Need help tuning my Vento GT-250 (Same as Hawk)

[skirmich]

This is the brand new spark plug I installed yesterday..

Running conditions:
From Idle to half throttle never going above 5000rpm.
Around 40miles in it.

[XLsior]

definitely not running rich or flooded.

nutmeg brown indicates the jetting is pretty close under current rpm ranges.

[skirmich]

Originally Posted by XLsior

definitely not running rich or flooded.

nutmeg brown indicates the jetting is pretty close under current rpm ranges.

Yeah the bike runs great..

My only issue right now is the inconsistent idle when coming to a stop, I cant seem to tune in the idle at all. Either is too low and stalls or its too high that rev hangs on 2000rpms. The idle screw is almost fully out to the point that it stalls if I go any lower but wont keep the engine running if I try to dial it in.

Perhaps the A/F screw is lacking some tune?

[krat]

I agree that the plug looks perfect, don't touch nothing! It will just continue to get that toasty look that we all know and love.

As for the idle problem, think about what you have, that carb is as simple as dropping a rock. The idle adjustment is a screw that impinges directly on the angled flat on the slide inside the tube. The slide pulls the needle out of the jet tube and releases fuel. That is a very mechanical issue, not a fueling problem. Slide in, needle up, faster/ slide out, needle down, slower.

Unscrew the top and pull that slide out and check it for burrs. Make sure the slide runs freely and that the spring is not binding the throttle cable. Make sure the cable is in its notch and the cable end is in its recess. Lots of times the cable gets tangled in the spring as you replace the slide and screw down the cap. Ask me how I know.

Something is probably blocking the slide from a full return consistently. That is where you get into burrs, sticky throttle cable, weak return spring....

You have this nearly worked out. It is now some little something that just needs a scrape with a pocket knife.

[skirmich]

Originally Posted by krat

I agree that the plug looks perfect, don't touch nothing! It will just continue to get that toasty look that we all know and love.

As for the idle problem, think about what you have, that carb is as simple as dropping a rock. The idle adjustment is a screw that impinges directly on the angled flat on the slide inside the tube. The slide pulls the needle out of the jet tube and releases fuel. That is a very mechanical issue, not a fueling problem. Slide in, needle up, faster/ slide out, needle down, slower.

Unscrew the top and pull that slide out and check it for burrs. Make sure the slide runs freely and that the spring is not binding the throttle cable. Make sure the cable is in its notch and the cable end is in its recess. Lots of times the cable gets tangled in the spring as you replace the slide and screw down the cap. Ask me how I know.

Something is probably blocking the slide from a full return consistently. That is where you get into burrs, sticky throttle cable, weak return spring....

You have this nearly worked out. It is now some little something that just needs a scrape with a pocket knife.

Ill try to look into a possible binding issue but I seriously doubt it..

When I think I have the idle dialed in I go for a ride and bamm again at 2000-2100RPM, I lower it on the fly (Seems to settle back to ~1700RPM) and then it suddenly dips into the low 1000s which either stalls it or seems like its going to but never goes back to 1700. I start to ride and stop again and repeat the same issue.

Right now I have to settle for the bike to hang the revs high for a while and if the light last long enough it will eventually (But taking its sweet time) go back to 1700rpm.. It also does this weird thing that once it has passed enough time it will idle high again completely on its own..

Ex:
>I stop for a light put on Neutral or Pull the clutch bike revs high on the low 2000s RPMs.
20 seconds pass
>Bike idles back down on its own to 1700rpms (My desired idle speed)
10-15 seconds pass
>Bike starts to idle high again to 2000s RPMs without me touching anything.
Rinse and repeat for as long as I am stopped.

This is the really the only issue I have at the moment.

Also I am running NGK DR8EA (7162) Plug which is what the manufacturer recommends, should I switch plug now that I have the mods?

[XLsior]

just keep riding it for now until things settle in.

Make sure your clutch cable adjustment has a a little bit of slack same goes for throttle cable..

the idle knob should only be a faction open from a fully shut/seated throttle slide before making the necessary a/f adjustment.

plug type seems fine.

[krat]

Originally Posted by skirmich

Ill try to look into a possible binding issue but I seriously doubt it..

When I think I have the idle dialed in I go for a ride and bamm again at 2000-2100RPM, I lower it on the fly (Seems to settle back to ~1700RPM) and then it suddenly dips into the low 1000s which either stalls it or seems like its going to but never goes back to 1700. I start to ride and stop again and repeat the same issue.

Right now I have to settle for the bike to hang the revs high for a while and if the light last long enough it will eventually (But taking its sweet time) go back to 1700rpm.. It also does this weird thing that once it has passed enough time it will idle high again completely on its own..

Ex:
>I stop for a light put on Neutral or Pull the clutch bike revs high on the low 2000s RPMs.
20 seconds pass
>Bike idles back down on its own to 1700rpms (My desired idle speed)
10-15 seconds pass
>Bike starts to idle high again to 2000s RPMs without me touching anything.
Rinse and repeat for as long as I am stopped.

This is the really the only issue I have at the moment.

Also I am running NGK DR8EA (7162) Plug which is what the manufacturer recommends, should I switch plug now that I have the mods?

Forget about your plug, and forget about your "mods", they are not relevant in this situation. You have changed your life by 1.22 cubic inches. That is not enough to factor in and everyone here has done the exhaust and cone filter thing. That is why we have a base line of jet sizes and adjustments to apply to the CG platform.

That carb is a tube with a slide to block the air and two holes to control the fuel flow. Those holes don't change. They certainly don't change randomly while you sit on the bike. The idle screw does not move in and out on its own either.

I do know one thing that will cause your symptoms.

Your air/fuel mixture screw has a fine point that closes the pilot jet when it is fully screwed in. It is a very delicate point and can be damaged if the adjustment screw is turned in too hard. It is easy to damage the point if you turn the screw in and out several times, going to full in, then turning out the desired number of turns from closed. I think you have made that trip several time now.

It also has a tiny spring to keep it from vibrating and changing the setting constantly, It can be mashed and become useless if tightened too hard,,,,, and that is the problem you seem to have.

I would check that A/F screw tip and spring.

Past that I am clean out of ideas.

[TominMO]

Are you running a good quality plug like NGK or Denso, or are you running a Chinese plug?

[J4Fun]

Tom, post #20 last sentence

[bogieboy]

In my line of work (small engine mechanic) an erratic idle is usually a vaccuum leak... i would double check the intake and carb are tight, no loose vac lines, and all gaskets on the intake side of things are in good shape. After that i would double check cam timing, and double check valve lash.

[skirmich]

Update:
Bike still runs great when hot.. Idle is still erratic (gonna redo the manifold and carb gaskets to check for any leaks as suggested by bogieboy).