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Author Topic: 1980 GN400 CV to VM swap.  (Read 926 times)

Offline 80gn4

  • Posts: 5
1980 GN400 CV to VM swap.
« on: Dec 27, 2017, 01:21:06 »
Well I've been all through the old posts and I haven't found much on this topic other than a few people fighting with vm36 carbs being to rich for their bikes. Here goes...

I've got a 1980 Suzuki GN400 all stock. The factory CV carb is a BS36SS and it's completely shot. All diaphragms and rubbers are dried and cracked thanks to being in Arizona all these years. I am not a fan of CV carbs whatsoever and I'm in need of a new unit anyways so I'd like to go with a round slide VM carb. I've been reading a lot of mixed reviews on sizing... Some people say I need a 36mm and others say when you switch from CV to VM you have to drop in size. Any help getting me into the right size carb will be greatly appreciated. Thanks in advance!

Offline teazer

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Re: 1980 GN400 CV to VM swap.
« Reply #1 on: Dec 27, 2017, 01:29:35 »
Basic rule of thumb is that a CV carbs has a big fat shaft across the outlet that disrupts flow and has to sized larger than the equivalent 'slide carb" by a factor of around 15% or so.  Assume that you will want a carb around 85% of the CV size and that works out to 30.6mm. 

So for a smooth bottom end and midrange, go with 30mm and for a littler more top end at cost of bottom end, try a 32mm.  So much for the theory.  Does anyone have any experience they can share of putting VM Mikunis on a GN400 for this poster?  (corrected)
« Last Edit: Dec 27, 2017, 20:15:30 by teazer »

Offline 80gn4

  • Posts: 5
Re: 1980 GN400 CV to VM swap.
« Reply #2 on: Dec 27, 2017, 12:21:22 »
Thanks for the help teazer

I'm thinking i'll go with the 32 since i'll be gearing the bike to run 30 min commutes to work on the highway. I'm not too concerned with bottom end but I don't want her to get winded real easy. Will the vm32 support modifications down the road? I know I'm going Pod filter and open exhaust right off the wing with possible high compression piston and valve work a season or two down the road.

Offline cxman

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Re: 1980 GN400 CV to VM swap.
« Reply #3 on: Dec 27, 2017, 17:53:23 »
to small that will choke out at high rpm

i have a 36mm on my xl350 it runs just fine an its what the guys run on teh sp400 and that is the same motor

white brothers used to sell the 36mm kit and it was a documented 5hp bolt on 

this is what i have on my sp400  i have a modded pipe but it should get you in range

pilot 30
air screw one and a half turns
slide 2.5
needle jet p6
needle 6dh3-1 [i1is the needle slot]
main 230

to go a tm34 if you want to putt around and have it haul in bottom to mid range and still good top end here is what i used to run

    Mikuni TM34    main 240    pilot 40    jet needle Q2    needle 6FJ41    air correct 2.0    slide 4.0    

1978 CX650 Super Deluxe
1979 XS1100 Special
1974 xl350
1983 cx650 Custom
1973 cb750
1980 cb750
1981 cb650
1982 cb900 c
1974 kawasaki 350 bighorn
1983 GL1100 aspy full dress
1983 GL1100 Nekid
and a bunch of others

Offline cxman

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Re: 1980 GN400 CV to VM swap.
« Reply #4 on: Dec 27, 2017, 17:55:03 »
teazer what do you need re the kawi carb kit?
1978 CX650 Super Deluxe
1979 XS1100 Special
1974 xl350
1983 cx650 Custom
1973 cb750
1980 cb750
1981 cb650
1982 cb900 c
1974 kawasaki 350 bighorn
1983 GL1100 aspy full dress
1983 GL1100 Nekid
and a bunch of others

Offline 80gn4

  • Posts: 5
Re: 1980 GN400 CV to VM swap.
« Reply #5 on: Dec 27, 2017, 19:59:23 »
Thanks cxman I'll order the vm36 then. Where is a good place to order the different size jets and needles? not sure what size it will come with stock.

Offline cxman

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Re: 1980 GN400 CV to VM swap.
« Reply #6 on: Dec 27, 2017, 20:09:35 »
here is what it is loaded with

Bore Size : 36mm
Spigot Mount : 43mm
Main Jet : 310
Pilot Jet : 35
Needle Jet : Q-5
Throttle Valve : 2.5
Jet Needle : 6FJ6
Air Jet : 2.0
1978 CX650 Super Deluxe
1979 XS1100 Special
1974 xl350
1983 cx650 Custom
1973 cb750
1980 cb750
1981 cb650
1982 cb900 c
1974 kawasaki 350 bighorn
1983 GL1100 aspy full dress
1983 GL1100 Nekid
and a bunch of others

Offline cxman

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  • Posts: 2689
Re: 1980 GN400 CV to VM swap.
« Reply #7 on: Dec 27, 2017, 20:10:32 »
see if the place you order it from will give you a deal and reload

or at least give uou a deal on the brass
1978 CX650 Super Deluxe
1979 XS1100 Special
1974 xl350
1983 cx650 Custom
1973 cb750
1980 cb750
1981 cb650
1982 cb900 c
1974 kawasaki 350 bighorn
1983 GL1100 aspy full dress
1983 GL1100 Nekid
and a bunch of others

Offline 80gn4

  • Posts: 5
Re: 1980 GN400 CV to VM swap.
« Reply #8 on: Dec 28, 2017, 14:41:25 »
cxman

I gave the place I ordered the carb from a ring and they don't do rejets. Looks like I'm going to have to order all the jets separately. I understood most of your previous post about carb setup except 2 parts...

what were you referring to when you said "slide 2.5"?
And also what did you mean by "needle 6dh3-1 [i1is the needle slot]"?

Thanks I can change jets and whatnot but i'm not too keen on the carb lingo  :o

Offline cxman

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Re: 1980 GN400 CV to VM swap.
« Reply #9 on: Dec 28, 2017, 17:54:00 »
in a mikuni vm carb the throttle (valve) slide comes in different cutaways the cutaway you need is a 2.5 that is wat should be in the carb when you get it so no issue

the needle 6dh3-1

has groove in the top a circlip fits in

i would order a 32.5 pilot and a 300 main to give your self a bit f range

in this case the needle is in the top groove or at the leanest position to start

as you raise the needle it gives more gas
1978 CX650 Super Deluxe
1979 XS1100 Special
1974 xl350
1983 cx650 Custom
1973 cb750
1980 cb750
1981 cb650
1982 cb900 c
1974 kawasaki 350 bighorn
1983 GL1100 aspy full dress
1983 GL1100 Nekid
and a bunch of others