Very slow return to idle after carb overhaul - XJ650RJ

Alright - I just recently finished doing my first carb clean and rebuild on my '82 XJ650 RJ Seca. The bike does have 34k miles on it, and appears not to have been as clean as I'd hoped when I bought it in the fall and so I've been wrenching and learning on it since. I installed the carbs last week, and got them to sync up pretty well (at least as close as my Emgo sync tool is accurate to, and enough to idle okay). Once heated up though, the bike is very slow to return to idle when I give it a little bit of throttle, and if I take it much over 4k rpm or so it seems to stick and I usually have to just cut the engine. Prior to pulling the carbs apart it would sometimes be slow to return to idle but nowhere near this bad.

Yes, I got my valves in spec first. And I re-checked them just last week in case any of the shims had settled.

-The throttle cable is definitely returning quickly, not hanging on anything.
-All four carbs passed the 'clunk' test solidly.
-I've checked for vacuum leaks with starter fluid - didn't find anything.
-I re-set the idle mixture screws (covers were already off) to 2 3/4 turns out. Though I'd played with them at 2 1/2 turns and a full 3 just in case - no difference.
-I re-set and have double checked the float levels.

Since it was my first time, I read up as much as I could on these forums first and went slowly and carefully, as well as using a couple of general maintenance books and the Haynes guide. Thanks to Rick for his detailed write-up and chacal for the parts. I think I did pretty well, no major snags. I didn't dip or boil the carbs, and don't have an ultrasonic cleaner, but did as thorough of a job as could be had with piles of blue paper towels and tiny q-tips, a few cans of carb cleaner, and various probes and drill bits. Everything has come out pretty shiny. Replaced float needles and seats. The diaphragm rubber looked okay. Cleaned out the cold-start enrichment circuit (tested with spray). I didn't break the rack.

Any thoughts? Thanks guys.

 

Sure.

When you cleaned the carbs, did you pull the pilot mixture screws and replace the tiny o-rings?

How did you set the floats? Dry, or wet-set individually using the clear-tube method?

If the carbs are truly clean and the floats are accurately set, then it sounds like you didn't have the bike fully warmed up when you did your vac sync.

It's good that you didn't dip or boil the carbs, that only endangers the throttle shaft seals (which unfortunately, could also be the problem anyway.)

 

I'd suggest doing a thorough test for Air Leaks.

If the Manifold Bolts are seized, ... not allowing you to replace or fully seal the Manifolds to Head connections, ... SEAL the Manifolds with the Carbs OFF.

Squish Silicone or RTV Sealant into the Joints where the Manifolds mate to the Head.

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"Bench Sync" the Throttles using the thinnest "Feeler Gauge" available.
3X5 Card Strips
Resume Paper
35mm Film Negative

Achieve a Bench Sync that will result in the Throttles getting Mechanically Synchronized :::=➤ CLOSED. (Virtually closed).

Get the IDLE to depend upon the adjustment of the Pilot Mixtures without needing to OPEN the Throttles.

 

Check your vacuum line from the carb to gas tank,If it is too soft and collapsing it will make the rpms stick or be slow to return to normal as it regains its shape.....

 

the vacuum line from the intake manifold to the petcock is what signals the petcock to feed fuel.

I'm not sure how starving the carbs for fuel would cause a hanging idle, but ok.

However, it is true than if it's not actual vacuum line it can collapse in on itself when hot and under vacuum.

Now, about the questions I asked...?

 

Its what happend with my 550,I changed the vacuum line and it wasn't flexable enough so it end up kinking,And the bike hung at 3000 rpm,I changed the line back and voila it was ok again.I don't know why it worked that way .....

 

When you dunces the bike was the choke closed and the engine warmed up all the way. Could be notthing more than the idle needing to be turned back a little. I say this cause it happened to me.

 

Dunces is supposed to be synced. Damn auto spell

 

I pulled out and inspected the pilot mixture screws but did not replace the O-rings. Looks like I'll be paying some shipping on just a few of the world's smallest rubber rings =)

I used the clear tube method for final setting of the float heights.

Bike was definitely warmed up, with the "choke" all the way off.

And I'm sure it wasn't the vacuum tube since I've still got the bike in carb-tuning setup, with the tank off and using an external tank to run it. Though on that note, all 4 vacuum ports are still hooked up to the sync tool, don't know if there could be leaking air in through the tool or those rubber lines though?

I've pretty thoroughly tried starter and carb spray testing for leaks but I can give the propane a shot and when I pull the carbs to get the washers in I'll try smearing some silicone the manifolds.

