Power boost at high RPM 8K

[firro]

Hey all,

So i got all new spark plugs, new intake manifolds, Stage 7 kit with VBOOST removed. Carbs revision done, ultrasonic cleaning etc, high octane fuel, full exhaust system Doma Racing.

Bike runs well, however i do notice that power is Ok'ish at low and mid RPM , i have never ridden another vmax so cant really compare, but something is telling me the bike should have more power at low and mid rpm, because at around 7,5 - 8k RPM at full throttle i get a insane power boost.. its like fast and furious when they hit the nitro button xD

I think i should have similair power also at lower rpm's ,, wondering what holds it back.. mixture screws?? or is this completely normal?

ps. db-killers are removed.

Thanks!

 

[Traumahawk]

Hey all,

So i got all new spark plugs, new intake manifolds, Stage 7 kit with VBOOST removed. Carbs revision done, ultrasonic cleaning etc, high octane fuel, full exhaust system Doma Racing.

Bike runs well, however i do notice that power is Ok'ish at low and mid RPM , i have never ridden another vmax so cant really compare, but something is telling me the bike should have more power at low and mid rpm, because at around 7,5 - 8k RPM at full throttle i get a insane power boost.. its like fast and furious when they hit the nitro button xD

I think i should have similair power also at lower rpm's ,, wondering what holds it back.. mixture screws?? or is this completely normal?

ps. db-killers are removed.

Thanks!

That is Normal. The whole point of Vboost was so you had basically 1 carb per cylinder. With Vboost opened, It allows basically a carb and a half per cylinder. (the intake is nothing more than an H that the carbs sit on) on top of the cylinders. So basically its over carbureted at the lower RPMS.

 

[gentsvmax]

And I think the Premium isn't necessary, I'm sure others will share on this.

 

[MaxMidnight]

Agree with both of the posts above.
How do you synchronise the carbs without V Boost?

 

[CaptainKyle]

Agree with both of the posts above.
How do you synchronise the carbs without V Boost?

I have a tool for it but it's a pain ! I always reccomend keeping the Vboost with stage 7 . It does not kill your low and midrange that way .

 

[Fire-medic]

I have Stage 7 and also kept the VBoost.

 

[maleko89]

Can use a Weber tool but why. Agree, no reason the remove vboost

 

[apsolus]

so maybe i can piggy back off thise post but my bike is mostly stock minus a slip on and k&n. i seem to have the idle set well at 40 pilots at 3 turns out. idle is pretty strong and steady. synced them aswell. ive installed 150 mains and top end feels good. but midrange is a tad soft and i also get a small rush as rpm increase. not normal vboost but earlier in the rev range and ive noticed when cold it take a little while till fueling is perfect going down the road in the midrange. ive got stock needles with one small shim to richen it up. i was going to add another shim to the needles and see how that feels. i also took out the dyno jet slide springs and went back to stock. is there a hot set up yall are aware of for midrange?

 

[Radioguylogs]

Vacuum pistons drilled for Dynojet springs?

 

[apsolus]

Vacuum pistons drilled for Dynojet springs?

they were but when i went back to stock springs i installed new china slides. they look identical to stock. i put the dynojet springs back in and removed the spacer real quick. bike seems better. im nitpicking here but i want it right right. i used to run that stage 7 kit but my bike ran really rich. and would stall out at lights. maybe i should reinstall the kit and retest. ive learned a lot about carbs sence then. do you have a good set up i can try?

 

[AndrewT]

