Bike Carb Conversion - Vacuum Questions/concerns

[johnhenry 20]

Afternoon!

 

Am in the middle of a bike carb conversion - 38mm choke CBR900 Fireblade CV carbs

Looking at Vacuum lines and how best to ensure a strong, usable and stable vacuum for all applications.

 

Current Vacuum Layout

 

Proposed Vacuum Layout - I am also removing the sunroof so can do away with that whole system.

 

I have included the Vacuum plenum at the bottom of the diagram.

 

My concerns;

1) As the brake servo comes off the inlet mani plenum (large volume) - am assuming there is a strong vacuum assisting it, if I'm reducing that I'm going to experience less assistance under braking?

2) is combining all vacuum systems (advance and servo) off essentially 1 small plenum going to ask too much of the whole system, would you split it somehow?

3) Is the fact the advance vacuum comes off the throttle body, and the brake servo comes off the inlet mani plenum significant (see point 1)

4) The inclusion of a anti pulse valve - location 1 or 2, my feelings are 2, because then the gauge will read more stable.

http://www.ebay.co.uk/itm/181713625230?ssPageName=STRK%3AMEBIDX%3AIT&fromMakeTrack=true

 

Any constructive input welcomed

Cheers

John

[Andy205 2]

Hi John,

 

1) The brake servo supply is larger just because you can exhaust the vacuum out of the servo quite quickly with a few pumps of the pedal. The vacuum pressure will be the same regardless of the inlet being standard or bike carbs. I'd suspect using all 4 vacuum pipes from the bike carbs would give you the same volume of air anyways, if not more.

 

2) Same again with the volume of air. You should have enough air to supply all. It will be a case of trial and error I expect. Maybe replacing the plenum block with a larger capacity one.

 

3) I don't think their is any reason for that, just convenience and the larger output for the servo supply.

 

4) I'd definitely fit the anti pulse at point 2 as you say, to keep the gauge steady.

 

Hope that helps,

 

Andy

[Andy205 2]

P.S

 

Someone else with specific experience of fitting bike carbs to a 205 may have better knowledge. I'm just going off my own experience from building cars.

[welshpug 1,662]

which engine?

 

duuh, 8v im guessing!

 

you only need the one takeoff on the manifold, that's all my mate DCC has on his 8v.

Edited September 27, 2015 by welshpug

[johnhenry 20]

Thanks both so much!
- I am looking into larger plenum just to be on the safe side - there is a 'vacuum plenum' which uses 4 take off's on eBay, and that has a much much smaller plenum than this, so we'll see if this works and run from there.

- Welshpug - good to know about only needing one, I work part time in a bike shop and from their advice, bike carbs can be pretty pulse'y and spikey so taking off from as many as possible and then adding an anti-pulse would be beneficial and it will provide a more stable vacuum and then regulate that more stable vacuum. could well be overkill, but I've read a lot of reports of bike carbs being 'un-usable' and a nightmare to get working, so £15-20 spent now to make it better will be invaluable.

 

Hoping to get a really nice setup finished sometime in the next 2 months, will try and write a guide as its a real learning curve and may be helpful for others if the results come out good!

 

Cheers again

John

[welshpug 1,662]

the elbow into the servo is a valve

[johnhenry 20]

Little update! Spoke to Bogg Bro's yesterday; they mentioned that a vacuum advance unit would need to have a measured amount into it as too much would wind too much timing on? - they also said it would probably make sense to keep the advance and servo separated as above for reasoning.

 

-Anyone know how much vacuum an effective advance would require? Boggs Bros said a 4-6mm take-off off 1x inlet, so I would get 2x cut on 1 'set' of the inlet to cover this, if 1 wasn't enough, I can add the second to it.

 

- Brake servo, they said 2x6mm or 2x8mm would cover it easily. Any thoughts?

- My final thoughts with all this in mind is to have 4x6mm take off's on each inlet. Initially 2x cover my Servo system, 1x for the vacuum advance, 1x spare. I can then add this spare to the servo for more vacuum if required, or add it onto the advance system and get more advance. Thoughts?

 

cheers

John