Carb Mods

Well I started to clean my carbs and after a few talks with Chacal we came to the conclusion that my carbs came from a non xj bike. They are Mikuni 32s. My bike is a 85 XJ700n (air head). (Supposed to have Hitachi 33s)
I would like to remove the 4 crappy CV carbs and put 2 Flat Slide carbs on. 1 carb for pistons 1&2 and 1 carb for pistons 3&4.
So what size carbs would be suitable to run 2 pistons?
And what problems could arise?
I tought about making manifolds out of copper pipe to fit the rubber manifold boots and then somehow attach a carb. All thoughts and wisedom would be appreciated. 8)

 

Is getting the hitachi 33's out of the question?

 

If you go with a 2 carb setup I would think that you will loose the throttle response you get with 4 carbs. My thought is that by going with one larger carb for 2 cylinders you will loose air velocity ie. the air flowing through the carb will slow down making your bike feel doggy at best. Also wouldn't you want to join 1 to 3 and 2 to 4 so that the vacum pulse on the carb is maximized to one large pull instead of two weaker ones? OK there's my 2 cents worth

James

 

You could make a header looking manifold that bolts to the engine and have one large side draft carb bolt to a plate for all 4. Might even work with a pod.

 

I'm not ruling out 4 33s, I would rather deal with 2 carbs instead. I thought 2 carbs would work better than 1. As far as which assignment series is concerned I don't know, does anyone else know which is best? (1&2-3&4) or (1&3-2&4)?

 

Interesting idea... Would it fit inside the frame? Would need to be a small manifold to avoid having to chop the frame. Definitely wouldn't work with the factory airbox.

Be interesting to see how you get on.

 

Well I think I can do it without cutting the frame. Airbox doesn't matter because I don't have one. :wink:

 

One question remains, what size carbs would i need to accomplish the 2 carb mod? 36s? 38s?

 

look up some 350 twin and see what they use

 

Makes since,thanks Polock. Got any ideas on which carb assignment to use? 1&2-3&4 or 1&3-2&4

 

I did a google shearch on 350 bikes to see what size carbs where used and found out that most used 28s and some dirt bikes with single cylinders use 33s. My thinking is that I'll have to use at least 2 36s if I go with a 2 carb mod.

 

would the carb size or jet sizes be more important?

 

I've heard of peeps running one 38 mikuni on a xs650 (2cyl.).

Both equally important would be my best guess. You'll need the airflow of the bigger carb to be able to pull the fuel (through a bigger jet)for 2 carbs.

 

My bad the single 38 mikuni was run on a xs triple
Thread here:
http://www.thechopperunderground.com/fo ... opic=44043

Something for the wizkids;
http://www.thechopperunderground.com/fo ... opic=46447

 

Thanks guys for the replies. I too thought that the carb size and jet size would matter but I think jet size importance decreases with higher carb size. Correct me if I'm wrong because this is uncharted territory for me. Thank you Pauluminous for that CFM chart. I didn't know what value to inter for the last two so I left them as is and the result was 69 CFM. So I thought 2 36s would work well. What do you think?

 

there's a trade off, big carbs make big HP but low and mid range stink because there isn't enough vacuum at the jets to reliably meter the fuel
thats why back in the days of four barrel car carbs they ran on two barrels them vacuum or linkage opened the secondary two barrels up
jet size is important on any size carb the air/fuel has to be metered right no matter how big the carb is
a smaller carb is going to have better throttle response and "driveability" at the expense of some top end HP
a bigger one will have the HP but the "driveability" will suffer
if others use 28's why would you think you need 36's?
check this guy out
http://www.wgcarbs.com/

 

Since the original setup had 4 33s, I thought I would have to go up in size to use 2 carbs since each carb would have to fuel 2 cylinders.

 

If I were you I would seriously investigate using 2 of your existing carbs and see if it's possible to get the air fuel mixture right with different jets.

Failing that, make your manifold in a way that makes it easy to up the size of your carbs

 

Well, I've thought long and hard about it too. since long ago. But you will loose velocity when you go to bigger carb throats. since the engine fires 1,3,4,2 #Two will breathe right before the intake valve for one opens and share the air charge (during valve overlap) somewhat, 3,4 the same. Don't go too big, you've only got 33mm valves I think. Accelerator pumps would probably be a huge asset so it dosn't bog (lose airflow velocity and lean out) when you open the larger throttles. I'm thinking dual 35 mm Keihin cr pumpers. They are full race carbs with no cold start circuits though. Remember the valve stems are obsticles too and you'll do better (more velocity and vacum) with a little less carb than a little too much in anything but an all out full throttle all the time application.

