720form>function Posted June 1, 2013 Report Share Posted June 1, 2013 Ok, I need some advice. Out of the blue my 720 began to misfire so I started to pull spark plugs and on cylinder 2 both plugs were smashed in. So I am not sure but i am guessing the piston is hitting the plug on that cylinder. Ive checked compression it was Ok and I also removed the rod cap on cly 2 and crank shaft and the bearing didnt seem to have that much wear. What else should i check? Quote Link to comment
wayno Posted June 1, 2013 Report Share Posted June 1, 2013 Maybe some crap broke loose from the top of the piston, and beat the shit out of things in there till it finally exited the cylinder, or beat it's self to powder. 1 Quote Link to comment
MicroMachinery Posted June 1, 2013 Report Share Posted June 1, 2013 Broken porcelain in your cylinder? That's bad... Quote Link to comment
wayno Posted June 1, 2013 Report Share Posted June 1, 2013 Broken porcelain in your cylinder? That's bad... No, I am talking about the shit that builds up on the top of the pistons, carbon or whatever it is, if a big enough piece comes loose, it makes a lot of noise before being pulverized. Quote Link to comment
720form>function Posted June 1, 2013 Author Report Share Posted June 1, 2013 I actually throw new plugs in this morning and within a couple of seconds the plugs were bent so i guessing i going to have to take the head off and see whats up. Quote Link to comment
Ratwagon1600 Posted June 1, 2013 Report Share Posted June 1, 2013 Ok, assuming your are running the correct plugs? Have the plugs got a washer on them? Has the head had the beejeebus machined out of it? If the engine was running ok and this "just happened"' possibly something has gone awry inside. Pulling the head off to fix this sucks, but, it might be the only way to effectively diagnose the problem. Good luck with it. Oh, and post some pics!! Quote Link to comment
datzenmike Posted June 1, 2013 Report Share Posted June 1, 2013 Piston can't hit the plugs on a 720. The reach is wrong (thread depth) or something is rattling around in there. Could be something that fell in the carb or a piece of piston. Before going wild and the trouble of removing the head try replace with proper NGK BPR6ES intake and BPR5ES (cooler) for the exhaust. If you have something other than NGK possibly the wrong plugs were handed to you. Always run NGK. 2 Quote Link to comment
Ratwagon1600 Posted June 1, 2013 Report Share Posted June 1, 2013 Always run NGK. True this, Bosch plugs = pooh. Quote Link to comment
Dawa Posted June 2, 2013 Report Share Posted June 2, 2013 oh hell yeah. bosch supposedly makes japanese cars run like shit. this doesnt apply to you because its an 'all of a sudden' thing, but sometimes people have to cut reliefs in pistons to keep from smashing their plugs. that probably applies to frankenstein motors, high compression motors perhaps Quote Link to comment
WooHooMobile Posted June 3, 2013 Report Share Posted June 3, 2013 I think getting or borrowing one of those remote camera tools and have a look down the spark plug hole, and even measuring the stroke, comparing the cylinders with some sort of thin ruler or stick out the plug holes and turn over by hand. camera like this maybe, http://www.ebay.com.au/itm/Digital-2-4-Video-Car-Inspection-Borescope-Endoscope-10mm-Camera-1m-Snake-Scope-/190849564387?pt=AU_B_I_Electrical_Test_Equipment&hash=item2c6f853ee3&_uhb=1 Quote Link to comment
datzenmike Posted June 3, 2013 Report Share Posted June 3, 2013 With luck it injested something and passed it out the exhaust. Shit travels at over 20K feet per min in there so even a small nut will hit very hard. Quote Link to comment
Dawa Posted June 3, 2013 Report Share Posted June 3, 2013 ^ pretty sure thats what happened to my old head, #3 exhaust valve which is why i couldnt ever get the damn thing to idle right. new carb, rebuilt head, and it was smooth sailing from there. i think it was due to ingestion of a washer used on the exterior of the carb... Quote Link to comment
720form>function Posted June 3, 2013 Author Report Share Posted June 3, 2013 There was a nut the some how got in to the combustion chamber. Pulled it out with a magnet. Quote Link to comment
720form>function Posted June 3, 2013 Author Report Share Posted June 3, 2013 here is a pic of my 720 Quote Link to comment
datzenmike Posted June 3, 2013 Report Share Posted June 3, 2013 With luck it injested something and passed it out the exhaust. Shit travels at over 20K feet per min in there so even a small nut will hit very hard. . At 6K the L20B piston hits speeds of 56 feet per second (not 20K lol) Quote Link to comment
85 Nissan 720 Posted November 30, 2022 Report Share Posted November 30, 2022 85 Nissan 720 z24 Yep same thing happened to me today was just driving along it started skipping and a little bit of Popping remove spark plug from cylinder 3 and 4 spark plug was bent on top I had some correct used ones did the same thing again going to have to take head off it will crank up just fine but it just sits there and skips wish me luck y'all this is my daily driver I really love this truck it has helped me out a lot 1 Quote Link to comment
datzenmike Posted November 30, 2022 Report Share Posted November 30, 2022 3 and 4 had plugs that were hit??? That means multiple foreign objects (FOD) for you air force guys. If plugs replaced and happens again then still in there. Try turning engine till at TDC (look at the valve springs to see if closed) and remove both spark plugs. Use a vacuum cleaner and tape a McDonalds milk shake straw on the end and try to vacuum out the tops of the pistons. 2 Quote Link to comment
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