GX160 with flat spot at low revs

[T-man-Moore]

Hello,
Newbe Ty here.
Myself and a few friends have our own kart league. We have a private (real tight and twisty) track. Basic rules of engagement are GX160 (or clone) engine and thats about it. (nobody is allowed to be spending silly money with billet stuff)
Now, i've built a kart using an ond tony-kart frame. Have extended the frame by about 90mm to enable me to fit the engine real low down with drive to pretty much the middle of the axle. Handles like a dream.
Having trawled through various sites, i've found out how to up-rate the standard 5.5 power-plant. (springs, keyway, carb, plug, exhaust, air-filter)
Problem is, it's developed a flat-spot at low-revs. Engine is fine if the revs are brought up gradually but foot to the floor from idle and it dies.
Have already cleaned out the carb, then bought a new one just to be sure but still the same. Plenty of spark. Got a couple of engines lying around so have changed the coil and bought new plug. Same prob.
Anyone got any ideas? Would the valve gaps cause this? If so, what should they be? (seen too many different opinions)
Look forward to any feedback from my first ever forum post.

 

[landuse]

The valve lash should be be set at .002" intake and .003" exhaust. This is a general statement made by AGK HERE

This was for a clone

 

[T-man-Moore]

Thanks for that Landuse.
Do you think the valve lash would be the cause the flat-spot at low revs?
I've advanced the flywheel by 4 degrees. Could that be the cause? didn't think it would be an issue as it seems like quite a popular enhancement.

 

[OzFab]

Are you using a standard main jet? This sounds typical of fuel starving.

With all the other mods, the carb is now having trouble providing enough fuel.

It sounds like you have a few spares so, drill out a jet & see if it makes a difference

 

[T-man-Moore]

Thanks Fabroman.
I've got a 75 jet in it already. You reckon i should go bigger?

 

[CaptCanadia]

YES, especially with increased air flow.

 

[Doc Sprocket]

In addition, you may well need to enlarge your pilot (idle) jet, which does affect operation coming off idle to some degree.

The very first thing I would do is a plug chop to see if your main jet is appropriate. Once you've got that nailed down, you'll see whether you need to play with the pilot jet. Also- having your idle turned down a bit too low can have a similar effect. If you can, increase the idle slightly.

 

[T-man-Moore]

Thanks for the above info chaps. Have increased the idle and reset the clearances. Flat spot seems to have (mostly)gone. It's still apparent if i floor it from a stand-still but when driving round the track you never notice it. As a trade off, i have now developed an issue where by the engine revs don't immediately cut-off when i take my foot off the throttle.
I'm kind of thinking that there is still too much 'lingering' fuel after I take my foot off the pedal. So for that split second or so, it feels like the kart wants to keep accelerating. (a little bit un-nerving as you brake hard in to a corner)
I noticed that some guys recommend using a gx140 flow-tube (or emulsion tube). Would that aide cutting off the fuel when i take my foot off the gas?

**Not really seen it mentioned much but boring out the GX standard carb seemed to show good/ noticeable performance gains.

 

[CaptCanadia]

Boring out the carb does show increased performance. If you cant bore it out and you have a gx390 carb you can use that, it will be the same thing, though that can be kind of pricey if you have to buy the carb and adapter.

The gx140 e tubes are used because they increase the low end power on your engine. Im not sure if that would solve your problem though.