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'78 GS400 and K&N pod filters - which carbs do i need?

  • Thread starter Thread starter athom
  • Start date Start date
A

athom

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Hi all,

I know already this is going to be a headache but i'm 50/50 on aesthetics and ride-ability on this thanks to www.wrenchmonkees.com in fact maybe 60/40. I have to give credit because it was what spurred me on to get an old twin to do up. In my view an essential part of the stripped down look i've fallen for is the unexpected space under the seat and so no matter what the cost i have to have pods. So i'll have to learn something or another.

From what i've read CV carbs like the Mikunis on my GS are not happy with pods due to a lack of flow resistance/vacuum on the filter side, are not fully adjustable and availability of jets is a problem. So i noticed a lot of customs have Keihin CR specials on but read enough to see they are really only good for a lot of flat out racing, they look cool but i want something more street suitable. Then i noticed chat that Mikuni VMs are more suitable, cheaper and available, as are jets for them.

Am i getting this right so far?

I'll be getting some more open pipes on as well and so i'm wondering if i'm going to get other carbs with more open filters and more open pipes i really should just flow the ports while i'm at it (handy DIY guide http://www.pirate4x4.com/tech/billavista/PDFs/Head Porting DIY by Standard Abrasives.pdf )...... just so i can use K&Ns :rolleyes:

As you can see i've gone completely insane :dancing:

before you say get a bigger bike instead of trying to tune a small one i did think of it but i really want a thin light nibble thing to chuck around town and the tight country lanes here, and this really is an exercise in aesthetics.

I'd rather tune for low and middle grunt than top end so again from what i've read i probably don't want a very big bore carb which would give better top end. But how do i work out what i should try? How do i know what size/type carb to get without something to try first to get an idea and go from there.

Please help untangle this if you can :confused:
 
I'm doing pods and hopefully a custom freer flowing exhaust on my 450 and so far I've seen no reason to change out the carbs for something different...

I believe three sizes up on the mains and one on the pilot should be ok for good pods and a good free flowing exhaust, but I have yet to test that theory.

Let us know what you do...
 
I think we both have the same carbs from what i read though i guess you have a slightly different jetting from stock for the extra 50cc.

I found this guy with them http://www.customfighters.com/forums/showthread.php?t=22730 he says "I got some new cheapo carbs" so maybe he's not on CV carbs.

This guy has them http://www.bikepics.com/members/reetman/77gs400/ and very open pipes http://www.youtube.com/watch?v=KwSPnMwKgjs he's active on a dutch forum and I'll join up if i can work out how to and ask what his set up is.

I don't want to get the pods and then have to change carbs and need different pods so i'd like to get some ideas before just having a go. Good luck with yours, i'll keep an eye on the twin threads.
 
I haven't seen much in the way of twin owners doing pods yet, or if they have they haven't posted up results.

I think our carbs will probably be the same and I believe jetting should be all that's required.

I'm a little way away from doing mine yet, but I'll definitely be putting details in my rebuild thread when I get there.
 
athom said:
From what i've read CV carbs like the Mikunis on my GS are not happy with pods due to a lack of flow resistance/vacuum on the filter side, are not fully adjustable and availability of jets is a problem.
Click to expand...


Got me a chuckle over this comment.

There are a bazillon bikes running CV carbs and pods together. And jets are commonly available.
 
Nessism said:
Got me a chuckle over this comment.

There are a bazillon bikes running CV carbs and pods together. And jets are commonly available.
Click to expand...
I'm working on fragmented various forums chat at the moment which is not an ideal source of info. Am i right in thinking that CV carbs are more likely to be limited in their adjustment though? And are the bikes with CVs running well? Basically what i want to know is what is the best carb to use on a street bike with pods and i'll go get some.

I also read that a smaller bore carb helps throttle response at low revs so am wondering which bore to get as well.
 
Hi,

The stock carbs with proper jetting and K&N pod filters will do you up nicely. You'll find tuning tips and vendors on my website. The above advice is a good place to start. Read your plugs (plug chops) to fine tune. There will be some trial and error. Have fun!

Thank you for your indulgence,

BassCliff
 
I just received some manuals and in the Clymer tuning tips section it advises changing the CV carbs for "32mm slide/needle Mikunis" Not sure which one that is but it looks a lot like the VMs i see for sale now though this is a 1978 edition. It then goes on to advise getting rid of the airbox and replacing with K&Ns, well ok then:dancing:

Here's a link to a photo of the page http://img109.imageshack.us/i/dsc01294a.jpg/ which maybe too big to load in the thread (will follow FAQ advice and get a photobucket account)

So maybe I could get a Dynajet stage 3 kit and get the CVs to work but it's looking more sensible to put that money towards a pre-jetted replacement set of performance carbs. At least there is only 2 to buy.
 
Interesting indeed! I haven't looked at any of the tuning tips in the Clymer yet as there's not much tuning you can do when it isn't even in one piece yet... :rolleyes:
 
pete said:
Interesting indeed! I haven't looked at any of the tuning tips in the Clymer yet as there's not much tuning you can do when it isn't even in one piece yet... :rolleyes:
Click to expand...
We'll i haven't even started taking mine apart yet but this issue is so pivotal for the look i needed to settle it at the planning stage. It's a useful section actually, i'll be replacing a lot of things so i might as well consider performance upgrades as i go and it goes into handling and electrics too.

by the way you have the best tank, have you seen this 750 with your tank on?
GS750.jpg
 
I will definitely have a read through at some point for sure. The way things are looking at the moment, I may have some thumb twiddling time ahead as I've nearly run out of things to do before cleaning the carbs and giving it it's first kick in over 5 years...

I haven't seen that particular one before, but the 450E/S tank is definitely the best style GS tank for a cafe IMO :D
 
Just because i hate finding incomplete threads.....

Spoke to sales at www.allensperformance.co.uk and was talked out of buying Mikuni RS carbs and advised to buy Mikuni VM carbs, a bit more turbulent flow he said but didn't think i would really miss not having RS carbs on a non-race bike. Judging by the pictures on the site of other old bikes they've worked on and the fact he's recommending something a 1/3 the price i'm inclined to believe him.
 
A place that tells you to spend less money? Wow! Either they're insane or great :)

I just had a look through my Clymer and I see nothing about performance tips or anything like that at all. The tune up section is purely about valve adjustments, spark plug adjustments, etc. Perhaps I have a different Clymer?

What pages do you find the performance stuff on in yours? I'll see if I'm missing something...
 
It's the last chapter "Performance Improvement". 11 pages where it tells me to fit a big bore kit, hotter cams, louder exhaust (but not too loud), get the head professionally flowed, more powerful coils, heavy duty clutch springs, change the gearing, don't change the balancer, better shocks, weld a stiffner on the swing arm (but "only if you're a committed cafe style rider" :D), improve the forks, change to lighter alloy rims, drill the brake rotors (but not the rear drum backing), change to better jap tires..... and this book came out the same year the bike did. It's a 1979 reprint of the first edition 1978 book for the GS400 if that helps. Next time i've got the scanner out i'll post the pages.
 
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