high idle to no idle issues

Hey all I haven't posted in quite awhile. I sold my previous project and bought a different bike. It is a '81 xj550 maxim. the bike ran great last year after a little bit of work. I parked it over the winter and ordered some parts rebuilt a few things and reassembled a few weeks ago. since then it has been a fight every time I try to do anything with the bike. I need some help.

To start off with what I did last year to get the bike running.

- cleaned carbs (yes I did break the rack)
- oil change
- new filter
- new plugs

That's it. after I did all that the bike fired up great, ran great and I never had a problem with it.

Now for what I did over the winter and over the last few weeks.

- Cleaned and rebuilt the carbs
- broke the rack replaced all the fuel tube O-rings with new
- the center fuel T did not seal. found a replacement part and installed
- replaced the float bowl gaskets
- rebuilt the starter
- rebuilt the petcock
- stripped, sanded, and repainted the tank
- cleaned and oiled the gas cap
- reassembled the bike
- put high heat RTV in between the carb boots and the engine (the old seals were extremely thin and allowed some air through

Now after all of this, the bike fires up great seems to run ok. however it will not idle down below 2000 RPM without stalling out. Now if I hold the throttle just slightly it will idle at 1500 RPM very nicely. release the throttle and it will stall out.

On the same hand letting it Idle at 2000 RPM when I apply the throttle and release the bike revs up nice, but then hangs and takes a very long time to come back down to 2000 RPM.

I have been fighting with this for 3 days now and I am about to lose the last little bit of hair I have left. Any help would be appreciated.

Thanks in advance

 

Ok, if you hold the throttle it idles at 1500, how are holding the 2000 rpm idle? Choke or idle knob... Im confused
Are you saying idle knob will allow 2000 or nothing?

 

idle knob is holding the 2000 RPM but without throttle it will not allow under 2000

 

Then your butterflys are out if synch, holding the throttle pulls on a different part of the shaft.
Especially if you had the rack apart, everything moves a bit, needs to be resynched

 

wouldn't a bench synch clear this issue?

 

That will only get it close, it will have to be put on a sync tool to get it to really run well. You can either make one (no so much fun) or buy one. Just make sure if you are using the air only type to get those little restrictors in line with each hose. It helps keep the bounce out of the gauges.

 

I would test the seal of the rtv for leaks.
when you used it did you go to full torque on the bolts or snug them let the rtv cure then torque them?
broke the rack ?
did you replace the throttle shaft seals
what are you using for an air filter?
if you have no vac leaks and did replace throttle shaft seals.
2000 rpm is when the main jets kick in this would indicate pil0t jet circuit issues..

that's the pilot jet, its air jet and mixture screws along with the passages

mixture screws properly set?
the o rings for mixture screws replaced?


I had no issues with 550's running well on a bench sync and mixture screws set to 2-1/2 turns to 2-3/4 turns (better) up from soft bottom.

 

@Timbox
I made a Synch tool last summer that works pretty well and wasn't that expensive or hard to make.

@XJ550H
I snugged them and let them sit overnight before being torqued down.
I had all four carbs separated. stripped completely down. scrubbed every surface possible. sprayed out and scrubbed out every place possible.
I did not replace the throttle shaft seals. as stated before the bike ran perfectly fine last summer so I did not see a need too.
I have a stock airbox with a K&N air filter.
O-rings on the mixture screws replaced
mixture screws set at 2 1/2 turns out from soft bottom


I played with it some more today and noticed that cylinder number one (if I remember right that's the one all the way to the right when sitting on the bike?) the exhaust pipe was noticeably hotter then the rest of the cylinders. I didn't have much time was just trying to play with the mixture screws some.

I am planning on taking the carbs off again tomorrow and going back through them to see if I screwed anything up when I reassembled them.

 

If you got carb cleaner on the seals for any length of time, then they are done for.
If they are the original seals, they are done for.
It's highly likely that is where your hanging idle is coming from.

Did you replace the idle mixture screw o-rings?
Those are another common cause of vacuum leaks that lead to a hanging idle.

 

Also noticed that with the fuel line from the petcock running into a bottle it acts correctly and pours fuel out easily on prime however with it connected correctly it does not seem to flow freely into the carbs.

 

That can someimes happen after re-installing the floats. Crack the drains open and give the bowls a tap to flush out any trapped air.

Which brings up another question. Did you wet-set the fuel levels? The acceptable range of fuel height in the bowls is damned tight, and having them even 0.5 mm too low can cause a lean condition that will mimic a vacuum leak.

 

@k-moe I did wet set the fuel levels.

I believe I found a big portion of the problem. I don't quite know how it happened however I have a suspicion.

If you look at the pictures the spring in between 1 and 2 carbs (I think that's right someone please correct me if I'm wrong on that) jumped a ring and was caught which threw everything out of wack and would explain why cylinder 1 was hotter than the rest.

 

So obviously because of this, all adjustments I made attempting to fix the problem would do nothing but screw it up farther.

Either way the carbs are off the bike again. So I might as well go through them again.

 

That shouldn't happen. You are mssing the lower screws that serve as a guide for the lower spring.

