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Need to re jet my 1978 gs400

  • Thread starter Thread starter Ziggy400gs
  • Start date Start date
Z

Ziggy400gs

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I've been doing some searching on here and the internet and can't find anything straightforward about rejetting my carbs on my gs400. It breaks up above like 5 or 6k (have no tach so that's an estimate.) And also bogs a bit in mid range at WOT. Yesterday I had it go lean while cruising at 60mph and died when I got off the free way but got better once I filled the tank, (still bogged when switched to reserve and tanks was not fully empty.) Now I know what running out of fuel sou d's like and that's what definetly happened. The bike has pods and straight pipe exhaust and it runs like it wasnt rejetted. I'm new here and new to working on bikes but I work on A LOT of cars. I just want to see if someone can point me in the right direction for finding quality parts, and a thorough enough "how to" on rejetting and carb building. I believe I have 34mm mikuni carbs. I could be wrong. I will verify when I get home by searching a part number on the carb. ANY HELP IS APPRECIATED THANKS 😊
 
I cant remember the pods they look like your typical k&n cone filter
 
I could always try and find a stock airbox and see if that helps. If they are easy to come by... i dont need the 2 extra hp. I'm not sure what the power gains are with exhaust and pods. I just want it to run pristine
 
Ziggy400gs said:
I could always try and find a stock airbox and see if that helps. If they are easy to come by... i dont need the 2 extra hp. I'm not sure what the power gains are with exhaust and pods. I just want it to run pristine
Click to expand...

With an exhaust, pods and properly jetted carburetors, bikes can run great, look great and make more power. I had to have a professional jet mine on a dyno, and it ran perfect.

Good luck.
 
My bike was doing a very similar type thing. When I bought it the vacuum had failed on the petcock and the PO put an inline filter in place to shut off the fuel. Once I replaced the petcock the bike became a whole 'nuther animal.
 
Ziggy400gs said:
The bike has pods and straight pipe exhaust ... 
Click to expand...
I hate to be the Debbie Downer here, but you might have alienated some of the jetting expert here with the "straight pipe exhaust" statement.

Yeah, you may enjoy the noise whenever you choose to make it, but what about the hundreds of other people that have no choice? Many of the purists insist on ONLY a stock exhaust. Others of us like a good-sounding system, but straight pipes are not just pushing the envelope, they are turning the public against us.
 
I bought the bike with straight pipes. I'll probably put some baffles in it. But I'd rather get it running right before I focus on anything else. It's not lean. Pulled the plugs and they are kind of sooty. Fairly new plugs too. One more so than the other. Thinking they may be out of sync. But now I've verified they are running a bit rich I am not as worried about riding it to work and over heating due to lean conditions.
 
Thank you. This is all I was looking for as I'm new here and wanted to be pointed in the right direction for the information I need. I will figure this out as I work on cars EVERYDAY. The bike is my new toy. Maybe I'll make a thread on the jetting/carb tuning procedure.
 
Ziggy400gs said:
... But I'd rather get it running right before I focus on anything else. It's not lean. Pulled the plugs and they are kind of sooty. Fairly new plugs too. ...
Click to expand...
Between the pods and the straight pipes, it WILL need richer jetting. If you have plans to change either the pods or the pipes, do that first, then you only have to go through all the "fun" of jetting one time. If you jet for your current setup, then change the setup, you will have to re-jet.

Considering the described condition of the bike, be very careful when you re-jet. People that hack up a bike like that are also likely to drill out jets. While they might get close, anyone that comes in later (like YOU) will see the number on the jet and replace jets based on that number. Stock jetting on a 400 has a 112.5 main jet. Let's say the jets are drilled, and might be about 130 (which might be close for your setup). You find them a bit rich, so you want to drop down a size or two, so you get 110 or 107.5. Do you see the problem? You really needed 125 jets, but are now about six sizes too lean. I recommend buying NEW jets in the size range expected for your setup, then making adjustments based on them.

.
 
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