1976 GT 500 Engine issues

[Omaniac]

Thanks for the add, people.
I just bought a GT 500, 1976. It is in very good condition.
I picked it up this weekend. When I test drove it, I could feel it was running fairly well under power, but when I let up on the throttle, it was not smooth. I kind of felt like one cylinder was misfiring, and the ride was jerky. It also idled low then died when I stopped.
I accepted this as something I could deal with later. Set the idle screw a quarter turn in, that took care of the engine stopping at idle, but then it idles too high.
I drove the bike some 300+ miles to get it home. The previous owner told me he had cleaned the tank and carbs, and installed a fuel filter as well. Also told me to try to avoid using the reserve or pri functions as the fuel was fairly dirty then he cleaned out the tank.
Off I went. My thought on the jerky behavior was probably a need for new jets. The previous owner changed the carb needle to the 2 position (from 3). I don’t know what is the normal position.
So, after a longer drive through mountains (great ride) I was getting a bit low on fuel. 3-4 miles before the next fuel station, in a tunnell (of course), the engine started to die. Switched to RES. Nothing happened. PRI. Nothing happened. Just reached an emergency area in the tunnel before it stopped completely. Yay!
After fiddeling with the petcock and the choke, I got her running again. After a mile or so, she died again. Another round of this and that, and I got it to fire up, and managed to drive to the fuel station. Fresh fuel and it was running again.
When I took off, it was running really well. Quicker, faster and smoother. As it should run when properly set up. Whatever blockage I had earlier was gone.
This lasted a few miles, then it started to die, run again, run on one cylinder, then two. Kept it going like this until I got home.
I know this drive was a bit of a gamble. The bike has 15000 miles on it, and chances are it has been sitting for a long time somewhere. A bit like asking a 80 year old to run a marathon. It actually did well, considering.
The benefit is I got to really see if anything was wrong. Nothing, except for a blown lightbulb in the tach, and the engine issues. Possibly a need to rebuild the petcock.
So what I thought was needed, a rejetting and a tune up, most likely is dirty fuel and dirty carbs. The carbs have been ultra sound cleaned by the previous owner, and the fuel filter should have taken care of any particles in the fuel, but… ?
Any suggestions? If the filter in the petcock itself gets dirty, will this give problems like the ones I had?
And, before you ask, my son was driving my car as back up on this trip.
Unrelated, this bike has a grab rail, was this original on the -76? The owners guide has a picture of a red bike with a grab rail, but that is the only picture I have seen of it.

[karl pa]

You definitely want to open carbs and petcock to make sure everything is clean. When I clean carbs I use carb cleaner and compressed air through all jets and passages to check if there open. I believe the needle clip should be on 3.
The 500 has a tendency to surge when letting off throttle or riding slow speeds, this can be caused by to rich or to lean idle mixture. Sometimes you can adjust air mixture screw to solve, on my 75 T500, i went to 32.5 pilot jet to solve my surging issue.
Is the timing set correctly?

[jabcb]

The petcock has two filters on the tank inlets. They often have gone missing over the years. This allows crud to get into the petcock & cause problems.
The other filter in the petcock bowl is after the petcock mechanism.

Use a flashlight & look into the tank to see the petcock inlets. Those filter are likely missing, which allowed the reserve inlet to get plugged up with crud.

You should flush out the crud in the tank. And possibly de-rust or seal the tank. And rebuild the petcock. But do not unscrew the vacuum mechanism unless you have to.

[Omaniac]

Timing is probably good, since I had a few miles of no surging og bad engine behavior. The fuel filters should protect the carbs. They got cleaned well by the prevoious owner. His garage was abolutely spotless, so Intake his word for it.
He said he cleaned out the tank, but there could still be crud in there.
So the tank will come off and get cleaned again. The petcock as well. Pretty sure this will fix the problems. I also got a second tank and side covers set with the bike, so I can swap out the tank if needed.

[Omaniac]

Here she is!

[Omaniac]

I took off the tank and removed the petcock. The inlet for the reserve/pri was full of small pieces of tank sealer. A lot of this stuff inside the tank.
So I need to clean out the tank completely. That is not so easy.
The petcock has a filter on top of the standpipe for the regular fuel intake. Is there supposed to be another filter for the reserve intake?

I can use the tank and never let it go into reserve, that would work, but don’t like to have unresolved issues with the bike.

[jabcb]

Sweet GT500!

The two inlets use the same filter. Probably can get new ones from Suzuki.
You may need to use a smidgen of JBWeld to make sure it stays in place.

You should also take apart the selector mechanism & clean that out. And clean out the bowl.
Replacement 4-hole gasket & o-rings should still be available.

[joolstacho]

Oooh nice... just like mine, but I have the not-so-shiny original patina finish. That 'burnt orange/red' is a great colour eh?
BTW, removing and re-fitting the inlet/air-cleaner rubbers can be an evil thing.
I've found the only way to get the carbs and rubbers back together and aligned properly is to release everything including the airbox so you can move it back to get space for the boots to slip on and align. Tighten the big circlip at the airbox first then the rest. This means (hopefully) you won't need to use glue, which may tear the rubber when you next want to take it apart.
It seems like a lot of work but in the end it'll be less work. That air cleaner rubber boot distorts over time making it hard to align.

And... don't you hate those so called tank 'liners' - when they break down (which they all do at some stage), it's nightmare to get the tank cleaned out.

[Omaniac]

The petcock has one filter on top of the standpipe and one filter in the RES inlet, not one filter for both. A new filter is on the way from Germany.
Thanks for the description on how to remove the carbs, but they will stay in place. The fuel filters the previous owner installed between the petcock end and the carbs, have done their job.
I took the petcock apart and cleaned out all the stuff stuck inside. The bowl was full of stuff.
The red tank needs to be restored inside, but the good news is that my spare, blue tank, is in good condition inside. It will be repainted soon.
Then I will deal with the red tank later this year.

[Omaniac]

This will be the new color. The tank has a small dent, so will be fixed and painted. The oil tank is as new, and the side cover needs new paint.

[Omaniac]

The blue set has been delivered to the paint shop. Still working on how to clean out the red tank. Thinking of a method to vibrate it mechanically with some kind of pellets inside. I actually checked with a shop selling house paint, they have a mixer that vibrates, but it’s not big enough.

[joolstacho]

The old-timers method it to plug the outlet tap threads, wrap it in Bubblewrap or similar, and quarter-fill it with small sharp stones/screenings, add some sort of paint stripper? (Methylene chloride?) and tie the tank inside a cement mixer and let her rip!

[Omaniac]

That would work, but I don’t have a cement mixer.
Got the filter for the petcock today, but it was too small. New filter in the mail.
The blue tank will be ready in a week or two.

[dollydog]

i just fill my tanks up with dilute citric acid and leave it for a couple of days - after plugging the tap hole first of course - it's always worked well for me, and a couple of tanks had A LOT of rust inside. i did that, then used the metal prep, washed out, dried and applied my por 15. tank turned out perfect and has never leaked
cheers, dd.

[Omaniac]

I need to get rid of the bad liner first. I don’t think citric acid helps with that. I have sent an email to a UK company that restores MC tanks. I may send it to them to take care of the problem.
Is diluted citric acid paint safe?