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Gs425 rejetting help!,

  • Thread starter Thread starter Axehand
  • Start date Start date
A

Axehand

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I have a 79 gs425 with mikuni bs34s, pods and open exhaust. I've cleaned them, replaced o-rings and gaskets. I can get the bike to start and idle fine but as soon as I get on the throttle the slightest bit it starts bogging and backfiring. I understand that I need to go up in jet sizes because I'm running extremely lean. Can anyone give me an idea of jet sizes that would be a good starting point for this application?

Any help would be greatly appreciated!
 
Can't offer any suggestions other than "several sizes up", but be aware that if you have no filter on the intake of the carbs, they will NOT respond to any throttle input.

.
 
Thanks Steve. I'm really hoping to find someone who has a similar setup that runs right, lol! Are you saying that with pod filters the bs34s will not function properly regardless of how they are tuned?
 
I'm assuming that your running 112.5 or 115 mains stock. In that case the next sizes up would be 117.5 and 120.

These carbs will run run properly when tuned although its not as simple as putting a set of pods on and only jets.

You may have to change the air jet or needle or drill the slide . These kind of things can be found in dynojet instructions if you were to get a kit.
 
Mekanix said:
I'm assuming that your running 112.5 or 115 mains stock. In that case the next sizes up would be 117.5 and 120.

These carbs will run run properly when tuned although its not as simple as putting a set of pods on and only jets.

You may have to change the air jet or needle or drill the slide . These kind of things can be found in dynojet instructions if you were to get a kit.
Click to expand...


I contacted dynojet today and they told me they didn't make a kit for my application. Do you know something I don't?
 
Mekanix said:
I'm assuming that your running 112.5 or 115 mains stock. In that case the next sizes up would be 117.5 and 120.

These carbs will run run properly when tuned although its not as simple as putting a set of pods on and only jets.

You may have to change the air jet or needle or drill the slide . These kind of things can be found in dynojet instructions if you were to get a kit.
Click to expand...

Actually I'm running 110 mains, not sure pilot size, adjustable needle set in middle position. Any suggestions based off of this info?
 
I don't know of any specific kits for that make and year but I have seen instructions for dynojet kit's in general for those carbs. One thing I've noted is what they change in order to go up in stages.
It usually involved increasing main jet sizes and drilling the slide using a specific size bit.
The other change was with the supplied needles and air jets.

Your going to need larger main jets because of the exhaust and larger again because of the pods. So maybe up to 4 sizes larger. 120's or more in order to get the right amount of fuel it needs.

I wonder if a kit for a 450 would work? They do make a kit for that.

They don't list the instructions for it but here is an example for an 1100 with the same carbs and jetting that is close to yours.
http://www.dynojet.com/pdf/3133.pdf
 
Last edited:
Would I have better luck if I switched to a VM style carb? I'm going crazy trying to get these bs34s to work!!!
 
I believe the 450 carbs are a bit different and trying to find a kit for them is hard too.

Go to www.motorcyclecarbs.com, look in the Support section under Jetting Rules of Thumb.

That should get you a starting point, then being CV carbs you need to tune from the main jet down.

Do plug chops or get a Gunson Colortune unless you're rich then you can get someone to dyno tune it.

Once you've got the mains sorted, then do the needles, then do the pilots.

If it's as lean as you think, I would raise the needles somewhat as a starting point also, you don't want to hole a piston while trying to tune it!
 
So I went to 130 mains and set the needle clip to the last position raising the needle. Still running lean, but able to ride. These changes made a big difference and I feel I'm pretty close.
I wanted to thank you all for your help! I'll update tomorrow, I'm going to pick up my new pilot jets.:)
 
Nice one!

Have you confirmed the changes with plug chops or something?
 
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