WHAT would prevent bike from firing?

[baz666]

Sorry about all the posts but I'm going nuts. My 83 GS750ES cranks like hell but not a cough or stumble.
I get spark on a 4 cylinders when laying the plugs on the valve cover.
Fuel runs out the petcock, no problem.
I changed the vacuum hose since there was a tiny crack in one end.
The signal fuse was blowing. Traced it to a bad ground on front left turn signal.
I put a few drops of gas directly into cylinders, as BikeCliff suggests, and cranked it. Nothing.
I hooked up a little test light to the fuse box. The light stays on when plugged into headlight circuit.
The light also stays on when plugged into the signal circuit.
The light comes on then GOES OUT after a moment when plugged into ignition circuit.
Does this mean I've got power to signal and headlight but not to ignition?
I don't know what else to try.
I am going nuts trying to get this bike to kick over! I feel like if I can just get it started, things will begin to make sense.
WHAT the heck would prevent it from starting when it's got fuel, spark and air?!
ANY help would be MOST appreciated. I'm tearing my hair out!

[SVSooke]

Did you at some point disconnect the coils?On mine the PO had the leads wrong so just swapped the plug connectors.Worth a double ckeck.

[TheCafeKid]

I am assuming it has clean carbs? And I mean clean ??
Airbox is sorted assuming you have one?
Fuel/air ratio must be pretty close to correct for it to fire. What are your adjustment screws set to on your carbs? Float level? If you're getting spark it's not the ignitor. Have you checked the battery has good charge? Especially when you hit the button? If the batt drops too low when the starter button is pushed (below 11 is not great) then you may not have hot wnough spark. Are your plugs wet with gas after all this cranking? Gaped correctly? Yada yada yada.

[baz666]

I figured that a few drops of gas in the cylinders would at least give me a cough or stumble and that would let me know if it's the carbs or not but maybe that's not enough. I was hoping not to have to pull the carbs off but I guess there's no choice.
I supposed if I'd done that at the beginning instead of screwing around trying this and that, I'd have a clean set of carbs now, if nothing else.
Okay, here goes, carbs coming off today. Wish me luck!

At the risk of starting to sob openly, I have to say this site in an incredible resource and that 'resource' is the people who spend their most valuable asset - their time - to help others for no monetary return.
thanks,
baz

[spyug]

There are two things ( well likely more but two stand out in my experiences) that will cause a problem....fuel and spark. It looks like you have spark so its got to be fuel so pulling and cleaning ( a full dissassembly and soaking in carb cleaner, then blowing out all the passages and cleaning all the holes in the jets) is an absolute must. The carb cleaning series is an amazing resource and takes you through it step by step. Do a through job.

Once you clean the carbs, starting for the first time can be a bit of a chore. Some fire right up, some labour. What I have taken to doing is adding a second auto or marine battery for extra cranking power. It gives plenty of power to crank it over for many minutes plus extra juice to insure good spark.

Make sure that you properly bench synch the carbs so that the butterflies are open about the width of a paper clip. This is important since if they are totally closed you will have a bugger of a time getting them to work. It is also a good idea to have your airbox properly fitted as they seem to work best when they're on.

You realise too that once it runs you will need to "dynamically synch." the carbs which requires the use of a manometer ( vacuum synch tool).The best one is the Morgan Carbtune which is pricey at around $120 but invaluable if you are going to be working on bikes. The bike may run well enough without it but they really do have to be synched to get full power and good fuel economy.

As the others mention also the quality of spark is important and its very common in these older bikes to have a weaker spark caused by a voltage drop to the coils. As connectors get older they corrode and this causes resistance in the circuit. Your battery may show 12+ volts but when measured at the coils it may have dropped to 11 or even less. The coil relay mod was designed to overcome this and as its a relatively cheap and easy modification I heartily recommend it.

Good luck with the cleaning. Don't forget to order your o-ring set from Mr. Barr.

cheers,
Spyug

[Dodik]

well i havent seen one other common suggestion yet but did you check your valve clearances?

[baz666]

Thanks again for all the suggestions and help, you guys.
I just checked the valve clearances but maybe I messed up somehow so I'll check them again today. I've been using BikeCliff's directions for that.
thanks again for all the help. It really is great to feel like I'm not totally alone in this.
baz

[Dave8338]

well i havent seen one other common suggestion yet but did you check your valve clearances?

