High Idle Problems

[ActiveRusty]

Hey Everyone
I have a high idle and this occurs with the idle screw all the way out and not throttle cable bindings no air leaks anywhere found. and I'm a bit curious of what could be the problem running about 4k rpms at idle without choke and with choke it settles all the way down to about 1k rpms and is very strange. I'm running a runtong HP Carb with a 65 jet in it as my height above sea level is about 170m and runs perfect with that jet its just the high Idle concerned about

Thanks everyone

 

[allen standley]

Hello Active,
I can't think of anything which could cause this other than an air leak. If you can remove the carb and take it to the hardware store - find an "O" ring which will sit in the throat of the carb that may do it. Follow this link
https://motorbicycling.com/threads/o-ring-at-carb-intake.37303/

 

[ActiveRusty]

Thanks Allen maybe worth a try

 

[ActiveRusty]

Just thinking of it now i dont think that could be because i sprayed everything down with WD:40 to check for air leaks if there was a leak then the idle wouldve dropoped hey maybe its the carb lol idk but its weird **** though choke on idles perfectly then choke off braaaaaap 4k rpms lol thanks for the help though

 

[allen standley]

2 screws holding the bowl in place tight?

 

[ActiveRusty]

yep tight pretty tight

 

[allen standley]

Check head bolts / nuts ?

 

[ActiveRusty]

torqued down pretty well same with exhaust and intake bolts

 

[allen standley]

With a little more thought... try this - remove the carb, remove the bowl then look at the jet and tube ( the brass tube the jet is screwed into). These have been known to unscrew, not tightened fully during assembly and fall out into the bottom of bowl. That's all I got!
If you get it fixed please report back. Good Luck to you.

 

[ActiveRusty]

Just switched the jet out yesterday from a 70 to a 65 so i know its their pretty well but thanks for your help

 

[FOG]

Diagnosis over the internet is difficult at best, but I'll have a shot!

Methinks the clue is your choke telling us your idle mixture with the choke "off" is way too lean. This has nothing to do with the main jet. Idle mix is normally controlled by a pilot jet with an air screw for fine tuning.

Nominal setting on an air screw, and I hope you have one, is 1 1/2 turns out. I'd start there. Turning it in results in a richer mix.

Good Luck and I hope that helps.

 

[ActiveRusty]

Ill try that but I think will just create a higher idle. I remember when i had the choke disconnected it idled normally and when i put it back on it goes up really high so yeah the idle screw is all the way out and throttle slide is all the down to so yeah I dont really know

 

[FOG]

I'm not knowing your carb but an air screw and an idle screw are two different things. The idle screw mechanically raises the slide. The air screw has a tapered tip to control air flow in the idle circuit.

Does your carb have a air screw? Sometimes called an air/idle adjuster. I'll never own a carb that doesn't, but I know they're out there.

 

[ActiveRusty]

The Carb is a Runtong HP Carb with a 65 jet in as I'm at 170m elevation and it runs good with that jet a tanned spark plug which indicates a perfect mixture its just the high idle. The idle screw is set basically all the way out and the throttle slide reaches the bottom and still has a high idle and no the carb dosent have a air screw as its a 66cc 2stroke motorised bike. As I'm pretty sure 4 strokes only have air screws

 

[ActiveRusty]

as well it will idle fine with the choke on to

 

[FOG]

Don't take this wrong, I'm only trying to help, but I'm gonna take ya to school on how a typical motorcycle carb works at idle.

At idle and very small throttle openings your motor is running on the idle circuit. I'll repeat that. Idle circuit. It's a separate passageway fed by a small opening in the bell or mouth of your carb. Look for it. You'll see it. In that passageway is an air screw that controls how much air reaches your pilot jet.

What's probably happening here is it's been pre set too lean at the factory and then covered by a plug. Usually brass. Lean mixtures run hot and fast. Especially on 2 strokes. That lean mix will cause a high idle that will go away when the choke makes the mixture richer.

If you can find the plug, get it out, and then turn the screw in. It'll probably richen up and idle right. If not, throw the thing away and get a decent carb that has an air screw.

I hope that helps.

 

[ActiveRusty]

Could you possibly reccomend a carb please and Ill look for that screw

 

[ActiveRusty]

Here are the photos would the air screw be that flat head one?

 

[FOG]

Now you're on it Rusty. U da Man!

I'm not seeing it in your pics. Usually it will be on the same side of the carb as the opening that feeds it. You can sometimes follow the casting the passageway needs to find it. Maybe on the left side?

 

[FOG]

Looking again, 1st pic, and I'm not even seeing an opening to feed an idle circuit. Could be I'm all wrong and it ain't even got one. I dunno. Chinese.

The 2nd pic looks strange too. The slide isn't normally exposed out the back like that. That makes me think sloppy parts could cause an air leak in there somewhere. On a conventional carb even a loose cap will leak air. How do they get away with that?

I dunno Rusty, but I ain't impressed with that carb.

This is more typical.



The idle circuit's fed thru the hole on the left.

Here's a sideshot showing the air and idle screws. Air's in front. Idle's in line with the slide.



And frankly I ain't the best person to recommend a carb for you. I've been messin' with Japanese for something like half a century now but they're too expensive these days. Other guys here know better than me.

Help him out guys! I haven't been any help at all.