Need help with custom intake!

[Racing Geek]

I've got a 1974 Honda CL360 that I swapped a GS750 engine into. The GS engine is the air cooled 16 valve version. The stock carbs won't fit the CL360 frame so I plan to make some 2-1 intake manifolds like the CB750 guys can use. My issue is with choosing the size of carb. The stock carbs are 32mm and I've been told 34mm is a good upgrade for these motors. However, since I'm using a single carb for two cylinders I would assume I need a larger carb. From some quick math it seems a pair of 38mm carbs would be ideal and I was recommended a pair of 36mm carbs by an engine builder. Since my math was based on the 9,000rpm redline it would make sense that he would recommend 36mm when I told him it will be a daily rider. What do the GS experts think I should do? Has anyone even done this to the GS engines?



To show I'm serious about doing this I present you with my CL750!

 

[Dave8338]

The volumetric max of the 38mm carbs may be better for a drag bike situation as your intent would be to run the thing pinned. As you stated, you will be using this as a daily rider and the 9000RPM redline will be rarely if ever hit unless you miss a gear. I would go with the smaller carbs.

Many tend to over carb engines and this leads to a poor air/fuel mix as the velocity of the air going through the carb continues to slow down as carb size goes up and displacement remains fixed. Even with the 36mm carbs on it, you will have to rejet (you knew this already), as the volume of the system will change as your intake runners are longer to sneak around the frame. This can also cause a "slight" issue as the intake tubes for the # 2-3 cylinders will be shorter than #'s 1 and 4. Exhaust will also play a large part. One thing to keep in mind... only one piston will be firing at a time on each carb, therefore, as for the need of a larger carb, I would be trying the BS32's first to see if you really NEED to go larger. The only reason there were four of them to begin with, was that it does provide better top end (hp) but usually at the cost of the low end (torque) and that Suzuki could package all four in a lesser amount of space required, that an intake/2-carb system.

Hope this helps...

 

[AJ]

Ok, the 360 engine was (IMO) step down from it's 350 ancestor, but I still have to ask WHY?

 

[koolaid_kid]

Ok, the 360 engine was (IMO) step down from it's 350 ancestor, but I still have to ask WHY?

And the 350 was a step down from its 305 ancestor. (Just stirring the pot.)
I find this a fascinating project, but I wonder if the frame can take the added torque. Some extra bracing may be in order.
At any rate, as previously stated the smaller diameter carbs will allow more low end power, and adding more of them will allow more top end power. What about the siamesed carbs I have seen on some mid-sized engines?

 

[BassCliff]

Greetings and Salutations!!

Greetings and Salutations!!

Hi Mr. Racing Geek,

I can't really help you with your modifications but I can inundate you with basic maintenance stuff. Let me dump a TON of information on you and share some GS lovin'.

I just stopped by to welcome you to the forum in my own, special way.

If there's anything you'd like to know about the Suzuki GS model bikes, and most others actually, you've come to the right place. There's a lot of knowledge and experience here in the community. Come on in and let me say "HOoooowwwDY!"....

Here is your very own magical, mystical, mythical, mind-expanding "mega-welcome". Please take notice of the "Top 10 Common Issues", "Top 15 Tips For GS Happiness", the Carb Cleanup Series, and the Stator Papers. All of these tasks must be addressed in order to have a safe, reliable machine. This is what NOT to do: Top 10 Newbie Mistakes. Now let me roll out the welcome mat for you...

Please click here for your mega-welcome, chock full of tips, suggestions, links to vendors, and other information. Then feel free to visit my little BikeCliff website where I've been collecting the wisdom of this generous community. Don't forget, we like pictures! Not you, your bike!

Thanks for joining us. Keep us informed.

Thank you for your indulgence,

BassCliff

 

[AJ]

And the 350 was a step down from its 305 ancestor. (Just stirring the pot.)
I find this a fascinating project, but I wonder if the frame can take the added torque. Some extra bracing may be in order.

Well, not really (305 vs. 350). The 350 was a marvel of its day. The 360 ushered in some improvements and manufacturing simplifications, but never achieved the overall reliability until evolving into the 400 series.

But your joke was well taken and you made the same point that I was trying to make, that the 360 frame and fork really isn't up to the task of the bigger motor.

 

[gearhead13]

IMO that frame will twist like a pretzel

 

[Buffalo Bill]

Not quite a bolt in?

Not quite a bolt in?

IMO that frame will twist like a pretzel

Sums it up for me.

Kind of a DIY crackpot idea to put a way too powerful engine in there.

