81 XJ650, 3 years of chasing a fuel delivery problem

Alright, so, I made a forum post maybe a few months ago with a semi detailed history of my bike and it's persistent issues, but have been unable to solve them. That being said, I am going to attempt to write up everything I have done and tried to do with such detail (or at least attempt to) that hopefully someone can just go "Ah, you forgot this, dummy!"

Back in late 2023, I bought the bike in non-running condition, gas was left in the tank and carbs for a year or two which turned into that lovely green gunk we all know and love. Got the bike home and did a once over to see what I needed before I ordered parts. Evidence suggests that the bike was indeed maintained before I bought it, previous owner parked it due to health reasons and gave me a tote full of extras (along with the aftermarket fuse kit from Chacal).

The problem I have been chasing for three years: High and hanging idle, slow rev down, intermittent firing, popping through carbs and exhaust, and burnt plugs. I can set the mixture screws to five turns out with almost no change.

Bike: 1981 XJ650H (Maxim)

Note: I only buy parts from Chacal.

The list, in order, of things I have done engine-wise:

- Checked and corrected valve clearances

- Ordered new intake manifolds to replace the crusty ones

- Rebuilt the carbs the lazy way (not breaking the rack), cylinders 2 and 3 dead.

- Rebuilt carbs the right way (attending church). Had lots of trouble setting float height but eventually got it. Still two dead cylinders at idle, still a whole basket of lean symptoms.

- Checked compression: 132, 123, 130, 133

- Bought a Coloretune and vacuum gauges

- Still no changes, so I let it sit for the winter

- Went back after it in spring 2024, bought new ignition coils and did the resistor delete thinking maybe it was electrical. Nope, still have cylinders 2 and 3 pipes warm, not hot, to the touch. The whole ignition system ohmed out within spec. Installed new fuse kit.

- Bought a set of Murray's carbs and installed them, little better but same issue. (Regretfully the stock airbox did not survive)

- Rode around for a little bit like that, but the bike would balk and stumble on take off. Decided that I would bite the bullet and do a re-bore.

- Had the cylinders bored out to 63.5mm and installed fresh pistons and rings, installed a new cam chain, made sure the timing marks lined up, buttoned the whole thing up and got it back into the frame. Goes without saying, all new gaskets and the like were used.

- New compression readings were and still are: 145, 144, 145, 147. Not bad for a first time, at home rebuild?

- Slapped Murray's carbs back on, and it ran okay, but not great. I still heard the distinctive slap of a misfire on cylinder 2 or 3 between 2000 and 4500 rpm while under load. Murray's carbs would prove to have their own slew of issues like hanging slides or unpredictable operation, so I ditched them.

- Late 2025 I bought a full rack of carbs, back to square one. Did the full proper rebuild, deluxe kit and all. Used an old pilot jet and a q-tip to test the pilot passeges and they were good, nice strong jet of carb clean from the opening. Used silicone grease on the throttle shaft seals and diaphragms, and made sure the air jets were NOT how they are pictured in the hynes manual.

- With the lack of an airbox (I know, I know) I bought #42 pilots and #114 mains, installed them, and set the mixture screws to 5 turns out. My colortune was on loan to a friend so I thought I'd just let it run stupid rich to be on the safe side. Vac synced and began riding around, cause it was finally running on all four.

- Oops, got my colortune back and lo and behold, white flames on all four. Pulled the plugs, all burnt. Removed the rack again, checked the passages again, made sure everything was were it was supposed to be, checked wet set, all good.

- Earlier this year, I noticed the retaining rings on my right two manifolds were damaged while continuing to look for the source of my problem, so I went back to the Murray's carbs, thought maybe if I had them retune and resync them they would work. They somehow came back worse than before.

- Now, once again, I am back to the stock carbs, ordered two new right side manifolds. Somehow missed the part about making sure the carbs were perfectly straight, which was a bear and a half, think my brackets might be bent slightly. They pass the glass test (unless I have crappy glass).

- Played a full round of musical throttle plates, no changes.

- Checked for vacuum leaks, and while my idle did climb once while doing so, I have not been able to repeat it. I think maybe the vapors from the carb clean I was using found their way to the intake side of the carbs.

To make a long story short, bike runs (if you can even call it that) but is on track to put a hole in a piston


Additional notes:

- Spark has been verified on all four cylinders.

- Colortune plug will sometimes show a dead cylinder, on ANY cylinder.

- Vacuum synced more times than I can count.

- Replaced petcock with a non-vacuum one from Chacal.

- I have put my hand up against all four of the the carbs while it's running, strong suction and my hand gets covered in gas.

- Stock exhaust is still on.

- All of my idle testing and tuning is done with no filters on, and from my understanding, it should be able to idle just fine like that.

- The pods I (temporarily) use have runners on them that do not cover any of the ports. I plan on making two 1 into 2 runners to solve some of the more glaring issues with using pods.

That's pretty much it, I am open to trying and checking anything at this point, even sending my carbs off to a much more seasoned forum member if only to check my work.

And remember, this is only a list of engine/carb stuff I have done, all safety related things like wheel bearings and brake shoes/lines have long been taken care of.

Side note: If anyone is curious about my experience with the Murray's carbs, I can do a full write up on those in a separate post.