Intermittent High Revving at idle

GoldenMotor.com

fugit

New Member
Mar 5, 2012
176
0
0
Oregon
Hey anyone :)

So got 2nd motor fired up and it really runs great. I am having alot of fun! But I noticed that when I let go of throttle sometimes ( while riding) the engine rev's high and I need to goose the throttle a few times to get it back to normal idle. Or if I am idleing down the motor after a run around town, before shutting off, it will be idleing fine then rev high just out of the blue.

Anyone got an idea where that problem may be?
 

vachon644

New Member
Nov 27, 2011
95
0
0
Quebec city, QC, Canada
Sounds like an air leak.
Start the engine up and spray some ether (Quick-start) or carb cleaner around on the engine to pinpoint the leak. Assuming the engine sucks in air, the RPM should change when spraying on the leak. Chinese 2-stroke engines are notorious for leaks between the carburetor and the intake manifold.
 

2door

Moderator
Staff member
Sep 15, 2008
16,302
175
63
Littleton, Colorado
Check your throttle cable to make sure it isn't binding. You didn't tell us what engine or carburetor you have so offering other suggestions is hard. Provide us with some information to work with.
Tom
 

fugit

New Member
Mar 5, 2012
176
0
0
Oregon
Motor is 66 cc and the stock carb on it. Throttle seems fine got a lot of free motion in it and it snaps back seemingly perfect. I am also using the upgraded manifold with the o ring on it. From the first suggestion I did tightem the manifold screws ands I was able to turn them about 1/2 or a bit more turn. I also tightened the screw that holds the carb onto the manifold and was able to get a 1/4 turn on that too.

I will run it here in a bit see if that helped :)
 
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dragray

New Member
Mar 10, 2012
278
2
0
Indiana
to me, it sounds like possibly the float may be set too low.
this will cause the float bowl to run out of gas, making the engine run lean until it gets more gas into the float bowl.
a lean condition will make the idle go up.
an air leak would cause the high idle all the time, not just every once in awhile. plus, with an air leak, the engine would run terrible all the time because it would be too lean.

it's possible that your carb slide is binding up in the carb every once in awhile.
beleive it or not, the shape of the slide and the carb throat can change with heat / cold which can cause the slide to not be the same exact shape as the throat, which may cause it to slightly bind up.
 

fugit

New Member
Mar 5, 2012
176
0
0
Oregon
So I bumped the throttle needle valve up one and of course as mentioned tightened the manifold and carb screww that attaches manifold and it seems to of helped alot. Still get it a little right after an extended WOT but it is momentary then settles back down just fine. :)

Thanks guys everyone here is such a big help !
 

rustycase

Gutter Rider
May 26, 2011
2,746
5
0
Left coast
f, you didn't mention which 'stock carb' you had. :)
the way things go in china, it could be any of three???
or who knows what might happen mid way in the production run! lol
VERY competitive people, over there.

A few days ago I was reading expert tuning tips on Mikuni and keihin carbs on 2 stroke motors... of course they are far superior, and much more adjustable than what we run.

IIRC, there was mention of similar symptoms resolved by adjustment to the idle circuit.

I also read of another respected fellow who advised a 5mm x5mm notch to be cut into the piston skirt, at the intake port location, to improve the intake 'signature'.
Not a clue if that might apply to your intermittent thingy, but it might... :)

First thing I'd do, if the throttle cable wasn't somehow binding, somewhere, would be to put some rtv goop on the carb to manifold joint. Not a lot, just a little bit. I'd use rtv red.

Good luck
rc
 

fugit

New Member
Mar 5, 2012
176
0
0
Oregon
It looks like the NT carb but with out the upgrade. Or it is the carb that JNmotors uses then adds the velocity stack and better filter, Mine is just the stock one though without those modifications..

Does that help?
 
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rustycase

Gutter Rider
May 26, 2011
2,746
5
0
Left coast
Well, yes it does!

If it were a cns it could be almost anything under the sun malfunctioning!!!

but with the nt you don't even have an idle mixture adjustment...
Still, it's a better performing carb.

Do you have a good filter?
Both fuel, and air intake?

perhaps there might be a bit of transient crud in a passage, bouncing around in there, causing an intermittent lean situation, causing the rpm to elevate???

I used a little rtv red over the clamp on when I used the nt.
I think when I install it again I will put a bit on that O ring, also.

Interesting prob, f...
At least they are small and ez to work on!
and cheep!

Good luck
rc

Perhaps float is set too low, or petcock restricted, causing it to run out of fuel after a WOT run, making it lean, and causing rpm to race????

Or like the other feller with the hissing tank because the venting was faulty???

.
:-||

I replaced my crummy cheapo stock cns air filter element with some B & S blue foam stock... FAR better quality.
The motor fell flat on it's face!
Nobody home.

May have to take out that new filter foam and put the free flowing original stuff back in!

laff
 

fugit

New Member
Mar 5, 2012
176
0
0
Oregon
yea I use a fuel filter and the petcock filter. I though about taking that petcock filter out. But i got some good leads now on what may be going on and can isolate that problem eventually. Not so bad now other then right after WOT and only a tad - maybe 2 seconds then idle down nice.

I have an NT carb, CNS carb, and ordered a Modified carb from JNMotors ( velocity stack and better air filter) Oh boy im addicted :) WHen the MOdified carb gets here I will RV seal it at o-ring. Eliminate potential sources. I really need to get better gasket material too and redo all gaskets. But I just wanted to see how she ran from the box first.

Tomorrow, I go to 80:1 mix on the AMSOIL - Ya HOOOOO :) Ran two tanks on yamalube oil at 24:1 so I hope she's ready....

Thanks for the help!!!!!
 

blazingoutcast

New Member
Mar 28, 2012
51
0
0
st. cloud mn
Good luck with it all i hope u get her running right n do u let the engine warm up before riding cuz i've noticed on my grubee skyhawk gt5 angel fire slant head 66cc with n nt carb i have to warm her up or her speed and power will fluxuate ridiculouly also causing an extremely high idle now i gave my two cents someone wanna throw me two cents

I have my idle screw turned all the way in just to get it to idle m i runnin to rich or lean also i have oil coming out of my muffler at the gasket cant seem to make it stop its starting to irritate me lol any n all advice welcome
 

fugit

New Member
Mar 5, 2012
176
0
0
Oregon
Well I am rather new but seems if you bump up the needle setting in the carb one notch that may help adjust idle. I read that somewhere a guy had a similar problem. He went too low on needle valve setting and couldnt idle carb afterwards.

As for the oil out of the gasket well thats easy. Remove all stuff existing smooth out surfaces, and reapply a new better gasket. Use that copper spray gasket material too with a regular gasket. Just spray a few layers of that onto the new gasket let dry to tacky and replace. Make sure you tighten well and you should solve that. :)
 

blazingoutcast

New Member
Mar 28, 2012
51
0
0
st. cloud mn
Ok thank u ill look into it what about the screws that come out of the engine cuz i have a feeling one is stripped how do i replace then n its the screws or bolts i should sag that attach the engine and muffler?????
 

rustycase

Gutter Rider
May 26, 2011
2,746
5
0
Left coast
lol

New here?

I just ran across this at another forum where they REALLY razz newbies.

f, with the switch from your fat mix to the 80:1 your burn will be richer and that lil' problem just might go away!

Good luck
rc
 

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