It took me years of trial and error to get the carbs close to working in a reasonable way. Each carb set up is almost unique for the bike and all things done to the engine, exhaust, carbs and inlet tract and fuel you are using. Best thing to do is buy an air fuel ratio gauge kit and install it where you can see the gauge. I have mine next to the speedo. I have the sensor at the bottom of the front left header just before the crossover pipes.
Fuelling is 3 zone. Off idle is the idle circuit settlings and it sounds like you have this close. 1/4 throttle to 1/2 throttle is the slide springs, jet needle and needle jet. 1/2 throttle to wide open is the main jet size.
Air fuel ratio at idle and low throttle positions should be 15-17 and mid throttle 14-15 and WOT 12 to 13.
These are also roughly the 3 zones of the carb fuelling.
Usually you start with main jet size as this effect everything else a bit. this is best done on a dyno as there are not many places you can be WOT without full paying attention to what is happening - it is a Vmax after all.
Once the main jets are sorted. then the middle zone can be addressed.
These are CV carbs - constant velocity. The velocity, venturi pressure drop and air density all play a part here.
If your vacuum springs are too soft or have insufficient pre load then the piston will rise earlier, velocity will be lower more fuel is drawn in and fuel atomisation decreases resulting in poor performance as the air velocity is speeding up slowing down and speeding up again this is now efficient and the fuel does not burn properly as it is not atomised and mixed well with the air.
If the vacuum springs are to heavy or have to much pre load the slide dies not lift restricting the air flow and fuelling.
You can try different jet needles and lifting or lowering them but this does not solve the vacuum spring issues. How does this look on the AFR gauge? If the spring is too heavy the more throttle you give if will lean up the mixture or as RPM build at the same throttle it becomes leaner. Visa versa for too soft a spring.
If the vacuum spring preload is not correct if it is too light the opening the throttle small amount it will become rich and if the vacuum spring preload is too heavy. open the throttle it will become leaner.
If you get the correct the vacuum piston spring force and pre load. The velocity will be correct you get correct atomisation of the fuel which give the best performance, also using way less fuel, so it is then about the correct jet needle and needle jet to get the correct AFR through zone 2 at any RPM.
As these are CV carbs the vacuum piston rises and falls with each intake cycle. The needle jet is made of brass and the jet needle made if stainless. So this occilating action wears the needle jet oval which will mess up your AFR making it richer and the fuel atomisation poorer. So suggest these are replaced before you do anything else. If you had any Dynojet carb mod then the vacuum piston vacuum holes would have been drilled larger. Whilst this gives faster throttle response it also makes the vacuum piston occilate more which wears the needle jet faster, also fuelling rate will vary more during the intake.
With K&N filter the air density is increased through the whole inlet tract. If you also have the Y cover off the airbox it is a bit more again.
Higher density means higher mass flow rate. higher mass flow rate at the sweet spot velocity will creat higher vacuum so lift the vacuum piston earlier and faster. This requires a slightly higher vacuum spring preload and spring stiffness.
I have after much stuffing around standard vacuum springs, needle jets, jet needles and 150 mains with a 20 mm spacer under the vacuum springs.
My bike is higher compression, ported, Slightly more aggressive cam K&N airfilter with Y of the airbox, stockish exhaust. Running 98 petrol.
This works well AFR at all RPM and throttle. Bike goes, and fuel consumption is less.

 

[gentsvmax]

It took me years of trial and error to get the carbs close to working in a reasonable way. Each carb set up is almost unique for the bike and all things done to the engine, exhaust, carbs and inlet tract and fuel you are using. Best thing to do is buy an air fuel ratio gauge kit and install it where you can see the gauge. I have mine next to the speedo. I have the sensor at the bottom of the front left header just before the crossover pipes.
Fuelling is 3 zone. Off idle is the idle circuit settlings and it sounds like you have this close. 1/4 throttle to 1/2 throttle is the slide springs, jet needle and needle jet. 1/2 throttle to wide open is the main jet size.
Air fuel ratio at idle and low throttle positions should be 15-17 and mid throttle 14-15 and WOT 12 to 13.
These are also roughly the 3 zones of the carb fuelling.
Usually you start with main jet size as this effect everything else a bit. this is best done on a dyno as there are not many places you can be WOT without full paying attention to what is happening - it is a Vmax after all.
Once the main jets are sorted. then the middle zone can be addressed.
These are CV carbs - constant velocity. The velocity, venturi pressure drop and air density all play a part here.
If your vacuum springs are too soft or have insufficient pre load then the piston will rise earlier, velocity will be lower more fuel is drawn in and fuel atomisation decreases resulting in poor performance as the air velocity is speeding up slowing down and speeding up again this is now efficient and the fuel does not burn properly as it is not atomised and mixed well with the air.
If the vacuum springs are to heavy or have to much pre load the slide dies not lift restricting the air flow and fuelling.
You can try different jet needles and lifting or lowering them but this does not solve the vacuum spring issues. How does this look on the AFR gauge? If the spring is too heavy the more throttle you give if will lean up the mixture or as RPM build at the same throttle it becomes leaner. Visa versa for too soft a spring.
If the vacuum spring preload is not correct if it is too light the opening the throttle small amount it will become rich and if the vacuum spring preload is too heavy. open the throttle it will become leaner.
If you get the correct the vacuum piston spring force and pre load. The velocity will be correct you get correct atomisation of the fuel which give the best performance, also using way less fuel, so it is then about the correct jet needle and needle jet to get the correct AFR through zone 2 at any RPM.
As these are CV carbs the vacuum piston rises and falls with each intake cycle. The needle jet is made of brass and the jet needle made if stainless. So this occilating action wears the needle jet oval which will mess up your AFR making it richer and the fuel atomisation poorer. So suggest these are replaced before you do anything else. If you had any Dynojet carb mod then the vacuum piston vacuum holes would have been drilled larger. Whilst this gives faster throttle response it also makes the vacuum piston occilate more which wears the needle jet faster, also fuelling rate will vary more during the intake.
With K&N filter the air density is increased through the whole inlet tract. If you also have the Y cover off the airbox it is a bit more again.
Higher density means higher mass flow rate. higher mass flow rate at the sweet spot velocity will creat higher vacuum so lift the vacuum piston earlier and faster. This requires a slightly higher vacuum spring preload and spring stiffness.
I have after much stuffing around standard vacuum springs, needle jets, jet needles and 150 mains with a 20 mm spacer under the vacuum springs.
My bike is higher compression, ported, Slightly more aggressive cam K&N airfilter with Y of the airbox, stockish exhaust. Running 98 petrol.
This works well AFR at all RPM and throttle. Bike goes, and fuel consumption is less.

Excellent Post here!