1980 GS450 Carb query

[SteveTJ]

I am putting together a GS450 from a box of parts (storage locker find) everything was nicely labelled and have only had to replace a few parts along the way. Have the bike all together and running (of sorts) will only run on choke and bogs down, I?m thinking air leak somewhere? The pictures in the Manual are black and white and impossible to see in any detail but from what I?ve found online and comparing to some other Mikuni carbs I?ve got I think I?m missing a bit? Choke side carb has a hole just under the air mixture screw, put your finger over when running and revs increase. Looks like there should be a vacuum hose there, any clues?

[tom203]

Are you using a vacuum operated petcock? Is your carb sorta like this...?

image.jpg

[LAB3]

I am putting together a GS450 from a box of parts (storage locker find) everything was nicely labelled and have only had to replace a few parts along the way. Have the bike all together and running (of sorts) will only run on choke and bogs down, I?m thinking air leak somewhere? The pictures in the Manual are black and white and impossible to see in any detail but from what I?ve found online and comparing to some other Mikuni carbs I?ve got I think I?m missing a bit? Choke side carb has a hole just under the air mixture screw, put your finger over when running and revs increase. Looks like there should be a vacuum hose there, any clues?

My best guess is that the hole you're describing is where a fitting goes in, then a vacuum hose connects to the petcock from there. For the time being, you could put a piece of tape or whatever over it. The next thing is that there's an idle speed knob that bolts onto the carbs. When you separate the carbs it makes sense to loosen this as it makes taking them apart easier. In that process, it causes the carb butterflies to close all the way. Try screwing that in clockwise a bit and get some tension on it to open those butterflies up a tad. Having recently gone through a 450 myself, that's what did the trick for me.

[sam000lee]

On the GS450 the nipple that holds the vaccum line going to the petcock can be pulled out with finger-strength and does sit underneath the screw on the left side. If it is missing you will certainly have an air leak and be missing out on the vacuum operation of the petcock.

Here's what it looks like on my GS450:

[Robbiem1961]

That hole in the side of the carb body is where the vacuum pipe elbow locates, from memory it's only a push fit, I fitted a new '0' ring and used silicon (RTV) to hold it, I've not had any further issues since