gk76a unable to idle (urgent)

[ben2306]

hi guys, my friend's gk76a now is in workshop. after cleaning the carbs twice, its still unable to idle w/o dying off. changed new air filter & sparks plug & o-rings. what could be the problem? settings should be ok.

[Marsbar]

same problem with mine, itll idle for few seconds n then just die, even if i turn the idle screw to about 4k, even after a good turf when its about 1/2 temp i stop at lights n revs will just drop and die, not really that bothered at the moment as i just trottle it abit but would like to know how to fix it,

like you i have had the carbs out twice, new 'o' rings, fully stripped, balanced, new iradium plugs, air filter looks clean.

[dobbslc]

I had a similar thing on my 76A, try adjusting the pilot screws out to about 2 turns each from fully in, it helped with mine.

[Marsbar]

check your compression, my 3rd cylinder was at 40PSI....no wounder it wouldnt idle

[ben2306]

what actually causes the compression to be lower? mine was like that before but fixed up by workshop, never asked how or why. so now i'm quite interested... thanks.

[Marsbar]

rings, valve seats.

im replacing all 4 rings, all valve seals, but tuning the head whilst its off, had it skimmed and using chemical metal to make the ports flow better. i also need 2 new inlet valves.

[dobbslc]

Ben the low compression could have been due to tight valve clearance's, that's an easy fix for the garage.
Otherwise it's worn rings, burnt valves, head gasket etc.

Dobbs

[Zanin22]

I know it's not up to date but I had same problem with my gk76a sp, hope this will helpfull to others in same boots. Changed all oriings, balanced, idle mix screw 1st 1.5, not worked, 1.8noope (from in) 2.0nope, 2.5 nope, idle for few seconds and then just die or sometimes went up & down, specally when was on side stand straight died, then tried to play with the mix screws I was found out I had to turn 2 carb mix screw all IN and then my idle was perfect, still cant believe it, then I went for a quick ride, in some gears below 14rpm have abit flat spot I will play with it tomorrow to make it 100% but the bike is now run like a dream

somewhere I found how to set your pilot screw adjustment. I just copy here because it was helped for me to understand the theory but abit over complicated

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1) Warm up the engine to full operating temp.

2) Turn up your idle a few hundred RPM using the throttle stop screw (basically you want a fast idle). This will make it easier to hear small changes in RPM. Watch for overheating--pointing a big shop fan at your engine will help it from getting too hot during the fast idling. The whole procedure shouldn't take too long though.

Each time you change the screw setting 1/4 or 1/2 turn or so, wait about 5 seconds to let the idle speed normalize. It usually take the carb and engine a moment to react to the change.

3) Turn the fuel screw IN until the idle starts to drop and miss. The engine should die if you bottom out the screw. Your pilot jet is too big if it doesn't die when the screw is bottomed out--it should die before it gets that far in.!!!!!

4) Then begin turning the fuel screw OUT. The idle should peak and become smooth. Keep going and look for the idle to begin to drop/miss again.

5) The goal is to find the setting that provides the highest and smoothest idle. If it's unclear exactly were that point is then set to the midpoint between step #3 and step #4. For example, if the idle starts to drop at 1 turn out and starts to drop at 2 1/2 turns out then 1 3/4 of a turn out should be the correct setting.

If the peak/smoothest RPM is reached somewhere between 1-3 turns then your pilot jet is correct (the 1-3 turns applies to most carb types). If you end up less than 1 turn out then your pilot jet is too big and you need a smaller one!!!! If you end up more than three turns out or the fuel screw seems to make little difference as you continue turning it out than you need to go up (bigger) on your pilot jet.

To re-emphisize: If the idle never drops when you're turning the fuel screw in, you need a smaller pilot jet. If the idle never drops when you're turning the fuel screw out, you need a bigger pilot jet.

Typical fuel screw settings are in the 1 1/2 to 2 1/2 range.

6) Once you've got the fuel screw set, re-adjust your throttle stop screw (idle screw) to an appropriate idle speed.

And that's it! Your pilot circuit is now VERY close to ideal. From here you can experiment with how small adjustments affect low-end (i.e. small throttle openings) response and make adjustments for weather. The hardest part is usually gaining access to the screw while the engine is running. Also, you may really have to listen carefully to detect the rpm changes in the idle. 100 or 200 rpm differences can be tough to detect when the change happens over several seconds.





And lastely, a little bit about the difference between fuel screws and air screws:

Two stoke carbs normally have air screws and four stroke carbs normally have fuel screws. You can tell if a carb has an air screw or a fuel screw by it's location on the carb. An air screw will be on the intake side of the carb while a fuel screw will be on the engine side of the carb.

They sort of work opposite one another. An air screw adjusts how much air is being delivered thru the pilot circuit: in is rich (less air) and out is lean (more air). A fuel screw adjust how much fuel (or air/fuel mixture) is being delivered from the pilot circuit. In is lean (less fuel) and out is rich (more fuel).

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But simply if you turn the fuel screw all in and nothing happen then leave it all in and go to the next, if you hear little change then leave it where you think is the smoothest and go to the next

now my setting looks like 1st carb: 0turn out, 2nd carb 1turn, 3rd: 2turn 4: 2turn. I will have to look the 1st and 2nd carb pilot jets whats going on.

I will put my beer in the fridge... useless girlfriend
make sure you ride safe, peace