1979 GS425 Building more power

[bwanna]

... chop 20mm off the slide springs, and take the vacuum port on the slides out to about 3mm which has made the throttle a lot more responsive.

Pete, Do you mind expanding on what you mean here? I understand the slide spring that you cut 20mm off, but the vacuum port... can you describe that more?

 

[Guest]

Firstly, I'm no carb guru, so don't fully get what it does... I just know it works

If you look at the bottom of the slide, in addition to the hole for the needle, there is another hole.

I believe this is where the air coming in is caught to cause the slide to lift.

I enlarged this hole to about 3mm which causes the slide to lift quicker, which in addition to the shorter slide springs makes the carbs quite a bit more responsive.

I'm sure someone who knows what they're talking about will be able to explain the whole situation a lot better than I can, but that's what I did and I like the results

 

[dude99]

So since I have to rejet anyways for our future pods and pipes I'm looking into carb options. I'd like to put VM carbs in it, both for simplicity as well as performance.

The stock carbs are 34mm CV and from what I've read, if you want to fit VM carbs you go 6mm smaller. This would mean I'd be fitting 28mm carbs. Does this sound right?

I also assume I'd need to make some sort of adapter for them to fit the stock intake manifold rubbers? Is there a favorite set of VM carbs off of a different bike that would fit or is it a matter of just finding a set and making them work?

 

[dude99]

These seem like a good deal.. thoughts? My only concern would be the choke knob being obstructed by the carb next to it, but it looks like it should work.

http://www.ebay.ca/itm/2pcs-MikuniV...Parts_Accessories&hash=item4ac0b81c02&vxp=mtr

 

[Big Rich]

Those are VM24's, which means the slide is 24mm. Just curious, where did you hear about they should be 6mm smaller?

I think even VM28's might be on the small side. But that's your call. Mikuni makes their VM carbs in left and right sets (the idle screw is on the outside of each), and they sell different kinds of choke plungers for whatever you might need.

And I'd be really surprised if VM28 or VM30 carbs didn't fit perfectly in your intake boots.

 

[dude99]

I read the 6mm rule over on a honda forum, I had heard before that you want to go smaller with a VM carb, I just didn't know how much. So the number refers to the diameter of the narrowest part of the carb, ie) the slide?

if vm28 or vm30 might fit my boots then I'll look for those.

 

[Big Rich]

Correct - the slide diameter. If you search around online you should be able to find dimensions for the VM mounting diameter.

But here's what I can tell you for certain: the GR650's carbs have the same mount as VM34's. And the GS450 is the same as a GR650.......and GS500 carbs (all years) bolt right onto a GS450 head. Make sense?

 

[Big Rich]

Correct - the slide diameter. If you search around online you should be able to find dimensions for the VM mounting diameter.

But here's what I can tell you for certain: the GR650's carbs have the same mount as VM34's. And the GS450 is the same as a GR650.......and GS500 carbs (all years) bolt right onto a GS450 head. Make sense?



Edit: I have no idea why it posted twice because of a smiley face......

 

[dude99]

Ok, so lets say that for arguments sake that the 425 has the same diameter as the 450, which it more than likely does, a vm34 would be bolt on? Wouldn't that be too big?

I don't suppose there is a thread where someone has actually done this?

 

[Big Rich]

Oh, a VM34 would be way too big. I was pointing out that all those carbs have the same mounting diameter. Look here:

http://www.mikunioz.com/dimensions1.htm

Are the carbs on a gs425 the same as a GS450?

 

[Agemax]

Those are VM24's, which means the slide is 24mm.

Actually, you are wrong. the size of the carb is determined by the ID of the bore where it enters the engine, NOT the slide diameter.

A VM26 and the VM28 has a slide diameter of 31.45mm

 

[dude99]

Thanks for the clarification.

 

[Big Rich]

Well I'll be darned...... all the years I've had them and I never measured that. Thanks Agemax.

Dude99, I think the easiest (and relatively cheapest) carb swap for you would be GS500 carbs. There is all kinds of jetting info for intake and exhaust changes on the GSTwins site for them, and they probably just bolt right onto your head.

 

[dude99]

What years did the GS500 have slide carbs? or did they all?

 

[Big Rich]

They all have CV carbs with flat slides. The simplest ones were used from 1989-2000. The carbs from 01-02 are kinda odd and hard to find parts for IIRC, and from 04+ they have more jets and emissions equipment mounted.

 

[dude99]

thanks for the info... I think I'd still like to fit slide carbs though.

 

[dude99]

I feel I should clarify my last comment. Since we'll be running pods on this build I don't want to run CV carbs as they don't take particularly well to pods.

 

[Big Rich]

All good man. But K&N makes an air filter specifically for the GS500 carbs. It's called a lunchbox filter and mounts to both carbs at once, and is a popular filter for folks that have a GS500. Just food for thought....

 

[dude99]

Interesting, and good to know... I'll have to mull this over some more with my buddy who I'm building the bike with.... Having googled gs500 carbs just now I have to say that I prefer the look of slide carbs, less bulky and overall cleaner looking.

Just out of curiosity, would slide carbs give us a snappier throttle response?

 

[Big Rich]

That I can't answer... it's been a while since I've ridden mechanical slide carbs. And it's been even longer since I've ridden stock CV carbs on any bike.