78 GS750 wont ever start cold

[Guest]

Hello all I am EJ,

I amd the owner of a 78 gs750. I originally bought the bike as a get me to the next CB750 but ended up falling for it haha. So after two years I've learned a great deal about not only this bike but how to repair most anything on the bike. I've fixed so many issues that had been on the bike and feel so proud of myself. I am now down to the very last and one of the biggest issues its ever had: Not starting when cold/hard to idle

So heres the things I've fixed and or replaced on the bike to find the root of the problem:
-Valves checked
-upgraded to dyna s ignition and timed just how the instructions stated
-spark plugs have been replaced
-plenty of good oil
-battery has plenty of juice at 12.6 volts
-spark plugs make a nice blue spark
-carbs have been taken apart and cleaned very well
-carbs have been properly synced
-fuel screw and air screws are set to stock settings
-all jets are stock from what I read in the shop manual
-Spark plugs are nice and brown in color
-Stock air box
-Vance and Hines exhaust

So the only time I ever have this problem is when it first fires up. I HAVE to bump start it. No option when its cold. Once I have it bumped I have to hold the throttle a while when its warming up as it will just die. Doesn't matter if the choke is on or not. Once it starts to warm up I let of the choke till it will hold an idle with the choke all the way off. After that, never an issue again until I let it set again overnight. I've been banging my head trying to figure this out and the last step I've been told is that either I may need to go up a jet size(but it pulls hard no matter what I do)or that I need to raise or lower the jet needle. I've not done either of the last two steps but wanted some insight into this from the experts on here.

I thank you all greatly in advance and hope to learn much more about this fun bike

[chuck hahn]

Turn the side MIXTURE screws in 1/8 of a turn on all 4 carbs. This will richen the cylinders and may make it catch easier. By "stock" settings what are they set at now...both the bottom pilot s and them side mixture screws??? Nn My 77 750 and all my 1000s i find that the bottom pilots at round 7/8 out and side mixture screws at 1 1/4 to 1 1/2 out seem to do the best.

EDIT..once its running spray some starting fluid on all the intake manifolds and see if the RPMs go up as your hitting them. If so its a vacuum leak problem. If not its most likely a mixture problem.

[CrazyCloud]

Also double check the passage inside your float bowls, it is easy to overlook. There is a small brass 'choke tube' that comes down from the carb body and sits in the hole in the bowl, if the passage beneath is clogged it can't suck fuel well on choke. Soaking does not always clean the passage, you need to poke it out with a small wire and carb cleaner and verify that it's open. I think you're pilot jets should be size 15, you might want to check that they are correct. If it has sat for a while make sure to put the petcock on prime for ~30 seconds before you try to start it. Good luck!

[chuck hahn]

True. i have taken a drill bit and had to twist it by hand to get the crap out of the pick up tube in the bowls before. sometimes a good heating will make the crud bubble out the tiny hole too. good to have these cleaned well also. Very good point made here.

[eil]

Have you checked your charging system? Is the R/R stock or upgraded? How old is the battery?

We've seen cases like this where the battery isn't providing enough power to the ignitor while also turning the starter. (But once the bike is up and running, everything is fine since the R/R brings up the voltage.)

Put your battery on the charger for an hour before trying to start it. If it starts fine, you need to troubleshoot and fix the charging situation. Probable causes in order of likelihood: dirty electrical connectors, battery, R/R, stator.

posplayr has a charging system quick test, search the forum for it.

[78GS750_Cafe]

Bike has been ridden about every day since I bought it back to years ago. The jet sizes are 102.5 main, 15 idle, and . I'll check on spraying the carb boots again. I'll give it a good bump start and get it running. Just hope it isn't the boots. It's so hard to tell if they are worn or not. They don't have any cracks but you never know.

The battery is less than a year old. It holds a charge quite well and can crank and crank. So I don't think it is the issue. Oh and the R/R are stock but in good condition. The bike overall is well taken care of. If it looks like its going out or time to be replaced, I replace it ha-ha

I'll check that passage once more but I only rebuilt them about a week ago. I actually use a very small guitar string to clean the passages and then blow compressed air to clean out the gunk. But I'm not against doing this again haha

[chuck hahn]

What exactly do you mean by BUMP start here??? Push starting, using the kick starter, jumper cables...what???? Maybe some clarification in what your describing will help.

[78GS750_Cafe]

Sorry about the confusion. Guess that's a term we use here in the southwest ha-ha. I'm push starting it when it's cold. After that I can kick or electri start no problem

[allojohn]

Still sounds like a choke/valve issue.

[78GS750_Cafe]

So I'm out in the Walmart parking lot spraying the starting fluid on the carb boots and well it kills the bike?!

[chuck hahn]

That makes no sense that it will start push starting it but not off the starter. The starter fluid shouldnt kill the bike unless its sucking a crap ton in all at once and its flooding it almost instantly. try just s little squirt on each one and listen for the RPMs to change.

[78GS750_Cafe]

So it turns out after a VERY close inspection it is the carb boots. They have very small hair line cracks and are wet from where the starter fluid was sucked in. Time to order some new ones along with new intake boots as well. Once these come in I will test it and retune as needed. I have good faith that this is the issue.

[78GS750_Cafe]

One of the boots
IMG_20151031_172825-01.jpg

[chuck hahn]

May want new clamps as well..they do stretch after a while.

[78GS750_Cafe]

Already bought those figuring they were the issue. I'm glad the lighting is lower right now or I couldn't of seen these horrible cracks. Funny thing is they aren't on the outside but are literally peeling from the inside