It will still buck and snort with REALLY tight valves.


You say you have spark.

Plug wires all going to the correct jugs?

Are the pistons wet when you look through the plug hole, PRIOR to throwing some fuel in the hole?

When was the last time it ran?

Fuel in the float bowls?

Compression?

Carbs?

[baz666]

No bucking, no snorting.
Wires going to correct coils, according to wiring diagram.
Pistons were dry as a bone PRIOR to putting a few drops of fuel in each jug. I mean dry as a desert bison skull, despite all my choke and full-on throttle and cranking.
Last time it ran was Nov. 9/10
Yep, fuel in the float bowls.
Compression between 149 and 151 across the board.
Carbs - now that's the question. I stupidly forgot to add Stabil last fall and now maybe gummed up. But to not get a single snort or rough running or ANYTHING at all? WTF?
The thing cranks like mad. But here's something I just thought of. Normally, you'd think that after all that cranking and wide open choke you'd smell even a little unburned gas. In this case - NADA, not a whiff, so maybe that's a tip.
Electrically, it was blowing a signal fuse but that was a bad ground on front left signal. However, I rigged up a test light and plugged it into headlight fusebox circuit - light remained on.
Then plugged it into turn signal fusebox circuit - light remained on.
Then plugged it into ignition fusebox circuit - light came on then went out after a moment.
Huh? What does all this mean? I sure don't know but she keeps on cranking like the dickens. Just won't fire.
The consensus seems to be carbs, not getting fuel.
If I can't get it started today while trying some earlier suggestions, like spraying carb cleaner into airbox while cranking then it's time to pull out the carbs and do the super clean. Was hoping to avoid that but as I said somewhere else, if I'd got down to it istead of carping on this forum, that would be done by now... Oy vey, the trials of the ham-fisted home wrench...

[waterman]

What shape is your battery in? How old and what type? A test light isn't going to be sensitive enough to tell the state of charge. Can you jump start bike using a car battery and jumper cables. If you try, leave automobile engine off, just run cables to existing battery and try starting.

If your bike was running okay last fall, short of living in a desert, I doubt your carbs are completely gummed up. The battery, if left alone, has steadily lost potential and is most likely borderline charged. My bikes will happily spin over but not start without a decently charged battery. Automobile batteries with their higher amperage capacities will quickly spin over a bike engine and provide the extra oomph to fire off less than perfectly tuned engines.

Once it is running, then you can worry about state of tune. I find it odd that all of these carbs need cleaning after essentially a short period of inactivity, months of course, not years. My brother's gs1000g that he purchased new, had never had the carbs cleaned until last year. He let it overwinter, inactive, ever year. Every year it would start and run okay. Since I have cleaned, tuned, and synchronized it, it runs better than it has run for many years but that is another story.

[chuck hahn]

When i tore down my 78 1000 C this winter it ran like a champ..then the same problem as your having. Like it was 180 out of time. Had spark and lots of gas flow. I switched the coils in the wiring harness and magic. I hooked the coils back up into the wrong sets of wires.. try that and see what happens.

[spyug]

Gummed carbs after a few months seems common enough up here. Its happened to me even with a healthy dose of Seafoam in the tank so I wouldn't rule that out.

No one likes doing carbs but after about the 10th time you do it you could do them in your sleep Actually the more you do it the better at it you get. The hardest part is really getting them out and in again. A couple of tips for you:

On taking them out remove from the right side and leave the cables in place until the carbs are mostly all out.
On reassembly put the cables back on prior to mounting them to the boots. It makes it alot easier.
To get the carbs back on with a minimum of fuss heat up the boots with an electric paint stripper gun ( Crappy Tire $20 or less) until they are soft and pliable. Shoot a little WD40 or other lube in the throats of the boots and use a long handled screw driver or pri bar & a rubber mallet to get them to pop in. The best method I've found is to insert them from the bottom at an angle and lever them upright and forward with the pribar . One side usually goes in first so tap the other end with the rubber mallet to drive them home. works every time.

Now quit stalling and get them stripped, dipped and rebuilt.

Good luck.
Spyug