Not sure about this one...

So a couple days ago I noticed that when I started my 1980 xj650 maxim, with choke on, it starts no problem but does not seem to "warm up" like it used to. I left it running choke on for about 5 mins and it just sputtered around 1500 rpm, whereas it used to start revving up to about 3000 then back down, then I could release the choke and ride no problem.

With it starting like this, when I remove the choke and ride I must move into first right away with a decently strong throttle pull otherwise it dies. When running like this the exhaust noise is choppy and makes little "poof" noises and also it has greatly reduced power. Once I ride a bit the engine seems to kick in and start performing normally. Only to happen again if i shut the bike off for a half hour or more.

After a few short rides and the problem persisting, I decided to put it away until I get some advice from here!

 

when did you last do valve shims and clean your carbs?

 

Yep does sound like both are contributing to the problem. Gum & Varnish in the carbs, and some out of spec valves. Yes and Yes.

Bet on those pilot circuits needing attention (longer warm up and dying at idle), and warm/cold changes point towards valves. No Poof sounds allowed!

 

Get some new sparkplugs

 

Would out of spec valves be something that suddenly happens? I had ridden it everyday without this problem since I got my carbs cleaned and fixed about 1000 km ago. Then almost overnight it seemed to happen.

 

An XJ with too-tight valves will run FINE right up until the day it simply won't anymore.

So the problem appears to be "sudden."

That being said, your issue does sound more carb-related; HOWEVER you can't properly set up freshly serviced carbs unless the valves are in spec.

How many miles on the bike, and more importantly how many since the last valve clearance check?

 

31000 km on the bike, about 6000 km since valves were checked. When I brought my bike to my mechanic for issues 1000km ago, he thought it was valves but said compression test showed no problems and the problem was carb related so the valves weren't checked with most recent service.

 

I think most people here would agree that taking it to a mechanic is part of the problem. And a compression check doesn't tell you what the valve clearances are. Check the clearances yourself if at all possible. Even if they are all perfectly in spec you will know where you stand.

Nobody will clean your carbs like YOU either. And sometimes I think even a torn seat is due to dirty carbs on these bikes! ;-)