Idle RPM

[JPaganel]

My idle seems to be somewhat temperature dependent. On a hot day in traffic it can get to 2000, but will drop readily if I start to engage the clutch. I've read that this can be caused by carb synch being off. Is this correct?

I also noticed that today on the way home from work the idle stayed put at 1000. When I got home I realized that the choke  was not all the way in. It was out just a tiny bit. Could this indicate that I need a richer idle mixture? Or is today being cooler the more likely explanation?

This is the sort of stuff that makes me wish for FI...

[FJmonkey]

The last click on the choke is not likely a real setting. You may have a vacuum leak, while at idle spray some WD-40 or other solvent type spray around the intake boots. If idle changes you found your leak. If not, then keep looking....

[JPaganel]

It wasn't the very last click, more like two or three. I think it's where the enrichment valve just starts to engage. I have a can of starting fluid, I'll try that.

I am scared to take off my plastics, they have cracks and I'm afraid they will fall apart and I'll spend weeks trying to put together the puzzle...

[FJmonkey]

Yea, I fell for you, but you gotta do it some time...

[Rhino]

I had an issue where the motor would rev to about 2000 when warm.

after spending some time with the carbs it seemed to correct itself.

I balanced the carbs and played around with the idle mixture using the spark plugs as a rough gauge or where i was.

[andyb]

Does sound like sync if they're all clean internally and you don't have a vacuum leak.  If you can throw a sync into it, you'll possibly need to sit down and do a good session of blip testing with the engine good and hot to really dial it in.


<rant>
FI?  Fooey on that.  Carbs can be fixed with a screwdriver without a laptop, $400 interface box, a dozen pricy sensors to break or get jacked up from crappy or leaded gas, and that much more wiring to screw with.  I will admit that there's a couple of things that I really like about FI, but am very cheerful that my motorcycles don't have it!
</rant>

[JPaganel]

Does sound like sync if they're all clean internally and you don't have a vacuum leak.  If you can throw a sync into it, you'll possibly need to sit down and do a good session of blip testing with the engine good and hot to really dial it in.

I'll give it a try.

I don't really know what's inside them, and I don't want to take them apart just now. The bike is my daily transportation, and the last time I dived into a rack of carbs I wound up with a yearlong project instead of a running bike.

<rant>
FI?  Fooey on that.  Carbs can be fixed with a screwdriver without a laptop, $400 interface box, a dozen pricy sensors to break or get jacked up from crappy or leaded gas, and that much more wiring to screw with.  I will admit that there's a couple of things that I really like about FI, but am very cheerful that my motorcycles don't have it!
</rant>

There's FI and there's FI.

My personal favorite in the FI world is the old GM TBI systems (OBD-I flavor). They are mostly self-diagnosing, pretty simple, and components aren't silly expensive. There is a minimum of vacuum tubes and diaphragms to go bad, and wires fail a lot less than rubber. Also, wires and most sensors can be easily tested. The only thing that screws them up most of the time and is hard to diagnose is the EGR system, and that's a vacuum operated bit, just like CV carbs.

The GM Rochester TBI has a throttle position, intake manifold vacuum, exhaust oxygen and engine temp sensors. That's it. There's a knock sensor, but it's part of the ignition.

There are no idle adjustments, no mixture adjustments, no rejetting (and associated guesswork and ripping apart half the vehicle for every change) for altitude or exhaust mods, it doesn't care if you change air filters to a different style and shape.

I am amazed that carburetors lasted in the cycle world as long as they did. I'm pretty sure in 1986 no major auto makers made a carbed car any more.

[andyb]

I am amazed that carburetors lasted in the cycle world as long as they did. I'm pretty sure in 1986 no major auto makers made a carbed car any more.

Ford did.  Arguably Mazda, but it was branded as Ford.  In a body made by Kia, if memory serves.

The Ford Festiva had a whopping 58hp and used a carb for induction through the very early 90's.

[JPaganel]

It's resolved.

I used a homemade synchronizer to synch up the carbs and replaced the vacuum hose to the petcock that felt hardened and loose on the nipples.

My idle now looks like this:

FJ1200 idle after carb synch