carburator issues

[radon]

Hi

I'm working a my 2002 gs500 that I'm rebuilding and I'm having some carb issues that I have no idea how to fix or what to do

(some backstory)
when I bought it was running fine but it had a tendency to die while sitting at a red light but that went away after riding it for a bit then it started bikefiring thought the carbs at times so done new fuel lines, cleaned the carbs, did a compressions test and 130 ish on left cylinder, 90ish on the right, stripped it down and got the frame powder coated, replaced the crankcase because of a crack, new engine bearings, new piston rings and rehonded the cylinders, got new intake boots, did the valves to the correct spec, fully rebuilt carbs with new gasket and o-rings, corrected the float hight and the tested the jet needle for leaks they it holds decent compression and also rebalanced them which brings us to now

the bike now dosen't want to ideal and still backfires every now and then but I found out that if I blow into the vent hose/overflow hose the revse go up and ideals for about 10 seconds or so until it wants to die again, I have taken the carbs apart atleast 10 times now trying to figure out why its not running but everything looks pristine, tried running it with the stock air filter and nothing really changes ,I'm using a fuel thing that you hang up to fuel the carbs so I have blocked off the vacuum vent since I have taken off the hose that leads to the petcock because I want to get of all the fuel lines because of the petcock and I bought the appropriate petcock on the fuel tank

I'm at my whits end with the carbs and I have no idea what to look for since I cant find anything on the internet about my carb situation. I just want to get it running so I can go do my full license and enjoy the nice weather, any help is much appreciated

ps. I looked into buying different carbs but the only ones I found are a pair of refurbished carbs from the US for £400 not including shipping which I wont do since I have already spent enough money

[chris900f]

If you only have 90lbs compression on one cylinder and 130 on the other the problem is not likely the carbs. You either have a damaged cylinder or piston ring or you have a valve clearance issue. If the clearance drops to zero the valve will not fully close leading to a lack of compression on that cylinder.

Your next step will to be the valve clearance check. If you're lucky, you just need to replace a valve shim.

[haybaler]

I dunno, but maybe it's not the carbs.

Please check for spark with a strobe timing light or simple inline spark
tester. Maybe one of the coils is weak, or something else electrical?

[radon]

If you only have 90lbs compression on one cylinder and 130 on the other the problem is not likely the carbs. You either have a damaged cylinder or piston ring or you have a valve clearance issue. If the clearance drops to zero the valve will not fully close leading to a lack of compression on that cylinder.

Your next step will to be the valve clearance check. If you're lucky, you just need to replace a valve shim.

compression is fine now after rebuilding the engine, after putting the vacuum petcock back on the bike started up as normal and it ideals nicely after its warmed up

[radon]

I dunno, but maybe it's not the carbs.

Please check for spark with a strobe timing light or simple inline spark
tester. Maybe one of the coils is weak, or something else electrical?

spark is fine and after cleaning the coil connections i have better spark, also checked the signal generator and one of the pins was bent a bit and after bending it back and putting the vacuum petcock on, the bike starts up and ideals nicely with no back firing