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GS750 no start

  • Thread starter Thread starter theinsuranceguy
  • Start date Start date
T

theinsuranceguy

Guest
Been having some trouble with this one and I feel it's simple, but I can't seem to get this thing fired up. Bike was running well for two years, however it had some bad leaks from the carbs overflowing. I adjusted the float bowl height, cleaned the carbs meticulously, installed new air boots, new battery, new plugs, new valve cover gasket (old one was leaking) and rebuilt the petcock because it was leaking.

The bike sat all winter and the battery was no good come spring so I went down to local motorsports store and replaced it. This morning I checked for spark at all four plugs by removing the plug and grounding out on the rad fins - I reconfirmed that each wire was correct from the dizzy to its respectable cylinder - I could see a healthy spark at all four plugs.

The starter just cranks and it doesn't even want to sputter. What am I missing!!:p

Thanks all!
 
"... cleaned the carbs meticulously,.........and rebuilt the petcock because it was leaking. "

Were the carbs cleaned just before this problem developed? And how did you clean them? Sounds like the choke system is not "enriching" as you crank. My bike will not cold start without a functioning "choke" system.
What year is this 750?

did you check petcock for correct operation? Petcock rebuilds are an iffy proposition.
 
Cleaned the carbs then let it sit all winter?

You might need to clean them again if they had fuel in them.
 
theinsuranceguy said:
... I checked for spark at all four plugs by removing the plug and grounding out on the rad fins - I reconfirmed that each wire was correct from the dizzy to its respectable cylinder ...
Click to expand...

You have "rad fins" AND a "dizzy"? You sure you are working on a GS? :-k

.
 
Did you reconnect the vacuum tube to the petcock? I forgot to do this once on my GS750 and was all upset that the bike wouldn't start after putting the carbs back on. I was cranking and kicking and itching my head and once I reconnected that tube it fired right up.
 
tom203 said:
"... cleaned the carbs meticulously,.........and rebuilt the petcock because it was leaking. "

Were the carbs cleaned just before this problem developed? And how did you clean them? Sounds like the choke system is not "enriching" as you crank. My bike will not cold start without a functioning "choke" system.
What year is this 750?

did you check petcock for correct operation? Petcock rebuilds are an iffy proposition.
Click to expand...

Ya I saw that he did that, but it doesn't mean the spark plugs were getting fuel. Keep in mind the bike only needs 3 things to run well, fuel, spark and air. Sounds like the spark thing is working, move on to the next... I am assuming you primed the carbs before trying to start it after sitting so long.
 
The year of this bike is 1977. I have a clear fuel filter and the petcock in the open position fills that unit pretty quick. I unscrewed one of the float boat drains yesterday to check for fuel in the float and it was dripping out when unscrewed it a little.
 
I did reconnect the vacuum to the petcock. Is there a decent vacuum diagram out there that shows where the vacuum line connects to on the carbs? I'm pretty sure its in the right spot however I want to make sure. Thanks.
 
Steve said:
You have "rad fins" AND a "dizzy"? You sure you are working on a GS? :-k

.
Click to expand...
Excuse my vocab. Engine cooling fins and ignition coil. Old VW engine engines are on my mind sometimes.
 
azr said:
Ya I saw that he did that, but it doesn't mean the spark plugs were getting fuel. Keep in mind the bike only needs 3 things to run well, fuel, spark and air. Sounds like the spark thing is working, move on to the next... I am assuming you primed the carbs before trying to start it after sitting so long.
Click to expand...
I did not prime the carbs. Is there a proper procedure on how to do this?
 
Switch the petcock to `Prime' for a few seconds to allow fuel to flow freely and fill up the bowls. Just don't forget to put it back to `On' afterwards.

Did you check the vacuum tube? If it's not hooked up to the petcock or plugged with something it is drawing a lot of plain old air into the system and will make it hard to start.
 
Last edited:
Vacuum line from the petcock goes to the port on #3 carb.

Your old VWs had radiator fins?! Just messing with you...
 
It feels like there is no combustion happening (obviously)... the starter is cranking away (sounding healthy) when using the button start, but when using the kick starter there is no resistance on the way back up. It feels like everything is in place (fuel, spark and air) but no combustion...I'm truly at a loss here. I don't feel like doing a whole engine rebuild here but is that what I should be doing to figure out the problem?

Is there a way to check fuel is actually making its way into the combustion chamber?
 
If you crank for a while with "choke" system activated, then remove spark plug, you should see signs of fuel on plug. No fuel likely means your enricher (aka,choke) is not working - not supplying extra fuel dose.
 
It feels like everything is in place (fuel, spark and air) but no combustion
Click to expand...

"Feels like" ain't gonna be enough. :)

So, as usual, the things required: Spark, Fuel, Air(/Compression). You confirmed Spark, so that leaves Fuel and Air (and the proper mixture of them).

Old trick here is to drop a teaspoon of gasoline down each cylinder. By the time you've screwed in the spark plugs back in, you should be able to get at least one to two firings.
 
roeme said:
"Feels like" ain't gonna be enough. :)

So, as usual, the things required: Spark, Fuel, Air(/Compression). You confirmed Spark, so that leaves Fuel and Air (and the proper mixture of them).

Old trick here is to drop a teaspoon of gasoline down each cylinder. By the time you've screwed in the spark plugs back in, you should be able to get at least one to two firings.
Click to expand...
I did the trick as suggested and sure enough she fired right up. It was the first time, I have heard the bike running in over 5 months.

I take it this confirms there is no fuel entering the cylinder? How can I narrow down the problem from here?

Thanks
 
If the bowls have fuel but the fuel isn't reaching the engine, then the carbs could have some blocked passages.

What are your fuel and air screws set at? I think 3/4 tirns out on the fuel screw and 1.5 turns out on the air screw is what's recommended. Hopefully, one of the more experienced guys will correct me if I'm wrong.
 
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