Engine won't start when cold.

[nategs550]

Hey guys I'm needing some opinions on this issue I'm having. If the bike sits for over a few hours its almost impossible to restart. Turns over acts like ive got no fuel. Spray a shot of starter fluid in and she fires right up like she should. I've replaced the petcock, cleaned the carbs, new o rings on carbs and airbox boots. Checked for air leaks and found none. She runs great with no hesitation, pulls to redline. If I shut her off warm she fires back up. Wait a few hours and usually requires another shot of starter fluid. The only thing I haven't don't yet are the valve clearances. Does this sound like my issue or are there some other ideas I can try before I tear her down?

More specifics to help out.
1980 GS550E. Mikuni BS32 carbs. Pretty much stock except for a 4-1 exhaust the previous owner made.

Main Jet: 97.5
Pilot Jet: 40
Air Jet: 150
Idle Mixture Screw: 3.5 turns out

Hope this helps.

 

[hillsy]

Sounds like it might be a vapour lock in the fuel line if it fires right up with starter fluid.


How is the fuel line routed? Does the line loop around anywhere? It needs to be as direct as possible from the fuel tap to the carbs. Do you have an in-line filter?

 

[nategs550]

It's as short as humanly possible. I had a vapor lock issue before with the inline filter so I removed it and am now only using the stock petcock screen and the tiny screens in the bowls. Thought maybe the gas cap but removed it for testing and still no luck. Restarts fine when warm so a hot fuel line should be ruled out no? Only trouble when its sat a few hours.

 

[HerrDeacon]

Are you are using the choke/enrichener when cold?

Also, typically on Mikuni's the mixture screw should only be in the range of 1 to 3 turns out, anything outside this range indicates you have the wrong pilot jet installed. I'm not sure if this applies to the BS carbs or not, but worth looking into.

 

[nategs550]

Yes I am using the choke. I turned it to 3.5 turns per stock specs I found on GSResources.com. I thought about possibly turning it back to 2.5 and see what happens. Probably attempt that today. On a quest for a front fender at the moment.

 

[nategs550]

Followed this as best I could.

 

[Big R]

Until you check your valve clearances, it's tough to rule out anything else. Any time mine were out of spec, it was tough to start when it was cold.

 

[nategs550]

On my list of things to do today. Never know if it was hard to start beforehand because I bought it in pieces. Lol. Thanks guys ill get back to you with clearance checks and go from there.

 

[hillsy]

It might be that the fuel is evaporating out of the bowls as it cools down.


Do you still have the breather pipes connected?


Try switching the fuel tap to PRI next time this happens and see if the bike fires up.

 

[nategs550]

Breather pipes are not connected. I switched to prime yesterday and she did indeed fire right up.

 

[TheCoffeeGuy]

Sounds like your bike have a vacuum actuated petcock, did you replace that hose?

 

[hillsy]

Re-connect the breather tubes. You can use clear tube if you want. Route them so they go down between the gearbox and the swingarm pivot.


It sounds like the fuel is evaporating out of the bowls via the breather ports.

 

[nategs550]

Yes I replaced the vacuum line for the petcock. I'll hook up the hoses today and see what we get and reply back. Thanks for the help guys.

 

[znsullivan88]

yeah. id look into the petcock. my gs400 had a similar problem. rebuilt mine and it solved my starting cold issue.

 

[fury413rb]

if my bike sits longer than a day or two it needs to be on prime for a bit before starting.

 

[nategs550]

Here's the problem. 2 of those set screws that activate the choke apparently backed out and fell out I assume while testing it around my neighborhood. Pull the choke cable and only 2 of 4 were working. Grabbed some from the local salvage shop, pulled the choke cable up, fired within 1/2 second of touching the button. Been running great now.