Ninehigh
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| posted on 28/2/10 at 09:48 AM |
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Why bike carbs
I've heard of a few people using bike carbs on their car engine, but what does this actually do? I have to ask now because my tutor at college
wants to know (after we did carbs and injectors last week).
From what I understand you'd be getting a mix of double fuel to air... although I've no idea how that's beneficial to anyone except
BP.
(Btw if this should be somewhere else please move it thanks)
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nitram38
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| posted on 28/2/10 at 09:53 AM |
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I think most people use bike fuel injection because of cost.
Jenvey bodies can cost a couple of thousand pounds.
You can buy bike throttle bodies/injectors for £100-200 and buy aftermarket controllers for a few hundred more
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mookaloid
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| posted on 28/2/10 at 09:56 AM |
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people use bike carbs as a cost effective alternative to upgrading to twin 45's or the like. there is also a school of thought that they
actually perform better, giving smoother running, better idle etc than webers.
"That thing you're thinking - it wont be that."
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deezee
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| posted on 28/2/10 at 09:57 AM |
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Because they are cheaper and flow more than car carburettors. Why would you be getting double air to fuel ratio? They are all balanced and tuned.
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Ivan
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| posted on 28/2/10 at 09:58 AM |
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Don't understand what you mean by "Mix of double fuel to air"
Most petrol engines run on air to fuel ratios of around 13:1 and all carbs are set up to provide something like that, never 6.5:1.
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Ninehigh
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| posted on 28/2/10 at 10:03 AM |
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Well the Llambda (sp?) ratio is 14.7:1, but seeing as cars with fuel injection already do this, and carbs are having their attempt at getting that
fuel mixture before it gets as far as the injectors... I'm trying to understand how they actually work because a carb mixes fuel and air, but so
do the injectors. If carbs are set up for this is what I'm thinking correct or do the carbs not let fuel in and are just used for the air
control?
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mookaloid
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| posted on 28/2/10 at 10:06 AM |
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you either have carbs OR injectors not both.
If you are home brewing your fuel system, it is easier to use carbs because they require no electronics. on the other hand if you can build your own
fuel injection system then you don't need carbs
"That thing you're thinking - it wont be that."
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zilspeed
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| posted on 28/2/10 at 10:06 AM |
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Bike carbs are modular, as are bike throttle bodies if chosen correctly.
This makes it very simple to have a setup with one choke per cylinder, just like webers / dellortos.
The fact that they are modular means that spacing to suit the car engine is also very simple.
Regarding carbs, they are mostly variable venturi (sort of like an SU, but more like a Stromberg because of the diaphragm).
This makes them more tunable than a fixed venturi carb like a weber where you need access to lots of chokes to get it right.
With the variable venturi, it will pretty much open as much as it needs to and the tuning is all done with the jet and needle.
With the throttle bodies, as said previously, they're cheaper than Jenveys is the only advantage.
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Steve G
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| posted on 28/2/10 at 10:06 AM |
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quote:
Originally posted by Ninehigh
From what I understand you'd be getting a mix of double fuel to air... although I've no idea how that's beneficial to anyone except
BP.
Why would they be doing double fuel to air?? The engine would immediately flood if that was the case.
Just think of them as carbs of a different design (closer to old SU carbs than Webers / Dellorto) that just happen to have come off a bike.
At the end of the day all you want to do is inject a certain amount of fuel into the engine for any given engine speed and load to get the correct air
/ fuel ratio (AFR) which will be around 14.7:1 (air : fuel) plus or minus a couple or so depending on engine load. This wont change by much no matter
how you get the fuel in there so a properly tuned set of bike carbs will be putting in very similar levels of fuel into the engine as Webers or Fuel
Injection.
Editted to correct the stoichiometric ratio which is 14.7:1 dohhh
[Edited on 28/2/10 by Steve G]
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Ninehigh
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| posted on 28/2/10 at 10:09 AM |
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Ah so it's an alternative not an addition?
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mookaloid
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| posted on 28/2/10 at 10:10 AM |
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quote:
Originally posted by Ninehigh
Ah so it's an alternative not an addition?
You've got it 
"That thing you're thinking - it wont be that."
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Toltec
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| posted on 28/2/10 at 10:22 AM |
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The other thing that occurs to me is if someone has suggested using a set of bike carbs as throttle bodies. You make up a manifold with injectors and
use the carbs without any fuel supply to control the air flow. I am not sure how well that would work with vacuum slide carbs though, I suppose you
could fix the slides fully open and just use the butterflies. Would that save much money over just getting some bike throttle bodies?
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MakeEverything
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| posted on 28/2/10 at 10:34 AM |
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quote:
Originally posted by Toltec
The other thing that occurs to me is if someone has suggested using a set of bike carbs as throttle bodies. You make up a manifold with injectors and
use the carbs without any fuel supply to control the air flow. I am not sure how well that would work with vacuum slide carbs though, I suppose you
could fix the slides fully open and just use the butterflies. Would that save much money over just getting some bike throttle bodies?
I reckon its possible, but an expensive way of doing it?
Kindest Regards,
Richard.
...You can make it foolProof, but youll never make it Idiot Proof!...
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DRC INDY 7
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| posted on 28/2/10 at 10:41 AM |
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yep its been done mate of mine with a indy zetec has bike carbs for the throttle bodies just fix the slide in the up postion
https://www.facebook.com/groups/462610273778799/
Puddle Dodgers Club
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Ninehigh
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| posted on 28/2/10 at 10:58 AM |
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quote:
Originally posted by Toltec
The other thing that occurs to me is if someone has suggested using a set of bike carbs as throttle bodies. You make up a manifold with injectors and
use the carbs without any fuel supply to control the air flow. I am not sure how well that would work with vacuum slide carbs though, I suppose you
could fix the slides fully open and just use the butterflies. Would that save much money over just getting some bike throttle bodies?
I'm sure that's what I was told last time I asked..
Thanks for all the replies guys, I've learned something today 
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woolly
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| posted on 28/2/10 at 11:27 AM |
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choke size is a compromise on fixed choke carbs (weber etc.) small for low down control big for power. bike carbs have variable choke in an attempt to
equal out the air speed over the jets and gives better low down control. yet a large choke size for power.
old stromgberg carbs were called CV carbs constant velocity. SU carbs same principle different approach.
HTH
woolly
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BenB
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| posted on 28/2/10 at 02:04 PM |
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Most people fit bike carbs because
1) they're cheap
2) they flow well
3) they're easy to adjust (shim the needles and job's a good'un [usually] )
4) they can be easily re-spaced
The reason they are quite good for converting into TBs is the above and that they don't have a fixed choke in them (unlike say Webers) so you
don't get the same restriction. With EFi after all, you don't need a choke to create the pressure change needed to atomize the fuel.
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