Idle creeping up to 1.5/2k RPM

[Maurice]

Hi,

disclaimer: I am new to this, but mechanically inclined and have read the carb guide.

My bike is a 78 SE, got it completely stock except a Boyer ignition and it still mostly is. The bike had been sitting for some years but started well enough, although it was evident the carbs needed a good cleaning.

I took the opportunity and moved to UNI pods, mostly for the convenience of taking the carbs in-out as I knew I'd be doing this a lot. I now have the chinese puzzle motion down with respect to pulling the carbs, takes 5 minutes from bike to bench tops.

In any case, cleaned the carbs, re-jetted to 1 size up on pilots and mains, new o-rings and gaskets, and now the bike runs nicely, except for idle: it would site there at 1.2k, then if a red light took too long it would go up to 1.5k, 2k and stay there. Bit puzzling until one day I forgot to move the petcock from prime to on and saw the carbs leaking.

Float valves: with new ones and float height to 24mm and now idle is constant, pulls off idle fine and the throttle action seems perfect (to me...) across the whole RPM range.

Just wanted to share in case this could help someone, this specific symptom did not exactly yield many search results here when I googled.

Cheers

 

[GreasyC]

If your carbs leak you need to fix that for obvious reasons, but that's unlikely to be related to your idle climbing. The idle rpm is affected by many different variables. Some stay constant over the course of a ride like the idle screw and pilot screw settings, and others are always changing like the engine and air temperature. It's impossible for a "dumb" system like a carburetor to hold one specific rpm under all those changing conditions so some change is to be expected but 2k is more change then you want so some things to check:

1. Check the advance unit for sagged out springs. If the timing isn't being fully retarded at idle this can cause the rpm to hang high.

2. Pilot mix and idle screw settings. A slightly richer pilot mix or a slightly lower idle setting or both. When the bike is fully warmed and exhibiting the rpm creep set the idle down to where you want it. If this makes the choke required for longer than you feel is sensible after a cold start then enrich the pilot setting some.

 

[Maurice]

Hey thanks for the feedback, but maybe I didn't explain it clearly: idle is no longer creeping up with new float valves (OEM ones, not knock offs). Leak was likely caused by the bowl overflowing.

Oh and ignition is electronic (boyer), and I set it with a timing light just a hair below full advance so it won't break my ankles.

I still do have some popping at idle and decelerating (does it more when cold, I'll figure it out...) but it's fairly stable/smooth otherwise.

 

[GreasyC]

Sorry my brain must have tuned out and missed that you already fixed it. An excess of fuel bowl should throw the mixture towards the rich side, which if anything should cause a dropping of idle rpm when the bike gets hot (possible stalling at lights etc). Strange that putting less fuel in the bowl would fix that symptom but it's a strange world. Glad you got it fixed.

 

[Maurice]

No problem. It could have been a combination of things, since I figured out my popping issues were due to leaky holders. In other words: bad float valve + air leak at holders = RPM creeping up when bowls start filling faster than idle drinks.

At any rate I have no more popping now, transitions are smooth, off idle is snappy and power seems good (for the bike...).

Now I could go and put the bad valves back in to see what only that does, but somehow I don't feel like it

One thing I wish for is a progressive choke. This on/off one sucks: it's 3k RPM or die, nothing in between. Well, unless you hold the thing carefully. Luckily now it runs well after only a few seconds of choke, but I wish I could set it half way seems like it would be more than enough...