Back Fire & Stumble Stock 240Z

[Miles]

My stock 72 240Z w/SU carbs stumbles when taking off in first gear then catches and everything is fine through second to fourth gears. Cruise is fine. Problem imalmost goes away once the engine is warmed up. Also, the engine will backfire (popping sound) through air cleaner and it occasionally runs on when the key is turned off. The stumble problem started after I replaced the points and condensor. The engine runs smooth and normal once I get the car through the initial hicup in first.

 

Thanks

 

Miles

[Guest Anonymous]

advance your timing some. Mine starts doing this every once in a while if I havent been watching my ignition timing. As the points wear, the timing gets retarded. This will also solve backfiring probs. As for the car running with the key out, it might be an old diode in the alternator. Do you have the internally regulated alternator swap? this is a common problem after that swap.

[Miles]

I'll try advancing the timing.

 

Thanks

 

Miles

[Mike C]

hiccups out the carbs is usually from being too lean (which is why it does it cold) Also stumbles can be because oil level is low in carbs.

[Miles]

Which way do you turn that nut on the bottom of the SU carb to make the mixture richer? Looking from top down, is clockwise rich and counter clockwise lean? I dialed in max advance and it helped some and increased power some too.

 

Thanks

 

Miles

[Mike C]

It raises and lowers the emulsion tube just like the choke. So screwing it in richens it up, and out leans it. In is clockwise looking from the bottom.

[DAW]

1) You have to ask yourself why the ignition timing is suddenly retarded once you changed the points. The point gap directly determines dwell, which directly affects ignition timing. In your case, you set the gap too narrow (lengthening dwell) and retarded the ignition timing. I know that adjusting the timing helped, but you need to go back and properly set the points, then proceed.

Using a dwell meter is best. Make sure you've put a dab of distributor lube grease (comes with points)on the side of the points' rubbing block, so that there is a constant supply of lube to the dist. cam/rubbing block. Remember, as normal wear occurs, the rubbing block will wear and the gap will narrow. Anyway, set dwell (and not toward the higher limit of the specs), the reset your ignition timing. Bump up the timing a bit more than stock, as this will increase throttle response. If you get a ping (with good gas), pull off the road and slightly turn the distributor (in the same direction as the distributor rotor turns)then drive it and listen for ping. Once it's gone, stake mark the distributor base where the small index marks are, to indicate where you want it in case you have to remove it.

 

2) Before adjusting mixture, set dwell and timing. Then make sure the chokes are off, which means checking at the carbs also to make sure that the choke linkage doesn't have the jet base withdrawn any. You should push up with your finger on each of the tubes (they are in the center of the knurled nuts by which the mixture is set. You can really get lost with idle mixture adjustment, and remember that the adjustment you're making is affecting the entire rpm range re mixture. It's best to raise the jet fully (turn it clockwise seen looking up from the ground/counterclockwise looking down at the carb tops), until it stops back each off the same number of turns to a starting point. Try two turns of the knurled ring. Usually the is a small and large BB pressed in the ring which you can feel and keep track of your turns. Start the car, make sure carbs are synchronyzed (flowing the same air at part throttle and at idle, per Unisyn), set idle rpm screws, then adjust mixture. For performance, tend towards rich rather then lean, and keep track of where you are at re turns of both carbs. Don't get way out there with 3 total turns out on one carb, and 1 1/2 turns out on the other. Better to let it idle a little ragged at first, if that's what it means in order to keep the mixtures similar over the rest of the rpm range. If you still get a lean sag off idle acceleration, you could try lighter piston springs in the SUs, or richer needles. This gets a bit involved, but is well worth it.

3) Re points problems, if you get to the junkyard (on a budget) look for a Petronix type ignition points conversion from a Datsun 6cyl or any other 6cyl. Actually, you can just take the optical unit, wiring and module from any engine, then buy a 6cyl optical shutter from Crane.

[Miles]

DAW

 

Very helpful. Thanks for the info.

 

 

Miles

[DAW]

Quick P.S., alternative to the Crane/Petronix/Allison optical trigger conversion into your distributor, you can take the entire electronic distributor from an '84 Maxima (or earlier, but with the black module box attached to the dist.). Simple bolt-in swap and less hassle than the other way.

DAW