MOTORCYCLE THROTTLE BODIES

[jhancock]

Rich,

MC engines come in so many variants that I am not sure you can get a general
answer to your question(s).  The only general one that I know is that modern MC
designs almost ALWAYS use seperate carbs...(fuel supply) for each cylinder (note
this does not apply to Harleys). This is usually due to one of two reasons,
engine geometry (i.e. a single carb would be too far from the cylinders, e.g.
Beemer opposed cylinder engines), or "flow" control.  For about 5 years MC
performance engines (which change complete ENGINE design every 2-3 years!) have
been dynamically tuned from the tip of the air-box-intake, to the tip of the
Exhaust canister output to maximize flow in certain rev ranges. (This is used to
"pack" fuel/air mixture, using the charge's mass, into cylinder at high revs.  I
wish I understood the valve timing aspects of this better!)

Most MCs are not "mass produced" like cars, and sport-machine venturi diameter
leaps about 1-2mm every 2-3 years, and each engine-intake-exhaust system is
virtually custom designed, there is little commonality of parts.  If you try to
"reach-back" into the parts bin from 3 years ago, you are so far off the cutting
edge that you might as well buy a new bike, or you goof up the flow of your bike
by getting it out of flow-balance.  This does not however stop intrepid souls
from trying, especially since the EPA and DOT noise restrictions force MC makers
to take carefully tuned engines and put doofey restrictors on the I/O ends to
keep intake and exhaust noise in certain sonic range and DB limits.  Removing
these usually produces useful results.

If I were to look for the cutting edge of MC fuel injection I would look to the
new-for-97 Suzuki TL-900S Vtwin, the Ducati superbike, I don't have a good I4
motor example, the latest generation BEEMER Boxer (opposed twin which I THINK is
EFI), or for the twilight-zone... the Bimota V-Due ("Vee Du-ay") which is a
direct injection (street legal) 500cc two stroke.  Note that the Twin-cylinders
(flat and V but not most Harleys since they can't take any more heat-load
anyway!) are pushing forward with EFI because they need that advantage to keep
up with the I4 designs which are mostly still carbureted.  P.S. Watch out for
the Suzuki TLS EFI since early 97 models were rushed into productions and had to
have the map chip replaced in a mfr recall.

To answer some of the simpler questions: MC engines usually use plastic "boots"
bolted to the engine intakes, and clamped to carbs (slip fit...).  Thus the Fuel
system is completely seperate from the engine, and unique, identical, and
synchronized for all cylinders.   I know of few exceptions to this rule (Harley
being an example).  The intake runners/venturi size of modern bikes vary with
cylinder capacity (that "flow thing rearing it's ugly head).  I will address the
sport-ier machines; The low end is 24mm for I4 600cc bikes, whereas the upper
end is more like 40mm+ for 900cc Vtwins (I may try to look up the latter and get
back to you since this 900-1000cc vtwin motorcycle engines are more like
automotive applications anyway).

These ramblings are generalities (since I am not all that good with specifics)
across the MC spectrum and, I am sure, instances could be found to refute every
point that I made.  Needless to say MC intake configuration is an extremely
complex subject fraught with point solutions rather than general approaches.  I
do, however find it interesting that the highest tech motors thar you can buy
off a showroom floor, in a vehicle that costs less than a Japanese family sedan,
still are predominantly carbb'ed instead of EFI.

I will be interested to see if you get more specific answers.

Now if someone can help me EFI my 89 Jeep Wrangler I6 4.2L, I can go back to
rebuilding my carbs for my 1985 Honda VF1000R which will then be used to go out
and embarass some unsuspecting C5 EFI Vette!

John

Rich Mauruschat wrote:

> At the risk of sounding like the spanish inquisition, are there any 'bike
> enthusiasts who can enlighten me on the subject of motorcycle injection
> throttle bodies?
> 1, Are they generally individual with individual spindles linked together
> externally?
> 2, Are the injectors fitted into the body or into the head/manifold?
> 3, What style of injector do they use; o-ringed Bosch type?
> 4, What size are they generally: 40mm or so?
> 5, Do they have individual air-bypass adjustment screw or the like?
>
> I have never seen these things and am just interested to know what they look
> like, what models they are fitted to and whether they could be adapted for
> use on automotive applications (presently using adapted cut-down Weber
> sidedraught carbs!).
> Thanks for help
> Rich.