What size carbs do I need? (odd setup)

R

racing geek

Just a kid with a welder
I put one of the later air-cooled 16 valve GS750 engines in my CL360 but the stock 32mm carbs don't fit. I've been told that 34mm carbs are a good upgrade but those also wouldn't fit. I plan to make some 2-1 intake manifolds and run two carbs to feed the four cylinders. I'm not sure what size carbs I would need though.

I did some quick math a couple nights ago based on the 9,000rpm redline and found that a pair of 38mm carbs would be ideal. However I was told by an engine builder on another forum that 36mm carbs would be the way to go since I mentioned it would be a daily rider. Any GS or carb experts have some input?
 
maybe you could model what you're thinking of after cyclex's kit for the DOHC CB750? This sounds like an intriguing project btw.
 
I have seen their kits and mine will end up looking sort of like that but I don't think the 34mm carbs they use would apply to me. IIRC the cb750 uses 29mm carbs stock instead of 32mm like my gs750 motor. Based on that I would think I will need at least a pair of 36mm carbs. Anyone have some detailed pics of those cb750 intKe manifolds. That would help me out too. All I can find are pics of the outside. I'm interested in what the inside shape is like.
 
I know you want to get it right the first time, but there is no data like Empirical data. The very best way to answer your question is to build it, tune it, and test it. Pick a carb size, and do it. If you find you started with carbs that were too big and get poor low-end performance, you can always put them up for sale on eBay and try again.
 
Hold on a moment. Let's talk carb sizes and types.

CB750 used 28 (notional) slide carbs. Your GS and later DOHC CB750 use 32mm CV style carbs which flow about the same as a 28mm slide carb.

In theory, you could use one 28mm slide carb per pair because there is not a lot of overlap between the two cylinders i.e. they are not both sucking at the same time. That would be a little restrictive for several reasons. Assuming that you are planning on using a slide carb, 32 to 34 should be more than large enough.

More to the point though, GSX750 into a 360 honda frame....... That sounds a little unsafe at any speed (to crib from Ralph Nader). Suzuki double cradle frame handles well. Honda 360, not so much. :-(

All joking aside, I'd rethink that plan.
 
I want pictures ;D
I've got a spare frame for 360, AND, I got GSX600F motor (with a 750 top end ::) )
It sounds totally stupid, I want one ;)
 
teazer said:
Hold on a moment. Let's talk carb sizes and types.

CB750 used 28 (notional) slide carbs. Your GS and later DOHC CB750 use 32mm CV style carbs which flow about the same as a 28mm slide carb.

In theory, you could use one 28mm slide carb per pair because there is not a lot of overlap between the two cylinders i.e. they are not both sucking at the same time. That would be a little restrictive for several reasons. Assuming that you are planning on using a slide carb, 32 to 34 should be more than large enough.

More to the point though, GSX750 into a 360 honda frame....... That sounds a little unsafe at any speed (to crib from Ralph Nader). Suzuki double cradle frame handles well. Honda 360, not so much. :-(

All joking aside, I'd rethink that plan.
Click to expand...

I suppose I could try the 34mm CV carbs from my TX650 project since that will need larger carbs anyway.
No thanks. I'll be ok. ;)

crazypj said:
I want pictures ;D
I've got a spare frame for 360, AND, I got GSX600F motor (with a 750 top end ::) )
It sounds totally stupid, I want one ;)
Click to expand...

The engine is about 3/8" off center in the pictures. I haven't moved it over that last little bit yet. I'm 6'3" and when I sit a little behind the upper shock mounts my knees are 1.5" away from the valve cover.

The frame is being braced and it will be plenty strong so those of you that feel the need to tell me it will be a pretzel or I'll kill myself, save your time.

P2040857.jpg

P2030849.jpg

P2030842.jpg

P2030839.jpg

P2030841.jpg

P2030852.jpg
 
That's completely nuckingfuts, I love it.

Your madder than me ;D 8)
 
crazypj said:
That's completely nuckingfuts, I love it.

Your madder than me ;D 8)
Click to expand...

Thanks for the compliment. ;D

I haven't been updating my build thread on here for a while but this is what the CL360 looked like before the engine swap.

