Theres a tradeoff with your logic.
If you increase airflow, youre increasing how much air goes through the venturi ports on the carb and youre actually increasing the fuel you get through a jet, size for size.
The general rule of thumb in 2 stroke world is in fact (Ive personally confirmed this), if you go up on the carb size, you jet down.
Example given, I got my bike with a 36mm Keihin PWK. It had a 160 main in it and a 45 pilot. It was WAY too rich. It would run out of gas WAY before everyone else on the ride, and it would foul plugs badly.
Got to the point where I couldnt even get the thing to not foul plugs even running a 120 main. I dropped to a 110 main and a 30 pilot and it was STILL too rich. I ended up trading it away for a 28mm PWK.
Jetting on the 28 proved to be a 145 main and a 40 pilot. I had to jet back up for the smaller carb.
Just some food for thought.
I agree with you somewhat on this fact. But at the same time the actual size difference between a mikuni vm26 and tm28, are not the what you would think them to be. So for comparison I have taken a pic. The I.D. to the manifold is exact same size as stock, the I.D. on the air side is 2mm larger than stock. The amount of air difference is not anywhere equal to a stock vs. 36 where I agree you would jet down.
There is no accurate way to convert mikuni sizes to keihin as one are based on size and one on flow rate. PWK jet sizes are not accurate to a mikuni.
Ok, so the main reason a flatslide is better is because it does have higher airflow velocity when compared with a round slide. You could be correct if in fact the carb was the only difference at the time I changed but I suspect only one jet size. But you failed to read all the posts, there were other mods from the point that I was at a 260 main and 32.5. Mainly being in air delivery by the CFM.
Now, we could argue all day, but let me tell you the actual facts. My buddy and I both did this carb mod. He had FMF, CFM/UNI, VF3's. No head work. I have the exact same except for DMC. I am currently putting mine together but we did his when we both got our carbs last week.
He had 290 main and 32.5 pilot before carb, but had his CFM already on bike. After putting on carb we started at 300 main, 35 pilot, After plug chops he ended up with.......wait for it. 280 main and 32.5 pilot.You are right here by one main jet size I:I
Ok, so I am adding my CFM and previously (without it) was running 260 main, 32.5 pilot. So what will is a safe bet to start 280 main and 32.5 pilot. This is two sizes in main above 260, which is going up in jets, not down.
These are the facts, you are only right to a point, if you increase airflow through venturi ports, you get slight increase in fuel through jet. But it does not fully compensate for airflow or you wouldn't have to ever re-jet.
Adding the amount of airflow from CFM vs. stock you have to go up in jet sizes.
And for the last time keihen jets cannot be compared to mikuni. those posting keihin sizes to someone who does not know the difference and has a mikuni is going to blow up their motor by being lean. I really wish people would just read all, before adding their $0.02X( , is this not a place for techs to answer?
So if in fact you traded a TM28 straight across for stock, what you had in stock is going to be the best starting point. But as always Plug Chops not forum suggestions will tell you.
