Sorry for the long post but I want to give all relative info for this problem.

'05 BF750, 105 hours, fuel always Sta-Bilized. has dyantek CDI.

Starts and idles perfect. In neutral and in gear it has smooth throttle to mid range, then bogs. If I spray carb cleaner into the throats of the carbs while revving it she runs great. Looking down the throat while in the mid range position, the fuel from the main jet is entering the throat of the carb more "splashy" then spray-like.

I put the stock CDI back in, same problem. I opened up the carbs, cleaned them perfectly. There was quite of bit of crystallized fuel in the bottom of the bowls. #38 jet on both carbs are clean and I see light through a nice round hole so they are clean. The pilot SCREW on the rear carb was stuck and I drilled it out, chased the threads and put a new one in. Its idleing fine. FYI, the pilot screw tap is M6x.50 threads.

The only thing I didnt check, because I dont have a fuel level gauge (graduated vial) is the running fuel level. I was thinking that maybe the level is low and the carbs are sucking it dry at high rpm? I dont see how the float level could have changed over time by itself. But I do add Chemtool B-12 (2 oz.) into the tank on each fill up. Maybe a float warped and its effecting my fuel level? I had that problem on a Honda after I soaked the carb and float in carb cleaner.

ANY help would be appreciated before I end up buying 2 new floats to try. Thanks in advance and again, sorry for the long post.

 

You are mr additive arent you,

I would change that drippy jet out with a new one, and / or try dial a jet kit if you really want to be able to tune

 

So it's lean at the upper end. I wouldn't think that the floats would change height over time. The only thing a float usually does is absorb fuel, but this is usually a characteristic of older floats. If you can press your fingernail into one and it squeezes out fuel, the float is bad. There is the possibility that YOU inadvertently changed it upon reassembly. (I've done it.)
You don't need a pro graduated tube for float settings. A clear vinyl tube works just fine and you can mark it yourself with a good ruler and permanent marker. So long as the fuel level is between the second graduation (15mm above the bottom graduation) at the punch mark on the carb, the level is OK. It is good to check this before you go further, though.
It's got to be in the main jets as long as your plugs are good. Dynojet kits are the bomb and fix a lot of problems with these lean running carbs. The dial-a-jet kits offer more flexibility. But don't rule out fuel delivery either. If you have a kinked line, weak pump, or clogged filter somewhere it can starve it at high RPM, too. Start at the tank and work forward. Just some ideas.
And I'm with kukerdan. Lay off all the additives (except the stabilizer if you don't run it much). It's OK once in a while, but fuel has enough cleaning agents in it to take care of itself. Sorry for the long post.

 

Here's an update. I put a fuel line on the carb and put a funnel in the open end. I put the fuel line from the tank in the the tank fill. Filled the funnel with fuel and fired her up. Ran the same. Easy start, responsive throttle with bogging on the top end. Checked the fuel line in the fuel fill and its pumping nicely, no problem there. I then shot some carb cleaner into the carb throats and she revs nicely up to full throttle. Definitely a fuel issue. I have to check the plugs but I dont have hopes that they are the problem. I never ran into that. I've rebuilt some many carbs of different types over the years and never had a problem like this.

I will check the running fuel level height. Can you explain further about the markings I should make on the hose. Second graduation?

 

Just a short length of clear hose, say, 8 inches. One end has two marks 15 millimeters apart. This is your fuel height range. Plug the unmarked end into the fuel bowl drain and support the marked end next to the carb so that the top mark (the second graduation) is just above the punch mark in the middle of the carb body. Keep it there. If you lower the upper mark (the second mark graduation on the tube) below the punch mark you will get an inaccurate reading and the test will have to be restarted. Open the bowl drain screw and the hose will fill with fuel. Start it and do your testing. If the fuel level remains in between the two marks while running, you're gold. If not, adjust your floats. Good luck.

 

Po, thanks for the details. Heres what I have. I made up the hose, lined it up with the punch marks and opened the drain screw. Fuel flowed into the hose and then behind it was an air bubble about 1 inch long in the tube. So I pulled the tube off the drain, fuel flowed everywhere and I quickly put the hose back on. No more air bubble. I was careful not to move the second graduation from the punch mark. I then started her up. Fuel in the hose did not increase or decrease. The rear carb level was about a 1/2" lower than the bottom graduation. The front carb was about 1 1/2" below the bottom graduation.

While doing the test, I figured once I cracked the drain screw it would push all the air right out of the hose. I didnt think I would have to remove the hose to "purge" the line. What do you think. Readings are low (if I did the test right). How could the floats get so out of spec. Theres only that little metal tab on the float to adjust and I only removed the float and put it aside. Any ideas??

 

Update: I tested it again. This time I put the hose on, kept the graduations well above the punch mark, started her up, cracked the drain screw and slowly lowered the the top graduation to the punch mark. Minor air bubble at the carb nothing big. Same results. Still 1/2" low on the rear and 1 1/2" low on the front. I understand how they got out of spec. Seems to me, when I eye up the level on the front carb to the float bowl that there is a possibility that the bowl is getting sucked dry. What do you think?

 

Smoothtpd, I apologize for my memory lapse. I was thinking of the Prairie. Your low measurements for the carbs are likely correct. The fuel level should NOT be between the two graduations, but below them. The front carb should be 20.4 mm (.80 inches or just over 3/4 inch) below the top graduation when it is even with the punch mark, and the rear should be at 19.5 mm (just over 3/4 inch also). Your front carb is indeed low, but the rear looks spot on, since 1/2 inch is about 13 mm. With a front carb that low, there is a possibility that the pressure differential in the carb can dry the bowl since it can't refill quick enough at upper RPM.

 

Ok,here is what I have. I got the manual for the quad. I pulled the carbs. The front one was futher out of adjustment than the rear. I set the floats as per the manual at 3MM with the float just touching the needle pin. Both look good now. I did the fuel level check with the hose again. The rear still shows about a 1/2 inch below the bottom graduation but the front is about an inch.

I changed both spark plugs. the front one was carbon coated but nothing excessive. The rear one has carbon as well but the grounding tip is slightly tan.

I started her up and tried to rev her. She still bogs but not as severe. If I hold it floored she is bogging, backfiring through the exhaust and then sometimes catches and goes full throttle. So I do seem to have some progress.

The manual says they should be .80 front and .77 rear (both about 3/4") from the bottom of the punch mark. So, take my rear carb, is a 1/2" lower than the bottom graduation which is 15mm below the punch mark to begin with. So the rear would be about 1" lower than the punch mark. (Looks like 13mm is about a 1/2" on my slide rule). So wouldnt my fuel height still be low?

 

I've edited my former reply due to a math error. 13mm is 1/2 inch and not 1/4 inch. Ignore the bottom graduation and only measure from the top graduation for your fuel level as long as the top graduation mark is even with the carb punch mark. So the answer is easy. 15 mm+13mm (1/2 inch) is 28mm. I'd call it low and in need of a richening adjustment.
This all is not to say that all this will guarantee to solve your bogging problem, but without it the bike doesn't have a chance. You'll likely need to fine tune it after it runs much better. Good luck