PB030641.jpg
 
Seriously dude, you are nuts.. :). I'm not a big fan of 360's but that looks good and has some interesting mods. FZR(swigner) and what okk likes a Ducati/Brembo disk and caliper set up squeezed between non OEM fork legs. Is that na FZR600 front end by any chance?

BTW those clip ons are upside down but I'm guessing you already know that.
 
teazer said:
Seriously dude, you are nuts.. :). I'm not a big fan of 360's but that looks good and has some interesting mods. FZR(swigner) and what okk likes a Ducati/Brembo disk and caliper set up squeezed between non OEM fork legs. Is that na FZR600 front end by any chance?

BTW those clip ons are upside down but I'm guessing you already know that.
Click to expand...

A quick run down of some of the parts I've swapped onto the bike:
FZR swingarm
XS650 front hub (I'll be lacing it to a 18x2.5 wheel)
Ducati 916 front brake (320mm rotor and 4 piston caliper)
modified CBR600 F2 front forks and triples
15" eye-to-eye shocks
VTR1000 clipons (upside down like you said)


Here are the specs of the bike:
54.75" wheel base (center of adjustment)
25* rake
4.5" trail
4.25" front suspension travel
3" rear suspension travel
7.5" ground clearance
31.5" seat height
350# estimated wet weight (I was at 300# wet with the stock motor)
~80hp
 
That is one mean operation your building there. I will be very intrigued to see what frame strengthening methods you employ. Keep on trailblazing!
 
I just measured a GS500 swing arm pivot, looks like same length as CB360 plus a couple of inches longer in the 'arm' section ;D
Can't get even close to spare SACS motor and measuring one in Katana isn't very easy with fairings on, but, it looks like it will be 'reasonably easy' ;)
SACS motor is a bit smaller than the TSCC
Another project (eventually ::) )
I should hate you for posting those pics ;D ;D ;D ;D ;D
 
Geek,
Good that you used those forks with so much less offset. That brings trail back to where it needs to be with steep rake angle.

PJ,
We can hear you getting all excited. Is that good for your blood pressure? LOL . :)
 
What do you guys think about using a pair of VM34's on the bike instead of the stock 34 CV carbs from the TX650? I think I found a good deal on the VM's.
 
I think a pair of 34's may be a bit small, you could probably get some 36~38mm CV carbs
Harley use Kei-Hin or Mikuni, they have accelerator pumps and would probably be close to correct jetting.
Lot of people do 'performance' upgrades so carbs are quite often available at reasonable prices with very little 'mileage'
Check eBay
 
I hope you do no more than take this to the local coffee shop,,,,,,,,,,,at posted speeds.These bikes fitted with GOOD tyres will flex more than your local yoga instructor...be carefull
 
Wee Todd said:
I hope you do no more than take this to the local coffee shop,,,,,,,,,,,at posted speeds.These bikes fitted with GOOD tyres will flex more than your local yoga instructor...be carefull
Click to expand...

I plan to ride this thing all over the place. I have several rides planned for this bike ranging from ~900 miles up to ~5,000 miles and I'm sure I'll have it over 140mph on more then one occasion. The frame won't flex with how I'm building the bikes. ;)
 
Sudco carbs come set up with "2 Stroke" needle jets and air jets too.

36 is a little large, but in a VM series, if 34 works on a CB750 it should probably be Ok on yours.
 
OK, I'm plotting ;D
I have most of the parts for a rolling chassis, got a Ti exhaust off a GSX-R 1000 I picked up cheap a few years ago (couple of scratches, guy said it wasn't worth putting on eBay as no-one wanted stock exhausts 8) )
Got the blue GSX-R forks, pair of early 3rd gen Katana wheels, 3" & 4" x 17")
I still think a pair of HD CV carbs would be the best way to go
Running one 36~38 on a pair of cylinders will make them work more efficiently than individual carbs.
Suzuki used 4 x 38mm CV on one of the GSX-R's so a pair on lower tuned motor should work real well.
The HD carbs are set up for a 4 stroke motor and will probably be real easy to tune.
The other advantage, they will be far cheaper than buying 'new' VM's then swapping out all the brass
 
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