81 GS450 with 89 GS500 carbs

[Majikrat]

First post on here, great site and I've learned a lot about my first bike. I read on here about the troubles with 81 GS450 carbs the mikuni bs34ss are rare and parts are not cheap. I saw a few guys recommend upgrading with 1st gen GS500 carbs so I found a pair off an 89 GS500. Now the fun part is what do I need to do to get them working on my 450? I see the choke is different and the throttle cable is a little different also. Any advice is appreciated. I got this bike from a family member who was going to donate it to a charity, we turned it into a Father Son project and got it running. It's been a ton of fun and looking forward to many more miles.

Thanks,
Jay

[chris]

If I were going to put 30 year old carbs on my bike, I would clean them. This site has a excellent carb cleaning tutorial. Just because the outside looks good the inside might be a completely different story. The choke is the least of your concerns, I would also confirm spacing and inlet/outlet O.D. Good luck with your project.

[grcamna2]

subscribed.
There's members here that know that have swapped the top-ends from a GS500 onto their 450s including carbs.I don't know which thread yet but they are on here.

[pete]

Hey Jay and welcome!

I run 2001 GS500 carbs on my 82 450 along with running the same year GS500 camshafts, everything else is stock GS450. I swapped the carbs out due to the diaphragms having little tears in them and encountered the exact issues you mentioned. These stock 450 carbs were only used on the 80 - 82 450s and replacing diaphragms is indeed an expensive affair! JBM Industries do make rubber diaphragms but I couldn't find any in their range that suited and I tried the only ones that were close with no success.

I'm not all that familiar with the 500s in general so I'm not sure on specs or differences between years, but I can tell what I did to make the 2001 carbs work on mine.

Firstly, while they slip straight in to the stock GS450 intake boots on the head, I found the part that inserts into the rubber boot just a little shorter on the 500 carbs which made them a little awkward to seat properly. I replace the intake boots with the GS500 intake boots, problem solved (replace the O rings while you're at it).

Next, unless the intake diameter on the 89 carbs is different to the 01 carbs, you'll find you'll need to do something on the air filter side. Stock 450 from memory is 50mm diameter, the 01 500 carbs are 60mm, so you'll need to do something there. I ended up putting some aluminium EFI intake trumpets on there but that ended up with a huge 99mm bellmouth which almost didn't fit, and I've put Ramair filter material over the top. It's not a great solution and I'll probably change it. The alternatives are pod filters (but please get good ones) or put on the stock GS500 rubber inlet boots and put some sock filters over the top (that will likely be my next step).

I can't remember if I'm using the stock GS450 throttle cable or if I swapped that out to use the GS500 cable unfortunately, but you'll soon find out if you need to swap it out. I'm using the stock GS450 control block so if you need to change anything it will just be the cable itself. Incidentally I use a throttle tube from a Yamaha R1 to get a bit shorter throttle action.

With the choke, I run a GSXR 600 left control block (can't recall the year but an earlier one from memory) that has provision for a choke lever on it, and I use the stock GS500 choke cable to connect to that. You could likely use a GS500 left control block to do the same and adjust the connectors to suit.

Finally, jetting! For the life of me I can't remember what jetting I ended up with, but I know I'm a bit rich in there somewhere as it feels a little sluggish when it gets hot. I never spent the right amount of time on it and I don't have an bung in the exhaust for an O2 sensor yet which I'll address next year.

You can likely just start with the stock GS500 jets and go from there, but it's all going to depend on what you end up with for intake filtering and how restrictive your exhaust is or isn't.

Don't forget also to make sure your valve clearances are spot on to start with, then get the carbs balanced, and then you can get tuning.

All the best and enjoy!

[Majikrat]

Thanks for letting me know what you went through to get them working. I will post an update with what I did to get them working and tuned.

[pete]

No problems at all, definitely interested to see what your end result is.

[LAB3]

What parts are you needing for the BS34ss carbs? The only thing I couldn't find where emulsion tubes for mine but it wasn't like anything really needed replacement in the first place. Yes, the diaphragms are expensive but when you look into everything you need to convert to another set it's still cheaper to get them.