weird idle issue...

Over the last couple of days, my XJ750 has done something I thought a little strange. I was sitting at a stop, with the bike idling normally, and then the idle started to climb. At the point it did it I did not have my hand on the throttle. Now here is where it gets really weird. I checked the throttle to make sure it was rolled off and it was. I then grabbed the front brake and the idle dropped back to normal. I let off the brake and the idle would start to flutter up again, only to drop when I applied the brake. It did not do this the next two time I rode, but it came back tonight on the way home. The idle would climb over 500rpm if I didnt do anything.

The only work done on the bike recently was installing new sterring head bearings. The throttle, clutch and choke cables are operating freely and do not seem to be pinched or binding.

A couple of other things I have noticed since this idel thing began are...

Engine braking does not seem to be as effective as it was before.
The engine sometimes does not seem to come down from higher revs as quick as it should when I roll off the throttle.

Its time for an oil change, I have a new valve cover gasket, alternator brushes and colortune on the way. So I have a little work to do, but was wondering what might be causing the idle issue?

 

I am not an expert, but my first guess would be the carbs. When was the last time they were serviced, rebuilt, cleaned or synch'd?

 

I think you h ave Diaphragms sticking open.

That will make the Idle rough and ... depending upon how much the throttles are opened to make your bike Idle ...

Some bikes are running on "False Idle". The throttles plates are open enough to allow the engine to "Idle" off Main Jets. You know if this is happening by hesitation and needing to crank-up revs getting out-of-the-hole.

I think you have a combinations of factors working to give you that condition.

I don''t think your Pilot Mixtures are correct across the board ... and I bet at least one of your Diaphragm Pistons is sticking.

With the Piston not falling during closed throttles ... additional fuel will upset the ratio and the engine will hunt for a correct mixture.

With one Diaphragm Piston sticking ... the additional fuel will cause a fluctuation as it revs higher with the supply going in before it corrects itself as the engine vibrations work with the returnn spring to close it.

It wouldn't hurt for you to pull the Tank ... pop-off the Carb Tops and get inn thehre with some ScotchBrite and "Scrub" the Inside Diameters of the Bores.

Remove any aluma-oxidation and check that the Diaphragm Pistons rise and fall without any resistance.

Search: "Clunk Test"

 

Carbs were done several months ago, full dissasembly and cleaning by a local shop. Bikes been running fine up til now.

No hesitation on throttle up from stop.

I believe the "warmed up" idle speed is too high. Which is what prompted me to purchase the colortune. I'll see i f I can do the cluck test this weekend.

 

You can get a quick look at your spark plugs and they will tell you plenty of info about the mixture. You might also want to synch your carbs if you are going to change or adjust the mixture, also take care of one of the possible problems.
Here is a good thread for a cheap alternative to the expensive carb sticks if you need it:

Old School Method of One Vacuum Gauge Carb Sync

 

Thanks Rick, My bike has that exact symptom, guess I'll give it the 'clunk test' and a quick color tune

 

Keep in mind the the Pilot Mixture Screws have to be set Rich enough to Idle the Bike ... and, ... sustain the Mixture for the brief moment that the Throttles are opened until the Main Jet Supply kicks-in.

Too Lean and the Bike "Waits" until the Main Jet Supply supplies the Fuel to get you off and going.

The Ideal situation is having the Pilots Rich enough to give you a very steady Idle and just enough to sustain the Power-on to get rolling from a stop.

The Colortune Plug will Dial-in Idle real nice at the Lean-end. Just where the Plug gets Blue from too Lean. You need to Tweak through that "Early Blue" and find the "Other side" of the plateau of the Blue and come-back into it.

THAT's where there is enough Mixture to sustain the rev's when the Throttles are opened and you get-going from a stop.

 

well, sucky (rain) weather left me no real chance to do anything today (no room to work in the garage) but I was able to pull the plugs and check them.

1 and 2 are white, and 3 and 4 are white(but not as white as 1 and 2) So I now know I have a lean condition on my hands. The Colortune arrived yesterday so if the weather cooperates, i'll be able to get in there with it tomorrow.

