.

I just run the premium because i wanted a good grade of gas and the closest gas station only has 93 for premium. Don't notice any difference in performance, just don't want to run any cheap gas in it. Also i use Star Tron gas stabilized in all my small engines.

Not too worried about the gas just want to make sure i am not too lean and hurt the engine, I have not changed any of the stock carb setting and it runs fine on what looks to be a genuine Keihin carb from Japan, I remember some post saying the 2020 Brozz carb ran good out the box, just wanting to make sure its not too lean.

What is making think it is too lean is that i always have use a full choke no matter what the temperature is outside, and have to run it for about a minute before it will idle with the choke off. I have my idle set at 1500 rpms

Also saw some posts about adjusting the airflow mixture but not sure if that would be enough either.

Once again not worried about performance just dont want to run a too lean engine

Im at 600' elevation and did not have any noticeable issues with the bike running lean when new and stock. I did later modify the exhaust and airbox and then had to turn the a/f screw out a bit as well as add 2 washers to the needle jet and go from 105 main (stock) to 115 main.
To modify the stock carb you'll need a hacksaw to cut a center groove in the tamper-proof screws and then late replace these screws.

It's a cheap, low performance bike, why use expensive fuel? I guarantee/have observed that in 3rd world countries, the fuel is much worse than even the 85 octane they sell in Colorado, and they're like cockroaches, you can't kill them.

It won't 'hurt' the engine, any hurt to one of these engines would be something you'd detect, something catastrophic. With the OHC motors, you change the oil and it'll probably outlive you, with the CGs, even if you don't change the oil, you'll be tired of it before it dies. If it's not pinging, even if a bit lean, you won't hurt it. Too rich is worse, as it causes the engine to carbon up more, and can potentially wash the lubricating oil off of the cylinder in extreme cases.

The pushrod engines were literally designed for the third world, with third world maintenance cycles, to boot.

Sure, they're jetted a bit lean, but more so lean for performance, not dangerously lean (unless you start modding it without changing jetting, but even still, you'd notice before something went bad). They want them to hold together.

This isn't some 80s 2t motor jetted so lean that if it's uncorked it'll seize a piston, they're safe to run as is.

The best indicator of an engines running condition is the plug. If it is lean the plug should give you an indication. Then the question becomes is it lean at idle, half throttle, or full throttle? When you own one of these carbureted China bikes there is no way to get around being a carb tuner sooner or later. I mean you may avoid it for a good long while but eventually you will need to tune or get someone to tune for you. Might as well dive in and learn to tune. To me starting with the stock carb is the perfect cheap way to begin. You have to tune a new carb too, so you might as well get some experience with the stock carb with which you are familiar, especially since it is running well it should need only small tweeks.

Pinging/pre-ignition is more likely at lower octane levels, but only if the engine compression and combustion environment is such that it may occur. If things are as they should be, lean or rich won't matter, it won't spark knock.

Gowzer, you're on the right track with that thought, but a few chemistry facts incorrect.

A higher octane fuel does not result in more of the fuel being burned, preventing pre-ignition.

Higher octane fuel actually has less BTU of energy and less actual gasoline in it per unit measured. The octane (which is not commonly known that octane isn't only a rating, it's an actual chemical- C8H18), does not by itself burn easily at all. So when octane (the chemical) is added to gasoline, it reduces the combustibility of the fuel as well.

So it isn't that more fuel is being burned in higher octane content fuels, it's that there is less fuel present and that the octane added makes the two combined chemicals (gasoline and octane) less likely to ignite under compression than the gasoline alone.

Now because octane is a carbon heavy chemical, if it can't be burned by high compression and hot flame fronts from the spark event, it can lead to large carbon deposits and build up in "normal" engines.

But again, none of this octane/ethanol-free blah blah blah talk has anything to do with being rich or lean.

OP, either open up the stock carb and pop a new, larger main jet in there, or buy a $25 HWBNDE branded vm26 from Amazon and a $!0 jet kit and plop that bad boy on there; tune for the best sized main jet (on an otherwise stock engine at your elevation, I bet a 110 or 115 would be good).