Don't anybody have any respect for all the testing and work hours the factory engineers put into a bike?

Take it for granted they already been there, and it didn't work.

 

[Racing Geek]

The volumetric max of the 38mm carbs may be better for a drag bike situation as your intent would be to run the thing pinned. As you stated, you will be using this as a daily rider and the 9000RPM redline will be rarely if ever hit unless you miss a gear. I would go with the smaller carbs.

Many tend to over carb engines and this leads to a poor air/fuel mix as the velocity of the air going through the carb continues to slow down as carb size goes up and displacement remains fixed. Even with the 36mm carbs on it, you will have to rejet (you knew this already), as the volume of the system will change as your intake runners are longer to sneak around the frame. This can also cause a "slight" issue as the intake tubes for the # 2-3 cylinders will be shorter than #'s 1 and 4. Exhaust will also play a large part. One thing to keep in mind... only one piston will be firing at a time on each carb, therefore, as for the need of a larger carb, I would be trying the BS32's first to see if you really NEED to go larger. The only reason there were four of them to begin with, was that it does provide better top end (hp) but usually at the cost of the low end (torque) and that Suzuki could package all four in a lesser amount of space required, that an intake/2-carb system.

Hope this helps...

All the intake tubes will be near identical in length with how I plan to make them.
Exhaust is taken care of thanks to a Kerker 4-1 header and Kerker muffler.
I have 34mm CV carbs that I could pull off my TX650 project for testing purposes. I'll need some 36mm carbs for that bike anyway.

Ok, the 360 engine was (IMO) step down from it's 350 ancestor, but I still have to ask WHY?

I'll counter your question with another question... why not?

And the 350 was a step down from its 305 ancestor. (Just stirring the pot.)
I find this a fascinating project, but I wonder if the frame can take the added torque. Some extra bracing may be in order.
At any rate, as previously stated the smaller diameter carbs will allow more low end power, and adding more of them will allow more top end power. What about the siamesed carbs I have seen on some mid-sized engines?

The frame will be fine. I have extra bracing mocked up.
Not sure what you mean by "Siamesed carbs." Are you talking about taking the carbs from a bike such as a TX650 where they are linked together?

Well, not really (305 vs. 350). The 350 was a marvel of its day. The 360 ushered in some improvements and manufacturing simplifications, but never achieved the overall reliability until evolving into the 400 series.

But your joke was well taken and you made the same point that I was trying to make, that the 360 frame and fork really isn't up to the task of the bigger motor.

Like I wrote above, the frame will be fine.
I'm using modified forks from a CBR600 F2. The swingarm is from an FZR.

IMO that frame will twist like a pretzel

Thanks for the welcome BassCliff.


This is what the CL360 looked like before I tore it all down for this motor swap.

 

[Racing Geek]

Sums it up for me.

Kind of a DIY crackpot idea to put a way too powerful engine in there.

Don't anybody have any respect for all the testing and work hours the factory engineers put into a bike?

Take it for granted they already been there, and it didn't work.

Think about it this way.... The factory engineers kept trying all kinds of crazy things and only a few ever made it to production. I'll use the NSR500 as an example. The thing had OVAL pistons. I'm sure many people were thinking there was something wrong with the guys that came up with that but lone behold it kicked butt. Sure it wasn't a very practical design but I'm sure the engineers learned lots from that bike.

Think of my bike as a design exercise much like the NSR was to Honda. It's not like any of the bikes I'm using parts from are some super rare 1 of 100 or something. Who's to say it didn't work? Maybe they tried it and found that the 4 carbs was an overall better design so that's what they went with for the production bike. Keep an open mind.

 

[AJ]

This is what the CL360 looked like before I tore it all down for this motor swap.

That actually looks really nice. I've seen a lot of "conversions" in these old Honda twins, but not too many with a wicked swingarm like that.

I have no doubt you can brace the frame. I have no doubt of your overall skills to pull this project off. And I certainly meant no disrespect. It just seems to me that it would be easier to fit the Honda tank and bodywork to your Suzuki frame than to adapt the frame to to that engine. Don't forget to upgrade triple trees!

Those high pipes could be valuable. Hard to find.

Good luck with your project.

 

[Guest]

I'd go with the 34's. Mine has a set of 34mm flatslide cv's from an '87 gsxr 1100 on it with a yosh pipe, k/n pods, mildly ported head with an intake cam from an '85 gs700. I'm sure that 36's would be too much for mine.

