this misfire is becoming a PITA! Help?

[jbush.2010]

So I've searched and searched and searched. For about 3 days straight and have found nothing. My 1991 240sx is a 5 speed with the KA24E. Somebody swapped it before I got it. I'm new to 240's but I do know how to work on cars. I have had it about a week and fixed the first issue of the vacuum leak. Then the next day I had extreme lifter tap. Ended up having to buy a new head because the previous owner stripped out two of the bolts that hold the rocker arm shaft/cam caps down. The plug wires and pluga are new. Dist cap and all the hardware is also new. My issue is the car idles rough and you can hear a definite misfire from the exhaust. Also during light throttle the car misses like crazy but once I. Put more throttle into it, she comes to life and runs great! It gets aggravating because I deliver pizzas and that crusing low throttle is what I use most. There are no vacuum leaks that I have come to find using the method of spraying the engine with carb cleaner. The distributor is advanced all the way forward because its the only way it will run properly. Checked all the plugs yesterday and the are all firing. Any input would be nice! Thanks

[jbush.2010]

Nobody can explain this??

[RHDphil]

sounds like timing is off, check your timing

[jbush.2010]

Like the timing chain or the distributor itself?

[RHDphil]

timing chain, u said you had to get new head so i can almost guarantee thats your problem from what you said

[jbush.2010]

Okay ill check that out. I figured the distributor may also be a tooth off because it is all the way advanced and it only continues to get better if I turned it. If I turn it the other way it has no power at all. I have to take the valve cover off anyways so we'll see

[Cruz_240sx]

check one cylinder at a time to see which one is missing. while car is running pull the spark plug wire and see whitch one has no affect. i would check for a stuck or sticky valve if its a single cam its a common problem. is there any rattle while the car idles? tensinor could be worn out causing the timing to skip

[AsleepAltima]

quick way to check chain timing without removing anything.
remove #1 spark plug and disconnect distributor
crank engine by hand or slowly with key till you are at tdc compression on the #1 piston
easy way to verify: plug the spark plug hole with a rag and then bump the ignition till the rag gets blown out. then, looking down into the cylinder or using a long screwdriver or extension, insert the driver/ext into the cylinder and turn the engine by hand till the piston moves the driver/ext to its peak before it starts to fall again.
now, once you are at tdc compression, look down at the crankshaft pulley and check to see that the pointer is at the "0" mark. once you verify that, remove the distributor cap and check to see if the rotor is pointing at the #1 cylinder spark plug wire position. if it is, then the chain is timed right. it will be very noticeable if its not.

[AsleepAltima]

you can even check ignition timing this way, but it wont be as accurate as a timing light will be. you do this by doing all the above steps, then moving the crank pulley to the 20* mark. then you look at the rotor position and it should be pointing to the #1 spark plug. the above method will get you close to the #1 position but not dead center because of the ignition timing setting.

[jbush.2010]

quick way to check chain timing without removing anything.
remove #1 spark plug and disconnect distributor
crank engine by hand or slowly with key till you are at tdc compression on the #1 piston
easy way to verify: plug the spark plug hole with a rag and then bump the ignition till the rag gets blown out. then, looking down into the cylinder or using a long screwdriver or extension, insert the driver/ext into the cylinder and turn the engine by hand till the piston moves the driver/ext to its peak before it starts to fall again.
now, once you are at tdc compression, look down at the crankshaft pulley and check to see that the pointer is at the "0" mark. once you verify that, remove the distributor cap and check to see if the rotor is pointing at the #1 cylinder spark plug wire position. if it is, then the chain is timed right. it will be very noticeable if its not.

That is just the answer I've been wanting. So let's just say the timing is slightly off by a tooth or so. Is there any easy way to adjust it without taking the whole front cover off? Like maybe take the cam gear off and adjust it that way? I almost feel like it may be just a tooth off. But the part that baffles me is it isn't really rpm dependent, its really just throttle position dependent. About 10% throttle makes it miss, anything above that it runs great, wide open throttle it runs great even if I bog the engine and don't down shift as long as I'm at at least 1/2 throttle its fine

[AsleepAltima]

that sounds like a bad tps voltage setting to me. i made the big mistake once of setting my tps to .045 volts instead of .45 volts and it did exactly what you are describing.

[jbush.2010]

that sounds like a bad tps voltage setting to me. i made the big mistake once of setting my tps to .045 volts instead of .45 volts and it did exactly what you are describing.

Now that is what I thought it was because the issue was throttle related. How do I go about setting this throttle position? The big thing that baffles me is it just occured. Can the tps just started happening.

[AsleepAltima]

in the last 10 years, ive replaced maybe 5 tps's. 1 of them died for no apparent reason.
easy to check too - Throttle Position Sensor Adjustment (http://www.240sx.org/faq/articles/throttle_position_sensor_adjustm.htm)

[jbush.2010]

That was for the DOHC, is it the same for the SOHC?

[AsleepAltima]

its been a while but yes, i think they are the same. the voltage is the same im pretty sure - just check the back of each pin to see which one gives you the voltage.

[jbush.2010]

im hoping it fixes it, im all out of ideas. it sometimes idles up and down when i first start it then it smooths out. when its cold it seems to run better with almost no missing??

[AsleepAltima]

that could be the idle air control valve being dirty or just old. my sr does the same thing sometimes and its freshly rebuilt but is still running the old iacv. have you done a compression check on it yet?

[jbush.2010]

Update! Checked the timing and it was dead on, cleaned the MAF, cleaned the IACV, and cleaned the spark plugs up. Put it all back together and it still ran like shit. So i pulled a vacuum line off on accident and it smoothed out and ran perfect! problem solved.
Also unplugged the tps and nothing changed lol