...and we'll go from there.

Thanks

 

You have to determine what the condition that is allowing AIR & Fuel to get beyond the Throttle Plates.

1.) Assure its not "Tight Cable" syndrome. Ck the Throttle Grip and Control for Free Play and Tightness to the Handlebar.

2.) Inspect the Throttle Cable-end Mount at the Carbs. Not out-of-shape nor the Cable-end not inserted fully.

3.) KNOW that the Throttle Butterfly's are CLOSING when the Cable is Slacked.
Bench Sync the Rack using the Thinnest Material to guarantee the Butterfly's Close.
::: 3X5 Card
::: Resume' Paper
::: 35mm Negative

4.) Sync Screws not stripped.

5.) Sync Screw Tension Springs present.

6.) Rack is Plumb and Square.
::: No loose Bodies
::: Linkage not binding.

7.) Rubber Diaphragms are Seated in Locating Grooves and are Operating correctly.

 

A lean idle mixture can cause some hanging up of the idle speed.

Just for the sake of it, give the idle screws one turn out each and try it then.

If it does not help you have not wasted much time.

 

Got the carbs back off the bike, will work back through them this weekend. The cable definitely moves freely, but I'll reset the bench sync to make sure it's a narrow opening - I was using fine gauge wire on the first bench sync and it may well have wandered wider during all my repeated tweaking and syncing attempts on the bike.

I found that by some miracle the manifold bolts are not seized, so I pulled them off too, and while the gaskets look fine I'll go ahead and put some high-temp rtv gasket sealant in there (as well as some anti-seize on the bolts of course) and make sure they're bolted on nice and tight.

The idle sync screws aren't stripped, but they were also hard to clean, so I'll make sure they're clean and lubricated and moving nicely.

Here's hoping.

Thanks guys.

 

Nice and tight on those bolts is only about 6 ft-lbs. Go past 8 and you will probably be removing a broken bolt.

 

Looks like I might have found at least one culprit - I rechecked the float levels and was finding that I wasn't getting reliable measurements with the tube I'd been using before - think I was trying to get too short a length to make the bend, and kinking or causing some fluctuations in the observed level from compressing the tube. Got a longer length on and made sure to let it sit for a bit to ensure the level had stabilized, and lo and behold! Levels were a bit low and with some variance carb-to-carb. Spent the afternoon carefully resetting the levels and tripple checking it all, and carbs back on the bike. Looking forward to attempting to re-sync them tomorrow, because I've got a feeling about this.

And yep - anti-seize on the manifold bolts and torqued to exactly 6ft-lbs, thanks tskaz

 

Got her all synced back up and running today. Seems to be a decent bit better. Maybe still not the most responsive throttle/quick to return, but it's at least as good as it was before I tore the carbs apart if not a little better, and will hold a nice idle. It's definitely not surging or getting stuck at 4k rpm as before, and I would deem it safe and rideable (if only it would stop pouring rain here).

Definitely know I got that cold start circuit and well cleaned, started right up on the first crank.

I think it was likely a combination of things - possible small leak at intake manifold, slightly off float levels, perhaps a slightly inaccurate first attempt at syncing, with too high of an idle speed set.

Thanks again for the help guys.

 

i have a very similar issue but have not checked to cable kink. / Also have not done a valve adjustment yet either. I did clean a reassenable the carbs while back what is the real order things should be done in when it starts and runs but idle runs away once its warm and 2 cyl have a nice brown plug and 2 have a dark black plug. I want to go back the the drawing board and the order or repairs would be helpful. Or just point me to a previous post? thanks

 

Hey Jeff, sorry I didn't come across this post until recently. I hope you've figured out your bike well enough since to enjoy the riding season.

After my last post I actually continued to have some problems still with idle and the slow return to. What I learned is that you really have to be thorough, and that minor malfunctions in one system can lead you down a wrong path. I think every week or two I'd just spend a couple more nights crossing more items off my list, with minor improvements each time.

Eliminating cheap fixes, especially the ones that are overdue for maintenance in the first place, is the great place to start. Definitely get the valves in spec. The shim removal tool and shims themselves are cheap.

I think two of my problems ended up being an over-oiled foam air filter that changed back to an OEM paper filter, and a spark plug resistor cap that was out of spec and causing a weak spark in one cylinder. I don't know why I handn't busted out a multimeter on the bike earlier.

It took me a long time, but she idles and runs nice and smoothly now. Good luck!