 

PS: I think copper tubing would make a very poor intake manifold, Gasoline and ethonal are too corrosive, there are places that can make custom one off aluminum castings for the right price$$$ but I suspect you can use aluminum tubing and/or mold your own out of carbon fiber if you feel so inspired. Just a thought.
The biggest parasite you are facing if all out preformance is what you are after is the shaft drive hub, it easily sucks 15-20 rear wheel horespower. Not including the weight. Sorry!

 

Not sure,
using the calculator HERE
for a 700cc leaving the Maximum Safe Engine RPM and Volumetric Efficiency (VE) Of Engine as is, shows 74 CFM @ 100%VE and 55.5 CFM @ 83%VE (as left before), pl.

So at 69cfm ur getting close to running it at 100%, I -THINK- a consumer bikes normally runs around a 75%VE so 83% is already pushing it. bye bye fuel economy plus what Pollock and Padre said.

I haven't figured out how the req. CFM translates into a carb size in mm :? . So how did you figure out taht the 69 CFM translates into 2 36s?

*EDIT* left while typing and most has been said I guess. I'm with the idea of trying to run 2 stock carbs first if that doesn't work out size up.

 

:? Are you sure? What about copper fuel lines? They're not that uncommon.
Plus copper would be the sh!t, worth making a new one every once and a while. :lol:

 

Haven't you ever noticed that copper turns green when it oxidises and the 10% ethonal blend fuel sold here in the states is an oxidiser, I don't think that can do any good to a cylinder. If you bend the copper tubing the inner radius of the bend will reduce the volume of the tubing too, causing a restriction at the bend. Prebent tubing would flow better and be easy to solder together. But back on copper tubing it used to be "the kind" moonshiners used to make home stills but modern ethonal manufacturers sue exclusively stainless steel to reduce toxins such as cianyde (which is a by product of copper manufacturing) in their stills and vats. Ethonal is grain alcohol, like vodka. Copper would work on a temporary basis but long term I'd never consider putting any copper parts in my fuel system. I've never seen it used by any manufacturer either except for floats which eventually corrode and leak and sink. Brass yes, copper no.

 

remember, those carbs only have to feed one cylinder at a time, people have run 1 carb on an xs11,

cycle x makes 4 into 2 into one kits: http://www.cyclexchange.net/Twin%20Carb ... %20Pg..htm

they say your engine needs to be fresh

 

Thanks! I've never seen them advertised even at distributors like sudco. I know that going bigger isn't necessarily going to be better and I think dual 35s on a 700 or 750xj with stock cams and redline would be plenty.

 

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Early beetles came with copper fuel lines and they can be had at an aircraft parts supplier: http://www.aircraftspruce.ca/catalog/me ... tubing.php.
Obvious early beetles aren't ment to run on a 10% ethanol based fuel.
Copper fuel line doesn't come highly recommended but it isn't uncommon either. Although a lot of what's said to be copper might be cunifer.

The sytem from cycle-x looks pretty cool 8O , 4 into 2 34mm Mikuni carbs on a 750cc. For a standard 4 into 4 they sell 30mm mikunis.

Any1 figured out how the required cfm translate into a mm carb size? :?

 

Thanks guys for all the help. Thanks Schooter for that link, its exactly what I was thinking and looking for. Funny as always Rick, but I have to let the mad man out of me sometimes. :lol: I also thought copper would be fine because of copper fuel lines but aluminum would be better.

 

Dosen't the ID measurement of the throat determine the mm/cfm of the carb?

 

Nice pic Pauluminous, I love those velocity stacks. :twisted:

 

Pic's straight from Schooter's link

You're saying a Throat ID of 33mm will give 33 CFM? that would be way too easy, lol

 

Like I said earlier in this thread, I don't know much about carbs.
(Mama says, stupid is as stupid does!) :lol:

 

Naw, 33mm 'millimeters' not inches 1 cf equals 12 inches to the third power 25.4cm equals 10 inches. this does not compute.
Edelbrock makes oval bore carbs, kinda like the old spreadbore (i.e. Quadrajet) idea except for motorcycles. from 30x32 to 36x42

 

Thanks Padre I'll check that out.