BF90 Carb Sync

[rschichtel]

Can anyone advise the vacuum pressure to sync the carbs on a 99 BF 90? I had the usual problems, had the carbs rebuilt, but reinstalled them myself and need to sync them. Also, will a single vacuum gauge be OK or do I need the 4 gauge manifold?

Thanks,

Rick

 

[Flakeys]

Re: BF90 Carb Sync

You need the 4 gauge set up to sync them properly.

 

[rschichtel]

Re: BF90 Carb Sync

You need the 4 gauge set up to sync them properly.

Thank you. Do you know what the preferred vacuum pressure is, or is it just more important for them all to be the same?

 

[BaileysBoat]

Re: BF90 Carb Sync

All the same.

 

[hondon]

Re: BF90 Carb Sync

All the same is correct but getting there requires a little expierance,finese,and paticence,not to mention a tall pair of rubber boots.If those sync screws have not been disturbed,the valves are properly adjusted,and compression is ok,you should be resonably close after you bolt that mani back up.

 

[rschichtel]

Re: BF90 Carb Sync

Thank you. Do you know what the preferred vacuum pressure is, or is it just more important for them all to be the same?

I obtained my 4 gauge vacuum set and tried to sync the carbs, but the 1 and 4 cylinders are about 5 psi higher than the 2 and 3 no matter what the mixture is. Does this indicate the valves need to be adjusted first?

 

[hondon]

Re: BF90 Carb Sync

What do you mean by mixture?You are not syncing these carbs with the idle mixture screws but rather the throttle butterfly screws.You will need a tach ,a decent vac guage,and patience.Remeber that each time you tweek one of those screws you will need to rev the engine and let it settle down.If you do'nt go about it that way ,you'll be there all day.VAC specs are usually around 10 -12 but that can vary a great deal from guage to guage.The final goal is that all are even and the engine idles smoothly.

 

[rschichtel]

Re: BF90 Carb Sync

Thanks Hondon! It worked great. I don't know where I got the idea that the mixture screws were to be synced, but I did what you said and the idle is smooth and no sneezing on acceleration.

What do you mean by mixture?You are not syncing these carbs with the idle mixture screws but rather the throttle butterfly screws.You will need a tach ,a decent vac guage,and patience.Remeber that each time you tweek one of those screws you will need to rev the engine and let it settle down.If you do'nt go about it that way ,you'll be there all day.VAC specs are usually around 10 -12 but that can vary a great deal from guage to guage.The final goal is that all are even and the engine idles smoothly.

 

[mbfish]

Re: BF90 Carb Sync

I just synced the carbs on my carbed BF90 yesterday and I have to say it was easier than I though it was going to be. That said, I guess I was well prepared as I had done hours of research on boat and motorcycle forums.
The main tutorial than I used was from the C-Brats forum called "Honda 90 Blues - reprise and fixed". The author did an excellent job of explaining how he made the manometer. I did the same thing. I used 5/16" inside diameter vinyl tubing and 2-stroke oil. To balance the carbs, you don't need to know the actual vacuum value, you just need to make them all equal. There are also some very good motorcycle forums out there that I read as well that do a great job of explaining the process. The only part that wasn't spelled out was how to get the tubes attached to the vacuum ports on the manifold. I ended up using 1/8" (I think, maybe 3/16"...) inside diameter vinyl tubing between the manifold and the larger tubing. I had to chamfer the small tubing to get it to thread into the manifold. It made for a good tight seal at both ends of it.
It cost me $30 in materials and 3 hours on a Saturday afternoon. Both of which I think were well spent.

 

[mbfish]

Re: BF90 Carb Sync

Update:
The home-made manometer did a fantastic job and I couldn't be happier. There is ZERO hesitation when I hit the throttle. It's like a whole new motor. I have to admit, I was a bit skeptical, but she runs like never before. Silky-smooth idle and flawless transition to wide open.