1982 XJ550 Idle Runaway Issues

Hello All, I'm Back once again.

I've been having a problem on my 82 XJ550 that's been kicking my ass, Let me do my best to explain whats going on.

I believe the issue I'm having is caused by the OEM Jets (#165 Pilot Air Jet, #35 Pilot fuel jet, #112.5 Main fuel jet) Specifically the #165 Pilot Air Jet.

So around a year ago when I first bought the bike I purchased a XJ550 Carb rebuild kit from eBay instead of a proper kit (Oops). Fast forward. I rebuilt the carbs (Church of clean) with the eBay Jet kit not thinking much of it because this is the first time I've rebuilt carbs. Wet Set the float levels, bench synced them and threw em in the bike, started it up and it ran.

Took it for a ride around the block a couple times, everything seemed to be running good, I got home and pulled the plugs and all 4 plugs are dry and black as can be, so the bike is running extremely rich, I tried to fix that problem by adjusting the 4 Air/Fuel Mixture screws on the top of the carbs Inward from 2 - 1/2 to 1 turn from bottom to compensate for the richness but no luck, (cleaned plugs, took it for a ride and gave it some throttle and around 2-4k it was not happy at all, whole bike started acting weird, had to pull over). Reset the screws to 2 - 1/2 when i got home. plugs are still pitch black.

So I decide to pull the carbs because after reading online and learning more about jets I wanted to check the jet sizes again and swap the original jets back in to see if that fixes the richness problem.

I checked the jets, and all the jets are the same except the Pilot Air Jet.

The Pilot air jet I got in the kit from ebay was tiny compared to the OEM #165 Pilot Air Jet.

So I swapped the OEM jets back in after I cleaned them to see if that fixes the richness problem, the plugs look way better then they did before but now the bike has a runaway idle issue. When trying to set the idle, I back the choke off all the way and I give it a blip of throttle and it hangs a bit, and then just takes off on its own.

No sticky cables and everything returns to where it should

Could this issue be caused by the OEM #165 Pilot Air Jet being so much bigger then the ebay Pilot air jets and more airflow getting in may be lifting the slides & the springs aren't strong enough?

I'm confused because the bike ran alright with the old jets in (albeit rich) and the only big change is swapping back to the OEM #165 Pilot Air Jet.

I'm racking my brain here trying to figure this issue out so I can ride a little bit before the summers over :/ any help is greatly appreciated.


June - 2024 @ 27k Miles
-Carb Rebuild

• New throttle shaft seals
• New intake boots
• Wet Set
• Clunk tested
• Bench Synced

-Valve Adjustment
-New Front & Rear Brakes
-New Chain
-New Counter & Rear Sprokets
-New Clutch Cable
-Oil Change
-Oil Filter O-Rings
-New Gas Tank Lock & Key
-New Ignition Lock & Key

 

Have you tried for intake leak test? Either propane or carb cleaner/starting fluid.
Just to make sure it's not coming from there

 

I have not done a leak test yet due to the idle issues, I might be overlooking it but I don't think its a leak because with the ebay jet kit (Smaller Pilot Air Jet) it idles normal, but just to be sure I replaced the throttle shaft seals & intake boots but still no luck. I'm going to swap the ebay jets back in and leak test everything with a smooth idle and will let you know what I find. Thanks!

 

I would check for a leak first to get that of the way, since you've been taking the carbs in and out a few times, you could've maybe damaged a boot or even a shaft seal if you didn't pressed the 4 carbs straight on something perfectly flat while tightening them back to the rack.

I'll just do the simple things first, sometimes it's really worth it!

Also make sure the choke is fully closed by looking at carbs when closing it with the lever, I had to remove my cable because PO had wrong cable and it was still open a little bit when closed by lever. Now if I need the choke I can just use it directly on the carbs untill I get a new cable.

Hope that helps!

By the way I'm very surprised you got airjets with your kit, on the 650 and 750 of same year I never got any air jets. Are those hitachi carbs on the 550?

 

Update, Just did a leak test with the ebay jets (Smaller Pilot Air Jet) and wasn't able to find any leaks around the boot area, will try again tomorrow with a propane torch just to be sure.

The choke is fully closed and has just a bit of slack so i know its not being held open unfortunately, was hoping it was something simple but doesn't seem to be :/

The only jet I changed this time was the Pilot Air Jet and it idles perfectly fine (As is No High/Runaway idle, its still running rich) so its 100% caused by the Bigger pilot air jet, but just can't figure out why because the bike comes with #165 from factory.

Hmmm Interesting... And the 82 550 is rocking some Mikuni BS28's.

Thank you for the advice I appreciate any idea's you might have on what's causing the issue

 

For sure man! I'm glad at least you figured your issue! I have no experience with the mikuni carbs, both my bikes are hitachis.

Pretty strange that it runs fine with different jets than stock... is your airbox modified? Pods? Also strange that you get the bike running that rich, especially when turning in the mixture screw so much... floats set properly?

Have you compared the fuel jets size from the stock to the ebay ones, I mean the actual bore size and not the numbers written on it, it can happen a lot that cheap ebay kits have the wrong size despite what's written on it. I even had ebay jets that didn't have holes in them!

Have you rebuilt the mixture screw? Screw -> spring-> washer->oring. In that order? 100% sure that you removed old washer and oring before rebuild?

Just throwing things that could cause it...

All 4 spark plugs are looking the same? All full black right?

 

I haven’t read if you did a final running sync.
Very important

 

I only have experience with mikuni on my 550 but I potentially see a 650 or 750 in the future for myself and i believe those are hitachis so I'm excited to learn about those also.

