gs400...... thing just wont run....

[bigb3433]

okay so i bought this bike a couple of weeks ago and when i bought it supposedly it needed a new battery and a petcock did both of those and changed the oil....before this the bike was runnin fine other than the fact that it would not idle or it would idle very high.....then it flooded one day and its just never been right since wich is why i did the petcock and oil change.....so i tried to start it today and it fired right up revved itself really high and stayied at like 4500 for like 2 min then sputterd and fell and would not start for anything.....

trying to make this short i pulled the carbs and gave them a really good blow through with the air gun and carbsprayed them then jumped the bike (battery was totally dead from trying to start it) and it still wouldnt run until i finally let it crank on prime for a few minutes......

got it going rode around the neighborhood but i couldnt adjust the idle down and now its popping and misfiring really bad when i can even get it to fire (big black smoke) i changed the plugs too by the way....im just at my wits end i just wanna ride the thing and i dont know why its just decided to not do anything lately; anything i havent tried? the bike is being really inconsistent somtimes itll stay running on th"on" setting and somtimes it has to be in prime but it wont idle at any adjustment or pet cock setting i just need some guidence im very new to old bikes but i just dont want to mess anything up further.......


sorry for the long post but i wanted to give a very clear picture because its a problem i just cant seem to diagnose much apprecitation for any suggestions

[bigb3433]

okay quick update ive been searching (great site by the way) and i noticed that yes when i put the plugs againsnt the block it was barley a spark not a fat blute one what can i do about this or is this just because im trying to jump the bike on a basically dead battery? ....................also i dont have any vacum lines going to my pet cock and the bike didnt have any when i bought it (runs 2 k&n filters as well) so should it be running in prime anyway?? trying to figure this out on my own but who knows what ill come up with soo please respond even if u have the slightest idea

[Guest]

What year is it? Charge your battery before trying to start it. There should be a vacuum line that runs from your carbs to your petcock.

[bigb3433]

k ill make sure the battery has a good charge on it....where on the carbs does this vacum line connect too?

[kalessin]

If your bike is equipped with a vacuum petcock (it'll have two hose connectors on it if it does, and markings like "PRI" "RES" "RUN" (or "ON")), there should be a vacuum connection on the #2 carb (counting from the left side of the bike).

If the bike once had a vacuum petcock and doesn't anymore, then there should be a hose connector on the #2 carb that's plugged.

What exact year/model is your bike?

[Guest]

Year

That's why we need to know the year. Pre 80 have vm carbs and I don't know squat about them. 80-86 have CV carbs which I know too much about.

[bigb3433]

sorry that i keep forgetting to tell the year and model its a 78 gs400.....so this vacum port is on the petcock side carb? i saw a little black plug on somthing that u can swing back and forth a little i dont know if thats it or not its right by the choke lever....right now i stuck it on the vent to the bowls but i know this isnt right so im still runnin it on prime only right now..

[nabrams]

The good news: I believe the '78 has VM carbs (assuming they are the originals) and will run even if there are some serious air leaks (boots, airbox or exhaust).
The bad news: Even though it will run to some degree with serious air leaks, you still need to get those leaks resolved if you want it to run right in all RPM ranges.
Also, if those coils are weak you're gonna have problems at high RPM (coil performance degenerates at higher RPM).
Being a '78, it probably has points ignition - so they should be checked and adjusted after you get the air leaks worked out.
You need to know for sure whether or not that plugged-up hole on the carb is a vacuum draw or a vent/overflow - get a manual. If it's a vacuum draw then leave it plugged and run the bike on prime for now. If it's a vent/overflow then you need to unplug it.
If that petcock has an "Off" position then use it when the bike is not running.

[ptm]

Some random thoughts...

You're definitely dealing with the CV carbs not VM . The GS400 had the CV carbs from the start - years before the other models which didn't switch to them until '80.

As someone else mentioned, the stock petcock should have 2 hoses running to it - the fuel line and vacuum line. I can check tonight to see exactly where the vacuum line should come from on the carbs and maybe post a pic. If the line's not there the bike will run on 'ON' for just a minute or so until all the fuel in the bowls is gone. It should run fine on 'PRI' as long as the needles and seats are sealing properly. If they're not it will flood badly and either spill on the ground through the overflow hoses or contaminate the oil, or both. K&Ns should have no affect on the petcock setting.

The K&N pod filters would make it run lean if no jetting changes were made since the stock airbox is very restrictive. What do the plugs look like? white - tan - black - wet - ...

Be sure the battery is fully charged before trying to start it.

Does the idle adjustment screw have any affect on the idle speed? Doesn't sound like it from your description.

Are you using fresh gas?

Have you done a valve adjustment? Compression test?

That year does use points and condensors, have you checked the point gap and ignition timing etc.?

[ptm]

The vacuum hose for the petcock comes from the front left side of the left carb (as you sit on the bike). It's right above the throttle shaft.

Here's a pic

[bigb3433]

AHHHHHH.......that pic did it for me thats what was capped off that i was talking about ill try runnin my vacum hose to that tmrw and see if i can get this thing to run on the "on" setting finally....i think my problem was a combination of the bike being set up to run on esentially what would be the same as prime because it had a broken petcock before that would do nothing to stop the fuel from flowing....i really appreciate the help guys its raining bad here now but hopefully i can try to fix some things tmrw....

[robinjo]

So that's the vacuum tube set up, now it sounds like you have to adjust your points.

Check condition of the points, need to be flat and not too thin.
Set the height between 0.3 and 0.4 mm (0.11-0.15").
Set the timing with a timing light.
Check for sparking between the points.
If there is sparking (a small amount is not too bad) change the condensors.

If you don't know how to perform these procedures get a manual.