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Converting early gs400 to gs500 carbs... not a bolt on.

  • Thread starter Thread starter theironlung1976
  • Start date Start date
T

theironlung1976

Guest
All concerning my 78 GS400X. Dunno when the change occured in the head spacing but id have to guess 80. Chuck initially told me about this mod n ive been pumped to do it since he told me. After he emailed me the info i looked into it on the forum of course to see what was involved. Everyone was sayin its a bolt on. I was stoked to come up w the cabbage to buy em! Couldnt wait. Found a set on ebay for 81.99 on ebay (got my 500 cams, front axle, n weak ass stock fork brace from the same dude). Sold my old BS34s for 75... 6.99 in em! Yeah!!! Any whoooo... TORE the box open so excited to fit em on the bike n see that MIKUNI logo stickin out the side... BUM BUM BUM. No go. After i dried my tears i got out my trusty verneer calipers n found that the 500 carbs r .474 narrower than the stock 400 carbs.
 

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One cool thing about this mod... The old 400 carbs...
 

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The new 500 carbs... 3.2lbs lighter!
 

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I got the spacing difference figured out. 2 custom "t"s r needed. One for the vent, one for the fuel inlet. U could probably get away w takin the oe lower fuel inlet "t" n use it as the upper vent " t" w some tubing n pinch squeeze clamps after the width is lengthened. No pressure, just a pathway for fumes. 2 tie brackets n choke link rod need to b lengthened .474". The lip on the left side carbs throttle shaft needs to b lengthened .474" n shaped to allow for sync adjustment. My "t"s r being turned as we speak. Ill send a pic when theyre done. The rest is simple weldin. The bottom tie bracket holdin the carbs together hits the cam tensioner by the way. Needs to b relieved to clear it. The later carbs have a different bracket system n choke setup... N kinda looks easier to overcome. More to come...
 
Or you could make 1/2 " thick aluminum adapter plates to mount the manifolds to the head to line up with the 500 carbs & port the plates to match the head. That's how I do them. MUCH less work that way!!
Ray.
 
I thought about that n talked about it w a few other members. I have a friend that does lost investment casting. Was thinkin about doin it that way to really get a nice curved graduation goin into n out of the offset... n also cast the opening of the adapter to meet the 500 carb outlet n 400 head perfectly as they r different sizes. If u got any pics, by all means, please post em on this thread. Chuck n i r doin it for sure. If anyone else stumbles on this, itd b cool to have all the info up here. Thanks for the info ray!
 
This is the throttle shaft on the left side carb where the sync adjustment is that also ties the 2 carbs together. The carbs r put together at the stock spacing... as u can see the bracket is put on loosely w 2 PAIN IN THE NUTS JIS SCREWS. The left carb has a little lip that goes between the screw used to adjust the sync n the spring on the right carb.
 

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Pulled apart... close to the .474" that it needs to fit the 400 head. Little extension on the lip n yer good there.
 

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The lower fuel inlet T is the same inside diameter as the vents on the top of the carbs. If u cut the T down a tad n used some tubing n squeeze clamps to connect it all, u wont need a custom T there. I dont see any way to get around a custom T for the fuel inlet tho. This is the carbs pulled apart .474" as u can see from the bracket offest n the verneer opened up to it.
 

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Like this. Using tubing n clamps from the bowl overflow lines.
 

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And copy n paste .474" to these.
 

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As ray pointed out, the intake insulator / adapter would easier n thats why it was my first thought. And i still might go that route. Flow would hardly b interupted as its only offset by .237 per side. N over the port of the head its only .1185" on each side of the port... Not even an 1/8 inch. If u used the stock spacers n just moved them inboard, yer talkin about widdling a half moon outta the head n a half moon outta the spacer thats less than an 1/8" on each side. Not a direct line of sight to the back of the intake valve like the 400 carb / head setup is but the piston draws the a/f charge enuff that this little offeset wont b noticed... Horsepower-ly speaking. It might even help creating high pressure zones in different spots in the runner. The nice thing about it is it can b taken apart n everything put back to stock pretty quick, should u need to. Weldin the tab on the throttle shaft makes it a little more work but a grinder can remedy that. In case someone doesnt have the ability or resources to do the insulator plates, heres the only other way i can see it happening.
 
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