Pods + CV carbs - new thread

[atchbo]

Hi folks,

I'm starting a new thread to keep from hijacking.

I'm about to order some pods for my 1977 GS400 (BS34 carb, which I now hear is a CV type). Most of the time, the GS400 doesn't show up on lists (such as BassCliff's main list of manuals) but I did find it in the carb spreadsheet. I have picked up bigger pilot jets (25, or one size up) and one pair of main jets that are two sizes over (117.5).

I'm thinking of using a colortune clear spark plug to figure out the fuel ratio, but I'm picking up between the lines that others have had problems with pods/cone air filters and CV carbs. Can anyone explain what exactly makes them hard to work with when adding pods? Is there something that changes the way the intake suction raises the slides or something like that?

Thanks in advance for any help.

Atchbo

[hampshirehog]

CV carbs are way more difficult than slide carbs to tune in when altering the intake - you need a vacuum difference to make the slides lift and part of this difference is controlled by the airbox. I've never managed a faultless switchover - pretty good, yes, but everytime I've always considered there to be some 'fluffiness' somewhere.

Your carbtune will work very well for your pilot circuit (I suspect that you won't have to change your pilot jets - just adjusting the mixtute screw should bring things in line).

For tuning in the needle part of the circuit with a colourtune you'll need to disconnect a sparkplug and hold the throttle around half - three-quarters open. (If you don't pull a plug your motor will rev its nuts off). Set your colour by raising the needle.

Ditto needle position for your mains, though there it's difficult on a twin to stop the motor over revving. You're probably going to be better off doing plug chops.

[atchbo]

Ok, warning taken. I still don't know if I want to fight with getting the ugly airbox back in there, but maybe my extended air filter-to-intake idea will help a bit.

Because this is a wierd setup, I'll try to take pictures as I go. There are some early teardown pics now in my profile (tearing down the top end and starting the body work).

Atchbo

[curseboy]

I have pods and a free flowing exhaust on my 450 (i know, i know, not a 400) and I had some troubles. I went up on the mains to 125, pilots to 22.5, and raised the needles WAAAAYYYY UP (think about 4-5 washers...removed the backing spacer on the needle to make it fit), and I don't have many issues....No excessive cold blooded-ness, or mid-range stumbling, or anything of that sort.

[curseboy]

right...so to elaborate ...i went one size up on pilot jets and 4 (from 115 to 125) on mains...that should work as a starting point.

If trial and error isn't your thing, maybe leaving the bike stock is the way to go.

[76FJ55]

I agree with hampshirehog that it is more difficult to tune CV carbs than slide valves... the biggest difference is that with CVs there is not the direct link between throttle position and slide position. slide position is controlled by the amount of air passing through the carb throat, which is a function of throttle position and rpm. this relationship generally adds to drivability at the expense of throttle response.

I am not sure that the existence of an air box will have a profound effect on its own. my reasoning here is that the slide is controlled by the pressure differential on the slide diaphragm the upper side of the diaphragm is exposed to venture pressure via a hole in the bottom of the slide and the lower side of the diaphragm is acted upon by "atmospheric pressure". the "atmospheric pressure actually is taken from the bell mouth of the carb so whenthe air box is fitted it will due to it's restrictive nature drop the pressure in the throat of the carb but it will also drop the "atmospheric pressure" supplied to the under side of the diaphragm. this effect will esentially null out the effect on the slide position.

I would guess the greater effect would come from the decrease in intake tract length. the longer intake length associated with the airbox will most likely help stablize the intake charge flow and will also change the resonant frequency of the intake system.

I would not personally be that reluctant to attempt the pod set-up. Every GS since 1980 has had CV carbs and there are plently running around that have been sucessfully converted to pods.

[atchbo]

Hmmm. That sounds more like what I was expecting to hear. It will take me a while to get my bike back together, so I won't have answers real soon in terms of how this works out, but I'll post updates at some point with pictures of what I build. I'm hampered by $ and the time it takes to ship stuff back and forth (and by things like not having a 32 mm socket for the front sprocket... and taking a break to deal with a leaking roof).

If it's true that the airbox has tubes inside that are 2-3 inches long going to the carb intake, it would make sense that the bike would run best if you do the same thing with a pod conversion. To balance the vacuum between the two carbs, I think my breather T idea will come in handy. It's all just a matter of making it look reasonable in the end.

Atchbo.

[tkent02]

1977 GS400 (BS34 carb, which I now hear is a CV type)

Just curious, CV carbs didn't show up until 1980, by then weren't they all 425 or 450ccs?
You got a mix n match pooch-mutt bike like mine?

[atchbo]

I dunno. It seems like the GS400 was an abandoned child from the beginning. Z1 shows four models, but in 1977 it seems to have come with BS34 Mikuni carbs, which are CV style. Mine have the butterfly controlled by cables and the slider moves when you suck/blow on the appropriate air jet.

I bought the thing because the motor looks kinda like an English twin, and the PO had lots of extra gaskets, forks, clutch basket, rear shocks etc for me to screw around with.

Atchbo.

[atchbo]

Minor update... everything I have read says the BS34 carbs take 52 mm air filters. I brought my carbs in the the local shop and tried some on, and they took 48 mm air filters (filter boots). Good thing I didn't order them online.

I also bought a foot of hose that fit the carb intakes for my extension idea. I had to bring them in to special shop and try out different kinds of hose to find something that worked. Turned out that 1 7/8" clear hose with green coil was the winner. I think it's some kind of water suction hose. Maybe I can spraypaint the hose black before putting it on. We'll see; the guys at the shop said not to bother with this extension thing, but I may compare with/without to see if it runs any differently.

Atchbo

[Flaggo60]

............ I still don't know if I want to fight with getting the ugly airbox back in there...........

I think there are a lot of GS airboxes sitting abandoned on garage shelves for the very same reason...

[atchbo]

Here's a side picture of the intake extensions I made for my carbs. You can't see it but they have small metal hose barbs (nipples) that will be plumbed into the breather tube.

[andy]

Have you had any luck on this?
I want to pod up my 400 too