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Throttle results in decreased RPM

  • Thread starter Thread starter TylerM
  • Start date Start date
T

TylerM

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So after cleaning up the carbs on my GS550 and bench syncing them (pulled everything apart, cleaned thoroughly, replaced o-rings), I put them back on the bike and got some weird results. My bike will start but it is very hesitant. It takes a while to finally fire and when it does the idle is very rough. After about a minute it will die regardless of what I do to it. In this short window of it running I took the choke off and it still reacted in a similar way (so it runs with or without the choke on). After it shuts off it will not start up again, and I will have to wait a few hours to even try to get it back up and running. This was a problem I was having periodically before pulling the carbs.

The other issue is that when giving it throttle while running, the revs actually decrease.

My amateur diagnosis is the floats, because when cleaning up the carbs I looked at the float heights and saw they were pretty low compared to the manual specs. I quickly noticed that if I were to bend the tang to get them to the factory spec, the bend would have to be very extreme. Not being confident in myself, I thought that couldn't be right and I must have measured from the wrong spot. I decided that I thought they were good enough and put my carbs back together. After finding this thread, and the last few posts about the floats and needle valves, I'm thinking I made a terribly bad judgement call.

Unless I missed something, my next course of action will be to replace the needle valves, which should (hopefully) get the floats back to their correct specs, or at least closer. Am I doing the right thing? Does my logic make sense or am I missing something?

Would this be the same reason the bike struggles to start and won't stay on for more than a minute or two? Or is there another problem that I need to look into?
 
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Your needle valves are probably just fine. In that thread you linked, it looks like he simply had the wrong needle valves- they weren't worn out.
 
Did you totally dissemble the carburetors and soak them in dip for 24hrs per all the rebuild tutorials, replace O-rings?
 
I didn't dip them, but I thoroughly cleaned them with spray and compressed air, and used copper wire to poke through all holes and ensure they were clear. O-rings are less than a week old.
 
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I would wager that the carbs aren't clean enough. You could also have a vacuum leak.

Take them apart again, soak them overnight in something potent (carb dip or solvents, I use straight acetone), THEN blow out all of the passages. If you're still having problems after that, then we'll be here to help you figure it out. But there's a lot of guys here, myself included, who won't spend time trying to help you out until we are sure that the carbs are clean. We expect them to be soaked or ultrasoniced.

-Matt
 
lemonshindig said:
I would wager that the carbs aren't clean enough. You could also have a vacuum leak.

Take them apart again, soak them overnight in something potent (carb dip or solvents, I use straight acetone), THEN blow out all of the passages. If you're still having problems after that, then we'll be here to help you figure it out. But there's a lot of guys here, myself included, who won't spend time trying to help you out until we are sure that the carbs are clean. We expect them to be soaked or ultrasoniced.

-Matt
Click to expand...

I bought dip and will try that. I thought you were only supposed to leave your carbs soaking in dip for an hour or so?
 
TylerM said:
I bought dip and will try that. I thought you were only supposed to leave your carbs soaking in dip for an hour or so?
Click to expand...

Nah, just make sure you remove all the gaskets and rubber.

Solvent won't hurt the metal.

Really though, it's more than likely not the needle valves. The aftermarket ones are usually worse than the factory ones you (hopefully) have still.

Do set the float height.

The bike ran okay before you tore the carbs apart? Did you put the fuel and air screws back in the same spot?
 
lemonshindig said:
Nah, just make sure you remove all the gaskets and rubber.

Solvent won't hurt the metal.

Really though, it's more than likely not the needle valves. The aftermarket ones are usually worse than the factory ones you (hopefully) have still.

Do set the float height.

The bike ran okay before you tore the carbs apart? Did you put the fuel and air screws back in the same spot?
Click to expand...

The bike ran before I tore everything apart but did have a tendency to try to shut itself off without warning. I had to keep the throttle engaged slightly to prevent it from shutting off on its own while riding, and it had a significant hanging idle. I do still have the original needle valves, and I'll soak the extras (just in case) I have from a used set of carbs I bought to take some parts off of. The mixture screws are set at 2.5 turns from lightly seated.
 
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