GS450 Intake modification

Couple of myths that need to be dispelled.

Longer inlets do NOT create higher port velocity unless they are correctly sized and tapered. Increase length equates to more drag and less flow.

Carbs are not as good at creating a fine mist as to a good (modern) EFI system with multiple small discharge holes.

EFI does not inject when the valve is open. Even sequential systems can rarely inject enough fuel at high RPMs when the valve is open. They just fire often enough to work and that includes firing into the port or onto the back of the valve when it's closed.

Twin carbs typically make more power than a single carb all other things being equal but the difference isn't always large on a mild machine. Remember Triumph TR6 and T120. Single carb TR6 wasn't as fast as the T120 on the street but not a big difference. In a race bike that situation is different but on a mild mannered bike, not an issue. That's on a 360 degree crank.

With a 360 degree cam the carb can be stock sized or one size larger. Changing from 2 CV carbs to a large VM slide carb would be a significant difference. CV crabs only flow about 85% of the same size slide carb.

On a 180 degree cranks things are less simple.

Well I'm sticking to the source I trust on the fuel atomization issue.

Zookrocker said:

Well I'm sticking to the source I trust on the fuel atomization issue.

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Fair enough, but just to impart a little additional information: There are different types of fuel injections. For single point fuel injection (the most common type on first gen EFI vehicles), you are correct. A carb would probably provide better atomization. For the newer types of EFI (multi point and sequential), the atomization will be better. The reason has little to do with the spray pattern (as many people think), but more to do with where the fuel is applied. It is usually injected right onto the back of the hot intake valve. The fuel atomizes almost instantly due to the heat involved.

Regardless of personal opinions on the matter, the industry has switched over to EFI for one main reason, and that reason is emissions. EFI causes fewer hydrocarbons to be expelled through the exhaust, and as many of you may know, hydrocarbons are the result of incomplete combustion. One of the reasons for incomplete combustion is poor vaporization/atomization of the fuel.

Sonreir said:

Regardless of personal opinions on the matter, the industry has switched over to EFI for one main reason, and that reason is emissions. EFI causes fewer hydrocarbons to be expelled through the exhaust, and as many of you may know, hydrocarbons are the result of incomplete combustion. One of the reasons for incomplete combustion is poor vaporization/atomization of the fuel.

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Bingo. Auto manufacturers haven't switched to EFI on a whim - it's more efficient.

Yes, I completely agree with that Matt, and I do see your point about the newer EFI systems ability, however you kind of supported my argument there. The injection system is not what atomizes the fuel so well, but instead the heat of the intake valve vaporizes it. It's all neither here nor there. But yes, from an efficiency/eemissions standpoint EFI is the hands-down winner.

Zookrocker said:

I knew I had read this, and. Yes, not looking to start a pissing match, but from whath I consider the horses mouth:

Cycle World, August, 1995, in Kevin Cameron's article "Virtual Reality", "Injected fuel is less well atomized than carbureted fuel because carbs bleed air into the fuel stream, breaking it up finely. To counteract this, injected fuel must be given extra time in which to break up and evaporate".

Like I say, I'm not looking to argue, but when I found my source I had to post it.

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That's only true when you have emulsion tube 'four stroke' carb
The VM as it comes from SUDCO et.al. has a 'Primary Choke' which has completely different characteristics and delivers 'blobs' of fuel that will need breaking up..
As for fuel injection, it depends on the injector nozzle and spray pattern.
For the last few years Honda have been lazer 'drilling' 12 hole injectors, normal automotive are real crade in comparison and shoot a stream of fuel
As for carb size, a VM32 will support around 45bhp,36m will be plenty for a GS450 and work more efficiently as you have a more constant airflow. I'm not sure you manifold will be very efficient, pulsed flow from each cylinder will probably be moving in ways you don't expect at various rpm which negates any fuel economy 'savings'

What was the reason for doing this if you have two decent carbs to begin with?

Floegstad said:

What was the reason for doing this if you have two decent carbs to begin with?

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he is looking for more power,i hope he finds it ;D
if he finds it with a single carb it will be a miracle ,and it would make him a total genius who should be an engineer on some kind of top tier race team
but who knows maybe he has some design that has not been tried over the last 100+ years ?

xb33bsa said:

he is looking for more power,i hope he finds it ;D
if he finds it with a single carb it will be a miracle ,and it would make him a total genius who should be an engineer on some kind of top tier race team
but who knows maybe he has some design that has not been tried over the last 100+ years ?

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LOL! ;D

If the objective is more power, then twin carbs will always be a better option.

