Intermittent Idle issue

[fjbiker]

Hi Guys,

I'm trying to figure out the source of the intermittent idle issue on my 84 FJ1100. The bike has 80k kilometers on it and the motor is all stock except the 4-1 Supertrap and K&N filter in the original airbox.

The bike starts fine and runs good at all speeds.
The issue is while idling when the motor is hot. Intermittently the revs will momentarily drop for a slit second and then return to normal idle.

This can happen repeatedly every couple of seconds or sometimes many seconds apart or sometimes not all. If I don't do anything while it happens, the motor will continue running for a bit but will likely stall eventually. To keep it going I have to blip the throttle slightly when it's occurring.
This usually only happens after I've been riding for a while and the motor is hot; typically while stopped at a traffic light. Once I pull away from the intersection, all is good.

It started noticing it last year. Since then I've replaced the plugs, synced the carbs and run seafoam carb cleaner through it several times with no improvement.

Could this be a heat related issue with a coil (almost feels like an ignition issue) or is it most likely dirt in the carb? The only thing that throws me is that most of the time it idles fine.

Just wondering if any of you have seen this or have any ideas on which way I should direct my troubleshooting.

Thanks, Mark

[Pat Conlon]

First, make sure your carbs are sync'ed.

When your hot engine's idles drops, then rebounds like you described, it indicates a rich idle mixture so, the idle fuel/air mixture screws need to be leaned out...just a bit. Try turning the screws *in* 1/4 turn and see if that helps. I recommend trying the adjustments 1/4 turns at a time....although many folks do adjustments 1/2 turn at a time.
Search for "Idle blip test" and you will get some more stuff to read.

Report back and let us know how it works.

Cheers.

[fjbiker]

Hi Pat,

Thanks for the suggestion.

I synced the carbs last year while I was trying to figure out this issue but will certainly check them again. I had also checked the valve clearance and they were in spec.

I went for short 40 minute ride this evening and now I'm not so sure that the issue is necessarily heat related. About 2 minutes into my ride while waiting at the lights, the idle dropped again a few times a row then the bike stalled. The motor would have been close to operating temp but certainly not hot.

It always starts back up again fine but it does seem a bit hesitant just off idle when the problem is happening. When it is idling fine with no problems, I don't have any hesitation. Idle speed is set to 1k.

As for bleeping the throttle, it seems to return to the set idle speed but I'll try bleeping it again and pay more attention on whether it's dropping a bit or hanging or just right.

Can I get to the idle fuel /air mixture screws with the carbs in place, or do I need to pull them off the bike? There's not much room in there.

Thanks, Mark

[FJmonkey]

Welcome fjbiker, we will help you find your Kookaloo....

[fjbiker]

Thanks Mr Monkey.
I've been following the forum for a while now and have learned lots of good tips from the gang.

Hey Pat,
Disregard my last question about how to get access to the fuel/air mixture screws. I found one of your posts to another member stating that they are on the top of each carb. I always saw the rubber plug on the top of each carb but was never curious enough to see what was under there. Now I know.

Originally I was thinking they might be underneath the carb like on my 4 stroke thumper dirt bikes.

Mark

[ribbert]

Hey Pat,
Disregard my last question about how to get access to the fuel/air mixture screws. I found one of your posts to another member stating that they are on the top of each carb. I always saw the rubber plug on the top of each carb but was never curious enough to see what was under there. Now I know.

Mark

Mark, If I read your post correctly, the mixture screws are not under the rubber caps, they are for connecting vacuum hoses to a carb synching tool.
The mixture screws can be seen in the photo below, they are the small ones recessed into the carb body.

[fjbiker]

Thanks Noel. That's an excellent photo showing exactly where I was thinking the adjustment screws after reading the other post. I think on my FJ I have small rubber plugs covering the recessed screw holes; just going from memory. I'll know once I pull the tank off again.

So the plan is to resync the carbs again, do the blip test, check the results and also try leaning the air/fuel mixture by a ¼ turn in and check the results.
I won't be able to get to it for another day or so but I'll pass on the results once I get it done.

Thanks, Mark

[fjbiker]

Hi Guys,

I finally found some time last night to dig into my idle issue again. Unfortunately sometimes life gets in the way of our passions.

