I have recently bought a used 05 BF 750. It is running lean and I have cleaned the carbs good and still lean. I can wrap the air filter with a shop towel and it runs fine. I noticed there is supposed to a seal on top of the air box that seals the lid, it's missing so i have one on order. Could be the problem. Anyway could someone please tell me where the 2 vac lines on the left side of the carb go? Mine run into a plastic white cylinder with one hose coming out of the cylinder. I am thinking this connect to the bottom of the air box but was not connected to anything when I got it home. Also the cylinder has a open hole in the side. Is this supposed to be there? Thanks for any help!!!

 

The two hoses you are talking about and the white plastic bottle are for the carb vent systeim. As the carb float bowls (2, since twin-carb) fuel level go down, it creats vacuum (actually just lower pressure not really vacuum). Air must come in to displace. Also, as the fuel is coming into the float bowls, the air must go out (through those 2 hoses). The 3rd line is supposed to no where, just vented to atmosphere. Just make sure the white bottle is clean and the 3rd line is free of obstruction.

How do you know your engine is running lean other than saying it runs better with the shop towel in the air box? BTW, factory front carb setting is leaner than the rear.

 

It is backfiring bad. it just sputters when you get on the gas and runs about 18 mph. When I put a towel around the air filter it runs normal. Do you know if the white plastic bottle is supposed to have a hole in it? Since it is higher than the carb it looks like it would let water run into the carb through it.

 

Yes, I think the bottle has a small pin hole on the side for venting purpose. But that bottle/hoses got nothing to do with your running lean problem.

You should disassemble the carbs and check the current jet/needle setting.
If original, fatten up one size on the needle.
If not original, set it to original setting.

 

Thanks foru help with the vent lines. The seal on the airbox lid fixed the problem! Runs like a champ!! Also fixed the fan. The sensor in the radiator was bad.

 

my 750 brute force wont idle on fire up,when i take it for a quick chew then it will stay idleing ...would you know the isue with this....

 

You need to be more clear about your symptom. Is this what you are saying?

On cold start up, engine would not idle by itself and stalls. If went for a short riding, it would idle fine after warming up.

Is the engine stalling even with the choke fully opened (lever all the way to the left)?

 

thats correct a cold start up wont stay running even with choke on.after quick ride it will stay running.It's like its flooding..

 

Are you applying throttle when you do cold-starting with choke on?
If yes, try no throttle, just cranking only.
If no, then check the choke plungers (2) for sticking.

 

I have a 2005 brute force 750 started working on the bike because it would spit out of air box at only idle and low speeds and exhaust would glow at night, got the spit fixed but exhaust will still glow ???? Have shimmed needles cleaned carbs check chokes/valves/intake boots, everything is fine just can’t seem to figure out why it’s running lean ?