XJ650 Maxim high idle/hanging throttle

Hey all, the machine is a 1982 xj650 Maxim I resurrected and I've run into a bit of a wall with getting it to idle and run properly. I've searched the forum, found a lot of good info here - https://www.xjbikes.com/forums/thre...horrible-when-it-warms-up.128704/#post-646695 - but still buffaloed.

Carbs were split, fully cleaned, new throttle shaft seals. Started with a cheap rebuild kit but the parts were wrong so only ended up using the float needle and drain screws plus some seal/gaskets and a set of 112 main jets. Ordered and installed HCP15784SET4 Aftermarket Hitachi Y-10 MAIN JET NEEDLES, set of 4: $ 69.95 and HCP8418SET4 Aftermarket Hitachi brass MAIN NEEDLE JETS or EMULSION TUBES, fits all XJ650 (except Turbo) and 1981-84 XJ750 air-cooled models with the HSC32 or HSC33 series carbs, set of 4: $ 84.95 new mixture screw o-rings and a set of bowl gaskets from Len. Float level confirmed with fuel in the bowls and a clear tube. Did a bench sync and the throttle plates all visually seemed to be close to the same peeking through at a light. Valve clearance checked and returned to spec.

Used a set of gauges and the YICS tool to do a running sync. Started and warmed it up for a five minutes, used this time to connect the four gauges to a single vacuum source to confirm gauges sync. I used the idle screw to set the idle ~2000 for the first sync, matched 3/4 then 1/2 then 1/2 to 3/4. This (and presumably additional heat in the engine/carbs after running another five minutes) raised engine speed to ~3,500 and backing the idle screw out completely dropped it to ~3000. Which makes me think it's getting too much fuel. But the rpms are hanging when I blip the throttle, which points to lean.

Tried adjusting the mixture on cylinder 3 a 1/4 turn at a time leaner and 2.5 turns later it's bottomed out and still running, albeit a little rough and less vacuum so backed it out 3/4 of a turn. With all four mixture screws set 3/4 the idle is ~2000 but throttle is hanging worse. Shut down the bike, reset the screws to 2.5 turns out, restarted and the idle is back to ~3000. Which maybe point to a throttle plate issue?

As far as the sync goes, the gauges show all four cylinders within 1/4" of each other at 3000 rpm

Sprayed around with some WD40 but couldn't see any change in RPM so I don't suspect any vacuum leaks. Confirmed the throttle cable isn't binding, backed off the adjuster so there is slack. Enrichment circuit is closed and I pushed down on each to ensure it's sealing.

What next? I though maybe remove the carbs again and take a look at the throttle plates? = https://www.xjbikes.com/forums/thre...ly-supposed-to-be-at-idle.118960/#post-596127

Fall riding season is upon us and I'm dying to get this thing on the road.

Thanks

 

Edit/update - used a propane torch to recheck for vacuum leaks and no change to idle speed anywhere around the carbs or airbox/boots. Tried pressing down on the #4 throttle shaft screw and it dropped the idle a bit so reckon I'll yank them again and take look at the throttle plates.

 

Okay, yanked the rack to reset the throttle blades and I reset the mixture screws to 2.5 turns out. Reinstalled and did another running sync and was able to get the idle down ~1200-1400 rpm, high but not ridiculous. Took it for a spin around the neighborhood and as the machine gets hot the idle starts creeping up to 2500. Used the idle screw to lower back to ~1500. and rode for a bit longer and it keeps creeping up. Lower with the screw again, same thing - keeps creeping up to 2000-3000.

Other than the idle and hanging rpm on throttle it seems to ride okay. Good power up through ~8000 rpm with no hesitation, just the usual rattle from 5000-6000 rpm (which has gotten better with the carb sync).

I have no clue what to do next.

 

Do you have a colour tune? What colour are the spark plug electrodes?

 

The plugs look okay, mostly tan with a bit of darker shade on one side of the center electrode.

I was considering a colourtune plug but at this point I feel like it would be like buying a monkey a microscope. I've tried to be a systematic as possible but keep landing in the same spot.

 

Have you got an inline fuel filter?

 

Affirmative, no issues with fuel flow that I can tell.

 

The crappy carb kit came with the wrong size jets including a set of 43 pilot jets, so since everything that's happening points to a lean condition I swapped them for the stock 40 jets. The throttle is still hanging but not as bad and the machine seemed to run better with idle near the correct RPM in the garage but once I took it for a ride the idle rises to ~2000 even with the idle screw backed out completely.

Any suggestions are much appreciated. At this point, if I knew of a reputable shop in Dallas I would happily pay to make it right.

 

Don’t want to hijack your thread, but my ‘82 SECA is behaving the same way - even with new throttle shaft seals.

 

I have no idea why, but my carbs need to be in the neighborhood of 5 turns out (confirmed w/ ColorTune). Some machines just break the rules. *shrug* I hope you can get it dialed in.

 

Look, the engine can't run without air, and it must also have a tiny bit of fuel - sometimes this fuel comes from the sump gasses, so make sure you haven't contaminated your oil with "gas".
Take off the throttle cable - does it still happen?
Next look for air leakes - if your idle speed screw is right off and it still runs then your carbs aren't closing. Take them off, spend an hour just looking through the butterflies, maybe reseating by slackening off the plate screws, maybe re-aligning the carbs on the rack.
Check and change the mixture screw o rings.
Then do a bench synch using the edge of the first progression hole as a guide.
Fit, do another synch idle screw and mixture round, maybe two or three times to get it right.
There aren't any gremlins....

 

Like XJ 650 said, I had the same issue after a frustrating day and it would be nice if someone would post the exact procedure on doing this repair as I have never seen it on this site and had to have help with doing it on my bike over the phone with hogfiddles.

 

Repair? I'd hardly call the process a repair, just making sure every detail is correct.

 

Adjustments are repairs in the life of nuts and bolts and tuning engines.