BOV opening late, out of ideas

[rerb]

Anyone else ever run into this? Running a tial 50mm BOV on the lightest spring. I have 2 carb tops teed into a larger line that runs to the blow off. At low boost (Sub 10) it opens nicely, and I can hear the soft "woosh" but anything above that and I get compressor surge before the BOV opens. Sounds pretty sick, a loud chirp followed by the more traditional "woosh" but the whole reason I went for the 50mm was to eliminate surge. My HKS SSQV had no problems but it shat the bed. I'm pretty sure my watermeth ate the internal diaphragm which is why I selected for the piston and O-ring style tial. 

 

Any ideas? The BOV is fully open at idle (also not ideal, but this is still carb'd so it makes no real difference) so I don't think vacuum strength is my issue. 

 

[Maggotbreath]

You've connected your BOV to the carb tops? Shouldn't it be connected to the manifold side of the butterfly's?

[Gixer1460]

4 minutes ago, Maggotbreath said:

You've connected your BOV to the carb tops? Shouldn't it be connected to the manifold side of the butterfly's?

It's a BOV, not a waste gate and functions with vacuum not boost. Vacuum only available engine side of butterflies.

[badger]

My turbo smart bov wasn't opening on the lightest spring which basically turned out to still be way to heavy. My brother did some shopping for me and bought me a box of springs and even then the lightest one of those was still a bit to heavy. Cut 2 rings off and it's happy. You can shop for springs by dimensions and they're cheap cheap when you buy from a spring place. Bit of guess work involved involved by what spring weight you need though 

[Gixer1460]

3 hours ago, badger said:

My turbo smart bov wasn't opening on the lightest spring which basically turned out to still be way to heavy. My brother did some shopping for me and bought me a box of springs and even then the lightest one of those was still a bit to heavy. Cut 2 rings off and it's happy. You can shop for springs by dimensions and they're cheap cheap when you buy from a spring place. Bit of guess work involved involved by what spring weight you need though 

I'd say that BOV's aimed at the car / auto market are generally sprung too heavy as cars pull waaaaay more vacuum than a bike ever will - start soft and prepare to go softer, BOV should just about flutter at idle.

[Maggotbreath]

On 6/30/2023 at 10:15 AM, Gixer1460 said:

It's a BOV, not a waste gate and functions with vacuum not boost. Vacuum only available engine side of butterflies.

That's exactly what I was trying to say... should have said inlet manifold side of butterflies.

[rerb]

I've got 2 carb tops feeding into a larger vac line off a T, bov is completely open at idle. Under 12 psi opens beautifully, but above 12 I think there's some residual boost holding it shut briefly. I'll try cutting the spring and see if that helps. Pulls pretty strong vac using 2 sources, big improvement over just 1.

[Reinhoud]

Cut the spring, can use a pretty light spring as the boost pushes the valve down.

 

Looks like you connected it wrong too, should be connected to the vacuum points to where you synchronise the carbs / throttle bodies with, because thats the only spot you can get a vacuum, what sucks the valve open when going off throttle.

Edited July 16, 2023 by Reinhoud

[Gixer1460]

The carb top spigots work as well - this is were most vacuum operated fuel taps get their source from (usually only one though). Don't forget the diaphragm moves the slides with vacuum which is present above it ie. below the cap.

Edited July 16, 2023 by Gixer1460