GS450 Carburetor on a GS400

[Chroniverous]

Will a GS450 Carb work on a GS400? We seem to have gotten a GS450 Carb with our GS400 that was a basket case.

[solarae]

This link has been super helpful to me: http://gsarchive.bwringer.com/bikecl...eight.html#A01

[Chroniverous]

I have seen that website before, but I don't see anything about using a 450 carb on a 400 bike. I don't know what jetting or things we need to change to make it work. Currently we are dealing with a hanging idle and I'm not sure if it's because it's the wrong carb. The slides don't seem to be dropping back down.

[Steve]

OK, which 400? The only Penticton I know of is in Canada. You guys got a 4-valve 400 that we never got. The only 400 we had was up through '78, and it was a 2-valve engine.

If the slides are not dropping down, that will NOT affect any idling issues or a hanging idle. The slides are merely a variable venturi.

.

[Chroniverous]

78 GS400 Canadian Model, you are correct it's a 4 valve motor. We have adjusted the valves with new shims, adjusted points and checked for leaks. Factory settings on the car but the idle still hangs. When you rev it up it doesn't want to come back down quickly. Maybe 5-15 seconds for it to drop back to idle. We have also checked the fuel petcock and just about everything else.

The only thing we think it might be is the fact it's the wrong carburetor meant for a 450.

[Steve]

Have you changed the o-rings on the intake tubes? Many times, a "check for leaks" doesn't show anything, but the o-rings will be shot. Cheap enough to get from cycleorings.com.

Can you see a number on the side of the carb? Or maybe open up the float bowl and read the main jet size? You need to look at a main jet anyway, to see if it's a Mikuni jet or somem aftermarket crap.

.

[Chroniverous]

Ok, so... I am an idiot. This is a 400CC Carb, i matched it with the Caburetor in the Clymber Manual. Im on the Cycle O-Rings website, and which kit do I need for our bike. Is it the "Mikuni BS series ('80 and later) carburetor O-ring kits"?
Thank you!
And yes we have changed the intake boot O-Rings.

[Chroniverous]

Oh and we have matched everything up and all the internals seem to be factory settings.

[LAB3]

Oh and we have matched everything up and all the internals seem to be factory settings.

Get the carbs cleaned up and your new o-rings installed, stock settings should get you a decent running bike. The plug gaps on my 450 like to be a bit on the wider end, going from .025 to .035 made a lot of what I thought where carb issues go away.

[Chroniverous]

Thanks for the response! Ill look into the plug gap thing. We actually did a compression test last night and both were at 75psi.... We are going to change the oil and test again because we notice the oil was very very fuel polluted. Im assuming from the previous owner fiddling with the bike, he never actually rebuilt the carburetor. Ill report back after we recheck the compression on the cylinders with fresh oil.

For all we know the cylinders could be washed out. I dealt with the same thing on a BMW prior.

[Chuck78]

For future reference, the 77 - 79 GS400/425 use a different carburetor spacing than the 450 and 500 and the Canadian 1980 and later 8 valve GS400 tscc engine. Otherwise some of the offerings of GS500 carbs would be the best performance upgrade for the early two valves per cylinder twins

[Chuck78]

I believe the 450 rebuild kits pretty much work out for the 400/425 as well. They are both bs34 carburetors.