Hiyaa guyss, Adil here back from Pakistan.
Without wasting anyone's time. Here are the symptoms.
GS 500, 1991 Engine swapped 1997.

You start revving up from 4k to 6k. After 6k it starts choking. And it lasts until the rpm drops at 4k rpm.
You can take the bike to 7.5, if you revv very slowly. But after that same choke, and that choking is not jerking. Its just plain dying.

Mechanic says the valve springs are too weak. He showed me, that they should be so tight that you wouldn't be able to rotate them with hands.

Is that correct. Do I need a top-end rebuild??
Thanks Awesome people

Hi, how many km on the engine? Are the carbs standard? I would seriously doubt that diagnosis off hand.

You might need a top end rebuild. You definitely need a new mechanic.

Vacuum leak, slides not coming up. My guess.

Sort the carbs first, then see what is left.

Quote from: adrafiq.52 on August 20, 2021, 11:42:20 PM
Hiyaa guyss, Adil here back from Pakistan.
Without wasting anyone's time. Here are the symptoms.
GS 500, 1991 Engine swapped 1997.

You start revving up from 4k to 6k. After 6k it starts choking. And it lasts until the rpm drops at 4k rpm.
You can take the bike to 7.5, if you revv very slowly. But after that same choke, and that choking is not jerking. Its just plain dying.

Mechanic says the valve springs are too weak. He showed me, that they should be so tight that you wouldn't be able to rotate them with hands.

Is that correct. Do I need a top-end rebuild??
Thanks Awesome people

For sure sure sure - You need a new mechanic. I highly recommend GSTwin and yourself.
5-6K struggle - is almost 100% carb.
OK so post more details, carburetion is related to throttle position, not rpm.
Electronics are related to rpm not throttle position - and you are not having an electronic problem - That's for sure. Because the thing that affects it more than rpm is temperature. So it will act up when hot. So forget that.

Buckets rotate easily by hand when the valves are closed - so you have 1 problem solved. Your valves are adjusted right.

Now your issue is carbs - but I'll tell you exactly what part if you post details related to throttle position.

If its 6K when revved under no load - well that sounds like you've got a air flow problem - is the air filter nice and clean ??
And is fuel level in the floats right and are you getting fuel into vacuum line cutting out the left cyl ?

- I'm Srinath From India though I've lived in the US for 30 yrs now.

PS: Valve springs getting weak will cause valve float at lower than redline. Its heard as a loud rattly noise from the top end followed very very soon by all sorts of nonsense like piston hitting valves etc - and then you need a rebuild or a replacement motor.

After slowly reading your symptoms you really seem to be having fuel and air related problems. Slowly rev to 7500 and it chokes again

Is the carb rear vent for air open to air ?
And fuel flow and vacuum line filling up with fuel would still be an issue.
And yea, is the diaphragms good in the carb ? A tiny tiny rip in them can cause this problem.

You're really looking at a fuel and air problem.

Cool.
Buddha.

Ahh man. I am delighted as well as disappointed in myself.
This is one of the things that was hiding in the plain site.
Even though one other mechanic did mentioned that its a clogged air filter and you should replace them after a while. saying he has one in stock imported from UK and will cost mein
around 50 USD. I thought he was just trying to sell me air filter.
Well guess what, I swapped a new air filter and bwoah. the bike coughed like a sick got soup and then started idling perfectly. the throttle response improved. And I took it for a spin and WALLAHI
the problem gone.

I am grateful to everyone for suggestions. Thanks Srinath, Mr 72, herennow.

Note to self, air filters can be cleaned to only an extent. When they turn grey replace em.

Also investing in tools from now on. The mechanic was wrong this once but he has been right many times. And since there are not many mechs for bigger displacement here. I guess at least I need to learn
more on debugging the issues and filter facts from bull shaZam!.

And btw I ended up buying the air filter from local manufacturer for 3 USD.

Air filter can be cleaned with gasoline clean side to dirty side (outside to in) so you stream it in at an angle. Then follow it up with air.
Then - wait for it to dry dry dry completely before putting it on.
You can also take it apart and replace it with Uni type foam and wire tying it back together - Though this seems to be easiest with the "hi flo filtro" brand. This one you have is an aftermarket one - May be not too hard.

The Hiflo thing is held in with spot welds. Drill em out and the end cap comes off. Then soften the glue with a heat gun and out comes the filter paper.

Cool.
Buddha.