This is a discussion on Father's day gift Honda CB350 Build within the Project Builds forums, part of the Caferacer.net Forums category; Two strokes need to be jetted richer at WOT at high revs compared to a 4 stroke. A 4 stroke also needs to have the ...
Two strokes need to be jetted richer at WOT at high revs compared to a 4 stroke. A 4 stroke also needs to have the fuel flow broken up into smaller droplets to burn efficiently. A primary type needle jet achieves those for a 2 stroke and an emulsion tube (bleed) type does what's needed in a 4 stroke, but it needs much smaller main air jets.
So to use a carb set up for a 2 stroke on a 4 stroke might work, but it's not doing its best work. Change air jets and needle jets and you start to get closer.
Honda and their Keihin carbs used an oval venturi so that at small openings, it was like have a smaller carb and ass the slide rose, the overall area of the tall, narrow venturi was the same as a bigger sized carb. So they work great at small throttle openings and still work well at higher loads.
K0 carbs are better than K1 on because they take individual cables, but the top caps from a later model can be adapted. I used CB750 carbs on my Methanol burning 350cc CB77 racer for years.
This weekend I discovered that I got the blinker wires a little to hot causing them to melt while using shrink tubing. I was able to get the front of the bike wired, and built the battery box.
Next I will be fitting the rear fender and the seat, then wiring up the rear of the bike. so hopefully I will be tuning the bike by the end of the month.
Last edited by pappavol; 07-22-2019 at 12:26 PM.
Individual air filters and CV carbs can be awkward to set up, i had them on a XS650 i had.
I tried a few different pilot jets and mains in those old mikunis but it was never as smooth as with the air system on.
I made a air chamber from a old round lawnmower tank and one 54mm air filter. it ran great like this, and looked better than it sounds too.
Cv carbs need that air chamber space to let them work properly.
main reason is it makes sense
another thing not mentioned above but teazer opend my eyes to it is the path of fuel air mix is so much different on a 2 stroke
on a 2 stroke the mix is compressed twice (as well as being sprayed onto a rotating crank and rod)
the first staige of charge compression is what allows the mix to take the long path up stairs and it needs to be an somewhat energetic blast to maintain velocity up through the transfer ports and continue on loop charging the combustion chamber
loop charging is what was a revolutionary change in 2 strokes
and i remember very interestingly reading about it
as me dad had just got a brand new 50 hp twin evinrude for our little 14ft ski boat
and it had loop charging
no longer were 2 strokes going to realli on a hideously large shaped in a grotesque manner piston crowns in order to get the then straight shot transfer charge to change direction go up then down to chase the spent gasses out the ex port
loop charging meant pretty much flat top pistons and a simmetrical comb chamber with squish bands that can be fully effective
so the 2 stroke charge is really heavilly manipulated and hence a fine atomization is not nuber one
with the weaker signal just getting the gas in there is more important
4 stroke has a very stroing intake puls and it lasts longer in degrees of crank rotation than 2 stroke pulse
4 stroke has the charge exposed to lots of cylinder wall surface area
but it is simply dumped into the cyl the compressed with some vigourus mixing happenning when the squish gap closes up
if the charge is not fine enough
if it is to close to having droplets fall out of suspensinon then tmy thinking but i cannot provre it and have nevert read about it
but maybe too much fuel as liquid sticks to the cyl walls wetting them
anyway the other deal is the mikuni oem and keihin 4 stroke carbs all have a smaller inlet and vare using a stack to get a designed tuned intaske port length mn
and the mikuni designed for 4 t all have a tall oval variable venturi
the amals and del orto's do not but dells went to flat slides and they and amal have provision for proper stack design attatchement
the mic 2 t carbs have a huge bell mouth right there so to get a decently long intake port the carb needs moving away from the valve seat as a vstack wont be able to blend flow at that bell moiuth
there is prejetted this and prejetted that
but i looked at the sohc 750 and the only real diff to a cb350 is each 750 jug is 10cc's or so bigger
otherwise a simply 2 valve design
and trhe first xl175 is basically a cb350 cut in half and it uses same style keihin
honda put huge amounts of research and dyno testing developing these carbs with keihin
you look at them then look at a vm mikuni which was nevert meat to be used on 4 sdtrokes
and you see huge differences inb the design but still go trhe vm path ?
the dime skittle cycle parts prejetted route lol
and the sl350 twin uses an exact same design carb as the750 c albeit 4mm smaller (the sl port is 4-5mm smaller than a cb as well)
logic dictated that the 750 carb had to work perfectly on the cb350
and it does
i have fiddled with needle positions and mainjet but basically it runs perfect with no changes from the 750jetting specs
interestingly the 24mm sl carb and the 28mm sohc 750 carb y use same 120 main
and same pilot jet size
same slide cutaway number same float level etc
i know an accel pump can be a desirable feature but i feel 350 with the 2.2 lbs shaved offv the crank doesnt need a pump
its not like you are going to try to whack the throttle ope at 4k rpm anyway
here is a short shit vid of the beast
but nether is a hard whack to wot it really comes alive and responds even better
Last edited by XB33BSA; 07-24-2019 at 01:42 PM.
The Conventional carbs are a easier option to tune, with the CV carb the way they work you have a lot to take into account when tuning them.
The stock air boxes have the volume and filtration along with exhaust back pressure all worked out to meet emissions and run at or very close to optimal when they leave the factory.
CVs have real potential to play up as soon as you disturb anything, I imagine if you have a drawer full of springs needles main jets and pilot jets you might hit gold with oipen pipes and pod filters.
But sticking with the stock airbox or a home made box with similar characteristics will save a lot of time and frustration in the majority of cases.