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79 GS425 running rough

  • Thread starter Thread starter Anonymous
  • Start date Start date
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Anonymous

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I just started her up after replacing the carbs and it runs but is pretty rough. I bought it without the carbs and these carbs I put on it were from a GS400 of same type/year. Identicle carbs to the 425 model. As far as I can tell there are no vacuum leaks or anything. The motor has 17,000 miles on it and is very strong sounding. Its hard to explain the sound but its very deep and wimpy sound from the exhaust. Sometimes when I try to start her and after I attemp to and fails, she will puff out the carbs sometimes. She wont idle also, unless I keep the gas on her.
 
Does it sound like a timing issue or could it be the carbs are clogged? using a set of carbs from a GS400, is that my problem? At low revs it lacks power, but over 3k it seems to gain some response to the throttle. Any help is greatly appreciated.
 
Do you have the airbox installed? Did you bench synch the carbs before installation? Were the carbs worked on in any way?
 
These carbs are from a GS400 that were sitting in storage for months. Are they enough for the 425 to run right. Or are they totally different carbs? I do have the airbox on the moto, but they are not connected to the carbs. Is it that picky that it wont run good without the airbox on?
 
I don't know if they use the same carbs, but will ASSuME they do, so you're OK there.
The GS is very finicky about having the airbox hooked up. This includes having a filter, and cover. The is especially true for the vacuum carbs, the CV's.
Double check the timing. I'm going to guess that your ignition system has points, with a mechanical advancer. Check to make sure none of the springs are broken on it.
 
I'll hook up the carbs to the air box. The timing should be ok the motor has never been opened or anything. (plus I dont know how to check it). And then take a look at the points system. thanks for the help, :)
 
Wow, it seems like everyone has to go through this hell at least once.
Yes, you MUST have the air box on if it is a CV carb.
In my case (1980 GS450L), the air filter element had disintegrated and there were tiny air leaks in various other places. I had cleaned the carbs (several times) and it would not run right. It was only after I resolved ALL the air flow and air leak issues that the bike ran decently. The CV carbs on our bikes are VERY sensitive to improper air flow.
You will drive yourself nuts cleaning the carb a million times (as I did) only to finally realize that the air flow (or leak) problems were critical as well.
 
What do you mean by CV carb. The GS400 and GS425 did not have the same carb as the 450 model. Although I dont know the difference, the manual has two separate things from the 450 one. The 400/425 is a 34mm carb, I think. Are the jets different for the 400 and 425 model? I have the original 425 carbs, but its in pieces and some screws are missing. I'll hook up the air box and see how it works. Anything else I should look at?
 
The CV carb: The carb slide is raised by pressure differential (which is why the pressure on both the intake and exhaust sides of the carb is so important). There's a rubber daiphragm in the top of the carb - if this diaphragm is punctured you have no hope of the carb functioning properly. My GS450 has this carb type.

Mechanical slide carb: The carb slide is raised via mechanical linkage to the throttle cabe - so the pressure differential is not as important but still plays a role to some degree. My Honda has this carb type. It will run OK without airbox and air filter, but will run significantly better with them on.

That's about as much I can tell you without pictures. There are several web sites that have these pictures, but I can't recall them now.
At the very least, it would probably benefit you to figure out which carb type you have so that you can post questions on this site more accurately.
 
The GS425 is running perfect now. I just had to gap the points better. They were under .20mm. I gapped them at .35mm. So heres what I found out. The GS400 and 425L use the same exact carbs, and most engine stuff is interchangeable. The air box is not at all that critical to hook up on these models. It made no difference at all how it ran. Right off the storage shelf these GS 400 carbs, sitting for at least 6 months, bolted back onto a GS 425L run great and pose no problems with these models. I just wanted to let everyone know to check your points on these bikes if it is not running or running right. And I thank you all for your help. I can hop on her now and ride her and it sounds real good too. 8)
 
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