hot 4k build help

[oldcorollas]

it is more to do with changing the amoutn of rocker wipe, at high and low lift.
with standard cam, there is roughly equal wipe above and below mid lift.

with a higher lift cam, you only add wipe during that extra lift, and it can be substantial.

it is better to add more rocker wipe when the valve spring force is lower (ie.. 100lb instead of 300lb), so you machine the rocker towers by the increase in cam lift, and then the high spring force wipe is no more than standard, but the low spring force wipe is greater (ie, when valve is near closed).

the pushrod angle is also an issue, but a secondary one.

4KE TB size is 40mm, 7KE TB size is 50mm, i think i worked out the maximum needed throttle would only be around 43mm? (when comparing to rally suzuki 1300's). 7KE TB would be enough.
4KE and 7KE runner size is 30mm for both. you can always fit throttle from something to the lower half of the EFI manifold, ie with the injectors, or weld injector bosses etc to a webber manifold.

[briney]

how about motorbike carbs or itb's ? a set from a 600-ish cc bike engine might work ?

[4kforeva]

i was just looking at them r1 or gsxr 38mm carbs

[oldcorollas]

how about motorbike carbs or itb's ? a set from a 600-ish cc bike engine might work ?

GSX550E CV carbs have the perfect spacing for the K head
only 30mm throttles, but really won't be an issue. just means the port will onyl taper a little instead of a lot. you can taper the bellmouth instead

[4kforeva]

will they be big enough i thought maybe 40s , does anyone have some or know where to get them

[oldcorollas]

maybe not big enough, but might be a lot better than other options.
mount them very close to the head, so they are just a continuation of the port diameter, and then increase size with trumpet.
or maybe drop down a step in cam unless you plan to lighten the valve train have you heard a K valve train above 8000rpm? what max rpm are you planning?


also remember that at WOT, when the CV slide is up, there is basically no choke, unlike a weber.

[4kforeva]

max 7500 there is really not much point in more then that lol,

[2jzhilux]

A 2001-2003 yamaha raptor quad runs two 38 mm mikuni cv carbs and they are getting a bit older so parts like that are starting to appear off crashed bikes

[Rodger]

What are motorbike carb's throttle response like when on a slow car engine that does not change speed that quick (relative to a motor bike engine)? The reason motor car engine carbs have pump jets is to overcome the lag between air speed increasing and fuel draw. Do bike carbs have pump jets?

Do we on Toymods have any write ups about adapting and using motorbike carbs on car engines?

Regards

Rodger

[oldcorollas]

on a CV carb, the airspeed across the jets (under the CV slide) is always high, regardless of engine speed, and when you open throttle, the airspeed between slide and jet increases, increasing the fuel draw... which is why they work on bikes with faster response.

CV slides were used on cars.. in SU's... that would be another option.

but really, using an injection manifold with bike cars or car carbs as ITB's is a better way to go, as opposed to providing the fuel, unless it is a budget build


normal car carbies need a pump because you are transitioning from the idle jet below the throttle, to the main jet above the throttle...
so when you open throttle, you lose vacuum below, so idle jet stops working, and then it takes a bit of time for the airspeed to create enough suction through the venturi of the main jet...
some (most?) carbs have progression holes to help prevent the starvation between fuel delivery modes.

but with CV, you are always coming from the same jet, so there is no transition to think about.

[Billzilla]

What are motorbike carb's throttle response like when on a slow car engine that does not change speed that quick (relative to a motor bike engine)?

Just fine. They work the same no matter what engine they're on. The only thing you have to watch is the angle of he carbies as they don't like leaning over too much. (though some are built to sit like that)
My racing car runs four Mikuni 38m flat-slides and once it got tuned properly it runs far better than the Webers ever did. The only hassle was as I mentioned above, I had to make a new inlet manifold to make sure they sat vertically or it'd run out of fuel around right-hand corners.

[oldcorollas]

bill, would it be safe to say that the CV carbs are built for bike accel only, so they would be good in one direction of cornering, but poor in the other (ie, RH corner on yours)?

[Billzilla]

I don't know for sure.
The reason mine ran out of fuel is because I had a short inlet manifold on the head the carbies had to lean over at about 25° and that made them run out of fuel on right-hand corners.
Now they sit pretty much straight up and it's good.

[oldcorollas]

oh, well, carb is designed to be able to supply fuel for bike under accel, = fuel moving toward filter side of carb, since most bikes have carbs mounted to the rear of the motor (inline 4's).
as you can see from this diagram that means the float will allow more fuel into the fuel bowl under accel, than is there when not under accel.
under decel, the fuel sloshes toward the front, and closes the float needle more than usual, to stop more fuel coming into bowl under decel.

for your car, change accel to LH turn and decel to RH turn..
as long as it doesn't starve on RH turn, or in straight and doesn't piss fuel on LH turns, all good.

[4kforeva]

thanks for all the info guys, i forgot to mention that i am on a budget lol , i was talkin to a good friend he gave me a microtech m4 fuel only ecu .. i was thinking about getting a exel,mazda 323,dawoo, inlet and cutting to suit and just tig it to a std 4k5k inlet would this work ok or would moddin 20v itb be easyer