 

[Guest]

I went from a 305 to a 360 as a kid in the 70's. Neither one was anything to write home about. As for this project, i say go for it and keep us posted. I may do somthing simular after i get the gsxr motor i'm building into my 750...

 

[OLDENUFF]

You are using 1 carb for two cylinders, that is true but you are only feeding one cylinder at a time. Go with the smaller carbs.

 

[Dave8338]

Don't let the questions and doubts slow you down. I for one, keep the "rare" toys factory and the "not so rare" toys, modded to the extent my budget and wife, will allow.

Anything can be done. The impossible...just takes a bit more money.

 

[Racing Geek]

That actually looks really nice. I've seen a lot of "conversions" in these old Honda twins, but not too many with a wicked swingarm like that.

I have no doubt you can brace the frame. I have no doubt of your overall skills to pull this project off. And I certainly meant no disrespect. It just seems to me that it would be easier to fit the Honda tank and bodywork to your Suzuki frame than to adapt the frame to to that engine. Don't forget to upgrade triple trees!

Those high pipes could be valuable. Hard to find.

Good luck with your project.

I actually don't have the rest of the Suzuki bike. I got a deal on the engine, exhaust, carbs, and wiring so I got it to try the swap.
The entire front end is CBR600 F2. The F2 uses the exact same bearings as the CL360 too so it was literally a bolt on upgrade. The clipons are from a VTR1000.

I've actually got two sets of the high pipes with heat shields. I'm trying to sell one to fund the swap and I'm going to try selling the other with the complete CL360 engine (engine, carbs, exhaust, wiring) when I get the GS engine running.


This is the geometry of the bike and some of the specs...

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?




As for keeping you guys updated... I have a build thread on customfighters.com that I have been updating periodically. I also started on dotheton.com but kept forgetting about it since I don't go on there often. I'll probably be around here more since I'll probably be reading a bunch about the GS engines but I don't know if I want to keep two build threads going at once.

 

[koolaid_kid]

Lets face it, the Honda twins were nothing to write home about, performance-wise. But they were relatively reliable, considering.
Those pipes like pretty standard CL pipes, but nice.
I really like the swingarm, looks tough and durable.
The siamesed carbs I spoke of were two carbs joined together, perhaps they were more compact? IIRC they came on certain 550 Suzis, but I am sure other Japanese bikes had them as well.
AFA going with 2 instead of 4 carbs, it would be similar to the old carbureted 4s, 6s and V8s before fuel injection took over the automotive world. We are so immersed in the "one carb, one cylinder" thought pattern that we forget the 50s and 60s in-line 6s had a one barrel carburetor, possibly a two barrel. Even the V8s had a two barrel unless you bought the performance version.
You need to work backwards to calculate what you need if you go with two instead of four. Since the engine is basically an air pump, how much air do you expect to pump at full RPM? You know the volume of the engine (~750cc), you know the RPM you expect to reach, you can calculate the flow. Work backward to see if the carbs selected can match that. Then you can tune the lower ranges, knowing that you will lose a bit of low end grunt by using the dual rather than the quad approach.

 

[BassCliff]

"Siamese carbs"

"Siamese carbs"

Hi,

Not sure what you mean by "Siamesed carbs."

For a visual, see the parts fiche picture for an '84 GS550. A "siamese carb" has two throats and one slide.

Thank you for your indulgence,

BassCliff

 

[AJ]

My '69 CB350 could pop wheelies in 3rd gear, but top speed was not too great. Those motors could take a lot of compression without throwing bottom ends. Really durable little beasties for their day. Needle bearing cam caps and megacycle cams. And nowadays the vintage racers have added all sorts of upgrades.

Thinking about your carb challenge.... look at the intake manifold to a Harley Sportster 883. You might be able to use two of those and some plumbing to make a nice, smooth 1 carb into 2 jugs setup. The older ones were rubber (Gen1 Evo), not sure about the newer ones. Then all you need is a pipe with a small bend to go from the manifold to each of the intake rubbers on the Zuk motor.

 

[Racing Geek]

I do have an S&S Harley manifold to try but just by looking at it the angle will be way too wide. It will end up being really short (carb will stay close to the motor) which is good but the bends are going to be really sharp and hurt flow. I'm planning to use 3/16" plate for the flanges that bolt to the head and some 1.25" exhaust tube to make the intake runners. I'll probably end up rolling some sheet metal to match the OD of the carbs head side and then using a hose to attach the carb to the manifold. Since that hose will be flexible I plan to have a bracket come down from the frame help support the carbs.