Hello, I got this bike about a year ago (summer of 2019) and have been working on and off (since I have to be at school during the spring and fall terms) whenever I get the chance. Since getting it there have been an array of issues, some of which have been dealt with, but the bike has never run properly. I've browsed these forums but have not been able to find a solution, so I'll describe the current issues I'm experiencing and what I've already done to diagnose so far, and hopefully there are some new ideas on how to fix. To start off, the bike itself: Yamaha xj650 Maxim 1982 with aftermarket pod filters and aftermarket 4-to-1 exhaust (before I'm berated about having aftermarket exhaust/pod filters, the previous owner did it). I have bought an airbox but am hesitant to install after realizing that the full process requires taking off carbs, intake boots, starter motor, battery, cylinder head, and moving the engine forward an inch, all the while I'm not sure if this will even solve the problem(s). SYMPTOMS: The problems are as follows: bike has difficulty starting and only starts on mid-choke, and when it does start (usually after a quick squirt of starter fluid) it will do one of two things: If the engine is warm OR if I used too much starter fluid, the engine will jump to 5-6k rpm at idle (very loud and very scary) and stay there. When I throttle it in this state the engine rpms increase as expected. When I take off the choke in this stage the rpms also increase. every once in a while I'll ignore my natural instinct to kill the engine and watch it for a minute, and sometimes the engine seems to calm down a bit and go to maybe 3k rpm, but I have never observed it settle back down to a normal idle (1-1.5k rpm) after jumping up to 5-6k. In the second scenario, which is usually what happens the first time I try to turn the bike on from a cold start (and a bit of starter fluid, maybe a second-long squirt onto 2 of the 4 pods) it will struggle to run but will idle at 1.5k rpm. It usually dies after 5-15 seconds in this case, and if I take off the choke it immediately dies. Also, if I pull the throttle in this scenario the engine immediately dies (I assume it gets too lean). What I don't entirely understand is why in scenario #2 the throttle causes it to die whereas scenario #1 throttle increases rpm from 5-6k idle to 8k rpm as a throttle should. Maybe because when it idles at 1.5k it's already lean and opening the throttle immediately kills it with too much air, but in the first scenario the airflow into the engine is already so strong that it can more quickly atomize the fuel and suck it into the engine. I also noted that after pulling on the throttle at the 5-6k idle, it will hang a bit at ~8k rpm and slowly return to 5-6k rpm, which I believe is also a symptom of lean mixture. As for other symptoms I have little actual experience (this is my first bike), so my hearing isn't acute enough to try to "hear" whether the engine is lean or rich. If I had to say, the exhaust smells like gas (a fuel-rich symptom) but I could be wrong about how it smells, as I've never been shown what fuel-rich and fuel-lean exhaust smells like. Lastly, at the 5-6k idle the exhaust AND one or two of the exhaust headers is smoking. I'm not sure if this is normal at such a high rpm, but could be indicative of a vacuum leak at the header (I took off the exhaust manifold once to do an oil change, but when I put it back on I also replaced the old exhaust gaskets). I know there are some weird things that can happen with back-pressure of air into the engine if exhaust headers aren't on properly and this may be partly responsible. WHAT I'VE TRIED I did a carb overhaul with a kit I bought online, which included rejetting to the stock fuel jets, ie a #40 on the pilot fuel jet and a #110 on the main fuel jet, as well as gasket replacement, new needle jet, etc. I am considering purchasing a new set of main/pilot jets to account for aftermarket exhaust/pod filters, but I'm not sure that such a small difference (I believe the calculator I used said ideally I'd have a #42 pilot and #116 main) in jet size would change much/be able to cause the problem I'm experiencing. I have also tested for vacuum leaks after replacing the original crusty intake boots and their gaskets (between the carb and engine) and have not found any. I tested throttle valve seals, intake boots, and petcock-to-carb vacuum hose for any vacuum leaks by spraying starter fluid at each location and trying to start the motorcycle without result, meaning I shouldn't have any vacuum leaks. I have also done a compression test (good on all cylinders, between 93-100 psi for all four) and replaced spark plugs which I am certain are all firing. When I removed the original spark plugs I examined them and they all looked fairly normal, no obvious signs of overly rich or lean, but again I'm no expert. I haven't yet tried checking out the intake valves. I've seen a couple people say to check the clearances and do some other things but I have not done that. I have extensively looked at the carbs and boots. The only thing I haven't done with the carbs is adjust float bowl heights, but all are relatively close to each other in height (within 1-2 mm). Please, please, please if you have any ideas help me out. Apologies for the long post; I've spent a lot of time fixing up this bike and I refuse to take it to a mechanic! I've learned a ton about motorcycle maintenance and repair through this process but I think there are a few small things that I've overlooked, and I'm sure a couple of you with extensive experience could help a newbie out. Thanks in advance, Nick.