 

Start the Mixture Screws at 2-2/4 Turns Out.
Colortune the Mixture Out until you get the Blue.
Keep easing the Screw out BY DEGREES until you lose the Blue.
Come BACK to the Blue.
Tune for the RICH side of the Blue.

The RICH side of a Colortuned Blue is close to the Variant you want to have for Fine-tuning your Bike ... your way.

Once the Colortune "Finds" the optimum Air~Fuel Ratio for that Hole ... you will have to wait and put some hours on the Bike and folo-up the Colortuning with a PLUG READ.

The READ on the Plugs will be the Final Word on where you have brought the Bike in ... at.

The Window of Fine-tuning for either Fast Acceleration and Heavy Engine braking ... or, ... Smooth Acceleration and Engine "Coasting" will be within a very few DEGREES of Lean or Rich after the Bike is successfully Colortuned!

 

You said the idle settles when you grab the front brake. Is it possible you are actually rolling the throttle backwards a bit when you apply the brake? If you apply the brake by pushing on the lever only, not touching the grip, does anything happen?

This could all come down to a slightly mis-adjusted throttle cable.

 

Keven59 makes a good point.
There's a Knurl Throttle Cable Adjustment below the Throttle Grip.
Give yourself a little slack in the Cable and see if the situation improves.

You unlock the round Thumb-wheel and turn the Barrel while "Feeling" the play in the cable. Just don't have that Cable Guitar-string tight. That side needs a bit of play.

The Clutch side you want tight.

 

I thought about that. But the throttle was full off and has a little play in it already. So i have ruled this out.

Its strange. I'll be sitting stopped, not holding the brake and the idle would start to climb. I would pull the brake lever and see the dash lights get dim (this was at night of course) and as soon as the lights dimmed, the idle would come down.

 

Then I would have a peek at the Alternator Brushes and the connections at the Battery (Clean and resurface), Voltage Reg. (Clean contacts) and go over all the Grounds (Main - behind starter -- Coils - On frame near Coils).

Just as a precaution:

Measure Voltage at battery with Engine revving at 2,200 rpm's.
You need 14.2 -to 14.5 Volts at that Engine speed.

If you fail to see 14.2~14.5 Volts ... troubleshoot charging system beginning with Alternator Brushes --> 11mm Min Svc Limit.

 

I have a set of brushes on the way and should have plenty of time after Turkeyday to dig into it.

 

I can vouch for the relationship between ignition problems and idle quality. Recently one of the crank position pickup coils on my 1982 Seca 650 failed completely (had an open circuit). After replacing with a good coil, cleaning a few electrical connections in the ignition circuit but making no other changes, the idle quality is much improved. The idle speed used to fall off slower and slower until the bike would eventually die unless I cracked the throttle slightly.

Now, with the replacement pickup coil and good clean connections in the circuits to the TCI, the idle speed is rock steady.

My guess is your ignition is weak and the idle speed has been jacked up a bit to compensate. Under certain conditions when the ignition is behaving normally, the engine speed rises above normal. When you hit the brake and drop the voltage to the ignition, the idle speed drops back.

 

Pull the tank and do a number on the Harness-to-Coils connections. Clean them up with a Contact ReNew Spray and make sure they are tight.

Then, ... Make the Coils GROUND Contact clean and resistance free. Shine the Eyelet to the Grounding Wire with an Emery Board and make sure that it Contact Surface to the Bike's frame is Clean and Tight.

I soldered the Coil Connections when I was getting an occasional misfire happening and I haven't had any trouble like that since.

They are Stock Coils. I'm hoping Santa leaves me some of those DYNA Coils and High Performance Wire under the tree.

I have a feeling I'm not on the "Naughty List" this year!

(Ain't too proud to beg ... and, only my Momma loves me ... but, then she could be jivin' too!)

 

Right on, right on, right on....

 

I was hopin' somebody knew about "The Donnie Osmond Look-alike Contest"

Looks like you and me know good comedy, Justice ...

Ain't that right, "Blind Baby"?

 

We're never going to get that on the album cover.

 

Probably play jump rope wit it, though...