Need helping diagnosing

[Nahian]

So my bike (89 gs500) either starts up right away, or takes a few mins cranking and then starts. When it starts, the throttle response is very unusual. It will either bog down OR rev up very high and then drop, at which point the bike dies. The carbs have been cleaned with carb cleaner/air compressor/wire through the jets and the exteriors of the air filters were cleaned with the compressor as well as the vent holes in the gas cap. The fuel lines aren't kinked and neither is the vacuum line to the petcock. I noticed that the line going from the petcock to the lower T connection (carb) has air bubbles and can see that fuel is trying to move to it but seems like not enough suction is there. In light of this I removed this line from the petcock and fed straight fuel from a funnel through it (no vacuum needed). The bike then started right up but still bogged down and died like it wasn't getting enough gas. So then I put my hand partially over the exposed air box opening and the bike started to suck down the fuel from the line and revved normally and with a lot of gusto. Took hand off of of airbox opening and it immediately bogs down. What is going on?? I also noticed that the right spark plug was white when I took it out, but the left one was blackened, but not wet. Is this a vacuum problem, dirty air filter, or dirty carb, or what? My experiences with motorcycles are mostly two stroke dirt bikes, a ninja 250r and a vulcan 750. I need your help guys! What do you think lol?

Other notes: I did manage to ride it down the street in it's weakened condition but I had to keep the throttle wide open and was going 5mph the entire time till i turned around, came back to my house and bogged down. It didn't start after that and I called it a night. Tomorrow I am going to take out the carbs again and make sure everything was cleaned properly/no missing o rings and what not, and test my ignition coils with a multimeter.

[Snake2715]

where's the choke set?

spring on the carbs correctly placed?


bench synched at least since you need it to run to truely synch.

boots seated tight and crack free?

How old is the fuel? have you pulled the petcock to check its clean? inside of tank clean?

do you have a fuel filter on it?


did the bike run when you got it or is this a new bike for you?

[Nahian]

I have tried all different choke settings, the spring on the carbs should be set correctly (I am confident, but will definitely check them tomorrow), the boots are seated tight, and are brand new looking, the fuel is from a fuel can...no more than a few months old. I will try fresh gas tomorrow, should have thought of that. I have pulled both petcocks out and cleaned them, and inspected them, they are free of any sort of clogging. I do not have a fuel filter on it currently but I have lots of new ones and will put one on tomorrow if you think that will help. The bike was dismantled and came in 3 crates when I bought it. The one other thing I failed to mention in my previous post was that theres 2 brass bar things connected to each side of the carbs, and on one side it is broken. I have jb weld to fix it (previous owner told me that is what he was going to do). My friend who is a professional mechanic helped me with the bike, and is perplexed too. Here's a pic of the bar thing though. http://s205.photobucket.com/user/ben_2_go/media/Motorcycle%20Stuff/Mikuni%20BST33SS%20Carbs/rightside.jpg.html?sort=3&o=20

[GI JOE]

Look up in the saved posts at the cabs and their components in the orange area, it has lots of pics and labels. It sounds like you're describing the throttle shaft described in the first few pics. If that's the case my idea would be that it's sticking cause I believe that holds the flaps. I could be wrong as I'm still new and learning, but it looks like an idea. If I were you I'd make sure the flaps work without any issue at the very least. Again just my observation through pics.

[Snake2715]

a couple month old gas should still start but you never know.

I would leave the fuel filter off for now...seem to do more harm than good.

one carb has a vacuum port on the top right, the other carb has one but its blocked internally as well. were the carbs separated ever? the functioning port should be on the left carb and hose to the petcock.

one of the more experienced guys will hopefully be in to help.

[Nahian]

It's not the throttle shaft I was referring to, I have no Idea what it is, it's the very thin long hollow diagonal bar, the bottom of which is a few millimeters to the right of the Mikuni Corp logo. Anyway, yes the carbs were separated before, but I rebuilt the entire thing with new o-rings, gaskets, and jets. The vacuum port is functioning, I am not sure how well but it is definitely working, and it is indeed the left one connected to the petcock. Thanks for the thoughts guys. Here's a more detailed pic of the broken tube/bar. A piece of it is broken off inside the inlet at the top while the rest is separated.

[attachment deleted by admin]

[BockinBboy]

If covering the air box made it run better,  its definitely a mixture problem. That tells you it either has too much air or too little fuel. I'm inclined to think it is a vacuum leak because of the erratic idling. The lack of fuel flow points that direction as well.

I'd be willing to bet that your problem is stemming from that bar you repaired with jb weld. I 'think' that bar is part of the vacuum system.  That bar is an internal passage on the newer carbs as I don't have it on mine.  I have never heard of anyone successfully repairing carb parts with jb weld, carbs are just too sensitive to leaks with it.

I'd would source a set of carbs, or find someone who can repair that for less than a set of carbs.

- Bboy

[Nahian]

That bar is most likely from the choke system I noticed when I took apart the carbs and blew compressed air through everything. Its prob not that important... Also how can I have newer carbs? My bike is an 89 and I only have a main and pilot jet. Aren't the new carbs equipped with 3 jets? Anyway, the main problem was the pilots!! Somehow they got completely blocked, even though I cleaned the carbs no less than a week ago. Hope it doesn't happen again. The tank has been re-lined and is rust free, maybe it was random dirt or something since I used a fuel can.  As for the sputtering and irregular idle, it was a simple throttle cable adjustment. Go figure (missed the simple stuff). Now I don't have to put my hand over the airbox or anything, the bike runs as it should, and has got great throttle response. Thank you for all your consideration and time everyone and ride safe! 

[BockinBboy]

Yeah, you have the older carbs, I just meant my newer carbs don't have that bar. So I couldn't look at my extra set to figure it out what it's for.

Covering the airbox intake was a really good hint for a mixture problem. Had too little fuel. Congrats getting it worked out!

- Bboy

[Nahian]

Thank you, and ahh I'll remember the airbox tip. The more you know haha

[The Buddha]

That broken there part = feeding the choke in that carb.
You have to fix it before you go on.

You sound like you're super lean in 1 (the broken one I would guess) and a shade rich in the other.
Fix that gaping hole in the broken fitting then try it. Gotta stop the elephant before you think of the mouse.

Cool.
Buddha.

[Nahian]

Ok thanks Buddha, how should I fix it? Do I take it somewhere? If I could get the piece that's stuck inside out, could I order the bar piece and put a new one in? I cannot find that particular piece anywhere online. I do not even know what it is called.

[RossLH]

I ended up replacing the carbs when I found that very problem. Attempting to repair a small hard line is a fight you probably wont win.