79 gs550 only running on spray

[2wheelkid]

Hey guys, be nice to me, I am quite mechanically inclined but this is my first bike.. Just picked up this 79 gs550. Started and kinda idled but would die out so I figured I'd clean the carbs up. After putting the carbs back on I put fresh fuel in it and opened the petcock, heard fuel running into the bowls and when I tried starting it up she wouldn't fire. Dumped some fuel down the carbs and it would idle and rev until it ran out of the fuel I put in the carbs.. Any ideas of what to do next?! It has an 80 main jet and 50 pilot. 125psi all four cylinders.

 

[thehands]

Hello welcome to dotheton.com You might try cleaning the carbs passages again or looking at the floats and pins if they are sticking thy won't allow much fuel or any at all into the bowls.

 

[jpmobius]

You know two very important things. 1) the engine runs, so the ignition is ok and the issue is in the carbs. 2) the problem is insufficient fuel, you know the engine is not flooding. Take the carbs back off and remove the fuel bowls. Determine they are full of fuel - eliminates needle and seat/stuck float issues as the problem. If the bowls are empty, figure out why and try it again. If the bowls are full, it means that fuel can not be drawn up into the air stream for some reason. Take the carbs back apart and make sure you can spray a clean jet of carb cleaner through all the fuel and air passageways. You have to take the carbs completely apart to do any good. It is vital to understand that the fuel that enters into the air stream through the main bore of the carb is not just fuel, but fuel already mixed with air. Those separate small holes at the intake bell are air passageways that lead into the fuel passageways coming from the main and pilot jets respectively. The amount of air is controlled by a variable needle (pilot air screw) in the pilot circuit and a jet, sometimes permanent, in the main system. In both cases, the air mixes with the fuel in an emulsion tube directly above both the pilot and main jet. When the air stream through the carb draws out fuel, it pulls in air at the same time which mixes with the fuel before it emerges into the main bore. So you have to take everything apart and figure out what all the passageways do and make sure carb cleaner spray comes out all the right places. The pilot circuit usually has no separate emulsion tubes, the jets are integral to the tube which is why they have a perforated tube on the end. Take the pilot jet out and spray into the air passageway that leads to it from the intake bell and the stream should come out inside of the pilot jet opening. It should also come out of the pilot air screw opening. Spray into the pilot jet opening and you should get spray coming out in the main bore right where the throttle closes of the bore through a tiny hole(s). The main system works the same way. Take out the main jet and above it will be an emulsion tube - usually just a brass tube perforated with holes. Usually this tube is removed through the main bore and is held in by the main jet. Fuel flows through the main jet, then through the perforated tube. The outside of the tube is surrounded by air, which is supplied through an air passageway coming from the intake bell. Air flows through the perforations in the tube, mixes with the fuel, and then enters the main bore usually metered by a tapered needle. The pilot system works the same way except it is a bit simpler working at only small throttle openings.
So figure out what all the passageways do and where they lead, make sure they are clean and put the carbs back together. Common baffling problems are some big air leak like a vacuum port is uncapped and slides in backwards. The flat or straight bottom side goes toward the engine, the cut away side goes toward the air cleaner.

 

[fta_dave]

Does that model have a vacuum operated petcock?

 

[2wheelkid]

There is a line running from a brass pump to the petcock. There's two wires that run from the pump to under the seat. Possible that the pump isn't working properly? I have since been cranking on it with the electric starter and it melted a ground wire.

Does that model have a vacuum operated petcock?

 

[teazer]

That's odd. Could you post some pictures please. That bike doesn't have a pump originally but it should have a vacuum petcock which may be defective or may have a leak in the vacuum hose.

 

[2wheelkid]

My bad. Was looking at the wrong line. The vaccum line runs from the carb to the petcock.

That's odd. Could you post some pictures please. That bike doesn't have a pump originally but it should have a vacuum petcock which may be defective or may have a leak in the vacuum hose.

 

[teazer]

OK, so what was it that you were looking at? Was that just the starter cable perhaps? Might look like a hose .

So the real question is: Are the float bowls full of fuel or are they dry? If they are filling with fuel the issue is probably clogged pilot jets of the tiny passageways that fuel air mix is discharged through

 

[2wheelkid]

Fuel bowls are full of fuel.

OK, so what was it that you were looking at? Was that just the starter cable perhaps? Might look like a hose .

So the real question is: Are the float bowls full of fuel or are they dry? If they are filling with fuel the issue is probably clogged pilot jets of the tiny passageways that fuel air mix is discharged through

 

[teazer]

Time to take the carbs back off and clean them meticulously. Check each jet and take lots of pictures.

I use an ultrasonic bath and then I spray a can of carb cleaner through each jet and drilling one at a time and repeat on the next carb. Each circuit should spray the same on all 4 carbs. Do that with good ventilation and try not to breathe too much of that lung destroying crap.

If you don't have a manual, now would be a good time to get one and see if you can find any carb manuals. That will help you understand what you are looking for, but start with a parts diagram and stock specification and check all your jets against spec.

 

[runmikeyrun]

teazer hit the nail on the head with ultrasonic cleaning. Those things are a godsend. If you don't own one, most bike repair shops do and will be happy to clean them for you (for a fee). It's money well spent. Don't ever clean jets with a wire, you will ruin them.

Carbs can be frustrating, especially if you don't understand how they work. The internet is here for more reasons than cat pictures and political rants... there's a ton of good info out there! Make sure the info you're getting is the same from multiple sources to make sure it's not bogus. Read up on your brand of carbs and know how each component works, how to change the settings and how it affects other things. Educating yourself makes this kind of stuff a lot easier.

If your bike is stock, then make sure to get the stock jetting and setting specs. Simple things like mentioned before about carb slides being in backwards etc will only cause you to pull the carbs yet again. Work carefully, even if that means slowly. A few extra minutes will save you headaches in the long run. A factory manual is invaluable, and most can be downloaded for free for vintage machines.

Outside of the carbs, make sure to check other related components- vacuum lines, intake boots and other rubber components will age and crack, creating vacuum leaks. The cracks can be too small to notice, so once it's running if you're still having issues that indicate a lean condition, spray these areas with some WD-40 and if your idle speed changes you've got a leak there. Even an air filter that appears "clean enough" can be too plugged to let the engine breathe properly. Petcocks and their screens can clog with rust and dirt, even if your tank looks clean inside.

Once you get it running make sure to sync the carbs.

Edit: you can check your float bowl fuel levels with a piece of clear tubing. Connect the tubing to the drain port on the bowl, then hold the other end of the tubing above the level of the bowl to create a U. When you open the drain screw, fuel will fill the tube up to the level it is in the bowl. The manuals have a spec on how many mm it should be relative to the bowl, but if there's a big discrepancy or no fuel in the tube at all it will be obvious you have a problem. I would definitely check this because your problem is fuel related. Just because you hear fuel filling the bowls doesn't mean it's filling ALL of them.