NEED HELP. Starts (sometimes) but dies when I apply throttle

[JonathanPD812]

Hello!

I have a 1977 Suzuki GS400 that ran great until one day it just randomly stopped. I had let my brother ride it and he stalled it so I helped him walk it home and then it never ran again... Really weird, I figured it may have been because I left it out in the rain for a few days but that had never bothered it before... Hopefully I didn't make it worse but this is what I've done so far to try to fix it...

Took apart my carbs... They were filthy. Full soak and even ultrasonic cleaner at my work (lol) reinstalled and still nothing.. Noticed a weak apart so I got new points and installed them, set the gaps, and the timing. It now will start but quickly dies after idling for a minute. When I look at the plugs it seems that it carbon foals the left plug and appears to be only running on the right cylinder... I can unplug the left plug and it will run for a few seconds... I have no idea why that would happen or what that means...

Additionaly I ruined the tip of my air pilot needle so I'm waiting for two more to come in but I have no idea what to even check next but it's really frustrating where it ran fine and randomly didn't... I'll check anything I can I just don't know where to look. Thanks

[Big T]

Welcome aboard, Jonathan

Your problem seems to point to a fuel supply issue, so
Did you replace the condensers with the points?
Did you check the fuel flow from the fuel tap?
Did you fully disassemble the carbs and replace the O rings?

Just to clear up a few issues, and
Will it restart after it stalls?

[JonathanPD812]

Hi! Thanks for the response. To answer your questions,

I did not replace the condensers (I dont know much about those)

Fuel flow appears to be good from the tank, when I have it in the on position I can pull with a siphon and get fuel to flow and when I have it set to prime it flows steadily.

As far as the carburetor I did do a full disassemble with replacement of o-rings, however I disassembled since posting and found that the same side where it appears my plug was fouling behind the main jet it was gunked up again with some jelly like substance and a bunch of black stuff.

Another thing that I thought of when I did the first carb tear down and clean out I replaced the rubber boots with some straight boots which now that I think of it probably affected how much air I was getting (likely more because the stock boots went straight back from the carburetor and did a 90° turn to the air box, I still have the stock boots and they are in decent condition so I was thinking when I go back to reassemble perhaps I use those because maybe I was running into a jetting issue where I was getting too much air

I will say though it sounds like you may be onto something with the fuel issue especially if my main jet was clogged up I'm going to try to reassemble now that I cleaned everything out and will keep you guys tuned thank you

[JonathanPD812]

I realized I didn't answer your last question after it stalls and it fouled the plugs it would not restart unless I took the plugs out and wiped off the excess carbon deposits in which case it then would restart

[Big T]

Carbon deposits?
Like dry and caked?

Check your oil for gas contamination
It's sounding like excess oil in the combustion chamber

[phydeauxmutt]

Additionaly I ruined the tip of my air pilot needle so I'm waiting for two more to come in ...

Before you cram the new 'needles' in, find out FOR SURE that the tip of the ruined needle is not stuck in the carb. It is VERY easy to break them off, and can be VERY difficult to remove the broken tip.

If you cram the new needles in before verifying the passage is clear, you will just compound the problem and likely ruin another set of needles.

[Nessism]

Is the bikes maintenance all up to date? Valves properly adjusted?

You might want to check the newbie mistakes thread linked in my signature to see if anything rings a bell.

[JonathanPD812]

I have done most of the preventative maintenance. Have not checked the valves but I have the tank off now so I'll check.

The deposits in the carb are like dark brown / black but look like baby diaper stuff when it gets wet and when it had dryer it looks like black dust... I think I may have to drain my tank and clean it out... Might not be my only problem but it may be contributing....

When checking the carb to make sure the needles didn't break in there I blasted air through the passage and it comes out in the middle of the carb so it sounds clear and both sides look the same.. how else can I check to be sure?

[phydeauxmutt]

When checking the carb to make sure the needles didn't break in there I blasted air through the passage and it comes out in the middle of the carb so it sounds clear and both sides look the same.. how else can I check to be sure?

With the pilot fuel screws ("needles") removed, you can shine a light through the hole. Look at the other end of the hole to see if you can see the light. You can shine the light either way.

[JonathanPD812]

Checked the holes, looks clear! Cleaning the tank and waiting for needles to arrive in the mail.. then I'll be back to post when it's all back together

[JonathanPD812]

Okay so I want to reply with an update, cuz I feel like a chump but I want to verify if this may be my issue after all... Got new needles installed... Cleaned my gas tank and got everything out back together. Fires up, runs but only on idle... Tinkered with it for a bit and then went to go my plugs as per usual... At this point idk why it took so long but I thought maybe my plugs aren't carbon fowled but instead oil fowled... Looked up what would your motorcycle run like if over filled with oil and we'll, it's like all my symptoms..

Black smoke from the tailpipe, oily slick airbox, oil fowled plugs (which seam to get so bad that it doesn't produce spark until I clean them).

Thinking back, when it had trouble starting it a while back I first changed my plugs and oil but instead of draining all my oil I actually checked it and added some when the engine was cold and looking at the oil level the sight was all filled (way about the full level)

I drained some oil off and right away it worked better, so I'm going to drain it completely and fill it as I should .. but just wanted to see if over filled oil could cause the issues I spoke of originally. Thanks!

[steve murdoch]

Definitely the oily airbox is a symptom of an over-fill.

[Rijko]

Oil way above full and gunked up carbs usually points to gas leaking into the engine.
You mentioned 'adding some oil' - surely you checked oil level after that ?

I did not see a new tap mentioned - the 40+ years old fuel taps are known to fail.
Sometimes that's evident like a small trickle or a puddle on the floor, periodically.
Sometimes it goes into the engine.

Buddy of mine had one that would put a few drops on the floor once a week.
But he put it away for the winter with a topped off tank, and found the tank had lost over half it's content during the winter.
Guess where some of that ended up ...

Have you taken off the oil filler cap and smelled for gas ?

[JonathanPD812]

Oil way above full and gunked up carbs usually points to gas leaking into the engine.
You mentioned 'adding some oil' - surely you checked oil level after that ?

I did not see a new tap mentioned - the 40+ years old fuel taps are known to fail.
Sometimes that's evident like a small trickle or a puddle on the floor, periodically.
Sometimes it goes into the engine.

Buddy of mine had one that would put a few drops on the floor once a week.
But he put it away for the winter with a topped off tank, and found the tank had lost over half it's content during the winter.
Guess where some of that ended up ...

Have you taken off the oil filler cap and smelled for gas ?

That's interesting. Admittingly I didn't check the oil level after. It appeared low when I check but I think it was when the bike was cold..? I can't recall I just know I added some. I think there was still an issue, what it was I'm not sure, but the oil in the airbox then into the carb and finally into the engine / plug certainly had something to do with my running issues and misfires.

No smell of gas from the oil fill cap so I suppose I'm okay?

I removed the oil filter and drained it entirely... The eye on the side is still black though.. why is that?

[Big T]

If the eye is black, the engine case is filled with oil
The oil filter housing does not drain the oil
You need to remove the oil drain bolt on the bottom of the engine
You should drain all of the oil from the engine and refill with the correct amount