 

there are no lower screws to guide the springs on mikuni carbs
carbs are numbered left to right as you sit on bike . carb 3 is adjusted with the idle adjustment knob

 

your spring missplacment was keeping carb 4 butterfly open to far

 

k&n filter may be part of the problem

that would be cylinder 4 and it is hotter because the butterfly for that carb is open a lot wider than the others.
correct the spring issue, bench sync carbs and reinstall

 

DOH! forgot about which bike he has.


OP, anything other than a stock-type paper filter will lean the mixture out (enough to need changes in jetting). The bikes already came running on the lean side to meet emissions regulations (and boost MPG).

 

@k-moe thanks for that info I'll keep it in mind when I'm trying to tune the bike some.

Got the spring set correctly and took the float bowls off just to make sure everything there looked good. Didn't see any problems so they are reinstalled. Got the carbs bench synced again and will reinstalled the carbs tomorrow.

 

Alright so bike is back together and its got another issue.

with the choke on the bike fires right up no hesitation at all. As soon as the choke is shut off the bike dies. Now if I hold the throttle ever so slightly open it will idle great.

Idle knob is turned up all the way open.
mixture screws are set at 2.5 turns from soft bottom.
does the same thing on all petcock settings.
with the fuel cap closed or open.

 

How long are you letting it warm up before shutting off the choke?
It takes several minutes for the engine to warm up.

My normal procedure is to start the bike, let it warm until the engine speed begins to increase (around 2,000 RPM), then partially close the choke to drop the idle speed back down to ~1,100 RPM and ride for a few miles before shutting the choke off completely.

 

I usually let the bike run for at least 5 min before I move the choke. However when I first started it today it jumped to around 5000 rpm and wanted to hang there on full choke. I shut choke down to half and it stalled right out.

 

Before it was warmed up nice and had been for several minutes before I started messing with the choke and it stilled stalled right out

 

That brings my mind right back to this...

 

Not questioning that it might need throttle shaft seals, but you can't expect it to idle if the idle adjustment is backed all the way out.

 

Hmmm.... I wonder now if he means that he has the throttle plates fully open.


OP, did you do a bench synch? If so then you won't need to bother with the idle speed knob until the bike is warm enough to run without the enrichment. Then you adjust it to get the idle speed (in your case ~ 1500 RPM is best in practice).

 

@k-moe

I did do a bench synch. And it's not throttle shaft seals.

Half the problem was that I had the idle adjustment knob wide open trying to get it to idle. So of course it's going to run high. That was my stupid mistake.

Got it running good on choke yesterday. Idled right at 1500 rpm on half choke for about 10 min. During that time I sprayed down everything possible with ether looking for a leak. Carb boots engine side, throttle shaft seals, intake boots, and nothing changed the rpms. I couldn't find a leak anywhere. This is also with the k&n air filter out but the filter box closed up.

Ran very well on a little less than half choke. Throttle response was great. Under load had a hickup between 4000-5500 rpms but smooth before and after that.

Now if I shut choke off it still stalls out. But it's not starting and shooting to 5000 rpm and I have the throttle response back.

My thought is that I am not getting enough fuel to the carbs so it still needs to run through the enrichment system.

Any thoughts?

 

I was making the assumption is was backed out too far (CCW) since the statement was made "Now if I hold the throttle ever so slightly open it will idle great." Seems it should be just fine if it idles OK by holding the throttle slightly open and he just needs to adjust the idle knob in (CW) to get it to idle.

 

@Rooster53

Which is the confusing thing that I don't understand.

 

It just seems logical that if the bike will idle OK when holding the throttle ever so slightly open, then the idle adjustment screw has not been set to the point where it would effectively do the same thing. Note also that it is common to initially set the idle after a carb rebuild so the bike idles when first started, however after a complete warm-up they typically race a bit and the idle adjusting screw will need to be backed off - the running sync would then be performed with the bike fully warmed, which will then require further adjusting of the idle knob to keep it in the specified range.

 

Here's the thing about the throttle shaft seals (and why so many of us hammer people about them) the vacuum leak will often not show up at idle. As the throttle plates open, and vacum changes, the seals move. Hardened seals will leak then, but seal well-enough when you're testing them without any load.
I don't keep mentioning them as a WAG. Even if you can resolve this issue without changing them you will be tearing the carbs back down when the seals do finally fail. They are not able to last forever, and unless you changed them you have no idea how old they are. Back in the day the throttle shaft seals would need to be changed as early as ten years after the original build date.

Re: the bench synch.
That is a must-do because the throttle plates are what controls the idle speed. That you can get it to idle with the enrichment on means that you're not far off from baseline (assuming no vacuum leaks). If you let the machine idle for about 10 minutes she'll be warm enough that you can start adjusting the idle speed while gradually backing the enrichment lever off. Indications are that the plates are too far closed, so turn the idle speed adjustment knob clockwise when you do the adjustment. You can do the running synch after she can idle without the enrichment on at all.

 

did you ever figure out this problem newguy? Having the same issue now; ive tried everything and cant figure it out for the life of me. Idles at 6000 and i cant get it down. Taken the carbs off, replaced throttle seals, wet set, bench synced, rejetted...