Meier Link

Might want to check the petcock diaphrams while you have it off. Should be 2 in it and a spring. If the filter is that dirty then the petcock might not be functioning correctly. Also look for holes in the diaphrams use a flash light to check them. There should be zero. If there is even a pin hole in one they need to be replaced. Just a few bucks on ebay for a rebuild kit.

Also when putting the lines back on the tank make sure to put the vacuum line back on the correct nipple on the petcock.

CruxGNZ

You hardly ever hear of the fuel filter getting clogged on these bikes. It is usually crap in the carbs causing issues or the petcock diaphram that fails and wont let fuel get to the carbs making the bike stall. The later bikes have the petcock attached to the frame. Close off the fuel supply line from the tank, a vice grips works by pinching the line. Remove the petcock and check for correct operation.

Another thing, if you were working on the bike, make sure you didn't knock the vaccum line off the petcock. In case you weren't sure of how it works, when the engine turns over, it creates a vacuum. This vaccum pulls (for lack of better words) on the diaphram and lets the fuel flow to the carbs.

smokinjoe73

Man if that is clogged then you have way bigger problems, like someone left the cap open for a load of leaves to fall into it.

Like all fuel delivery relate issues. Drain the gas tank to bone dry, remove and check the petcock, replace, use new gas then retry.

Most likely your issue is carbs being dirty or if its at high rpm the ignition coil being bad (one or both).

Oh and yes if your vacuum line is messed up then that's the issue.

After you've done these steps, read the thread on adding an inline fuel filter and do that. It saves you countless hours removing carbs and messing with them.

duron600

Thanks a lot guys, I have a few things to try out now. I want to start with the simplest/cheapest/easiest ones, of course. How does this order sound:

1. Empty tank completely, take petcock off, remove filter, try to clean it. Add new fuel.
2. Check diaphragm in petcock, replace if broken. Probably combined 1 and 2 for practical reasons.
3. Replace petcock with a new petcock (any links to where I can buy one?)
4. Replace one or both spark plugs
5. Disassemble and clean carbs

Read somewhere about emptying tank, addind some carb cleaner, so it flows down into carb ... sumps? ... and leave it for a few hours to work its magic, then suck it out from some tubes under the carbs (no idea how to find these), add new fuel. Anything to that?

Thanks again!
Daniel

duron600

One thought: Since the engine runs fine at low power/speed/acceleration, does that indicate that the fuel filter/petcock is ok, but something is blocked in the carbs?

Meier Link

The simple check is the petcock and filter. If both are clean and undamaged it is the carbs.

As stated above, unless the tank was left open if probably not the filter but it's also a simple check along side the petcock. If both of those are good then it points to carbs.
The petcock is an easy rebuild. No reason to replace the assembly if it is the problem.

From what I know of these carbs is that if they are dirty then clean them properly then sync them. Don't try to do a quick fix method. More heart ache then solution.
Eliminate those 2 first and make sure the petcock is connected properly and it helps us refine the problem for you.

Edit: I'm drunk. Sorry if this doesn't make sense hahha.

smokinjoe73

It sounds like fuel starvation. Search the threads for the vacuum operated petcock. If you have the vacuum on the wrong nipple you can get this same symptom.

Putting carb cleaner probly wont hurt but also may not help. You drain the float bowls from underneath each. Its pretty simple.

But dont sleep on that vacuum line thing. That could be your whole issue.

CruxGNZ

I agree with Smokin'. It sounds like the petcock isn't working correctly. Super easy to rebuild it. Rebuild kits are found on eBay for very little money.

But first, make sure the vaccuum line leading to the petcock hasn't fallen off. If it is connected, make sure it is connected to the correct nipple on the petcock. If it is connected to the correct nipple, then check the entire length of that vacuumm line for cracks/holes. If everything checks out, then your petcock needs to be checked.

Killemall