To PJ's point about emulsion tube carbs, we had a pair of VM26 carbs on a CB160 race bike and with the standard 2 stroke Primary needle jets, the amount of unburned fuel in the exhaust was extremely high even when the Air: fuel ratio was spot on. In other words the fuel that was burned was in the right proportions to the air flow but large droplets were not being burned.

We found a pair of Bleed type needle jets and that allowed us to make more power with less unburned HCs.

Any suggestions on what i could use for a carb setup on my gs400 with open filters and 2-1 exhaust?

Floegstad said:

Any suggestions on what i could use for a carb setup on my gs400 with open filters and 2-1 exhaust?

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why not use a pair of slide carbs off an 2 valve gs750 or gs1000 ? it will take some
fab work but i always thought this would be the logical solution when replacing a CV carb lashup

Or go with some GS500 carbs - they are still CV carbs, but have flat slides (slingshot carbs).

Big Rich said:

Or go with some GS500 carbs - they are still CV carbs, but have flat slides (slingshot carbs).

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look into it when time COMES. Thanx again Rich

teazer said:

If the objective is more power, then twin carbs will always be a better option.

To PJ's point about emulsion tube carbs, we had a pair of VM26 carbs on a CB160 race bike and with the standard 2 stroke Primary needle jets, the amount of unburned fuel in the exhaust was extremely high even when the Air: fuel ratio was spot on. In other words the fuel that was burned was in the right proportions to the air flow but large droplets were not being burned.

We found a pair of Bleed type needle jets and that allowed us to make more power with less unburned HCs.

Click to expand...

I'm making a high speed drill press to drill the cross holes and will be experimenting with a new emulsion tube to use VM30 on CB350/360
Eventually I'm going to do an 'Instructable' building on the work others have done for DIY PCB drill press (http://www.instructables.com/id/Cheap-precision-drill-stand/)
I'm using aluminium box section instead of wood and a lead counterweight instead of spring (drilling about 7mm dia brass is slightly more difficult than 1.5mm fiber glass ;D )
Couple of years ago I made a pair of emulsion tubes for the VM32's on XS 650 that worked real well but then burned out speed controller for mini mill so couldn't make any more.
Shipped all the parts off for repair, guy had a heart attack just after fixing it and his wife trashed everything (he didn't survive )
probably blamed all the solder fumes for years of repairing stuff?

Thanks for all the input guys, I have not had a lot of free time over the past few weeks with the holidays and day job "Engineer- not a race team engineer,for a high paid race team which is a huge bummer". I have a coworker who builds high performance two stroke race snowmobiles up here in MN for ice drags and flat track. He has machined a few cool engine designs for high end racing teams and has been giving me a bit of insight into my project. He is helping me track down the VM36. The design has changed slightly into a dual diverging nozzle throat thing. lol I don't have a technical name for it yet. Ill just go with DN Intake. Updates to come. The goal is create something different, more power would be great, but equal performance is the only base-line.

good luck !! you would actually be better off with a purpose built 4 stroke carb with a squrter(accel pump) in my opinion

Yep, get a 38mm CV off an older 883 Sportster, plenty of parts available

So i know this is a little old, but i was wondering about the GS450 CV carb swap to the GS500 CV flat slides. What year GS500 would be the best for that kind of swap? I see a lot of 04-06 models on ebay. I planned on upgrading to a brand new set of Mikuni's but if the GS500's flat slide set work as good or better, i would rather get those; As i see those everywhere and they are not that expensive.

Here's what I can tell you:

First off, stay away from the 01-02 carbs. Replacement parts for those years can be tricky to find. But as I'm sure you remember, I have the early style 500 carbs on my 450. With any GS500 carbs, you can't use the stock airbox (I don't think that's a problem, but worth mentioning .....). Luckily though, there are jetting charts for them that will give you an idea where you should be. The intake boots are fairly interchangeable, but you'll want the boots to match the generation of carbs. If you use first generation GS5 carbs, you will also need the choke cable, left hand control, throttle cable, and right hand control from the same generation bike. And the same goes for the 04+ carbs, because the choke & throttle changed with the carbs.

Now the problems I ran into. The idle adjustment screw on the first generation carbs is on the bottom, and it is VERY close to the 450's cam chain tensioner. So be sure to wear gloves when adjusting the idle or you'll burn your fingers. I can't speak for the physical size of the later GS500 carbs since I don't have any. If the idle adjustment screw clears the CCT, you should be good.

As far as performance goes, the later carbs are said to perform better than earlier carbs, since they added a mid-main jet. I know they have added emissions hookups, but they can be disconnected with no issues.

Did I miss anything?