GS400 idle rise when warm, how much is too much?

[angrypants]

Hi GS gurus,

hope you can help, this is my first bike and while i have followed the 10 tips and GS happiest , newbie mistakes advice to the tee ( i think), i have no basis for comparison at all.


everything stock 1978 GS400 BS34 CV carbs.,
Completed , rebuilt petock, derustand lined, dipped and thouroughly cleaned the carbs incl cycleorings, new engine side boots & orings, new air filter and airbox sealed. bench then carbtune pro synced carbs used the highest idle method when setting the idle with the air screws/pilot screws the sweet spot is around the reccomended 1 & 1/4 turns out, value clearence in spec and charging sytem passes the tests, elec connections all cleaned, new points/ timing spot on. no plug chops done, they seem a little tanned??

THE ACTUAL QUESTION: im wondering what the acceptable rise in RPM is from just off off choke after cold starting to the a set of lights 20 minutes away ( well warmed up)?
wetaher conditions say 70 deg F, 20 deg celcius:

After starting with choke it sits around 2000 rpm and after around 1 minute the revs start to die down to about 900- 1000 and i need to turn the choke off to stop it stalling, with no choke it will need the odd blip or hold a bit of throttle to stop from stalling before i ride off (i did have to fiddle with the choke a while back to stop it revving too high after rebuilding the carbs and syncing them, can recall exactly what i did though ;-).
After i ride off if i get stuck at the lights 700 yards away it might still need a blip & still sounds a bit grumpy still idling around 1000rpm, but next set of lights 2 mile away its happy at 1100rpm. after i complete my little circuit giving iot a good go and stop at lights 15 minutes later the revs are 1200 or 1300 running very happily. though it has a few times just died at idle when warmed up, (and you could feel it coming )

i have run into the trap of turing the idle crew up when cold then turing it down when at running temp to make life easier at the time, but it bites back as next start it wont idle when it comes off choke and then if i adjust so it does idle cold i go to high it revs at 1400-1500 when fully warmed up.

is this normal behavior ?
does this sound acceptable range of RMP from cold to opertaing temp ?
am i coming off the choke to early?
should my rpm to drop like that so quick on choke wanting tio die .. . is my idle circuit rich making that happen

grant

P.S ive check for air leaks with sprays but found none , maybe they are very small on the intake side and show when hot . im not adverse to getting new intake new boots/tubes and clamps, pulling the carbs and starting again but dont want ti chase something that is not a problem

PPS sorry for the long detailed post , thought it may save questions

 

[Steve]

Don't worry about the "long detailed post", some of us really appreciate it. :encouragement:

It prevents playing a game of 20 Questions.

In my experience your 1 1/4 turns out on the mixture screws is a bit lean, but each bike, even each carb, can be different. Mine are almost never less than 2 turns, usually about 2 1/2 turn out. I would suggest turning them out about 1/4 turn to see if that makes a difference.

I belive the specified idle speed for your bike is 1150 ?50 RPM, so you are pretty close to that. A warm engine will run a little better than a cold engine, so speed about 5 minutes after turning off the "choke" is going to be a bit higher than 20 seconds off the "choke". How much of a difference? Could easily be 100 RPM, but that's only about one needle-width on the dispaly, so its sort of hard to tell.

.

 

[Gorminrider]

Sure, yes, they idle better when warmed fully. Yours sounds fairly normal to me on my 4 bikes. But each of mine has a slight difference...some are better than others when the choke/enricher is taken off. So I don't worry when each bike runs well when warm and tune to that.

You really don't want to be tweaking idle knob at the first light. Starting off with a bit of choke is better, imo but Still, I never take off with any choke on. Because: I will forget! at least until it occurs to me the bike is running a bit high and/or stinky.

By the way, the tachometer is not all that accurate a design. It's merely an indication so use your ears and choose "useability" rather than the tachometer. I find you don't want too low an idle at lights..even though I could pung---pung---pung-- if I wanted to. Yours does all sound a bit high to me..if my tachometer indicates a too-low starting idle around 800 and somewheres close to 1000 after a run. But the range of change is familiar ..it's a very hard thing to pin down as an overall fact on every bike. They develop personalities, especially across different makes and carbs...

At some point, you'll have to include the Old Bike Factors: wear in the jets,needles seals, passages a bit chalky- any and all of these. And even the alcohol in modern fuel which these bikes weren't made for..(float height is slightly affected and mileage-this modern stuff is really for computerised fuel injection)

 

[Gorminrider]

the oil thins a bit when warm too, at least it feels like it. Less friction everywhere when it's warm throughout.

 

[angrypants]

Thanks for the advice I appreciate it.

 

[Gorminrider]

Hey you're welcome from my personal perspective (= runs well?-don't fix it!)

and don't forget even black box-controlled fuel-injected cars rev higher until they warm up.