I synced the carbs again. They were slightly out but not by very much.
Bleeped the throttle many times but did not notice the revs hanging or dropping low when returning to idle. That part seemed ok.

Then I checked the air/fuel screw settings. This is where I found something strange.
I counted the number of "turns in" so I would know what they were initially set to. The settings were all over the map.

Cylinder #1:    3.5 turns out
Cylinder #2:   4.75 turns out
Cylinder #3    3.5 turns out
Cylinder #4:    4.0 turns out

I've owned this bike from 1987 and never adjusted the air/fuel screw before.
Could they have been like this from the beginning or could they have unscrewed slightly over the years due to vibrations?

The OEM service manual says the pilot screws should be 2 turns out. I've also gathered from forum that 2 turns out is a good starting point.

So I set them all to 2 turns out. I had to increase the idle adjustment quite a bit afterwards because the idle speed dropped too low after changing the air/fuel screw settings.

If it doesn't rain today, I'm planning to take it for a ride and will let you know if I still have my idle issue.

Mark

[Mark Olson]

when was the last time your valves were adjusted?  87?

[fj11.5]

The mixture screws originaly had / have small plugs covering them, at least we don't have to drill anything to find ours unlike some bikes

[movenon]

"This usually only happens after I've been riding for a while and the motor is hot; typically while stopped at a traffic light."

Could this be related to a fuel delivery problem due to vacuum in the fuel tank, fuel shut off valve?  Please forgive me. My knowledge of the 84 fuel delivery system is limited to only theory and what I gleaned from reading other posts. 
George

[fjbiker]

I took the FJ for a ride yesterday after the air/fuel screw changes. The problem happened twice again while waiting at a light; it idles fine for a few seconds then rpm drops and starts to sputter. I couple quick bleeps of the throttle are needed to prevent it from stalling. Once I'm moving again, it is fine.

I'm almost starting to wonder if the battery may be weak and while at idle, there is not enough charge to fire the spark plugs when the alternator output is low. The battery is old.  I'm going to check the voltages at idle.

One thing it did notice after setting the air/fuel screws to the factory recommended 2 turns out, is that the motor has more vibrations from 1k to 3k rpm. Above 3k, the engine vibrations smooth out. I think this change is due to my changing the screw settings.
There were certainly less engine vibrations with the screws at their original 3.5 to 4.75 turns-out setting. I'm really puzzled why these were so far off the suggested settings!

And the valves were checked last fall and are within specs. That thought had crossed my mind also.

Mark

[ribbert]

I took the FJ for a ride yesterday after the air/fuel screw changes. The problem happened twice again while waiting at a light; it idles fine for a few seconds then rpm drops and starts to sputter. I couple quick bleeps of the throttle are needed to prevent it from stalling. Once I'm moving again, it is fine.

What you describe could also be mechanical. Next time you have it idling push down directly on the throttle linkages on the carby to make sure it has returned all the way to the stop. For example a cable without enough slack can do this as can a partially frayed cable. It can hang a bit giving you a false idle when you close the throttle then creep the last little bit to the stop while you're sitting there.

These things are not as common but if your running out of ideas, it only takes a second to check and then cross of the list of possibilities.

Noel

[Pat Conlon]

Good point Noel, check the linkage for binding and....Yes, an old battery can cause all kinds of gremlins...

[Flynt]

I'm really puzzled why these were so far off the suggested settings!

Let me start by saying I'm the outlier with this opinion I think.  Most set to 2.5 turns and work out the rest from there.  I believe tuning each carb independently has big benefits that racers will never see and therefore discount (sorry Randy).

I have quite a bit of multi-carb experience and I believe the optimal running conditions will be with each screw set for that particular body's flow at idle.  You will see differences in flow, and more importantly in throttle plate vacuum, based on fine details of each individual throttle.  The reason you have a screw instead of an orifice is to allow fine tuning of the idle circuit to the throttle body.

In short, put them back and enjoy.  You can get lots more info about fine tuning multi-carb setups based on airflow (best done on a flow bench), but it sounds like you had a reasonable tune before going to a rough approximation, really meant as a starting point.  To get the best running at low rpm, you need to get those individually adjustable carb bodies dialed in for each cyl/throttle combination.  So go back and you'll likely be closer if not right there.

Frank