[jwhombel]

Currently rebuilding a 1972 CB350 K. Just inherited a pair of Keihin CV carbs that ran on a 74 CB450. I am installing aftermarket MAC mufflers and new K&N air filter and would like any comments on whether or not installing the new carbs would be worth the effort.

 

[LiLBull]

worth the effort ....listen any engine is a pump the more air and fuel you can get into it to fill the cumbustion chamber(requirments and this is the key the engine designed requirments) the better ,then you would need to get that larger volume of air and fuel out ,hence headers.So its not a matter of throwing on another carb.

You have to think in terms of engine requirements ,Ussually a change of carbs is needed if you change the pistons ,head,or cam shaft that will require more air fuel mixture.

ussually jetting (more fuel) or boring throats and bigger butter flys (more air)is done to acheive this with out a carb change.

So do you need it is it a replacement because you cant find the stock carbs .Stock (original equipment) is the best for smooth dependable non quirky dailey driving. If you put headers a jet kit is ussually recommended to increase fuel but ussually it will be fine without the jet kit just most people selling headers sell jet kits ...so.. you know.

Remember if you have ever pulled up beside a biker with it poping and farting and running ruff its ussually because his timing is off/points closing up or he has a bad combination of parts, that add up to a NOT FUN to Ride Bike.

believe me when I say that bike engines are very efficient from the manufacturer Period! They are performance built engines. a good fresh set of points and tune up adjusting your timing spot on and point gapped properly.Will make you think you have a new carb upgrade.

One other point is most people dont understand carbs at all,and blame the carbs first sign of trouble and start tinkering with them and finally they have to walk there bike to the corner garage for help.

Number one with vintage bikes is check your POINTS and TIMING always FIRST . Once the carbs are set very little can go wrong they DO NOT GO OUT OF TUNE (unless some one has tinkered with them or some thing has vibrated loose but this is the reason you will see little blobs of yellow or red or blue paint on carb adjustment screws it locks them into place ...when you see the paint is broken you know some one was in there tinkering ..!

Think of carb swap like this always ENGINE REQUIRMENT or NEEDS for the amount of air you engine needs .Do you go to a resturant and order the intire menu, no your body only requires one meal on the menu ....now think if you ordered and eat every thing on the menu your body didn't and doesn't require that much intake of food ..so now your slow,stuffed..you cant move, your feeling Ill...why because you over stuffed yourself ...so dont over carb your efficient bike either.

So look at the engine they came off of the internal specs and look at the internal specs of the engine your fitting .CC chamber shape ,compression ratio,cam and degrees of cam advance or retard all these things and more should be looked at to make a wise decission,look at the carbs side by side look down the throats meassure the differents look at the jets and their sizes ...You see all this makes the difference of some one that builds stuff and some one that bolts on stuff you have to really get into things to answer such a question its easy for someone that went thru all the above to tell you yes or no but if you want to learn you should find out for yourself Grasshopper and welcome to the Temple of Mechanics.

Alot of the guys here are like me we have learned thru the knuckle busting,fustration of trial and error and spent billions of time figuring this stuff out and I dont think any of us would trade or journey ....so they may fit, but Id go with the very effective stock carb,hey I dont know they may work, I havent looked into it.





Edited by - LiLBull on Oct 29 2005 08:16:26 AM

 

[kenessex]

Excellent advice that Lilbull has given. So the answer to your question is Maybe yes or maybe no.
Here is the story of my racebike 350. I have a mostly stock engine with the exception of a mildly ported head and a Type 1 cam (which is a little higher lift than the later years), and a 2-1 pipe. On the racetrack It will run out of breathing at the end of a longish straight. But, with stock carbs with no choke plates it pulls nicely and has great throttle response off the corners.I am going to fit bigger carbs for next season because I need more top end and am willing to sacrifice some bottom to get it. For a street bike, I would run with stock carbs. Unless I just wanted to do something different or trick, in which case I would go with some Mikunis or a couple off of a CB750 or something else that looked cool.
Plus you may have some difficulty getting the 450 carbs to even fit without messing around with cables and choke linkages and such.
Ken

AHRMA 412
Vintage racing - old guys on old bikes

 

[Geeto67]

the cb750 carbs when broken down individually are actually smaller than the cb350 twin carbs. They are also not constant velocity carbs. The cb750 811cc big bore kit pistons are actually the same size as std 350 pistons and you can use 350 blanks to make new 811cc pistons by relocating the wrist pin location.

As for the cb350 the biggest mod you can do to get the most out of these bikes is to make sure the carb diaphram is in good shape. If you get a hole or they become brittle you will get uneven carberation. The onyl way to fidn out if it is worth the effore to install larger carbs is to install them and see if your lap times or ET improve. it is very easy to "over carbureate" a cb350, unless you have some head work it may not do you any good. Instead of a bigger carb you may want to try a more efficient carb like a vm series mukuni roundslide or a mukuni flatslide.