Stock Airbox, No pods, Floats Wet set @ 2mm +/- 1mm (.08" +/- .039")

and I know isn't that strange? What's even more strange is that today I started it up and went for a little ride around the block a few times, and came home and the plugs are no longer black, (These are NOT new plugs) they have been cleaned & reused multiple times so part of the blackness on the inside of the plug is just where i wasn't able to reach when I would clean the plug before I tested something new. I need to get a fresh pair of plugs and do a plug chop to check the true condition.

With that being said how do the plugs look to you? Rich/Lean? It felt good on the road. Should I be worried about riding it? I've read that a lean condition is what you really need to watch out for cause stuff gets hot, but I didn't like riding it knowing how rich the plugs looked before but now I think they look a hell of a lot better. (they were as black as outer space beforehand)

I have compared the fuel jet sizes and they did have the an extremely small difference in bore size (Except the Pilot air jet, Massive difference from OEM). The difference was so small I almost didn't even want to change them but I figured I should. But dang no hole? that's crazy! Will definitely double check all jets from now on if I ever order from ebay again lol.

Yes, The screws were rebuilt but once again with that darn ebay kit so going to double check for a leak coming from the O rings/Something not sealing properly.

So Just so I don't confuse myself or anyone. The bike is currently running

Main FUEL Jet Size: #112.5 (OEM)
Pilot FUEL Jet Size: #35 (OEM)
Pilot AIR Jet Size: About half the size of the OEM #165 Pilot AIR Jet
Mix Screws: 2 - 1/2 turns out from fully seated

And my hanging idle that would turn into a runaway idle is no longer happening and the plugs are no longer pitch black.

If anyone has anymore ideas on why the OEM #165 Pilot air jet makes the bike runaway feel free to respond.

I appreciate the reply's/advice, thank you

 

No final sync yet, Only a bench sync. I didn't want to do a final sync on it before because it was running so rich (Pitch black) I knew something wasn't right and would probably have to change the jets back (which ended up happening) Now that I swapped the Pilot air jet back to the smaller one from Ebay so the idle isn't running away and the plugs aren't pitch black anymore I will be looking to do a final sync in a couple days.

 

Why o why o why do you lot over there buy potential rubbish from ebay sellers? The grief it causes is immense. Just click on the forum sponsor's emblem at the top RH of these pages, you will get exactly what you want.

 

Plugs looks ok to me, keep checking on them, you want a nice light brown color on them ultimately, I would definitely get new plugs soon tho.
The one thing you don't want is to run lean tho, if plugs get gray I wouldn't run the bike before being sure issue is fixed, that could cause damage.

I would 100% use oem plugs if you have them, a good cleaning will always be better than ebay jets. Yep I wish I had a pic to show you but I did get jets that literally had no holes in them, that caused a lot of troubles, im never using ebay jets again.

 

Are you sure the choke/enrichment plungers are all the way down when you have the choke lever all the way off? There should be a tiny gap between the plunger and the 2 tong forks lifting up the plugners.
are you sure you have the idle adjustment screw (the one that opens the butterflies) all the way closed enough?
If you take one carb and turn the idle screw all the way in, does that carb shut down? (it should go lean then start to shut down).

 

Already asked about the choke and he said it's properly set, that was also what I thought ^^

Curious about the idle screw all the way in tho!

It seems like he fixed the issue and that the bike is running well now, waiting for him to confirm!

 

Yes I've double checked that the choke plungers are all the way down and that the tiny gap is present, even has some slack in the choke cable.

Thats part of the issue i was having when trying to set the idle with the OEM Pilot air jets installed. I would back the idle adjustment screw all the way off so the butterflies are closed, I would start the bike, holding the throttle open around 1200 rpms till the engine has warmed up then with my hand on the idle adjustment screw slowly turn the idle screw in while letting off the throttle until the rpms are being controlled by the idle adjustment screw only. Id bring it up to about 1200, rev it to test throttle response, it would start to hang and then engine would runaway on its own so i would kill it and back out the screw all the way again and the same thing would happen.

I will update you with the info on the mixture screw, now that the bike seems to be running good I'm going to tune/sync it so I'll have some more info in the coming days

Appreciate the reply

 

Okay sweet! I'm still new to working on bikes/engines so everything is a bit of a learning process so my apologies for redundant question's haha, and alright I'll be picking up a new set of plugs for tuning and will update with pics. Will keep an eye out for a lean condition.

For sure will always keep that in mind, and definitely will not be using the good old ebay for any more carb parts... lesson learned

 

I start with the idle screw all the way backed out and slowly turn it in until its barely holding them open so I don't think its that unfortunately.

And took it for another ride today cause its been so nice out and the plugs are still looking good, bike feels good, so I'm going to go ahead and proceed with tuning and get it fully dialed in.

Thanks for sticking with me through it I appreciate it man!

 

Alright a little update, did a running sync yesterday and got the carbs all synced up using the 2 bottle sync method. Don't know if its all in my head but everything is feeling way better.

I tweaked with the mixture screws a bit but currently do not have a colortune (will be getting one in the near future) so can't get it dialed in 100% so I'm just trying to tune by plug & ear. Looks like #2 is rich and is #3 a little lean or is #3 looking good? Just trying to find the best looking plug and so i can have a baseline to reference

Also the plugs are still the old ones, still need to get some new ones

 

It's hard to say and I'm no spark plug expert, but I think the #3 base color is what you wanna get, a light brown color!